The demand for refrigeration has substantially increased since 1990s. The refrigerants employed in conventional cooling systems are known to cause ozone depletion and subsequent environmental impacts. Thermoelectric (TE) cooling and magnetocaloric (MC) cooling technologies face some challenges in replacing conventional vapor compression systems due to their relatively low coefficients of performance (COP) and heat transfer efficiency. A combination of TE and MC cooling could be a promising way to improve the heat-exchange efficiency. Recent studies have focused extensively on thermo-electro-magnetic materials that simultaneously exhibit TE and MC properties. These materials include both MC/TE composites and intrinsic thermo-electro-magnetic materials. However, inherent heterogeneous interfacial reactions in MC/TE composites simultaneously degrade both MC and TE performance. Intrinsic thermo-electromagnetic materials as single-phase compounds can effectively avoid such a degradation. Fe₂VAl-based Heusler alloys exhibit promising thermoelectric properties due to their unique electronic structure. The incorporation of magnetic Co atoms is expected to simultaneously enhance the Curie temperature (T_C), and synergistically regulate both MC and TE properties in Fe₂VAl-based Heusler compounds. This study prepared a series of Fe_2-xCo_xVAl alloys via arc-melting and characterized their phase composition, MC properties, and TE performance. This study demonstrated that controlled Co doping could be critical for optimizing the thermo-electro-magnetic properties of intrinsic Fe_2-xCo_xVAl alloys, thus providing an effective material design strategy for solid-state hybrid MC/TE cooling technologies.

A series of Fe_2-xCo_xVAl (x = 0, 0.5, 1.0, 1.5, 2.0) ingots were prepared via arc melting in argon (Ar) atmosphere with high purity Fe (99.99%, blocks), Co (99.9%, pieces), V (99.95%, pieces), and Al (99.99%, granules). To ensure a homogeneity, the ingots were subjected to five remelting cycles with intermediate flipping. The ingots were cut into desired shapes using wire electrical discharge machining for the measurement of thermal and electrical transport properties. The phase structure of the materials was determined by powder X-ray diffraction (XRD, SmartLab, Rigaku Co., Japan). The microstructures were determined by scanning electron microscopy (JSM-IT800, Japan) equipped with a back-scattered electron detector and a Bruker energy-dispersive spectroscopy detector. The lattice constants were refined according to the Rietveld method using the Fullprof program. The Seebeck coefficient (α) and electrical conductivity (σ) were measured simultaneously by a standard four-probe method (CTA-3, Beijing Cryoall Science and Technology Co., Ltd., China) in a helium atmosphere at 300-750 K. The thermal diffusivity (λ) was measured by a laser flash method (Netzsch LFA-467, Netzsch Co., Germany). The thermal conductivity was calculated according to κ = λρC_p, where C_p was the heat capacity according to the Dulong-Petit formula, and the density ρ was measured based on the Archimedes prinicple. The magnetization versus magnetic field (M-H) curves and thermomagnetic (M-T) curves were measured with a multi-VersaLab vibrating sample magnetometer (VSM, VersaLab, Quantum Design INC., USA). The magnetic entropy change curves (ΔS-T) were calculated using the Maxwell relations.

The results indicate that the incorporation of the magnetic element Co into Fe₂VAl Heusler alloy can synergistically regulate its MC and TE properties, showing a potential to become a high-performance intrinsic thermo-electro-magnetic material. The XRD patterns show that the Bragg diffraction peaks of all the samples match the standard cards without impurity peaks detected. The lattice constant increases with the increase of Co doping, indicating the continuous substitution of atoms Fe by atoms Co. The SEM images reveal that Fe, V, and Al are homogeneously distributed in the materials without any impurity phases. The incorporation of Co significantly elevates the T_C of Fe₂VAl-based Heusler alloys to industrially viable ranges, which is consistent with the Slater-Pauling rule predictions and the Bethe-Slater curve validation. At a magnetic field of 2.5 T, the maximum magnetic entropy change (ΔS_max) increases to 1.04 J·kg^-1·K^-1 for the sample with x of 1.5 J·kg^-1·K^-1 and 0.81 J·kg^-1·K^-1 for the sample with x of 2.0. The sample with x of 2.0 achieves an enhanced relative cooling power (RCP) of 110.7 J·kg^-1, benefiting from its significantly broadened full-width at half-maximum (δ_FWHM). The introduction of Co significantly increases the electrical conductivity (σ), shifting a transport behavior from semiconducting to metallic. The σ of the sample with x of 2.0 is the maximum vaule of 4.2×10⁵ S·m^-1 at 300 K. The substitution of Co introduces extra electrons, switching the Seebeck coefficient (α) from positive to negative values, indicating that the majority carriers shift from holes to electrons. The α of the sample with x of 0.5 is the maximum vaule of -68 µV/K at 550 K. The sample with x of 0.5 sample exhibits a peak power factor (α²σ) of 1.14 mW·K^-2·m^-1 at 550 K. The decrease in α²σ at high temperatures is due to the combined effects of reduced electrical conductivity and the decrease in the Seebeck coefficient caused by intrinsic excitation. At 300 K, The lattice thermal conductivity of the sample with x of 2.0 is the minimum value of 4.1 W·m^-1·K^-1. The maximum zT value reaches 0.073 K at 600 K for the sample with x of 0.5, which is 260 times higher than that of the sample with x of 0.

The introduction of the magnetic element Co could synergistically regulate the magnetocaloric and thermoelectric properties of Fe₂VAl Heusler alloy. The sharp increase in T_C could be attributed to the intense Fe-Co exchange interaction. The sample with x of 1.5 had the ΔS_max of 1.04 J·kg^-1·K^-1. The sample with x of 2.0 had the maximum RCP of 110.7 J·kg^-1. The sample with x of 0.5 had the maximum zT value of 0.073 at 600 K. The existing research showed that the T_C could be further increased and a better balance between MC and TE properties could be achieved via optimizing the Co doping content. To further enhance the TE performance without affecting T_C, doping heavy elements at the V site to increase phonon scattering or introducing V/Al antisite defects to improve electrical transport properties could be considered. This work could provide an effective material design strategy for developing intrinsic thermo-electro-magnetic materials based on Full Heusler alloys.

Textured ceramics have attracted much attention in functional and structural applications due to their intrinsic anisotropy and superior physical and chemical properties. Among them, textured transparent alumina ceramics are particularly promising for optical windows, high-power lasers, and harsh-environment engineering components because of their superior mechanical strength, chemical stability, and light transmittance. The templated grain growth (TGG) method emerges as a powerful route to fabricate textured ceramics, as it exploits the preferential orientation of seed crystals to induce large-scale alignment in polycrystalline matrices. However, despite its application in producing highly oriented alumina ceramics with remarkable transparency, the fundamental sintering mechanisms underlying texture development remain incompletely unclear. Conventional TGG research is hindered by two intrinsic challenges, i.e., the high number of template particles leads to frequent collisions, masking the true dynamic growth behavior; and the concurrent coarsening of templates and matrix grains at high temperatures diminishes size advantages, thereby weakening the thermodynamic driving force for template-guided growth. Consequently, the existing models, which largely attribute orientation development to the Ostwald ripening and localized grain boundary migration, cannot adequately explain the experimental observations reported, such as the persistence of abnormal texture alignment even beyond direct template-matrix contact regions. In this work, we systematically investigated the sintering mechanism of textured alumina ceramics via introducing large sapphire single crystals with controlled orientations (i.e., A-(110), C-(001), M-(100), and R-(012)) into high-purity polycrystalline Al₂O₃ matrices. This sapphire template-ceramic composite strategy could enable a direct observation of template-matrix interactions, while avoiding the kinetic distortions induced by template collisions in conventional TGG. In addition, we also analyzed a dual mechanistic framework that could integrate localized template coalescence and long-range stress-induced orientation via employing advanced electron backscatter diffraction (EBSD) and microstructural analysis, thus providing theoretical insights into the texture formation of alumina ceramics.

High-purity α-Al₂O₃ powders (purity > 99.99%, D₅₀ ≈ 200 nm) were used as a matrix material, and MgO was added as a sintering aid in selected experiments. Sapphire single-crystal plates of four orientations (i.e., A, C, M, and R) with the dimensions of 10.0 mm×10.0 mm×0.5 mm were embedded into compacted Al₂O₃ powder beds. Composite green bodies were prepared via uniaxial pressing, followed by cold isostatic pressing at 200 MPa. After a binder was burnout at 800 ℃, the samples were sintered in vacuum at 1300-1860 ℃ for 6-12 h. The microstructures were examined by scanning electron microscopy (SEM), and the crystallographic orientations were characterized by electron backscattered diffraction (EBSD). The sapphire-ceramic composites and dual-crystal systems (A-ceramic-C combinations) both were analyzed to evaluate the microstructural evolution, grain growth distances, and orientation gradients with and without MgO additives.

The results reveal that the embedding of sapphire single crystals induces a distinct microstructural evolution in surrounding alumina grains. For undoped systems, the radial growth distances of A- and C-oriented templates are comparable, reaching approximately 200 μm at 1800 ℃, with microstructures dominated by template-driven consumption of neighboring grains. However, the introduction of MgO significantly reduces the coalescence distances to ~118 μm, even at elevated temperatures of 1860 ℃, and effectively suppresses an abnormal grain growth, highlighting its role in grain boundary pinning and microstructural homogenization. The EBSD mapping unveils a striking divergence in orientation fields around different template planes. C-oriented templates generate orientation gradients extending up to 400 μm, which is far beyond their physical coalescence range. In contrast, A-oriented templates induce alignment only within ~200 μm. This discrepancy originates from anisotropic thermal expansion. In detail, α-Al₂O₃ exhibits the largest thermal expansion coefficient along the C-axis, producing residual stresses during sintering that drive surrounding matrix grains to rotate and align toward the template orientation. As a result, template-induced texture formation encompasses two complementary mechanisms, i.e., localized template-matrix coalescence driven by grain boundary energy minimization, and long-range orientation induction mediated by anisotropic thermal stresses. The dual-template experiments further substantiates this framework. In A-ceramic-C composite systems without MgO, matrix grains between the templates are fully consumed, adopting the orientation of the A-plane due to its higher surface energy and faster growth rate. In contrast, for MgO doping, abnormal grain growth is suppressed, and matrix grains between the templates display a similar preference for A/C-plane orientation. These results reconcile inconsistencies in earlier TGG studies and explain previously puzzling observations, such as the progressive increase in grains with small misorientation angles relative to the plane (006) during high-temperature sintering. Collectively, texture evolution cannot be solely attributed to the Ostwald ripening and template impingement, but must also consider stress-driven long-range orientation effects.

This work elucidated the sintering mechanism of textured alumina ceramics through a sapphire template-ceramic composite strategy. We established a dual mechanistic framework for texture development via systematically analyzing the microstructural evolution at different template orientations, sintering temperatures, and MgO doping conditions. The localized coalescence was obtained via grain boundary energy minimization and long-range orientation induction mediated by anisotropic thermal expansion stresses. The synergistic mechanism of template coalescence and non-contact orientation induction could inherent to the classical TGG. Among different orientations, the C-plane template demonstrated a superior capacity for long-range texture control, extending its influence up to 400 μm, compared to ~200 μm for the A-plane. Besides, MgO additives could play a critical role in suppressing abnormal grain growth and reducing template coalescence distances, thereby enabling a finer microstructural control.

Carbon nanotube fibers (CNTFs) are regarded as a promising candidate material for use in high frequency AC transmission line, electromagnetic shielding cable, antenna, lightweight high-strength composites due to their advantages in low-density, high conductivity and high-strength. Recent advances are made in the fabrication of CNTFs, especially those spun by a wet-spinning technology. However, CNTFs fabricated by a conventional wet spinning method generally have irregular cross-section and non-uniform radial structure, resulting in a rapid double diffusion between chlorosulfonic acid (CSA) in the spinning solution and acetone coagulation bath and a significant radial stiffness gradient in the fiber, thus easily leading to core-shell and wrinkled structures. In this work, we selected N-Methylpyrrolidone (NMP) as a coagulation bath to decrease the diffusion rate between CSA and coagulation bath. Single-wall carbon nanotube fibers (SWCNTFs) with a circular cross section could be continuously spun in NMP. We further introduced ethanol into NMP coagulation bath to control the double diffusion rate and viscosity to improve the draw ratio of SWCNTFs. As a result, SWCNTFs with circular cross-section, high orientation, and desirable electrical and mechanical properties were fabricated.

Single-wall carbon nanotubes (SWCNTFs) with a high crystallinity and a high purity were synthesized by a floating catalyst chemical vapor deposition method. The SWCNTs were put into a beaker, and transferred to a glove box in Ar atmosphere. CSA was then added to the glass bottle, and agitated for 30 min to obtain a liquid crystal (LC) phase. The LC spinning solution was transferred into a stainless steel syringe in Ar atmosphere and extruded through a spinneret into coagulation bath by an injection pump to form macroscopic fibers. An appropriate draw ratio was applied to the SWCNTFs through a drawing device, and fibers were then wound on a PTFE roll and dried under 30 ℃ for 12 h. In this work, all the fibers were spun at a maximum draw ratio that they could endure.

The SWCNTFs spun from NMP coagulation bath have a higher circularity of 0.80, while their alignment and packing density is relatively low due to the smaller draw ratio. We therefore introduces ethanol into the NMP coagulation bath to tune the double diffusion rate to increase the applied draw ratio. The results show that the maximum draw ratio applied to SWCNTFs increases with the increase of ethanol content and reaches to the maximum value of 169% at the ethanol content of 75%. The obtained SWCNTFs have a high circularity of 0.87 and a high orientation Herman factor of 0.987. We analyzes the mechanical behavior of the nascent fiber in the coagulation bath, clarifying how the coagulation bath affects the formation of wet-spun fibers. The results show that optimizing the coagulation bath viscosity effectively reduces a frictional resistance during fiber formation, while an optimized double diffusion rate promotes a denser packing within the surface solidification layer. These synergistic effects collectively enable higher fiber draw ratios and facilitate the development of a highly oriented microstructure with a circular cross-section.

This study investigated the radial structural heterogeneity and surface wrinkling in conventional wet-spun CNTFs via developing a novel NMP-based coagulation bath. The NMP coagulation bath significantly reduced the double-diffusion rate of CSA, enabling the fabrication of SWCNTFs without a core-shell structure, while improving radial structural uniformity. Ethanol was then introduced into the NMP coagulation bath to improve the draw ratio and orientation of the fibers. The optimized SWCNTFs showed a high circular cross-section of 0.87, which was twice more than that of previously reported SWCNTFs (i.e., 0.37). The fibers simultaneously had high orientation Herman factor of 0.987, high mechanical properties with tensile strength of (1047±44) MPa and Young's modulus of (24.0±3.26) GPa, as well as high electrical characteristics with conductivity of (5.74±0.28)×10⁶ S/m and ampacity of (3.99±0.07)×10⁸ A/m². The developed SWCNTFs with a homogeneous structure and a regular morphology could have a promising potential for applications in high-frequency transmission wires and fiber reinforced composite materials.

Despite significant progress in biomimetic material design, the trade-off between performance and energy consumption remains a major barrier to industrialization. Traditional strategies rely on harsh conditions (high temperature, high pressure) or suffer from insufficient mechanical properties and long curing cycles. ELS address these limitations by integrating mild preparation conditions, rapid curing, high strength-toughness synergy, large-scale scalability, and CO₂ sequestration. By combining structural and process bionics, this strategy breaks through traditional performance limits and provides a sustainable solution for biomimetic material commercialization. Future research should focus on optimizing the carbon mineralization reaction efficiency, expanding the range of CO₂-sequestering raw materials, and developing multifunctional composites for extreme environments (deep sea, polar regions) and advanced applications (sustainable infrastructure, carbon-neutral buildings, energy-efficient construction). This integration of bionics and green manufacturing not only advances material science but also contributes to global climate goals and the transition to a low-carbon economy.

The evolution of rheological properties in cement-based materials is governed by a complex interplay of colloidal interparticle forces (i.e., the van der Waals and electrical double-layer forces) and cement hydration, altering the solid volume fraction and C-S-H interactions. While hydration dominates static yield stress growth after the acceleration period, the relative contributions of colloidal versus hydration effects beforehand remain debated. In a macroscopic scale, concrete rheology is determined via a synergy between the paste properties and the aggregate volume fraction. The system transitions from being paste-dominated to governed by aggregate contact forces once a critical volume fraction is exceeded. The plastic viscosity and yield stress are quantitatively described by the Krieger-Dougherty and Chateau-Ovarlez-Trung models, respectively. Solid waste materials further modulate rheology by altering the water film thickness via changes in particle packing and specific surface area and by influencing the yield stress development rate through modifications to surface energy, liquid phase ionic strength, and hydration kinetics. The existing research sufficiently addresses the impact of mix proportions. However, a significant knowledge gap persists for the time-dependent rheological evolution under the complex shear history inherent to 3D printing processes. Future work should thus focus on elucidating the underlying microscopic mechanisms and developing quantitative models that couple shear effects with multiple material parameters to enable a precise rheological control.

Ultra-high temperature oxide eutectic ceramics have superior oxidation resistance, corrosion resistance, microstructure stability, and high-temperature mechanical properties. They are expected to be one of the new high-temperature structural candidate materials, serving for an extended period in high-temperature oxidation-corrosion environments. However, the engineering application of oxide eutectic ceramics faces key bottleneck problems, including the immature forming technology for large-size, complex eutectic ceramic components, the difficulty in inhibiting solidification defects, and low toughness. It is thus necessary to continue to promote the research process of the integration of shape control and property control of large-scale complex components of oxide eutectic ceramics, mainly in the following three aspects, i.e., 1) It is urgent to develop efficient solidification forming technology for large-scale complex ceramic components, and explore the solidification characteristics of high melting point multi-phase oxide eutectic ceramics under high temperature gradient; 2) The effects of process parameters and component size on eutectic structure and solidification defects under non-equilibrium solidification conditions should be explored, and the methods of microstructure refinement and solidification structure inhibition should be established; and 3) It is necessary to clarify the failure mechanism of oxide eutectic ceramics and develop a multi-scale strengthening and toughening technology suitable for high melting point ceramic materials to achieve a synergistic improvement of the strength and toughness of eutectic ceramics.

The focus and difficulties of M/HEC substrate/coating-modified C/C in future work mainly include the following aspects:

1) Efficient and rapid design of anti-oxidation/ablation ceramic components. With the rapid advancement of "Artificial Intelligence+Materials" and the proposal of the "Materials Genome Initiative", an increasing number of computational methods, integrated computing platforms, and databases are developed. This progress enables the use of AI-based high-throughput theoretical calculations (i.e., thermodynamics, finite element, first principles calculations, and molecular dynamics) and experimental verification to establish the M/HEC performance databases and optimize the M/HEC components with the superior comprehensive performance.

2) Construction of substrate/coating and their microstructure control. The structural designs (i.e., M/HEC zoned gradient modification, M/HEC nano-reinforcements, and M/HEC gradient coatings), stress distribution simulations (i.e., at interface and within gradient sublayer), and multi-scale heterogeneous interface control can improve the interface bonding and stress distribution state between M/HEC and C/C substrates, leading to a synergistic enhancement in bonding strength, crack suppression, and ablation resistance.

3) Clarifying the oxidation/ablation protection mechanisms under extreme coupling environments involving thermal, mechanical, and medium factors. A comprehensive service environment evaluation system, such as the plasma or arc wind tunnel, should be used to simulate multi-field conditions, including thermal exposure, mechanical loads (i.e., high-speed airflow and particle erosion), and corrosive medium (i.e., oxygen and water vapor). An in-situ visualization system is used to monitor the long-term service stability of M/HEC substrate/coating modified C/C composites under extreme service environments, which involve ultra-high temperature oxidation/ablation, high- and low-temperature thermal impact, and strong airflow denudation. In addition, multi-scale theoretical calculations and advanced in-situ characterization techniques are conducted to elucidate their oxidation/ablation protection mechanisms.

4) Fabrication of large-sized components and acceleration of their engineering applications. The existing preparation of M/HEC substrate/coating modified C/C composites remains largely confined to laboratory-scale research. Producing large-sized components is a complex process, which requires a large-scale processing equipment and precisely controls over multiple steps to achieve uniform distribution of M/HEC internally and externally throughout the entire structure. It is urgent for overcoming the application bottlenecks of process stability, composition uniformity, stress control and structural/functional integration design of large-sized components to develop simple and efficient preparation techniques.

While visible-spectrum TCFs are mature, IR TCFs still face some challenges in reconciling transparency and conductivity. This review outlines two primary synergistic mechanisms that provide foundational design principles for a family of IR TCFs. However, this field remains nascent. Future development will likely be driven by two key thrusts, i.e., 1) The acceleration of material discovery via high-throughput computational screening, which demands the identification of accurate descriptors, and 2) The development of novel low-loss IR TCFs, characterized by low carrier concentrations and high mobility, which requires a deeper mastery of carrier transport mechanisms. These advancements are critical for enabling next-generation IR modulators, optical communications, and sensing technologies.

The durability of cementitious materials in coastal environments is fundamentally challenged via salt crystallization, and a prevalent deterioration mechanism alters pore structure and degrades transport properties. While total porosity is commonly considered, pore size distribution (PSD) proves more critical. It is indicated that large pores (i.e., 0.1-5.0 μm) facilitate salt solution ingress, while small pores (i.e., <0.1 μm) generate high crystallization pressures. However, the existing models often fail to capture the complex crystallization at wet-dry interfaces in cement-based materials. This study was to develop a comprehensive mathematical framework using log-normal distributions to characterize the initial PSD and model its evolution under salt attack. We established predictive methods for permeability and sorptivity that could account for pore structure evolution, with validation through independent experiments. In addition, this study also analyzed the performance variations under different testing conditions and investigated water absorption behavior during salt crystallization.

The research methodology was founded on theoretical modeling and third-party experimental validation. First, a mathematical model for pore structure evolution was proposed. The initial pore size distribution (PSD) of cementitious materials was described by a composite log-normal distribution function, dividing pores into four families, i.e., large interlayer pores, gel pores, small capillary pores, and large capillary pores. Two distinct pore-filling paths during salt crystallization were hypothesized, i.e., A uniform (U) filling model, where salt deposits uniformly across all pores with a constant corrosion amount, and a non-linear (NL) filling model, where crystallization is concentrated near the wet-dry interface, with a corrosion amount proportional to the initial pore radius. The position of this critical interface was determined based on ambient relative humidity (RH) using the Kelvin-Laplace equation.

Subsequently, a novel intrinsic permeability prediction model (i.e., the Pore Size Distribution-Permeability (PSD-K) model) was derived from the Hagen-Poiseuille equation, explicitly incorporating total porosity, pore tortuosity, and the full PSD. For unsaturated conditions, this model was extended to predict gas permeability via integrating the wet-dry interface parameter r_c. Further refinements accounted for gas slippage effects and the influence of water saturation based on the Van Genuchten-Mualem model, yielding an apparent gas permeability. Finally, a sorptivity and capillary rise height model was formulated via coupling the PSD-K model with capillary pressure described by the Young-Laplace equation. All models were rigorously validated with the published experimental data from various mortar specimens subjected to salt exposure and drying-wetting cycles.

The validation against MIP data from a PC specimen after Na₂SO₄ immersion reveals a porosity reduction from 25.9% to 16.9%. The nonlinear (NL) filling model demonstrates a superior capability in capturing localized pore structure evolution, compared to the uniform (U) model. For a calculated critical radius of 64.9 nm at RH of 96.5%, the NL model accurately predicts a pore modification in the size range of 20-100 nm, dominating transport processes. However, discrepancies occur in smaller (i.e., <20 nm) and larger (i.e., >100 nm) pore regions, primarily due to the oversimplified assumption of uniform hydration product deposition. This indicates a need for future models to incorporate non-uniform hydration effects for improved prediction accuracy.

After the establishment of a reliable pore structure evolution model, we further evaluate its impact on the key transport property indicator-permeability coefficient. The PSD-K model can predict an intrinsic permeability for six mortar groups, having reasonable agreement with gas intrusion data (i.e., 0.66-7.51×10^-17m² for M40-M60). However, the PSD-K and the Katz-Thompson (K-T) models both overestimate water-based measurements by 1-3 orders of magnitude, revealing fundamental test-method dependencies. This discrepancy stems from C-S-H gel swelling during water intrusion, which compresses pore space versus enhanced flow due to gas slippage effects in dried specimens. For unsaturated conditions, the modified model confirms a critical water saturation threshold (i.e., about 0.8), beyond which gas permeability decreases dramatically as a liquid phase blocks continuous pore networks. The PSD-K model effectively tracks the permeability-porosity relationship during salt crystallization, demonstrating the model capability in simulating the performance degradation under chemical attack.

Beyond permeability, capillary water absorption represents another crucial process driving salt solution ingress, whose evolution patterns are similarly investigated. The sorptivity model shows a correlation with the experimental data for mortar specimens with varying slag contents. The results indicate that the sorptivity of GBFS0 mortar (i.e., 39.4% porosity) exhibits approximately four times higher than that of GBFS50 mortar (i.e., 8.4% porosity) due to its greater volume of large capillary pores (i.e., >100 nm) that dominate absorption processes. The results of environmental analysis reveal a negative correlation between RH and sorptivity as a higher initial water saturation reduces capillary driving forces, particularly in finer pores. Furthermore, salt-induced porosity increases significantly an enhanced sorptivity, with this effect being markedly pronounced under high-humidity conditions (RH > 80%) where the wet-dry front occupies larger pores, making absorption more sensitive to pore volume changes. These findings can provide crucial insights for predicting salt damage risks in various service environments.

This study established a theoretical framework for quantifying salt crystallization impacts on cementitious materials. The key findings demonstrated that the nonlinear pore-filling model could predict pore structure evolution, particularly in the critical size range of 20-100 nm, with the MIP data confirming a porosity reduction from 25.9% to 16.9% and crystallization localization at wet-dry interfaces. The PSD-K model could predict an intrinsic permeability, showing a reasonable agreement with the gas intrusion data (i.e., ~7.51×10^-17 m² for M60) and identifying a critical water saturation threshold (i.e., ~0.8) for gas transport. The sorptivity model captured a fourfold difference between high-and low-porosity mortars, revealing that salt-induced porosity increase coupled with a high humidity (i.e., RH>80%) drastically enhanced water absorption. This integrated framework could provide an effective tool for durability assessment of concrete in salt environments.

With the expansion of human activities, the application of cementitious materials has gradually extended from normal air condition to low vacuum environment. For instance, in high-altitude regions, concrete structures are affected by low pressure due to increased elevation. Moreover, for recently proposed the construction of low-vacuum pipeline transportation systems and space bases, concrete infrastructure will also face a challenge of operating under a low vacuum condition. Compared to normal air condition, low vacuum condition can accelerate the evaporation of moisture. This rapid moisture loss can affect hydration and alter pore structure of matrix, thereby posing a serious threat to its long-term mechanical properties. However, the existing studies generally use relatively high air pressure values (i.e., above 50 kPa) and short exposure time when characterizing early-age cement-based materials. Moreover, there is still a lack of consistent conclusions regarding the time-varying laws of the mechanical properties of mature cement-based materials under long-term low vacuum condition, requiring a further research. This study was to investigate the effects of water-cement ratio, curing age, and exposure time on the mechanical properties, mass loss, phase composition, hydration degree, and pore structure of cement-based materials under a low vacuum condition. A multi-factor coupling prediction model considering water-cement ratio and curing age was proposed. This research could hold a great significance of the evaluation and prediction of the mechanical properties of cement-based materials under a low vacuum condition.

Portland cement P·O 42.5 was used. Fine aggregates were medium river sand with a fineness module of 2.96, and coarse aggregates were continuous grading gravels with the diameters of 5-25 mm. In addition, tap water in laboratory was used for blending, and polycarboxylate superplasticizer (SP) was adopted to reduce water. Besides, cement paste, mortar, and concrete specimens were employed at different water-cement ratios (i.e., 0.43, 0.53, and 0.63), respectively.

To investigate the effect of low vacuum condition on the concrete performance at different curing ages, cement paste and concrete specimens with a water-cement ratio of 0.53 were divided into three groups, and cured for 3, 28 d, and 60 d, respectively. All the other specimens were cured for 28 d. For the respective curing ages, the specimens in each group were further split into two groups, i.e., one group was placed in a self-designed low vacuum chamber, while another was stored in a constant temperature and humidity chamber (i.e., temperature: (20±2) ℃, relative humidity: 60%±5%, air pressure: 99.85 kPa). For the low vacuum chamber, the temperature was set to (20±2) ℃, and an automated evacuation process was performed every 6 h at 9-10 kPa. The exposure time was 0, 6, 12, and 18 months, respectively.

The effect of low vacuum condition on the mechanical properties and moisture loss of mortar and concrete after reaching the corresponding exposure ages was determined via testing the dynamic elastic modulus, compressive strength, flexural strength, and mass loss. Meanwhile, the evolution of phase composition, hydration degree, and pore structure of matrix under long term low vacuum condition were characterized by X-ray diffraction, thermogravimetric analysis, and mercury intrusion porosimetry. Finally, a multi-factor coupling prediction model considering water-cement ratio and curing age was proposed to achieve an accurate prediction of concrete compressive strength under a long-term low vacuum condition.

The compressive strength of concrete under low vacuum condition is negatively correlated to water-cement ratio, while positively correlated to curing age. Meanwhile, reducing the water-cement ratio or extending the curing age can improve the compactness of matrix, thereby mitigating the degradation degree of concrete mechanical performance after a long-term low vacuum exposure.

Although long-term low-vacuum exposure does not change the type of hydration products, it accelerates water loss, thus inhibiting a further hydration of matrix. In addition, the hydration degree of matrix increases with the increase of water-cement ratio or the extension of curing age, which corresponds to the variation law of the total amount of main hydration products in the matrix.

Compared with normal air condition, a long-term low-vacuum exposure leads to an increase in total porosity and most probable pore size. Moreover, the total porosity and most probable pore size of matrix under a low vacuum basically decrease with the reduction of water-cement ratio or the extension of curing age.

The compressive strength of concrete under a low vacuum condition conforms to Abrams' Law and maturity theory, with the fitting curve showing a high degree of agreement with the test data (R² = 0.94). In addition, the enhanced database constructed based on this equation shows that the GWO-LSTM model can effectively predict the compressive strength of concrete at different water-cement ratios and curing ages after a low vacuum exposure (i.e., R² > 0.96).

The strength evolution of cement-based materials could exhibit a nonlinear time-dependent characteristic under a low vacuum condition. A short-term exposure boosts strength, while a long-term exposure causes degradation. Besides, the compressive strength of concrete under a low vacuum condition conformed to Abrams' Law and maturity theory. Moreover, compared with normal air condition, a long-term low vacuum exposure could not alter hydration product types but could accelerate water loss of matrix, inhibiting hydration and increases total porosity and harmful pores (i.e., >100 nm). Meanwhile, under a low vacuum condition, the hydration degree of matrix increases at a higher water-cement ratio or a longer curing age, corresponding to the evolution law of the pore structure. Furthermore, the GWO-LSTM model based on enhanced dataset could accurately predict compressive strength of concrete under a low vacuum condition at different water-binder ratios, curing ages, and exposure time (i.e., R² > 0.96).

Saline soils are widely distributed in Xinjiang, China, with locally high SO₄^2- concentrations. Under the complex service environment, conventional cementitious materials are often difficult to effectively resist sulphate erosion, resulting in the insufficient durability. It is thus necessary to enhance their durability through modification techniques. Nanoparticles of calcium carbonate (NC) as an economical, environmentally friendly and high-performance nano-material can provide an effective way to optimize the performance of cementitious materials. The NC promotes the formation of hydration products and improves the microstructure of cementitious materials through microaggregate filling, nucleation, and chemical reaction activity, thus enhancing the performance of the materials. However, the mechanism of NC on cementitious materials under sulfate erosion environment still needs to be analyzed. Also, the diffusion pattern of SO₄^2- in the materials and its correlation with the microstructure lacks a systematic quantitative analysis. In this study, the effect of NC dosages (i.e., 0-3%) on the sulphate erosion resistance of cementitious materials was investigated by a multi-scale method combining macroscopic performance test, X-ray diffraction (XRD), ethylene diamine tetraacetic acid (EDTA) titration and numerical simulation, revealing the modification mechanism of NC and determining the optimal NC dosage.

P·O 42.5R cement and analytical pure NC were used as raw materials to prepare cement mortar specimens at different mix ratios. After 28-d standard curing, the specimens were subjected to wet-dry cyclic erosion tests in 5% sulfate solution. The cyclic system was defined as, soaking for 16 h-drying at 80 ℃ for 6 h-cooling for 2 h, with a 24 h cycle period, totaling 150 cycles. The flexural/compressive strength, mass loss rate, and dynamic elastic modulus of specimens were measured for every 30 d. The phase composition and microstructure were characterized by X-ray diffraction and scanning electron microscopy. specimens The SO₄^2- content at different depths of specimens was determined by the EDTA titration method. Based on Fick's second law, a chemical erosion model was proposed to simulate the SO₄^2- concentration distribution in the specimens for each 30 d period and compare with the measured data for validation.

The impact of calcium carbonate nanoparticles (NC) dosage on the mass loss rate, relative dynamic elastic modulus, and flexural/compressive strength loss coefficients of cementitious materials is investigated under dry-wet cyclic sulfate erosion. At 150 d erosion, the NC1 group with 1% NC dosage exhibits an optimal erosion resistance. The smallest mass loss rate is, the highest relative dynamic elastic modulus will be. The flexural/compressive strength loss coefficients are 19.11% and 23.7% lower than those of the NC0 group (without NC), respectively. This indicates that an appropriate NC dosage (i.e., 1%) can significantly enhance the long-term durability of the materials, highlighting a critical role of NC dosage in the performance optimization. Conversely, NC dosage of exceeding 1% triggers agglomeration effects, creating a more penetration pathways for the erosion medium and leading to a pronounced material degradation. The XRD patterns reveal that NC1 retards the excessive growth of ettringite (AFt), provides additional nucleation sites for C-S-H gels to enhance their stacking density, and forms a unique structure where ultra-high-density C-S-H gels wrap around high-density C-S-H gels. This process optimizes the microstructure and phase composition of C-S-H gels, while modulating the growth orientation of calcium hydroxide (CH) crystals-transitioning from hexagonal plate-like to prismatic morphology. The synergistic effect between crystal orientation optimization and C-S-H gel densification mutually reinforces a multiphase microstructural stability of cementitious materials. The SO₄^2- concentration at the same material depth increases with erosion duration, while it firstly decreases and then increases with increasing NC dosage (i.e., NC0 > NC3 > NC2 > NC1). At 150 d, the SO₄^2- concentration in the 15-20 mm depth of NC1 is 34.3% lower than that of NC0, and the maximum diffusion coefficient (D_max) is only 43% of the NC0 group, demonstrating that a low NC dosage (i.e., 1%) inhibits SO₄^2- diffusion. In contrast, the excessive dosages (i.e., 2% and 3%) weaken this inhibition due to the NC agglomeration. The simulation results show that the SO₄^2- concentration profile of NC1 aligns well with the experimental data. As erosion progresses, SO₄^2- penetrates from the surface to the interior, reducing the concentration gradient, with the internal concentration consistently lower than the surface. Note that the NC1 group maintains lower SO₄^2- concentrations than NC0 at all depths, further confirming a critical role of NC1 in enhancing the material erosion resistance.

The moderate incorporation of NC could improve the sulphate attack resistance of cementitious materials, as evidenced by the reduction of mass loss rate, the increase of relative dynamic elastic modulus, and the significant optimization of the loss coefficients of flexural strength and compressive strength. At the NC dosage of 1%, the optimum material performance could be obtained. The NC effectively optimized the microstructure of the material via inhibiting the excessive growth of AFt, promoting the high density of C-S-H gel, and regulating the growth orientation of CH crystals, as well as significantly inhibiting the diffusion of SO₄^2-, thus effectively improving the durability and mechanical properties of the material. However, the agglomeration effect of nanoparticles led to a significant decrease in the material properties with the further increase of NC dosage.

Low-temperature sulfate attack is a major degradation mechanism in concrete. Unlike conventional sulfate attack dominated by ettringite- and gypsum-induced expansion and cracking, an exposure below 20 ℃ promotes thaumasite as a primary product. Thaumasite directly decomposes calcium silicate hydrate as a principal binding phase in cementitious materials, converting concrete into a non-cohesive mass with a loss of structural integrity. This poses critical challenges for infrastructure in western cold regions and coastal zones where high sulfate levels coincide with low ambient temperatures.

Conventional prevention focuses on mix design. Strategies include densifying the matrix, reducing tricalcium aluminate, and consuming calcium hydroxide with pozzolans. However, the effectiveness is limited and can be even counterproductive at a low temperature. Sulfate-resistant cements that perform under ambient conditions can accelerate thaumasite formation in the cold. Some mineral admixtures also alter a hydration-product stability, producing variable resistance. These limitations motivate alternative protection technologies. Electrochemical approaches show a promising application potential (for instance, electrochemical chloride extraction achieves 30%-60% removal within 4-8 weeks), and the mechanistic differences between chloride and sulfate attack affect applicability.

This work was to investigate a preventive electromigration control method that applied direct current electric fields in the direction opposite to sulfate diffusion, inhibiting a sulfate ion penetration during early service life and weakening subsequent degradation reactions. Unlike conventional material-based methods whose effectiveness depends on cement hydration, electromigration could operate through electric field-driven ion migration control, maintaining effectiveness in low-temperature environments where hydration reactions could be retarded.

Cement-based materials containing 30% limestone powder (by mass) at a water-to-binder ratio of 0.40 were employed. Portland cement P·I 42.5 grade and limestone powder were used. To simulate actual reinforced concrete structures, specimens were embedded with 5 mm diameter stainless steel rods at their centers serving as cathodes for reverse electromigration. After standard curing, specimens underwent electromigration treatment. The electromigration system comprised a direct current power source, electrode system, and electrolyte. External titanium mesh anodes and embedded stainless-steel cathodes formed an electrochemical circuit with specimens serving as electrolytes. The degradation environment utilized 5% mass concentration sodium sulfate solution. The entire experimental process was conducted in a low-temperature chamber at 5 ℃ for 175 d. To investigate electric field intensity effects on sulfate attack control, two voltage parameters were established, i.e., 1.0 V (EF1 group) and 2.0 V (EF2 group). The comprehensive characterization techniques were employed to evaluate the protection efficacy and mechanisms.

The macroscopic performance evaluation demonstrates a significant degradation prevention due to electromigration treatment. Sulfate-immersed specimens exhibit typical low-temperature sulfate attack characteristics including progressive microcracking at 84 d, crack propagation with surface spalling and softened products at 140 d, and severe cracking with extensive spalling at 175 d. In contrast, electromigration-treated specimens maintain a superior appearance integrity throughout exposure, with EF2 specimens displaying only minor microcracks at 175 d, while EF1 specimens show a relatively more pronounced cracking.

The electrical resistivity evolution provides insights into degradation progression and protection effectiveness. Sulfate-immersed specimens exhibit a rapid resistivity increase during the first 60 d primarily due to pore filling by degradation products including gypsum and ettringite, temporarily increasing specimen densification. However, a continued exposure leads to a surface cracking, causing a resistivity growth to plateau after 140 d and declining to 18.63 Ω·m at 175 d as cracks expand and softening products accumulate. The electromigration-treated specimens, particularly EF2, demonstrate a more gradual resistivity increase and maintain relatively stable high values throughout testing, with EF2 retaining the maximum resistivity at 175 d despite minor edge cracking.

The depth-dependent sulfate content distribution reveals an electromigration effectiveness in suppressing sulfate penetration. All the specimens exhibit decreasing sulfate trends from surface to interior, reflecting external-to-internal penetration processes. The 2.0 V treatment achieves a 38.2% reduction in surface layer sulfate content, compared to sulfate immersion, with deeper regions approaching water-cured baseline levels, demonstrating an effective blockage of sulfate penetration for 175 d exposure period.

The thermogravimetric analysis and X-ray diffraction patterns with Rietveld quantification indicate a degradation product suppression. Sulfate-immersed surface layers (i.e., 0-5 mm) contain the maximum ettringite content of 11.95%, compared to water curing, while EF1 and EF2 specimens contain 6.25% and 4.98%, respectively. The calcium hydroxide content analysis reveals a significant consumption in sulfate-immersed surface layers (i.e., 9.27%) versus EF2 specimens (i.e., 12.9%), indicating that electromigration substantially inhibits calcium hydroxide reaction with sulfate ions. The attenuated total reflectance Fourier transform infrared spectra detects a characteristic absorption peak at 625 cm^-1 in sulfate-immersed surface layers, which is similar to silicon hydroxide group vibrations reported for thaumasite, indicating a minor thaumasite formation in severely degraded specimens.

The result by mercury intrusion porosimetry reveals the effect of electromigration on the pore structure evolution. Sulfate-immersed surface layers exhibit 7.6% reduction in cumulative pore volume relative to water curing (i.e., from 0.144 cm³/g to 0.133 cm³/g) due to degradation product pore filling, while EF2 specimens achieve a further reduction to 0.118 cm³/g, demonstrating an enhanced surface densification. The analysis by scanning electron microscopy with energy-dispersive spectroscopy shows a spatial visualization of elemental distribution and microstructural changes. The quantitative line scan analysis demonstrates that electromigration treatment fundamentally alters a sulfate spatial distribution within materials, reducing maximum sulfate content from 9.89% (in mass fraction) in sulfate-immersed specimens to 3.95% in EF1 specimens (i.e., 60.06% reduction) and 2.80% in EF2 specimens (i.e., 71.69% reduction).

This work could validate an electromigration-based control method for low-temperature sulfate attack on cement-based materials. The application of 2.0 V electric field significantly suppressed a sulfate penetration, reducing a surface layer sulfate trioxide content by 38.2% relative to untreated specimens. The electromigration treatment densified a surface layer structure, decreasing a cumulative pore volume to 0.118 cm³/g, while reducing a threshold pore diameter from 101.41 nm to 85 nm. The energy-dispersive spectroscopy analysis demonstrated that electromigration fundamentally altered sulfate distribution within specimens, reducing maximum sulfate content from 9.89% to 2.80%, effectively inhibiting degradation product formation. This proactive electrochemical control technology could offer an effective solution for protecting concrete structures in sulfate-rich environments in western cold regions and coastal areas.

It is essential for understanding sulfate-induced deterioration mechanisms to quantify sulfate ions (SO₄^2-) in cement-based materials accurately. Reliable sulfate measurements are also required for developing corrosion prediction models and for supporting durability-oriented design and maintenance strategies. Conventional analytical methods, including gravimetric analysis and ethylenediaminetetraacetic acid (EDTA) titration, have limitations in highly alkaline and turbid cement extracts that contain multiple interfering ions such as Ca²⁺, Mg²⁺, OH^-, Na⁺, and Cl^-. These characteristics complicate endpoint identification and reduce measurement precision. The difficulty becomes more pronounced when sulfate concentrations are low, which is typical in the early and intermediate stages of sulfate ingress. Therefore, a rapid, accurate and interference-resistant technique is required. This study was to establish conductivity titration as a reliable method for quantifying sulfate in cement-based materials.

The conductivity titration system used was constructed with a DJS series conductivity electrode, a DS18B20 temperature compensation module, an automated peristaltic pump and a microcontroller unit. Barium nitrate (Ba(NO₃)₂) was selected as a titrant. Standard sulfate solutions were first tested to establish the characteristic stages of the titration process. The titration curve of SO₄^2- conductivity could be divided into unsaturated section, crystallization nucleation section, crystal growth section, quasi-reaction end section, reaction end section and infinite titration section. The endpoint was identified by a slope transition between the crystal growth and completion stages.

Optimal operational parameters were identified through repeated tests. These included the appropriate titrant concentration, droplet rate, sample volume and working temperature. Interference-removal procedures were then developed to address the complex ionic environment of cement-based systems. Carbon dioxide (CO₂) was used to eliminate OH^- and Ca²⁺ during free sulfate measurements. A treatment involving silver oxide(Ag₂O), carbon dioxide (CO₂) and hydrochloric acid (HCl) was applied to eliminate H⁺, Ca²⁺ and Cl^- during total sulfate analysis. The accuracy, resolution, and repeatability of conductivity titration were validated using standard SO₄^2- solutions and compared with gravimetric and EDTA titration methods.

The optimized conductivity titration system achieves a minimum SO₄^2- resolution of 0.1 mmol/L, indicating sufficient sensitivity for detecting sulfate ingress in cement-based materials. Clear and reproducible inflection points appear on conductivity-volume curves even at low concentrations, ensuring accurate endpoint identification. For a solution containing 0.01 mol/L SO₄^2-, a titrant concentration of at least 0.033 mol/L Ba(NO₃)₂ produces a slope change greater than 60°, confirming a distinct crystallization transition. The results of repeatability tests verify the stability of the method, and 20 consecutive titrations yield an average titrant consumption of approximately 5 mL with a standard deviation of 0.41.

Interference removal proves an essential for maintaining measurement accuracy. CO₂ and HCl can be used to remove some interfering ions such as OH^- and Ca²⁺ in the test of free sulfate ions. In the acid-soluble sulfate test, HCl is used as a dissolution solution. Ag₂O, CO₂ and HCl can be used to remove a large number of interfering ions such as H⁺, Cl^- and Ca²⁺, and improve the stability of the titration curves. The results of comparative experiments show that conductivity titration provides more reliable results than gravimetric analysis and EDTA titration when samples are turbid, contain multivalent cations or exhibit a unstable color, or when SO₄^2- concentrations vary in a wide range. The gravimetric analysis produces large errors at SO₄^2- contents of below 0.5%, and the EDTA titration is strongly affected by Ca²⁺, Mg²⁺ and turbidity. In contrast, the conductivity titration accurately reflects changes in ionic composition regardless of solution appearance.

The results of tests on sulfate-exposed cement paste confirm that the method can resolve SO₄^2- profiles and diffusion gradients within the material. This ability demonstrates its potential for evaluating the progression of sulfate attack and for supporting durability assessment and corrosion monitoring.

The conductivity titration could provide an accurate and stable quantification of both free and total SO₄^2- in cement-based materials. The optimized titration parameters produced clear and reproducible inflection points on conductivity-volume curves, enabling reliable endpoint identification even at low sulfate concentrations. The pretreatment procedures designed for alkaline and ion-rich cement extracts effectively eliminated OH^-, Ca²⁺, H⁺ and Cl^-, ensuring a high measurement precision. The results of comparative tests showed that conductivity titration exhibited significantly better adaptability than gravimetric analysis and EDTA titration, especially for turbid or multi-ion systems. Application of the method to sulfate-exposed cement paste confirmed its capability to resolve sulfate profiles and identify diffusion gradients. This study demonstrated that conductivity titration could be an effective analytical tool for sulfate determination in cementitious materials and offer a promising potential for improving durability evaluation and corrosion monitoring in concrete structures.

The transport process of moisture in concrete is a physical phenomenon and a medium for the transmission of many harmful substances, having a far-reaching impact on the durability and structural safety of concrete. Moisture flows through pores and cracks in concrete that can provide penetration channels for harmful substances such as chloride ions, sulfate ions and carbon dioxide. Especially in humid environments, moisture promotes the migration of these harmful substances and accelerates the chemical processes such as chloride corrosion caused by chloride ions, thus significantly reducing the durability of concrete. The quantitative study of moisture transport can reveal the migration patterns of moisture in concrete, and identify/quantify various durability problems caused by moisture, providing a theoretical support for the maintenance, repair and life prediction of concrete structures.

In this study, the quantitative principle of moisture transport in mortar was derived based on Beer's law theory, and a triangular prism moisture layer sample mold was designed and conceptualized. Water could be directly infused into the prism, and in the process of X-ray transmission, a superposition model of dry mortar and water layer was formed. The relationship between known water content and X-ray transmittance was obtained by this calibration model, and then the calibration was performed through the change of X-ray transmittance of mortar sample before and after water absorption.

According to the standard GB 175—2007, China, mortar samples with the dimensions of 40 mm×20 mm×60 mm (h×b×d) were prepared with water, sand, and P·O·42.5 ordinary cement. The samples were the samples with different water-cement ratios of 0.4, 0.5, and 0.6 (W/C) (at a cement-to-sand ratio of 1:1), the mortar samples with different cement-sand ratios of 1:1, 1:2, and 1:3 (C/S) (at a water-cement ratio of 0.5), and the mortar samples with different sand particle sizes of 0.5-1.5 mm, 1.5-3.0 mm, and 3-5 mm (at a water-cement ratio of 0.5 and cement-sand ratio of 1:1), totaling 8 groups. Prior to the capillary water absorption test, the calibration model for each sample was measured by X-ray projection, and the X-ray radiography error was corrected for field inhomogeneity. A water calibration curve was then obtained, showing a relationship between X-ray transmittance (-ln(I_w/I_w0)) and moisture thickness, ranging from 0 to the maximum thickness d. Subsequently, a capillary water absorption experiment was conducted. After the absorption experiment, X-ray radiography and weighing were performed to obtain the X-ray transmittance (-ln(I_w/I_w0)) at different water absorption stages, with field inhomogeneity correction and normalization applied. Finally, the mass after water absorption was calibrated via the calibration curve and verified by the weighing results.

The relationship curve between water thickness and X-ray transmittance in mortar samples is obtained, having the fitting results (R²>0.99). During water absorption, the bottom of the sample begins to absorb water, forming a distinct water peak, indicating a one-dimensional water absorption process. As water absorption progresses, the water front gradually moves upward. However, the water distribution during capillary water absorption varies significantly due to the differences in composition and porosity among the mortar samples. The height of the water absorption front increases with time, though the rate of increase gradually slows. The water absorption front disappears when capillary water absorption approaches a saturation after a prolonged absorption period. For the total water absorption of each mortar, the maximum relative error between the calibration and weighing results is 4.8%.

A quantitative method for cement mortar was proposed. This method could allow for the quantification of moisture content in samples with different water-cement ratios, cement-sand ratios, and sand particle sizes through a single calibration process. The method could reduce both cost and time via eliminating a need to prepare different calibration samples. The method could accurately track a moisture transmission in mortar in real-time, capture the distribution and evolution of moisture in various mortars, and enable local quantification of moisture content in different regions of the samples. The experimental results showed that the smaller the C/S and sand particle size was, the better the water absorption uniformity would be. The seepage velocity and height of water surface peak increased with the increase of W/C and C/S. This method could obtain a reliable calibration for different mortar samples. Compared to the weighing method, the maximum relative error of all the samples during the water absorption process by this method could be less than 4.8%.

The utilization of alkali-activated materials represents an effective approach to energy conservation and emission reduction in the construction sector, having a strategic importance for supporting the achievement of "dual-carbon" goals. However, alkali-activated mortar is prone to substantial volume deformation at early stage due to the coupling effect of chemical shrinkage and autogenous shrinkage. In the long term, its insufficient durability adversely affects the service life of repair structures, thereby restricting broader engineering applications. Furthermore, conventional internal curing methods using polyacrylic superabsorbent polymer (SAP) exhibit a poor durability in high-alkali and salt-rich environments. Therefore, this study was to synthesize a modified polyacrylic-based SAP (MSAP) through starch grafting and diatomite incorporation, with optimized design parameters. The influence of MSAP dosage on the rheological properties, strength development, shrinkage behavior, and durability of alkali-activated fly ash-slag mortar was investigated. The findings could provide new methodologies and technical support for controlling shrinkage and enhancing the durability of alkali-activated mortar, thereby facilitating its large-scale green application.

The MSAP was synthesized via aqueous solution polymerization. The key synthesis parameters (i.e., the acrylic acid-to-starch mass ratio, acrylic acid-to-diatomite mass ratio, amount of initiator ammonium persulfate, and amount of crosslinker N,N'-methylene bisacrylamide) were systematically optimized. The water retention and release performance of the MSAP were evaluated by a tea-bag method in different solutions. Multiple batches of mortar samples were casted. Mix proportions with favorable mechanical properties and bond strength were screened, and the optimal internal curing water dosage was determined for the mortar system. Furthermore, the influence of MSAP content (i.e., 0-0.5%) on the properties of alkali-activated mortar was investigated.

The workability (i.e., flowability and setting time) and mechanical properties (i.e., compressive strength) of the mortar were tested in accordance with the JGJ/T 70-2009. The chemical shrinkage was measured by an expansion method based on the standard ASTM C1608-12, while the autogenous volume deformation was determined via a non-contact corrugated tube tester based on the standard ASTM C1698-00. Internal relative humidity development within the mortar for 0-28 d of curing was monitored by HKT-XY humidity sensors. The capillary pressure was recorded by a capillary pressure sensing system. The microstructural and pore structure characteristics were examined by field emission scanning electron microscopy and mercury intrusion porosimetry.

The MSAP samples synthesized under different parameters all exhibit a rapid liquid absorption within the first 30 min in deionized water, followed by a gradual decrease in absorption rate until reaching equilibrium after 120 min. Compared to deionized water, the absorption capacity of MSAP in 0.9% NaCl solution is significantly reduced. A commercial SAP shows an absorption ratio of only 23 g in NaCl solution, indicating its inadequacy for high-salinity environments. The modified MSAP developed in this study achieves a markedly improved absorption ratio of up to 110 g/g. In the dry state, MSAP particles display a dense network structure with a low light transmittance. Microstructurally, the particles exhibit irregular morphologies with abundant pores, wrinkles, and fine granular protrusions on the surface.

The MSAP demonstrates significant liquid absorption and release behaviors in the pore solution of alkali-activated mortar, helping to maintain a high flowability (>60 mm) even after 60 min. It delays the hydration reaction rate and inhibits the early formation of gel frameworks, resulting in prolonged setting time across mixes with different MSAP contents. For longer curing periods, however, the hydration degree improves and the microstructure becomes denser. The incorporation of MSAP at appropriate dosages (i.e., <0.3%) does not significantly compromise the compressive strength. Sample SO1 with 0.1% MSAP reaches a 28 d compressive strength of 52.4 MPa, which is slightly higher than that of the reference group.

The chemical shrinkage of alkali-activated mortar shows a multi-stage evolution, i.e., initial expansion, accelerated shrinkage, release-compensation, and steady phases. A greater MSAP content suppresses an early-age chemical shrinkage. However, at excessive dosages (e.g., 0.5%), notable negative volumetric effects emerge at later stages, with compensation rates varying widely from -77.4% to 73.6%.

A non-linear positive correlation occurs between MSAP content and autogenous shrinkage compensation, exhibiting a threshold effect. At 0.2% MSAP, the development of capillary pressure is significantly delayed, the autogenous shrinkage compensation rate reaches 52.2%-63.1%. Beyond 0.3% MSAP, the sensitivity of compensation efficiency to further dosage increases diminished, stabilizing within 65%-85%.

The physical filling effect and hydration promotion by MSAP increase a fluid flow resistance, thereby reducing the relative permeability coefficient and slowing moisture diffusion. At 0.2% MSAP, the relative permeability coefficient decreases to 4.29×10^-12 m/s, which is approximately 40.0% lower than that of the control group. Similarly, the chloride diffusion coefficient of sample S02 decreases to 6.0×10^-12 m²/s. Conversely, a greater MSAP content leads to interconnected pore networks and localized macro-cracks and voids, adversely affecting the durability of the mortar.

The optimal preparation parameters for starch-grafted/diatomite-modified polyacrylic acid MSAP involved the mass ratio of acrylic acid to starch of 6, the mass ratio of acrylic acid to diatomite of 10, the amount of initiator APS of 0.15%, and the amount of crosslinking agent MBA of 0.05%. An appropriate dosage of SAP particles effectively promoted the hydration process of alkali-activated gel products through the slow release of internally stored water, thereby improving the pore structure and enhancing both mechanical properties and durability. Meanwhile, MSAP could supply the consumption of free water in the system via releasing absorbed moisture, thus mitigating the development of capillary pressure within the alkali-activated mortar and reducing its autogenous shrinkage.

The existing Portland cement concrete cannot ensure the long-term service life of tunnel engineering in high geothermal and high salinity environments. This work was to adopt a calcined clay limestone cementitious (LC³) system as an alternative material and the microstructural evolution mechanism of LC³ mortar under high geothermal conditions was investigated. A chloride ion transport model for thermal damage accumulation and adsorption-fixation effects was established, and the accelerating effect of thermal damage degree on chloride ion penetration was revealed.

Raw powders included cement, limestone powder, calcined clay, gypsum, and fly ash. The cement was P·II 52.5 grade Portland cement. The limestone powder consisted of ≥90% CaCO₃. The calcined clay was produced from a clay with a kaolinite content ≥40%. The gypsum was dihydrate gypsum with a purity ≥90%. The fly ash was Class F fly ash. The aggregate was machine-made sand from tunnel muck. Mortar specimens were prepared according to the standard GB/T 17671—1999, China. The LC³ system contained 55% cement, 30% limestone powder, and 15% calcined clay. The specimens were cured under different conditions (i.e., 20 ℃-95%RH, 65 ℃-10%RH, and 105 ℃-10%RH). The microstructure was characterized by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The total and free chloride ion contents of the specimens after immersion in 3% NaCl solution were determined. A chloride transport model incorporating temperature gradient and the Langmuir nonlinear adsorption was proposed, and the related results were simulated by a software named COMSOL Multiphysics.

The results show that a high geothermal exposure leads to significant microstructural deterioration of the LC³ mortar. The porosity increases to 45.14% (i.e., an increase of 40.36%) and the average pore diameter increases to 115.25 nm (i.e., an increase of 356.25%) at 105 ℃, compared to those at 20 ℃. The XRD patterns and the NMR analyses indicate a decomposition of hydration products like portlandite (CH) and ettringite (AFt), and changes in aluminum coordination, reducing the structural stability and chloride binding capacity. The LC³ system demonstrates a superior resistance to chloride ion penetration, compared to plain cement and binary systems (i.e., limestone, calcined clay, or fly ash). At 105 ℃, the total chloride content in LC³ is only 83.9% and the free chloride content is only 73.6% of those in the plain cement system. Based on the prediction by the proposed chloride transport model, an increase of 92% in the total chloride content at 2.5 mm depth at 105 ℃, compared to that at 20 ℃, confirming the promoting effect of high geothermal conditions.

High geothermal conditions could deteriorate the pore structure of LC³ mortar and reduce its resistance to chloride penetration. The LC³ system exhibited the superior performance against chloride ion transport, compared to traditional cement and binary systems under high geothermal conditions, due to the synergistic effect of limestone and calcined clay. The chloride concentration decreased non-linearly with depth, and elevated temperature significantly accelerated the chloride diffusion due to the pore structure coarsening and reduced binding capacity.

It is vital to maintain the durability of construction materials since sulfate attack is a primary factor influencing the lifespan of concrete in marine setting. Sulfate ions (SO₄^2-) degrade concrete via attacking the calcium silicate hydrate (C-S-H) that is the primary hydration product of cement and the binding phase in concrete. This process primarily occurs through calcium leaching, where Ca²⁺ reacts with sulfates to form gypsum, altering pore structure and weakening mechanical properties. C-S-H exhibits a "long-range disorder, short-range order" molecular structure, consisting of nano-scale particles. It is indicated that sulfate-induced degradation involves a competition between SO₄^2- and silicate ions (SiO₃^2-) for Ca²⁺, leading to a material destabilization. The simulations by molecular dynamics (MD) and quantum chemistry (QC) provide insights into this phenomenon, but they have limitations, i.e., MD lacks chemical reaction accuracy, while QC is restricted in a scale. Understanding the molecular interactions governing Ca²⁺ transfer from SiO₃^2- to SO₄^2- is essential for decoding sulfate-induced degradation. However, the existing experimental and computational methods struggle to capture this process comprehensively. A more effective approach is required to accurately simulate chemical reactions and address the dynamics of Ca²⁺ transport in C-S-H degradation.

This study was to construct a C-S-H model based on tobermorite's unit cell structure, optimizing elemental balance to achieve a Ca/Si ratio of 1.5. The energy minimization was performed by the GFN-xTB method, with the model embedded within a 13 Å water sphere to simulate aqueous conditions. The simulations by Born-Oppenheimer Molecular Dynamics (BOMD) employing the GFN-xTB method investigated the C-S-H decalcification under sulfate attacks. Two 100 ps NVT ensemble simulations at 298.15 K were conducted, i.e., one in pure water and another in sulfate solution (with one SO₄^2- molecule analyzed for Ca²+detachment). The atomic structures were visualized using VMD. The calculations based on Complementary Density Functional Theory (DFT) (Gaussian 16) focused on critical decalcification steps, isolating structural regions (SO₄^2-, desorbed Ca²⁺, and Si—O tetrahedra). The PBE0 hybrid functional and def2-TZVP basis set were used for single-point energy calculations, with DFT-D3 dispersion correction addressing weak interactions.

This study investigates the decalcification kinetics and structural evolution of C-S-H under sulfate attack using molecular dynamics simulations. In pure water, C-S-H experiences minor structural changes, with surface calcium ions partially dissociating but remaining adsorbed. However, in a sulfate solution, calcium ions desorb rapidly upon sulfate introduction, leading to a structural fragmentation, including Q² to Q¹ transitions and severe silicate chain distortions. The analysis by reduced density gradient (RDG) reveals sulfate's stronger interaction with Ca²⁺, compared to silicate, facilitating calcium extraction. The analysis by electron localization function (ELF) shows a weak Ca—O_Si bonding, while Ca—O_Sul exhibits a stronger electronic localization, promoting calcium displacement. The analysis by bond critical point (BCP) indicates non-covalent interactions drive decalcification, with competitive binding between sulfate and silicate.

The analysis by electrostatic potential (ESP) indicates an increased C-S-H electronegativity post-decalcification, destabilizing the structure. The energy barrier calculations show that sulfate significantly lowers decalcification and silicate chain fracture barriers, weakening C-S-H integrity. Water further amplifies sulfate-induced degradation, enhancing polarity and self-expansion effects. These findings demonstrate that sulfate corrosion accelerates C-S-H structural failure through enhanced calcium extraction, increased electronegativity, and silicate chain fracture, ultimately compromising mechanical properties.

This study explored C-S-H decalcification and degradation under sulfate attack using quantum chemistry and DFT. Sulfate ions could extract calcium via the Coulomb interactions, confirmed by RDG and BCP analysis. The decalcification could destabilize C-S-H, increasing silicate chain electronegativity, expanding Si—O bonds, and reducing dissociation energy. Silicate chain fractures caused C-S-H gel collapse, weakening cementitious materials. Water could facilitate a degradation via lowering energy barriers and enhancing hydrogen bonding with silicate chains, accelerating structural breakdown. These findings could reveal the fundamental mechanisms of sulfate-induced C-S-H deterioration, thus providing insights into material performance under aggressive environments.

In marine engineering, reinforced concrete corrosion and deterioration in tidal zones are severe. It is thus important for marine concrete subjected to cyclic wet-dry conditions in tidal environments to investigate structural service life prediction. Corrosion of steel bar caused by chloride ion intrusion is one of the most important factors affecting the durability of marine concrete, and it is also a key to investigating the life prediction of concrete in tidal zone, while the influence of carbonation on chloride ion erosion cannot be ignored. In this study, the data were incorporated into a chloride ion ingress model considering carbonation effects on ion transport via statistically analyzing and fitting tidal cycle data from a coastal area in the Southeastern China. A probabilistic service life prediction model was established based on the coupled carbonation-chloride interaction, using critical chloride concentration for steel bar corrosion as the boundary condition. The corrosion probabilities of reinforced concrete at different elevation levels were investigated and compared with actual engineering data.

According to the statistical data, China, the tidal level information of the southeast coast was counted, and the tidal equation was obtained. The chloride diffusion coefficient D of concrete with different carbonation degrees was determined by RCM. According to the standard GB T50082—2009, China, the carbonation depth Δl and the change rate of chloride ion diffusion coefficient ΔD were calculated at 0, 3, 7, 14, 21 d and 28 d, respectively. To investigate the critical conditions of carbonization and steel corrosion, 5 groups of solutions with different pH values were prepared, the specimens after demoulding were immersed in the solution, a certain amount of NaCl was dripped into each solution for every 48 h, and the rust was detected regularly. Based on Fick 's second diffusion law, Darcy 's convection law and the model proposed, a life prediction model of chloride ion concentration C related to time t was obtained. The results of durability life prediction of reinforced concrete under different carbonization degrees were calculated by random sampling, and the Weibull probability model was fitted to analyze the influence of different carbonization rates at high and low water-binder ratios. The dry-wet ratio at different height positions was counted after setting H=[1,6], the influence of different height positions on life was analyzed. According to the test report of a bridge in the southeast coast, the chloride ion diffusion coefficient D at different heights of the bridge was calculated, and the 30 a corrosion rate of the calculated value of the model and the actual value of the pier was counted, and the height development law and redundancy of the calculated value and the actual value of the bridge model were analyzed.

The law of tidal change at different heights belongs to the typical harmonic. The fitted tidal level equation solves the dry-wet ratio data closer to the reality. The low water-binder ratio concrete' D firstly decreases, and then increases and decreases, and high water-binder ratio concrete' D firstly decreases and then increases with the increase of carbonization depth. Compared with the carbonation depth of 0 cm, the final carbonation can reduce low water-binder ratio concrete' D, but increase high water-binder ratio concrete' D.

The life prediction results of high water-binder ratio are mostly concentrated in 30-40 a with 0% carbonization. The frequency and distribution of life expectancy are approximately a normal distribution, and the 95% life expectancy distribution is 21.9 a. The predicted service life of low water-binder ratio concrete is longer than that of high water-binder ratio, the predicted service life of high water-binder ratio concrete is mostly between [25, 30], and its 95% predicted service life is reduced to [0, 19] with 50% carbonization. Compared with 0% carbonization, the predicted life of low water-binder ratio with 50% carbonization increases. The life prediction of concrete is gradually shorter, and the predicted life with 95% reliability is 22, 17 a and 7 a, respectively, as carbonization increases. However, the 100% carbonization exceeds the thickness of the protective layer, and the predicted life decreases significantly. The predicted life of low water-binder ratio concrete with 100% carbonization is greatly reduced, but it is nearly 3 times higher than that of high water-binder ratio concrete.

The high water-binder ratio' D firstly decreases and then increases, the low water-binder ratio' D decreases continuously, and the dry-wet ratio firstly increases and then decreases, the predicted life of high water-binder ratio concrete firstly increases and then decreases, and the predicted life of low water-binder ratio concrete firstly decreases, and then increases and decreases as the height position H increases. The durability trend of the two in the range of 2-5 m in the middle of the tidal zone is opposite.

Compared with the data of a bridge in the southeastern coast for the carbonization and concrete strength grade, the carbonization degree is far from the steel bar. In the case of 30 a service, the corrosion rate is 13.872%-17.386%, and the average corrosion probability is 15.5%. The relative risk coefficient of steel corrosion firstly increases and then decreases with H. The calculated value of the bridge model is consistent with the actual value, and the calculated value shows a high redundancy.

The tidal variation patterns at different elevations belonged to typical harmonic wave types. Carbonation reduced chloride diffusion coefficients in low water-binder ratio concrete but increased it in high water-binder ratio concrete. The initial carbonation had a minimal impact on the reinforced concrete durability, but the concrete durability decreased rapidly when carbonation penetrated the protective layer. The service life of high water-binder ratio concrete showed an initial increase and a subsequent decrease, while low water-binder ratio concrete exhibited a sequential decrease-increase-decrease pattern as the height increased. A comparison of actual engineering data and model calculation results indicated that the computed values of the model could be close to actual steel corrosion probabilities, and the relative risk coefficients of corrosion probabilities at different heights were consistent with statistical results from engineering practice, demonstrating a reasonable simulation effectiveness of the model.

Chloride-induced corrosion of steel reinforcement can severely damage the reinforced concrete in marine and saline environments, specifically accelerating steel depassivation, electrochemical corrosion, and structural weakening. To deal with these challenges, the microbially induced calcium carbonate precipitation (MICP) technique emerges as a promising self-healing solution for concrete. The existing experimental investigations are predominantly limited to post-healing strength recovery and durability enhancement. However, the chloride ingress mechanism under working conditions remains insufficiently explored, particularly regarding the temporal coupling between crack closure and crack propagation process. Furthermore, a dual inhibitory effect induced by crack closure is also identified, which simultaneously impedes chloride penetration and restricts the transport of healing agents (e.g., urea, calcium sources). In this work, two numerical modules were proposed. The crack self-healing behavior as well as chloride diffusion process were simulated by a lattice network model. The inter-module coupling was implemented by a diffusion coefficient model dependent on crack width. The inhibition effect of crack healing on chloride diffusion within microbial sample was quantitively analyzed by the proposed model.

A lattice network model discretized by Voronoi diagram was adopted to simulate the diffusion of self-healing component (i.e., urea) and chloride in microbial samples. The governing equation for urea hydrolysis kinetics could follow Fick's second law with an extended term of ureolysis rate. The calculated mass of calcium carbonate was obtained from urea concentration profiles and then used to iteratively update crack healing ratios. Based on this foundation, chloride diffusion was subsequently simulated by means of the dynamically adjusted diffusion coefficients that could correspond to the changing crack widths. For numerical implementation, a cylindrical microbial mortar specimen (φ 30.0 mm×50.0 mm) containing a parallel crack (20.0 mm×0.4 mm) was modeled. To systematically evaluate the chloride inhibition effects of MICP and reproduce the chloride concentration distribution under temporal coupling issues, three working conditions were defined, i.e., post-healing chloride diffusion (PHD), simultaneous healing-diffusion (SHD), and sustained cracking with healing-diffusion (SCHD).

During the early healing period (i.e., 10 d and 30 d), no significant variation in urea concentration distribution appears within cracks. This is generally believed to originate from the dynamic equilibrium between urea supply and mineralization consumption during the initial healing stage. However, after 73 d healing, a sharp decline in urea concentration occurs in the 0-0.5 mm region near the crack surface. The MICP-driven crack closure is likely a primary mechanism for the inhibition of chloride diffusion. The results of crack healing ratio and diffusion coefficient along crack depth show that the increase in crack healing ratio within the 0-5.0 mm range results in a reduced diffusion coefficient. At 73 d, cracks within the 0-1.0 mm range reach the 0.08 mm threshold defined in diffusion coefficient versus crack width model, leading to a linearly decreased diffusion coefficient as crack closure develops. In the 0-0.5 mm region, the numerical results of crack healing ratio ranging from 96.2% to 99.9% indicate that the surface crack is nearly closed. The MICP-based crack healing is able to inhibit the ingress of Cl^-. Under the PHD condition at 10, 30, and 73 d, a stepwise decline in Cl^- concentration distribution occurs along the crack depth, even for the Ref group at 73 d. Within the 0-5.0 mm range, MICP-induced crack healing causes a rapid decrease in Cl^- concentration reduction, and this range corresponds to the predefined mineralization reaction zone. For the zone ranging from 5.0-20.0 mm, the concentration gradient gradually flattens, which is attributed to accelerated Cl- replenishment facilitated by high diffusion coefficients. At depths of exceeding 20.0 mm, the migration of Cl^- from the crack region to the concrete matrix leads to an abrupt reduction in diffusion coefficient, steepening the concentration gradient. The results of quantitative analysis reveal that after 73 d healing, Cl^- concentrations at 10 mm and 20 mm depths are reduced by 47.65% and 58.29%, respectively, compared to the Ref group. The sustained cracking also compromises the healing efficacy and hence elevates Cl^- concentration. At the initial healing stage, the SHD group exhibits significantly higher Cl^- concentrations than other groups due to wider cracks and lower healing ratios. As healing progresses, Cl^- concentration under SHD condition gradually approaches that in PHD group. Despite the smaller initial crack width for SCHD condition, the sustained cracking greatly increases the diffusion coefficient in crack. At the crack tip (i.e., 20.0 mm depth), Cl^- concentration in SCHD group increases from 0.0062% at 10 d to 0.1396% at 73 d, exceeding the final concentration in SHD group (0.1131%) by 123.49%.

This work proposed a numerical framework via integrating crack healing and chloride diffusion modules to address the experimental limitations in analyzing temporally coupled processes of sample cracking, microbial self-healing activation, and chloride ingress. The permeability of cracked microbial sample was significantly inhibited by MICP-driven crack closure. Under the PHD condition, Cl^- concentration distribution exhibited a progressive attenuation along the crack depth, with reductions of 47.65%(i.e., 10.0 mm) and 58.29% (i.e., 20.0 mm) compared to the Ref condition. Moreover, in shallow surface regions (0-0.5 mm), the chloride diffusion coefficient was restored to levels comparable to intact matrices. However, the sustained cracking could have a compromise microbial self-healing efficacy. Under the SCHD condition, Cl^- concentration at the crack tip increased from 0.0062 wt%(10 d) to 0.1396% (73 d). These results demonstrated that the MICP technique could be suitable for non-structural cracks induced by environmental stresses.

In the saline soil environments of the western China, bridge piers often experience "root rotting" in the adsorption zone about 50 cm above the ground/water surface due to dual sulfate erosion. This phenomenon manifests as severe spalling and cracking, significantly shortening the service life of the bridges. It is thus urgent to take effective measures in the section to retard sulfate transport under capillary action and enhance the durability of concrete. Physical coating layers such as epoxy resin can effectively block sulfate ingress into the piers, but a long-term exposure to ultraviolet radiation and high temperatures can lead to aging and failure. Although fiber-reinforced composites can isolate erosion and restrain concrete, they exhibit a poor high-temperature resistance and may release toxic gases in case of fire. In contrast, steel tube-confined concrete improves load-bearing and seismic capacity and effectively inhibits expansion cracking of concrete caused by sulfate erosion. This study was to investigate the influence patterns and mechanisms of sulfate ion transport in steel tube-confined concrete. The effect of steel tube confinement on the sulfate ion transport behavior in concrete was investigated at different steel tube wall thicknesses, C₃A contents, and sulfate concentrations. In addition, a corresponding sulfate ion transport model was also established.

Specimens were cast using cylindrical molds made of PVC and steel tubes with the dimensions of φ100 mm×50 mm. After 24 h demolding, the specimens were standard-cured for 28 d. After curing, the specimens in PVC molds were removed, and their sides were sealed with epoxy resin to serve as unconfined specimens, ensuring one-dimensional sulfate ion transport.

All the specimens were immersed in a Na₂SO₄ solution (replaced every two weeks), and radial expansion and ion content were measured at specified ages. The radial expansion of confined and unconfined concrete specimens was determined by a ±0.001 mm electronic micrometer (Sanliang Co., Japan). The total sulfate ion concentration in the concrete was characterized by an inductively coupled plasma atomic emission spectrometer (ICP-AES). The pore structure of sulfate-eroded concrete was determined by a low-field nuclear magnetic resonance (LF-NMR) technology (Suzhou Niumag Analytical Instrument Co., Ltd., China)

Compared with unconfined concrete, steel tube confinement reduces the sulfate ion content and diffusion coefficient in concrete. Increasing the C₃A content, sulfate concentration, and steel tube wall thickness further decreases the sulfate ion content and diffusion coefficient in steel tube-confined concrete. This is because sulfate erosion causes a concrete expansion, and the confinement effect of the steel tube restrains this expansion, putting the steel tube in tension and the concrete in compression, effectively inhibiting expansion cracking of the concrete. This reduces ion transport channels and delays ion migration into the interior. A larger wall thickness (i.e., a higher steel ratio) provides a stronger circumferential confinement, thereby increasing concrete compactness and inhibiting ion ingress. However, the improvement in confinement stress due to wall thickness is limited, thus slightly enhanceing its hindering effect on ion diffusion. As sulfate concentration and C₃A content increase, the formation of expansive sulfate products such as ettringite increases, leading to a higher confinement stress. This effectively inhibits expansion cracking caused by erosion, thereby slowing down ion diffusion.

Under sulfate erosion, steel tube confinement effectively inhibits expansion cracking of concrete caused by expansive products such as ettringite. As the erosion age increases, the increase in sulfate ion concentration significantly accelerates the chemical erosion process, promoting the formation of gypsum crystals. This reaction consumes calcium hydroxide in the system and triggers the decalcification and decomposition of calcium silicate hydrate (C-S-H) gel, leading to a noticeable discoloration of the paste structure. As the stability of hydration products decreases, the microstructure of the material deteriorates, manifested by an increase in the proportion of harmful pores and total porosity. The evolution of the pore structure further alters transport pathways, forming fast channels for ion migration, ultimately enhancing the transport efficiency and erosion diffusion range of sulfate ions within the concrete. Based on this mechanism, a time-varying porosity sulfate ion transport model for concrete considering the effect of steel tube confinement can be established.

Under sulfate erosion, the steel tube could restrain a concrete expansion through the confinement effect, putting itself in tension and the concrete in compression, thereby inhibiting cracking. Steel tube confinement could reduce sulfate ion content and diffusion coefficient in concrete. Increasing C₃A content, sulfate concentration, or steel tube wall thickness further decreased sulfate ion content and diffusion coefficient.

A time-varying porosity-based sulfate ion diffusion model was established, accurately predicting sulfate ion concentration in steel tube-confined concrete under sulfate erosion.

Concrete is a fundamental material for infrastructure in coastal and saline-alkali regions, which persistently faces a severe challenge of coupled chloride and sulphate attack. Chloride ingress induces steel corrosion, while sulphate ions lead to concrete cracking and mechanical degradation through the formation of expansive products. Under real service conditions, dynamic variations in temperature and humidity significantly affect the transport and reaction processes of these aggressive ions within concrete, resulting in complex competitive interactions between chloride and sulphate ions. Although previous research explored the transport behavior of single ions under constant environmental conditions, the synergistic migration mechanisms and competitive effects under combined salt, temperature, and humidity multi-field coupling remain inadequately understood. It is thus crucial for accurately assessing the durability and service life of concrete structures in complex environments to establish a comprehensive transport model that integrates temperature, humidity, ionic competition, and material damage.

A chloride-sulphate ion transport model was proposed based on multi-species transport theory, explicitly accounting for temperature and humidity variations, ionic competition, and the evolution of material microstructure. In the model, ionic diffusion coefficients were modified via introducing a temperature-dependent function and a humidity-dependent function. A competitive correction factor was also introduced to characterize the competition between ions for diffusion pathways. Within the reaction term, a kinetic parameter was incorporated to describe the decomposition of Friedel's salt (Fs) by sulphate ions, representing the dynamic two-way competitive reaction. Furthermore, the model could account for the volumetric expansion induced by the formation of ettringite (AFt) and Fs and its subsequent impact on the pore structure and transport properties, coupled with a damage function to quantify the acceleration of transport due to cracking. Besides, natural immersion tests were conducted on mortar specimens under various conditions. The concentrations of chloride and sulphate ions at different depths were measured. Numerical simulations were performed using a software named COMSOL Multiphysics, and the simulated results were compared with the experimental data.

The model validation demonstrates a good agreement between the simulated results and experimental data with relative errors for chloride and sulphate ions of below 0.32% and 0.55%, respectively. This indicates the model's applicability under complex salt-temperature-humidity conditions. The further numerical simulations also reveal some key findings. The concentrations of both free and total chloride ions increase significantly as the upper limit of the temperature increases from 283 K to 308 K. The peak values of the competitive interaction-induced nett free chloride concentration [Δc(Cl-F)] and nett total chloride concentration [Δc(Cl-T)] reach 266.55 mol/m³ and 290.91 mol/m³, respectively, indicating that elevated temperatures intensify the competitive interaction between ions. The response of sulphate ions to temperature is less pronounced, without an increase in free concentration, although the total sulphate concentration increases with temperature. High humidity conditions (i.e., RH = 1.0) markedly promote chloride ion diffusion and the formation of corrosion products. The peak values of Δc(Cl-F) and Δc(Cl-T) increase with rising humidity. In contrast, the effect of humidity on the nett free sulphate concentration [Δc(S-F)] is relatively minor. However, humidity significantly affects the distribution of total sulphate ions, resulting in a deeper and wider peak for nett total sulphate concentration [Δc(S-T)] under a high humidity. An increase in the environmental SO₄^2-/Cl^- ratio reduces the penetration depth of chloride ions, but significantly increases Δc(Cl-F), up to 259.39 mol/m³. Conversely, increasing the Cl^-/SO₄^2- ratio reduces the sulphate ion concentration and intensifies the competitive effect on Δc(S-F), although the magnitude of this increase is lower than that occurred in the chloride system. For a 40 mm cover thickness, increasing the temperature from 283 K to 308 K reduces a predicted service life due to sulphate attack and chloride-induced corrosion from 22 a and 43 a to approximately 6 a and 7 a, respectively, compared to the associated exclusive attack. Under a coupled attack, the service lives are 11 a and 16 a, respectively, indicating that coupling retards the degradation process. Increasing relative humidity also substantially shortens the service life, highlighting a critical influence of temperature and humidity control in practical engineering.

This study established a numerical model that could characterize the coupled competitive transport of chloride and sulphate ions in concrete under complex salt-temperature-humidity conditions. The modelling performance was validated with a good accuracy against experimental results. The key findings were summarized as follows: 1) Elevated temperature intensified ionic competition, significantly inhibiting chloride transport, while its effect on sulphate ions was weaker; 2) High humidity promoted chloride diffusion and competitive effects, affecting the distribution of total sulphate ions; 3) Variations in the environmental ionic concentration ratio could regulate the intensity of competitive interaction, particularly affecting chloride ingress behavior; 4) The influence of temperature variation on the transport of aggressive ions was more pronounced than that of humidity, and sulphate attack played a dominant role in controlling the durability of coastal concrete structures. The proposed model could provide a theoretical tool and a design basis for durability assessment and service life prediction of concrete structures exposed to complex environments.

Shield tunneling is widely adopted in major underwater engineering projects, exemplified in Qingdao Jiaozhou Bay Subsea Tunnel, China. As the primary load-bearing component, the mechanical and durability performance of shield segments directly determines the lifecycle safety of subsea tunnels. For the development of "ultra-long distance, ultra-deep burial, and ultra-large cross-section" for subsea tunnels, segment structures face coupled challenges from high hydraulic pressure and aggressive ion erosion, posing severe threats to the long-term service reliability of shield tunnels. The existing research indicates that high hydraulic pressure environments accelerate the corrosion degradation of shield tunnel segments, compromising their load-bearing capacity and durability. Specifically, an elevated hydraulic pressure accelerates chloride ion migration in concrete, alters pore structure characteristics, and reduces microstructural elastic modulus. For underwater projects such as the Qiongzhou Strait Cross-Sea Tunnel, where operational hydrostatic loads can reach up to 2 MPa, the durability deterioration mechanisms, microstructural damage evolution patterns, and chloride ion transport behaviors of shield segment concrete under ultra-high pressure conditions remain unclear.

This study was to conduct an in-depth investigation into the ion transport mechanisms and microstructure deterioration processes of shield tunnel segment concrete subjected to ultra-high hydraulic pressure conditions. In addition, the effects of nano-silica coatings, migratory corrosion inhibitors, and nano-precursor materials on the concrete durability were also investigated, thus providing a theoretical and technical support for ensuring the long-term service safety of shield tunnel segments in subsea environments.

In this study, a typical concrete mix design for shield tunnel segments was used, with enhancements implemented through surface application of nano-silica coatings, migratory corrosion inhibitors, and internal incorporation of nano-precursor materials to improve concrete durability. The experimental methodology encompassed rapid chloride migration (RCM) tests, electrical flux measurements, capillary absorption tests, MIP analysis, and chloride ion transport tests under elevated hydrostatic pressures (i.e., 1-4 MPa). The post-testing evaluations included quantitative free chloride content analysis, high-resolution X-ray computed tomography (X-CT) scanning, and nanoindentation testing to characterize the microstructural degradation.

The free chloride concentration in concrete gradually increases under an elevated hydrostatic pressure. In the control group, the surface chloride concentration is increased by 8.3%, 20.8%, and 29.2% as the pressure is increased from 1 MPa to 4 MPa. However, chloride ingress is significantly mitigated in the specimens treated with corrosion-inhibiting measures. Unloaded concrete exhibits a uniform porosity distribution (1.8%-6.7%) across heights, while hydrostatic pressure induced porosity increases within 10 mm of the surface. A pressure elevation causes a nonlinear decrease in pore connectivity tortuosity by 1.3%, 4.3%, 9.2%, and 14.4%, compared to unloaded conditions, correlating to pressure gradients. Simultaneously, connected pore lengths and equivalent radii increase under a pressure. High hydrostatic pressure degrades the microstructural toughness of mortar and the ITZ, thickens the ITZ, and establishes a coupled deterioration mechanism involving "hydraulic penetration-chemical erosion-mechanical damage." The application of nano-silica coatings, migratory corrosion inhibitors, and nano-precursor materials significantly reduce the chloride diffusion coefficient, electrical flux, and capillary water absorption of concrete. This enhancement is attributed to the pozzolanic reaction between nano-silica particles and hydration products, generating C-S-H gels that densify the matrix by filling pores. Migratory corrosion inhibitors migrates to the steel-concrete interface via capillary action and vapor-phase diffusion, forming a protective adsorption layer. Meanwhile, nano-precursors generate calcium carboxylate compounds during hydration, occupying submicron pores through hydrophobic and densification effects that substantially improve chloride resistance.

Hydrostatic pressure significantly accelerated chloride ion transport in concrete, and the surface chloride concentrations were increased by 8.3%, 20.8%, and 29.2% as the pressure was elevated from 1 MPa to 4 MPa in the control group. Although chloride concentrations decreased with erosion-inhibiting treatments, their mitigation efficiency diminished under rising hydrostatic pressure. For the post-pressure application, porosity increased within 10 mm of specimen surfaces, correlating to pressure magnitude. Enhanced pore connectivity and reduced tortuosity were obtained, accompanied with increases in the equivalent radii and average lengths of connected pore segments. Elevated hydrostatic pressure degraded the microstructural toughness of mortar matrices and ITZs, while ITZ thickness progressively increased with pressure loading. External application of nano-silica coatings, migratory corrosion inhibitors, and internal incorporation of nano-precursor materials all effectively enhanced the chloride resistance and reduced capillary water absorption of shield tunnel segment concrete, with nano-precursor materials demonstrating the most pronounced effect. These corrosion-inhibiting measures reduced concrete porosity by 30.3% to 64.4%, while decreasing the proportions of severely harmful pores.

The underground concrete structure is in a complex and changeable service environment for a long time, and it is often affected by the combined effects of load, corrosive ions and temperature changes. There is a significant nonlinear coupling relationship among these three factors, which makes the concrete show a completely different damage evolution law from the single effect during the service process. Especially in the underground environment, chloride ions and sulfate ions will accelerate pore dissolution and micro-crack propagation, and temperature changes will vary the hydration rate and ion migration ability. Under external loads, the generation and propagation of microcracks further promote ion penetration and chemical erosion, resulting in a rapid degradation of material properties. Conventional durability tests are often difficult to quantitatively describe the damage law under the combined action of multiple factors, and it is difficult to accurately predict the service life of underground concrete. It is thus of great significance to reveal the deterioration mechanism and service life evaluation of underground concrete via constructing a method that can comprehensively reflect the damage degree of concrete under multi-factor coupling and realize a life prediction.

In the experiment, ordinary Portland cement was used to prepare concrete specimens, and the mix ratio of each group was kept consistent. The test was carried out under the accelerated conditions of simulated underground environment, and the service environment of different load, ion and temperature combinations was set up to investigate the damage evolution law under the coupling of three factors. During the test, the five indexes of concrete failure load, ultrasonic wave velocity, dynamic elastic modulus, rebound value and quality change were tested regularly. These indexes could reflect the mechanical properties, internal compactness, surface hardness and mass loss of concrete, respectively, as the important indexes for comprehensive evaluation of damage degree. In addition, the evaluation system of concrete damage degree was also established based on the entropy weight method, and the weight was automatically assigned by the dispersion of index changes, so that the damage degree evaluation could be more objective and comparable.

For the life prediction, four models were selected for comparison, including Even Grey Model (EGM), Even Discrete Grey Model (EDGM), Optimized Discrete Grey Model (ODGM) and Discrete Grey Model (DGM), based on the grey system theory. The grey model has a good prediction performance under the condition of small sample and weak information, and is suitable for durability research with limited test period and small data volume. The accuracy of the model could be improved by cumulative generation and parameter analysis, and the prediction performance of each model was evaluated by various error indexes, and the optimal model was selected for the life prediction.

The damage degree of concrete under four different conditions firstly increases and then decreases. The failure load, ultrasonic wave velocity, dynamic elastic modulus, rebound value and mass change trend under the same conditions are similar. However, the turning points of damage attenuation under the four conditions are not consistent due to the difference of hydration reaction and internal pore structure. At 50 ℃, the faster early reaction rate can result in the faster pore filling, thus improving the compactness.

Based on the entropy weight method, the weight coefficient of each index to the damage degree of concrete is obtained. The influencing degree from large to small is failure load, dynamic elastic modulus, rebound value, ultrasonic wave velocity, and quality change. Also, the accuracy ranking of the four prediction models is obtained based on the comprehensive score ranking method. The prediction accuracy from high to low is ODGM > EGM = EDGM > DGM.

Under the combined action of load-ion-temperature, the failure load, ultrasonic wave velocity, dynamic elastic modulus, rebound value and mass of concrete all firstly increased and then decreased, but the turning points of damage under various environmental effects were inconsistent. In addition, the hydration products under chloride erosion environment were mainly filled with micropores (i.e., ≤ 0.1 mm³) and small pores (i.e., 0.1-1.0 mm³), and the pores of concrete at 50 ℃ in the early stage were more dense than those at 20 ℃. The damage degree of each index was quantified by the entropy method, i.e., failure load (i.e., 80%-90%), dynamic elastic modulus (i.e., 5%-8%), rebound value (i.e., 1%-6%), ultrasonic wave velocity (i.e., 1%-4%), and mass change (i.e., < 1%). The ODGM model had the highest accuracy in predicting the service life of concrete under the combined action of load-ion-temperature. The service life of concrete was 82 months, 29 months, 24 months and 20 months under the four environments (i.e., 0.2Fmax, 3%NaCl, 20 ℃ and 0.2Fmax, 3%NaCl, 50 ℃ and 0.2Fmax, 5%Na₂SO₄, 20 ℃ and 0.2Fmax, 5%Na₂SO₄, 50 ℃), respectively.

As one of the most widely used engineering materials in modern infrastructure, concrete is prone to premature deterioration under chloride-rich conditions such as marine environments and deicing salt exposure, which severely compromises the service life of structures. The ingress of chloride ions involves complex physicochemical processes, including dissolution and precipitation of minerals, adsorption and transport of ions, chemical reactions among ions, and the self-induced electric field generated arising from ionic interactions. Moreover, concrete is inherently a heterogeneous multiphase material, which comprises the cement paste, aggregates, and interfacial transition zone (ITZ) phases. Traditional experimental methods are often time-consuming and difficult to quantitively characterize the coupling mechanism between the electric and chemical fields. Alternatively, various theoretical models at different length scales for concrete durability were developed based on the assumption of fixed concentration at the concrete surface, while their predictive results contradict the experimental findings, where the ion concentration is dynamically changed at the concrete surface. Herein, this work introduces a non-equilibrium thermodynamics-based electrochemical model for multi-ion coupled transport in exterior solution, interior concrete, and their interface. The model enables to capture the dynamically change in ion concentration at the concrete surface. The model is numerically solved by finite element method and validated with several experimental results, where good agreements are achieved. Afterwards, the model is employed to investigate the impacts of initial diffusion coefficients, aggregate particle size, ITZ thickness, and external electric fields on the chloride ion transport behavior, the evolutions of mineral amounts and electric potential.

Based on non-equilibrium thermodynamics, an electrochemical model at mesoscale level is developed to simulate chloride-induced degradation in concrete. The model starts from the derivation of reaction rates for chemical reactions between the minerals and mobile ions, where the forward and backward reactions are fully considered to reflect the real scenario. The diffusion of multi-ions is governed by the conservation law of mass and the electric polarization is achieved based on Gauss's theorem. The constitutive relations are derived based on the second law of thermodynamics. Regarding the free energy density, it is formulated with consideration of mixing and polarization effects. For direct calculation of ion concentrations at the concrete surface, the computational domains include the exterior ionic solution, interior concrete, and their interface. By setting the appropriate boundary and initial conditions, the model is then implemented into the finite element software. Afterwards, the effect of solution domain size is investigated on the ion concentration, where a optimum solution domain size is finalized via the balance of computational accuracy and cost. For ensuring the accuracy of the developed model, it is validated with experimental data in open literature, where satisfactory agreements are achieved. After that, the effects of initial diffusion coefficient, aggregate size, ITZ thickness, and external electric fields are carried out on the transport of multi-ions, the dissolution of minerals, and the electric field evolution.

The diffusion of Cl^- ion is required to span different spatial domains, for example from the exterior solution to solution-concrete interface, and then the interior concrete. Due to the difference in diffusion coefficients between the solution and the concrete, the ion concentration exhibits a sharp discontinuity at the solution-concrete interface zone. Moreover, the ion concentration at the concrete surface is dynamically changed with time, which is consistent with the experimental findings. Additionally, it is found that the ingress depth increases obviously with increasing diffusion coefficient, and the concentration distribution profile is significantly varied for different kinds of mobile ions. Under the condition of a constant aggregate volume fraction, the aggregate particle size and the ITZ thickness have a limited effect on Cl^- transport. It is because that the ITZ generally fails to form a connected network within the concrete, preventing the establishment of continuous fast diffusion pathways at the mesoscale. With the increase of the applied electric field, Cl^- continuously accumulates at the concrete interface and migrates more rapidly into the interior concrete. Compared with a 2 V electric field, when the applied electric field is increased to 6 V, the penetration depth of Cl^- increases by about 40%.

A non-equilibrium thermodynamics-based electrochemical model has been developed theoretically to capture chloride transport and reactions within concrete. The model reveals that chloride ingress is driven by diffusion coefficient, weakly affected by aggregate size and ITZ thickness, and significantly accelerated by the external electric fields. A sharp variation of ion concentration is found at the concrete-solution interface, and the ion concentration at the interface is dynamically changed. The model framework is derived based on the constitutive theory of non-equilibrium thermodynamics, offering a rigorous physical foundation. It is conventionally extended to couple stress, temperature, or moisture field, thereby simulating ion transport process and multi-field coupled degradation behaviors under complex environments, such as salt-freeze cycles and stress-corrosion conditions. This work may also provide a theoretical guidance for design and optimization of concrete durability.

In the northwest of China, the harsh service environment such as saline soil and large temperature difference leads to the serious salt corrosion and freeze-thaw damage of cement concrete structures, and even structural damage and eventually shortened service life. It is important to improve the durability of cement-based materials via introducing polymers into common cement-based materials. However, the shortcomings of high-temperature sensitivity and low compressive strength of polymer materials limit their further application. In recent years, the introduction of nanomaterials greatly enhances the durability of ordinary cement-based materials. However, the dispersion and dosage of nanomaterials in cement-based materials and their influence on the microstructure of cement-based materials still need a further study.

Combined with the advantages of polymer and graphene oxide, sulfate and freeze-thaw coupling were used to simulate the special natural environment in the northwest China. Ordinary cement mortar and ethylene vinyl acetate polymer rubber powder were prepared with graphene oxide (GO) and ethylene vinyl acetate polymer rubber powder (EVA), respectively. Polymer cement mortar, graphene oxide cement mortar and graphene oxide modified polymer cement mortar were tested for relative dynamic elastic modulus, mass loss and compressive strength under different freeze-thaw cycles by sulfate-freeze-thaw cycle test. The microstructure evolution of specimens was characterized by X-ray diffraction (XRD), Fourier infrared spectroscopy (FTIR), derivative thermogravimetric analysis (TG-DTG), low-field nuclear magnetic resonance (LF-NMR), and scanning electron microscopy (SEM).

For 0.03% GO modified with 4% EVA cement mortar, the freezing and thawing resistance of the sample can be improved. After the sulfate freeze-thaw cycle, the maximum mass loss rate of OCM group is obtained (i.e., 5.26%), and the minimum relative dynamic elastic modulus is achieved (i.e., only 56.27%). The minimum of G3ECM is only 3.75%, and the relative dynamic elastic modulus reaches 73.95. After 125 freeze-thaw cycles, the compressive strength of G3CM specimen is the largest, and the OCM of blank group is the smallest. When EVA is mixed alone, the compressive strength of the specimen reduces, while the synergistic effect of GO and EVA can increase the strength. GO can obviously promote cement hydration, and EVA can delay cement hydration to a certain extent, but GO and EVA can still promote cement hydration and optimize microstructure under the synergistic effect. 0.03% GO mono-doped has an optimum effect on the optimization of pore structure, and the polymer film made of EVA can still promote the durability via hindering water transport.

In this study, GO modified polymer was used to prepare GO modified polymer cement mortar. The results of sulfate freeze-thaw cycle test showed that GO and EVA could significantly improve the durability of ordinary cement-based materials via affecting the cement hydration of cement-based materials, optimizing the pore structure and improving the microstructure.

By the end of 2024, China's railway operating mileage has reached 162 000 kilometers, including over 48 000 kilometers of high-speed railway lines. High-speed railways, characterized by diverse structural types and ribbon-shaped distribution, are directly exposed to atmospheric environments. As the structure directly bears high-speed train loads, the durability of ballastless track determines the service life and safe operation of high-speed railways. With the gradual expansion of high-speed railway construction to high-altitude and frigid regions, the durability of concrete structures faces some challenges. The unique planar structure of ballastless track leads to significantly different degradation patterns under harsh environments, compared to vertical structures like conventional bridge piers. Under a prolonged exposure to surface rainwater accumulation, snow coverage, and freeze-thaw cycles, ballastless track concrete surfaces gradually exhibit typical freeze-thaw damage characteristics such as surface pulverization and layered spalling. Note that the cast-in-place bi-block ballastless track slabs demonstrate higher risks of surface freeze-thaw damage, compared to precast CRTS III track slab. Therefore, greater attentions should be paid to environmental impacts on the durability of cast-in-place concrete structures in ballastless tracks. The vacuum dewatering technology as a surface physical modification method offers distinct advantages for cast-in-place ballastless track concrete, including operational simplicity and significant frost resistance improvement. However, the effectiveness of this technology in enhancing surface frost resistance for low-air-content cast-in-place concrete under low-pressure environments remains unclear. This study was to focus on cast-in-place ballastless track concrete under low-pressure environments. The surface frost resistance of concrete with different air contents from 2.0% to 6.0% under different vacuum levels of 0.06 MPa and 0.08 MPa for different treatment durations of 1, 3, 5 min, and 10 min was analyzed. In addition, an evaluation model considering concrete air content, vacuum level, and duration was also proposed via the theoretical analysis of bubble influence mechanisms on vacuum dewatering effects. This research could provide a theoretical support for enhancing surface frost resistance of cast-in-place ballastless track concrete structures in high-altitude and frigid environments.

According to the requirements of TB/T 3275-2011 standard, all raw materials were first loaded into a mixer within 60 s. The mixer was then operated continuously for 180 s before stopping, and the mixture was poured into molding forms. A self-developed vacuum negative pressure dewatering device was used to perform vacuum dewatering on the surface of self-compacting concrete. The device was run at different vacuum levels of 0.06 MPa and 0.08 MPa for different treatment durations of 1, 3, 5 min, and 10 min, respectively, to prepare cast-in-place concrete specimens treated under different vacuum levels and durations. Untreated specimens were retained as control samples. Immediately after vacuum dewatering, the cast-in-place concrete specimens were covered with sealing film and transferred to a constant-temperature environment of (20±2) ℃ for 24 h of initial curing. After demolding, the specimens were moved into a standard curing chamber with ≥95% relative humidity and maintained until reaching the designated curing age. The vacuum-dewatered specimens were subjected to surface frost resistance tests and vacuum dewatering ratio tests to validate the enhancement effect of vacuum dewatering technology on the frost resistance of cast-in-place concrete surfaces.

Vacuum dewatering technology can effectively enhance the surface frost resistance of cast-in-place ballastless track concrete. For 2.0% to 6.0% air-content vacuum-dewatered concrete under low-pressure environments, the mass loss per unit area after 28 single-sided freeze-thaw cycles is reduced by 5.0%-94.6%, compared to reference group specimens. However, the "over-vacuum" phenomenon caused by excessive vacuum levels or prolonged duration demonstrates the existence of an optimal vacuum dewatering regime for cast-in-place concrete.

During vacuum dewatering, the pore water pressure generated in fresh concrete drives free water migration and drainage under effective stress. The dewatering ratio continuously increases with higher vacuum levels, extended duration, and increased concrete air content. The key mechanisms for frost resistance improvement lie in a reduced water-to-binder ratio (i.e., 3.8%-11.2% reduction) and an enhanced surface strength (i.e., 4.4%-13.5% increase compared to reference group) in the concrete surface layer. Note that the strength improvement becomes more pronounced with a higher air content. These modifications collectively contribute to the enhanced frost resistance of the concrete surface through a vacuum dewatering treatment.

Vacuum dewatering technology could enhance surface strengthening of cast-in-place concrete through compaction and consolidation. During this process, the bubbles ruptured under the pressure when the vacuum-induced a negative pressure in fresh concrete exceeded a critical threshold that air bubbles could withstand. This rupture increased the number of connected pores-acting as "water drainage channels" in the fresh concrete surface layer-thereby accelerating the expulsion ratio of internal moisture along with air. This work could establish a comprehensive evaluation model for surface frost resistance of vacuum-dewatered concrete that simultaneously considered vacuum level, duration, and air content in fresh concrete. The model enabled a quantitative prediction of vacuum dewatering effectiveness and surface frost resistance improvement for ballastless track cast-in-place concrete under varying low-pressure conditions. This could provide a theoretical support for applying vacuum dewatering technology to ballastless track cast-in-place concrete structures in low-pressure environments.

The shrinkage cracking of concrete structures in arid regions such as plateaus becomes increasingly complex under the context of global climate change. This study utilized climate simulation data to analyze the coupled evolution characteristics of temperature, humidity, and atmospheric pressure across China. Based on a comprehensive analysis of 4662 sets of concrete shrinkage test data, the influence of atmospheric pressure on humidity was introduced and a modified CEB-FIP 2010 concrete shrinkage model was proposed that incorporated temperature-humidity-pressure coupling effects. The spatiotemporal evolution patterns of concrete shrinkage deformation nationwide were investigated. For a bridge pier concrete, the strain-stress-damage evolution process was simulated, and spatiotemporal dynamic zoning maps for shrinkage-induced cracking damage risks were developed. A risk of concrete shrinkage cracking damage by 2050 was projected to intensify significantly, with medium- to high-risk zones expanding by over 15% compared to current levels with increasing temperatures and decreasing humidity in arid regions.

The CMIP6 climate model was used to predict future temperature and humidity trends, with data spatially smoothed using cubic interpolation and the Gaussian filtering. The coupling relationship between temperature and humidity was analyzed by the Pearson correlation coefficients, Spearman correlation coefficients, and mutual information. The CEB-FIP 2010 model was improved via introducing a linear relationship between concrete strength and shrinkage development rate, combined with a barometric correction factor. The FEniCS finite element library was used to simulate stress and crack propagation in typical concrete components under different shrinkage strains, thus establishing a risk zonation framework for climate change scenarios.

According to the CMIP6 climate model predictions, the average temperature in northwest China will rise significantly over the next 30 a, especially in summer, with some areas exceeding 305 K (i.e., 32 ℃). Simultaneously, the relative humidity will decrease markedly, with some regions of humidity below 40% increasing in the north. These trends will intensify concrete drying shrinkage, particularly in arid regions. The analysis using the Pearson and Spearman correlation coefficients and mutual information reveals a negative correlation between temperature and humidity. In the north of 45°N, the correlation coefficients both approach -0.9, and the mutual information values approach 0.9, indicating a negative linear and monotonic relationship. This coupling will exacerbate concrete shrinkage effects. Based on 4662 sets of concrete shrinkage experimental data, the CEB-FIP 2010 model is refined via introducing a linear relationship between concrete strength and shrinkage development rate, along with a barometric correction factor. The improved model shows an enhanced prediction accuracy, with a nearly 1:1 ratio of overestimation to underestimation, particularly in high-temperature and high-strength concrete scenarios. The results of finite element simulations using the FEniCS library show that concrete cracking initiates at a shrinkage strain of 5.5 ζ ×10^-4, with stress concentration at nodes at 7.5 ζ ×10^-4, stable surface crack development at 9.0 ζ ×10^-4, and widespread crack propagation at 1.1 ζ ×10^-3. Based on climate change scenarios, China is divided into five risk zones for concrete shrinkage and cracking damage. From year 2020 to year 2050, medium- and high-risk areas centered on arid regions show a significant outward expansion. By year 2050, the medium- and high-risk zones in the northwest plateau region are notably larger than in year 2020, more than 15%, demanding priority crack-resistant measures to ensure a long-term concrete structural stability.

Future temperature in arid regions could increase, while relative humidity decreased significantly, with highly coupled temperature and humidity changes that would an exacerbate concrete shrinkage. Atmospheric pressure changes nationwide could be relatively stable, but the low-pressure environment in northwest arid regions could further increase a concrete shrinkage. The improved shrinkage model showed a significant enhancement in prediction accuracy, with a more uniform distribution of shrinkage values and a nearly 1:1 ratio of overestimation to underestimation. Concrete cracking occurred at a shrinkage strain of 5.5 ζ ×10^-4, with stress concentration at nodes at 7.5 ζ ×10^-4, stable surface crack development at 9.0 ζ ×10^-4, and widespread crack propagation at 1.1 ζ ×10^-3. China, could be divided into five risk zones, with medium- and high-risk areas centered on arid regions significantly expanding more than 15% outward from year 2020 to year 2050.

In regions characterized by excessively high diurnal temperature variations and strong radiation (high-temperature difference environments), concrete structures face heightened risks of cracking during service. Excluding the influence of hydration heat released by the concrete itself, energy absorption from the external environment during service is identified as the direct cause of concrete cracking. Surface cracking represents one of the primary failure modes for concrete in plateau regions with large temperature differences. In such environments, concrete structures are susceptible to cracking not only due to wind-induced drying shrinkage but also from intense solar radiation and excessive diurnal temperature fluctuations. Notably, vertical structures like bridge piers exhibit complex cracking patterns, with cracks often forming a crisscross network on the surface. As a non-homogeneous composite material with low thermal conductivity, concrete experiences internal/external temperature gradients and thermal stresses arising from differences in the coefficients of thermal expansion between cementitious systems and aggregates, which constitute the principal mechanisms for structural cracking. Existing methods for detecting and identifying cracks in concrete structures include manual visual inspection, infrared thermography, and Digital Image Correlation (DIC) techniques. Manual visual inspection, while the most direct and simplest method, suffers from low accuracy due to human subjectivity. Infrared thermography identifies cracks based on thermal conductivity differences between cracked and intact concrete; however, its performance is significantly affected by ambient temperature, humidity, and wind speed, making it unsuitable for regions with dramatic climatic variations like high-temperature-difference environments. DIC, a widely used image analysis tool, precisely captures strain field distributions in concrete under load, enabling accurate crack localization and evaluation. Nevertheless, DIC is limited to surface observations and cannot monitor the development of internal micro-cracks or damage. Additionally, its stringent requirements for ambient lighting hinder its application in long-term field monitoring of actual structures.

The raw materials used in this study included cement, coarse aggregates, fine aggregates, and water. The cement was P·O 42.5 ordinary Portland cement. The coarse aggregates consisted of continuous graded crushed stones with particle sizes ranging from 5 mm to 10 mm and 10 mm to 20 mm. The fine aggregates were natural river sand with a fineness modulus of 2.8 and graded in Zone II. To accelerate the simulation of concrete cracking characteristics under extreme temperature environments, a "square-inner-circle-outer" composite concrete structure was designed for temperature cycling tests. During the temperature cycling process, stress concentration induced by temperature gradients occurs at the edges and corners of the internal prismatic component, amplifying the thermal stress-induced damage to the concrete.

Acoustic Emission (AE) Technology exhibits unique advantages in monitoring concrete cracking under high-temperature difference environments. Compared to other detection/monitoring methods, AE offers real-time crack capture, high precision, crack localization, fracture type classification, and minimal environmental interference. Radhika et al. employed AE analysis to investigate fracture behavior and damage mechanisms in concrete beams under static and fatigue loads. Li et al. utilized AE to study damage degradation patterns and crack types in concrete under varying fatigue stress levels, categorizing the fatigue failure process into four stages. Jiao et al. applied AE to analyze the damage characteristics of steel fiber, basalt fiber, and glass fiber-reinforced manufactured sand concrete under load. However, current AE applications predominantly focus on damage assessment under mechanical loads, with its feasibility for internal damage monitoring and cracking state evaluation in concrete structures under high-temperature-difference environments remaining unverified.

First, a prismatic specimen with dimensions of 100 mm × 100 mm × 400 mm was cast according to the mix design and cured in a standard curing environment for 28 d. The cured prismatic specimen was then placed at the center of a cylindrical mold with a diameter of 200 mm and a height of 400 mm. Fresh concrete with the same mix design was cast into the mold. After 1 day of indoor membrane-covered curing, the specimen was demolded and transferred to a standard curing environment for an additional 7 d, completing the preparation of the "square-inner-circle-outer" composite concrete specimen. Prior to testing, concrete specimens cured for 7 d were dried for 21 d under constant temperature and humidity conditions (20±2) ℃ and relative humidity (60±5)%. Subsequently, the specimens were polished smooth and thoroughly cleaned to ensure effective sensor adhesion and facilitate observation of surface crack development. Eight sensors were attached to both the upper and lower sections of the lateral surface of cylindrical specimens, coupled to the sample interface area using high-temperature-resistant silicone couplers. To investigate the evolution of cracking damage in concrete under extreme temperature cycling, this study set the environmental chamber temperature to cycle between 0 ℃ and 80 ℃, with each cycle lasting 4 h. Concrete specimens standardly cured for 7 d underwent 40 consecutive temperature cycles in the chamber, with AE data acquisition covering the entire duration of each temperature cycle.

During the 40 temperature cycles, the acoustic emission AE equipment captured a total of 539 events generated by concrete stress release or crack propagation, which occurred throughout the entire temperature cycling process. High-amplitude signals indicate intense energy release at crack tips, typically associated with rapid crack propagation or unstable cracking. Most AE signals exhibited stable amplitudes between 50 dB and 80 dB during temperature cycling, reflecting the gradual accumulation of damage and crack extension under thermal fatigue stress induced by repeated expansion-contraction cycles. High-amplitude AE signals predominantly occurred during temperature rises to 60-80 ℃, demonstrating that concrete under thermal cycling is more susceptible to rapid damage accumulation or stable crack propagation at elevated temperatures. A concentrated increase in AE event counts was observed during the 16th temperature cycle, coinciding with a peak cumulative energy of 1943 aJ, indicating significant macroscopic cracking. Furthermore, the cumulative AE energy effectively characterizes the overall structural damage process. A sharp rise in cumulative energy over time correlates with the formation of fracture process zones, and abrupt changes in the energy-time curve slope serve as a criterion for microcrack coalescence into macrocracks.

Based on energy characteristics, the concrete damage and cracking process can be divided into three stages: microcrack accumulation, macrocrack initiation, and crack propagation. After multiple temperature cycles, internal microcracks interconnect during the cooling phase due to shrinkage stresses, ultimately forming macrocracks. This phenomenon is validated by the concentrated energy release and abrupt slope changes in the cumulative energy curve during cooling. Both microcracks and macrocracks in concrete under thermal cycling are predominantly tensile in nature, with over 90% of cracks attributed to thermal fatigue tensile stresses. In summary, the composite concrete structure designed in this study can effectively evaluate the temperature cycling resistance of concrete. The number of temperature cycles corresponding to the onset of macrocracks (Stage II) serves as a quantitative metric for assessing concrete cracking resistance.

This paper addresses the damage and cracking of bridge pier concrete under plateau high-temperature-difference environments. A testing method simulating concrete cracking in such environments was proposed, combining AE technology to achieve real-time monitoring and quantitative analysis of internal damage evolution. The damage mechanisms under high-temperature-difference conditions were elucidated. The key conclusions are as follows: 1) Micro-cracks initiate in bridge pier concrete during the early stages of temperature cycling under high-temperature-difference environments. The cracking process can be divided into three stages: micro-crack (damage) accumulation, macroscopic cracking, and crack propagation. 2) During the micro-crack (damage) accumulation and crack propagation stages, both temperature rise and fall exacerbate damage or crack development in the concrete. However, macroscopic cracking predominantly occurs during the cooling phase from 80 ℃ to 60 ℃. This is attributed to the faster surface temperature reduction compared to the interior, creating a non-uniform deformation effect characterized by "outer shrinkage and inner expansion." 3) Temperature fatigue tensile stress is identified as the primary cause of bridge pier concrete cracking under high-temperature-difference environments. Over 90% of the total cracks observed under temperature cycling are tensile cracks. Furthermore, the failure mode exhibits continuous damage accumulation and propagation along macroscopic primary cracks.

Although numerical modeling becomes a crucial tool in studying sulfate attack, focusing on key processes such as ion transport, chemical reactions, expansion force, and mechanical response, accurately quantifying the expansion force induced by the erosion product ettringite remains a challenge. This problem becomes particularly complex as considering the influence of temperature gradients present in real-world environments. Temperature gradients can accelerate the transport of sulfate ions and significantly alter the kinetics of the chemical reactions governing the formation of erosion products. Furthermore, the existing models for simulating these multi-field coupling effects and the corresponding experimental data for validation are relatively scarce.

This study was to develop a fully coupled numerical model for concrete under sulfate attack, integrating ion transport, chemical reactions, crystallization pressure, and damage, based on the crystallization pressure theory. The primary objective was to quantitatively elucidate the damage evolution of concrete under the coupled action of a non-uniform temperature field and sulfate attack.

This study employed an integrated approach combining numerical simulation and laboratory accelerated tests.

First, in the ion transport phase, the temperature-dependent Arrhenius equation was incorporated into the classic Fick's second law. This integration provided a mathematical characterization of sulfate ion diffusion under non-uniform temperature fields, thereby accurately describing the accelerating effect of temperature gradients on ion migration rates.

Second, in the chemical reaction investigation, a computational model for estimating ettringite formation under the influence of temperature fields was established based on chemical reaction kinetics. This method quantified the enhancing effect of temperature variations on the crystallization kinetics of erosion products via accounting for the relationship between reaction activation energy and temperature, enabling the prediction of ettringite quantities at different depths and stages of the attack.

Subsequently, for the crystallization pressure calculation, the conventional volume expansion theory was abandoned in favor of the crystallization pressure theory as the physical basis of the model. In this step, the amount of ettringite calculated by the chemical reaction module was converted into the crystallization pressure generated by its growth within confined pores.

Finally, in the damage computation study, continuum damage mechanics was introduced to define the damage variable and its evolution law for concrete. Through numerical solution, the spatiotemporal distribution of concrete damage induced by the accumulation of crystallization pressure was quantitatively computed. This ultimately established a quantitative relationship between the duration and depth of sulfate attack and the extent of concrete damage.

The diffusion coefficient of SO₄^2- exhibits an enhanced dependence on time, while showing a more pronounced reduction with depth, demonstrating a positive correlation with exposure duration and a negative correlation with penetration depth.

During the initial stage of exposure (≤ 30 d), no ettringite formation is detected. The ettringite content gradually increases as the exposure period progresses. However, a distinct transition point in ettringite formation occurs within the approximate range of 40 to 100 d, beyond which the rate of ettringite generation decelerates.

SO₄^2- continues to accumulate in the concrete pore solution and reacts with hydrated aluminate phases to form ettringite with prolonging exposure time, leading to an increase in the proportion of ettringite. At this stage, the amount of ettringite formed is controlled by SO₄^2- diffusing into the concrete. After the diffused SO₄^2- is fully consumed, residual aluminate phases remain in the hydration products. The ettringite subsequently formed within small pores (i.e., 10-100 nm) generates crystallization pressure, although the magnitude of this pressure remains relatively low (below the tensile strength of the concrete) at this stage, resulting in a minimal structural damage.

Under the coupled effects, the damage in concrete evolves through three distinct stages with exposure time, i.e., 1) An initiation stage without measurable damage, 2) An accelerated damage stage characterized by a rapid increase in damage extent, and 3) A stabilized damage stage where the progression of damage plateaus.

Based on the crystallization pressure theory, this study established a fully coupled transport-chemical-expansion damage model for concrete that could account for the synergistic effects of temperature fields and sulfate attack. The main conclusions were summarized as follows:

The diffusion coefficient of SO₄^2- in concrete under a temperature field exhibited a significant layered distribution characteristic, with the coefficient at the surface being approximately 1.5 times higher than that at greater depths.

The calculation results of erosion product formation based on chemical reaction kinetics indicated that the generation process could be dually controlled by the concentration of diffusing SO₄^2- and the content of initial hydration products. A distinct "turning point" existed, marking a transition from sulfate-ion limitation to aluminate-phase limitation.

The crystallization pressure theory was applied to quantify the internal stress generated by ettringite formation within nano-scale pores (i.e., 10-100 nm), providing a new paradigm for the application of this theory in studies of sulfate attack on concrete.

The sulfate-induced damage predicted by the numerical model exhibited a depth-time dependence and developed through three characteristic stages, i.e., an initiation stage, an accelerated damage stage, and a stabilized damage stage. The damage progression rate at the surface layer (i.e., 2-10 mm) was significantly higher than that at deeper layers (i.e., 14-20 mm), demonstrating a spatiotemporal differential evolution pattern.

In reinforced concrete (RC) structures exposed to chloride salt environments, the internal pore system provides pathways for chloride ion transport. Chloride ions that penetrate the concrete induce reinforcement corrosion and subsequent expansion, thus leading to concrete cracking. The newly formed cracks further accelerate chloride penetration and exacerbate structural deterioration. Numerical approaches provide an effective way for analyzing these coupled processes. However, conventional methods such as the finite element method, the extended finite element method, and the phase-field method are based on local theories, which makes it difficult to accurately capture displacement discontinuities during crack evolution. Consequently, the quantitative prediction of crack length and width remains a considerable challenge. Moreover, despite the development of numerous models, the most existing approaches are limited to individual physical processes and cannot fully represent their interactions. To address these limitations and investigate the corrosion-induced cracking behavior of concrete, it is essential to establish a coupled model of chloride diffusion and corrosion-induced cracking, enabling the quantitative prediction of the length and width of internal corrosion-induced cracks.

In this study, a diffusion-mechanics coupled lattice Boltzmann-peridynamic (LB-PD) model was proposed to simulate corrosion-induced cracking in concrete during chloride ion diffusion. A generation-placement method was employed to construct the concrete meso-structure, incorporating aggregates, mortar, and the interfacial transition zone (ITZ). Chloride ion transport within cracked concrete was simulated by using the lattice Boltzmann method (LBM), where cross-scale transport between the concrete matrix and ITZ was achieved via adjusting the LBM particle distribution functions. Meanwhile, corrosion-induced cracks at different scales were captured via categorizing the bonds in the peridynamic (PD) model into three distinct types, namely mortar bonds, aggregate bonds, and ITZ bonds. The processes both were accomplished without the need for additional mesh refinement. A stress-based PD bond failure criterion was further established based on the Peridynamic differential operator (PDDO). To accurately characterize the crack morphology, the Zhang-Suen thinning algorithm was introduced to extract the skeleton of corrosion cracks, and the crack length and width were quantitatively evaluated based on the crack skeleton.

The proposed LB-PD model is employed to simulate corrosion-induced cracking in concrete cover for specimens containing single and three reinforcing bars. In the single-reinforcement case, the simulations are conducted with and without considering the coupling between chloride diffusion and reinforcement corrosion. The results indicate that the coupling significantly accelerates crack propagation, although the crack initiation time remains largely unaffected. Cracks initiate predominantly around the reinforcement and extend outward along ITZ between aggregates and the matrix, highlighting a key role of microstructural heterogeneity in controlling crack paths.

For the specimen with three reinforcements, cracks similarly initiate near the reinforcements and propagate to form a network of primary and secondary cracks. The simulations reveal that the spatial distribution of aggregates affects crack evolution. The regions with sparse aggregate distribution exhibit a faster crack growth and a more severe reinforcement corrosion, whereas denser aggregate arrangements constrain crack propagation and reduce corrosion severity. This heterogeneity also leads to a non-uniform chloride concentration on reinforcement surfaces, further affecting the initiation and development of corrosion-induced cracks.

The results demonstrated that the proposed model could effectively capture the coupled processes of chloride transport and reinforcement corrosion, reproducing the multi-scale evolution of cracks in reinforced concrete. The model also provided insights into the interactions between microstructural features, chloride diffusion, and mechanical cracking, which were critical for understanding the degradation mechanisms of concrete structures under chloride attack. Cross-scale simulation of chloride transport and corrosion-induced cracking could be achieved without detailed modeling of ITZ, indicating a good adaptability of the model. Furthermore, the Zhang-Suen thinning algorithm was employed to extract crack skeletons, enabling a quantitative prediction of crack length and width. The simulations also confirmed the dual restraining effect of aggregates on both crack propagation and chloride diffusion, providing a mesoscopic-level explanation for reinforcement corrosion and crack initiation mechanisms. Noted that the current model could consider only chloride diffusion and did not account for multi-ion interactions or associated chemical reactions. Future work could extend the model to a diffusion-chemical-mechanical multi-physics framework to more accurately evaluate the durability of concrete structures and optimize protective strategies.

During the operational lifespan of nuclear power plants, the accidental leakage of boric acid solutions is a potential event that can lead to the corrosion of concrete structures. This corrosive attack induces degradation in the macroscopic mechanical properties of concrete, posing a significant threat to the long-term structural integrity and operational safety of the facility. Consequently, a profound investigation into the mechanisms and patterns of this degradation is of paramount engineering importance for the accurate assessment of the long-term safety and service life of nuclear power plant structures. The general deterioration of concrete in aggressive environments is a well-studied field. The specific effect of boric acid as a prevalent chemical in nuclear power plant primary circuits on the microstructural and macro-mechanical performance evolution of concrete needs to be investigated. This work was to systematically investigate the degradation of concrete subjected to boric acid environments via integrating experimental tests with theoretical analysis. The primary objectives were to quantitatively characterize the deterioration of both microstructural and macro-mechanical properties in order to elucidate the influence and coupling effects of key factors (i.e., immersion time, boric acid concentration, temperature and water/binder ratio) and ultimately to develop predictive models for assessing the service life of concrete structures under the corrosive conditions.

A combined approach of experimental testing and theoretical analysis was employed to systematically investigate the degradation of concrete in boric acid environments. Concrete specimens with varying mix proportions, specifically different water/binder ratios, were prepared and exposed to boric acid solutions at different concentrations and temperatures, simulating a range of potential leakage scenarios. A comprehensive set of tests was conducted at regular intervals to monitor the evolution of concrete properties for an extended period (i.e., up to 360 d).

The experimental program included non-destructive and destructive tests to capture both microstructural changes and macro-mechanical degradation. Ultrasonic pulse velocity measurements were utilized as a non-destructive technique to probe the internal microstructural integrity and its evolution, providing an indicator of the extent of damage and micro-cracking induced by boric acid attack. Destructive tests were performed to quantify the mass loss, axial compressive strength loss, and splitting tensile strength loss. The mass loss rate was calculated to assess the leaching of cementitious components and overall material loss. The axial compressive strength loss rate and splitting tensile strength loss rate were determined to evaluate the degradation of key mechanical properties critical for structural load-bearing capacity.

Based on the extensive dataset obtained from these tests, a detailed quantitative analysis was performed. The effects of individual factors, immersion time, boric acid concentration, temperature, and water/binder ratio on the performance degradation indicators were analyzed. Furthermore, the interactions and coupling effects among boric acid concentration, temperature, and water/binder ratio were investigated. For the current volume of experimental data, a bilinear function approach was adopted for the preliminary analysis of these coupling effects. Finally, empirical prediction models for mass loss rate, axial compressive strength loss rate, and splitting tensile strength loss rate were proposed based on the experimental data, forming a basis for service life assessment.

The experimental results reveal significant trends in concrete degradation under boric acid attack. The microstructural deterioration quantified through ultrasonic pulse velocity measurements shows consistent intensification with increasing boric acid concentration, temperature, and water/binder ratio. The relative ultrasonic pulse velocity is decreased by 15.49%-111.46% after 360 d, while the ultrasonic pulse velocity-time curve slope is reduced by 25.98%-134.37%, indicating an accelerated micro-damage. The factor interactions exhibit weak coupling characteristics. A distinctive two-phase behavior emerges across all the measured parameters. During initial immersion (i.e., 0-90 d), the negative values are obtained for mass loss rate and strength loss rates due to pore-filling effects of corrosion products and ongoing hydration. Beyond 90 d, all the parameters are positive and show monotonic increases, reflecting net material loss and structural deterioration. The degradation rates significantly amplify at higher boric acid concentrations, elevated temperatures, and increased water/binder ratios. Based on the comprehensive data, empirical predictive equations were proposed for mass loss and strength degradation, enabling quantitative service life assessment of concrete structures in nuclear power plants under a boric acid exposure.

The results revealed a clear two-phase degradation mechanism, driven by microstructural evolution. During the initial immersion period (i.e., 0-90 d), the formation of calcium metaborate crystals in the near-surface region that could not be fully corroded introduced a dominant filling effect. This led to a temporary increase in density and strength, manifested as negative mass and strength loss rates. Beyond 90 d, the development and progression of a fully corroded zone became a prevailing factor. This process could cause a sustained microstructural deterioration, which was effectively characterized by ultrasonic pulse velocity measurements. The deterioration intensified with increasing boric acid concentration, temperature, and water/binder ratio. Consequently, the mass loss and strength loss rates both became positive and exhibited a monotonic increasing trend. The empirical models were developed to predict the mass loss and strength loss, providing a reliable basis for assessing the structural performance and remaining a service life of concrete structures in nuclear power plants, thus offering valuable tools for durability and safety assessment.

The freezing and thawing (F-T) is one of the main threats for concrete in cold regions. The migration and phase transformation of moisture inside concrete induced by the F-T cycles is the origin of surface scaling and internal damage, which can induce the deformation of the matrix. The deterioration process of concrete can be further accelerated with the deicing salt. Previous studies usually followed the standardized methods. The deformation of concrete during the F-T cycles is free, which is different from the concrete in a service state. The concrete specimen is only equivalent to one unit in the concrete member. All concrete units undergo a deformation subjected to the F-T cycles, limiting the free deformation of adjacent concrete units. The limitation between adjacent concrete units is a form of restraint. Some restraint devices are developed to limit the deformation of concrete during F-T cycles. Conventional concrete frost resistance tests underestimate surface damage but overestimate internal damage due to the lack of consideration for restraint. However, the impact of restraint on the deterioration of concrete pore structure and the initiation/propagation of microcracks due to F-T cycles is unclear. Characterizing the internal damage of concrete is an important way to investigate the mechanism of concrete frost damage. The common method is to use digital image processing (DIP) to analyze the deterioration of concrete microstructure induced by F-T cycles, which has the advantages of convenient sample preparation and unrestricted sample size. In recent years, DIP methods have been also combined with morphological image processing to form a method of segmenting, extracting, and quantitatively analyzing for pores and microcracks that rely on "shape factors" to identify pores and microcracks. This paper was to focus on the restrained and unrestrained concrete as research objects. The pore structure and microcrack parameters were statistically analyzed after the single-side salt F-T test. The residual strain of concrete was calculated. The comparative analysis of the differences between two types of concrete in terms of pore structure deterioration and microcrack initiation was carried out to elucidate the influence of restraint on the deterioration of concrete pore structure and microcracks. The results could be expected to guide the mix design of frost resistant concrete in cold regions, ensuring the long service life of concrete materials.

P. I 42.5 cement, natural river sand with a fineness modulus of 2.68, and graded stone by uniformly mixing the crushed stone with 5-10 mm and 10-20 mm in a mass ratio of 4∶6 were used to prepare fresh concrete at a water/cement ratio of 0.60. The 28 d cube compressive strength of the concrete specimen was 28.3 MPa. Two groups of moulds were used to cast concrete specimens. For unrestrained specimens, cylindrical plastic moulds with a diameter of 100 mm and a height of 70 mm were used. For restraint specimen, a restraint device was used. The curing and pre saturation of concrete specimens were carried out according to the salt freezing method in the CIF test and the GB/T 50082—2009. The CDF/CIF TESTER produced by Schleibinger Geräte was used to provide single-side salt F-T cycles. After each 4 F-T cycles, the DIP sample was prepared. The processing of digital images included shooting, binarization, segmentation and extraction of region of interest (ROI), and skeleton. The obtained pore parameters included pore diameter distribution, most probable pore diameter (d_M), mean pore diameter (đ), standard deviation of pore diameter (d_σ), and percentage of damaged pores (P_dam). The obtained microcrack parameters included the mean length (L_mean) and mean width (W_mean) of microcracks.

The d_M , đ , d_σ and P_dam of both restrained and unrestrained concretes increase with the F-T cycles. After the 32 F-T cycles, the d_M , đ , d_σ , and P_dam of the restrained concrete are 11%, 10%, 5%, and 7% smaller than those of the unrestrained concrete, respectively. The results indicate that the restraint limits the expansion of the pores due to the F-T cycles. During the cooling process, the restraint weakens the water migration inside concrete via limiting the shrinkage deformation of concrete, thereby reducing the pore diameter shrinkage induced by water migration, the saturation of larger pores and thereby reducing the crystallization pressure. The restraint reduces the pore expansion and microcrack initiation of restrained concrete. The L_mean of restrained and unrestrained concretes increases with the F-T cycles. After the 32 F-T cycles, the L_mean of restrained concrete is 7% less than that of unrestrained concrete. The W_mean of unrestrained concrete decreases, but the W_mean of restrained concrete increases with the increase of F-T cycles. The restraint changes the evolution of microcracks in concrete due to the F-T cycles, resulting in macroscopic differences in the frost damage of the two types of concretes. Furthermore, after the 32 F-T cycles, the S_n of restrained concrete is 30% less than that of unrestrained concrete. The grey correlation degree shows that the pore expansion and microcrack length induced by F-T cycles are the main factors leading to the increase of residual strain. A correlation between the expansion of pore diameter inside restrained concrete and the increase in residual strain is greater, which is related to the weakening of water migration under the restraint.

The restraint had a significant inhibitory effect on the pore diameter expansion and microcrack initiation of concrete subjected to single-side salt freezing and thawing. The restrained concrete was to release stress through the expansion of microcrack width, being a key mechanism for reducing internal damage induced by the single-side salt freezing and thawing cycles. The main reason for the increase in residual strain of concrete induced by the F-T cycles could be the expansion of pore diameter and the growth of microcrack length.

The plateau region is characterized by low air pressure, diurnal temperature, high annual irradiation, strong dry winds, and other complex environmental factors. These conditions have severe challenges to the shrinkage and creep of railway bridge girders, as well as to the control of their alignment. According to the engineering practice, the creep camber of precast prestressed simply supported girders in the plateau region increases significantly, adversely affecting track smoothness and train safety. It is thus crucial for providing a technical support for the design and construction of railway concrete simply supported box girders to clarify the long-term deformation behavior of beam concrete in this complex environment.

Two mix ratios and two curing methods were employed to create three sets of comparative working conditions. The JZ-NC was used for constructing simply supported box girders in Langcangjiang Special Bridge, China, with standard curing (NC). The JZ-SC shared the same composition but utilized steam curing (SC). The LSLC-SC incorporated a densifying modifier, primarily silica fume and calcium sulfate whiskers (3% by mass), along with a shrinkage-reducing agent (0.5% by mass), and also used steam curing. For each group, four scale-beam concrete specimens were prepared, i.e., two for creep testing and two for shrinkage testing. The scale-beam underwent a four-point bending loading mode, with upper and lower spans of 500 mm and 1500 mm, respectively, at a creep load of 2 kN. Strain gauges were installed in the compression and tension zones to monitor strain, while the LVDT displacement sensors measured beam deflection. The specimens were subjected to shrinkage and creep tests in a high-altitude exposure environment at 14 d.

A constitutive model for the long-term deformation of scale-beam concrete under the combined action of wet, heat and force fields was proposed using a software named ABAQUS and the secondary development function, and the long-term deformation of the model beam concrete was simulated.

Compared with standard curing, the scale-beam concrete subjected to steam curing shows a reduction of 6.2%, 7.1%, and 6.0% in shrinkage strain, compression zone creep strain, and tension zone creep strain for 365 d, respectively. Under the same curing conditions, the incorporation of dense modified materials and SRA effectively reduces the shrinkage strain of the concrete. The 365-d shrinkage strain, compression zone creep strain, and tension zone creep strain are decreased by 13.2%, 11.4%, and 19.1%, respectively, and the mid-span deflection of the scale-beam is decreased by 13.8%, further verifying the regulatory effects of dense modified materials and shrinkage reducing agent.

The simulation results of the shrinkage and creep of the scale-beam concrete are in a reasonable agreement with the experimental results. The maximum error of the shrinkage strain is no more than 3%, and the maximum error of the creep is no more than 1%. This indicates that the established simulation method can well reproduce the long-term deformation behavior of the scale-beam concrete under the complex plateau environment.

The change in environmental temperature does not cause a uneven deformation in the scale-beam concrete, nor does it result in the warping of the scale-beam body, indicating that the temperature field mainly affects the longitudinal stress distribution of the scale-beam. The change in environmental temperature intensifies the redistribution of stress in the longitudinal section, and increases the shrinkage and creep of the concrete. Based on the micro-strain distribution characteristics caused by relative humidity, the humidity change does not lead to the warping of the scale-beam, indicating that the humidity field only affects the longitudinal stress distribution of the scale-beam, while the deformation coordination causes the stress to be redistributed between different sections, affecting the water migration and stress balance within the concrete, and exacerbating the shrinkage and creep of the scale-beam concrete.

Steam-cured concrete was prepared via incorporating a dense modified material (i.e., mainly composed of silica fume and calcium sulfate whiskers, with a mass fraction of 3%) and a shrinkage reducer (with a mass fraction of 0.5%) could effectively inhibit the shrinkage, creep and deflection deformation of scale-beam concrete in the complex plateau environment, and its 1-year shrinkage, compression creep and deflection reduction rates could reach 13.2%, 11.4% and 13.8%, respectively.

ABAQUS subroutine was written, and a numerical calculation framework for the long-term deformation of scale-beam concrete considering the temperature-humidity effects of the complex plateau environment was established, and the error between the simulation data and the measured results was less than 3%.

The temperature and humidity changes could mainly affect the longitudinal stress distribution of the scale-beam concrete, and the temperature-humidity effects in the complex plateau environment could intensify the stress redistribution on the longitudinal section, thus increasing shrinkage and creep.

The long-term deformation simulation of the scale-beam concrete at 50 a further verified that using the steam curing process, incorporating the dense modified material mainly composed of silica fume and calcium sulfate whiskers, and the shrinkage reducer could inhibit the shrinkage and creep of the scale-beam concrete in the complex plateau environment with the reduction amplitude of >17%.

Freeze-thaw (F-T) damage, characterized by microcrack propagation and material degradation caused by repeated water-ice phase transitions, is a critical factor leading to durability degradation of concrete structures in cold regions. Fly ash (FA) as an industrial byproduct is widely used to replace cement, reducing carbon emissions, while enhancing mechanical properties. However, conventional prediction methods for freeze-thaw resistance of fly ash concrete (FAC), which rely on the empirical formulas or simplified mechanical models, often exhibit limitations in efficiency and accuracy. These limitations stem from their inability to capture the complex nonlinear interactions among multifactorial parameters such as material composition, environmental conditions, and loading histories. As modern engineering increasingly demands high-precision durability assessments for infrastructure lifespan prediction, there is an urgent need to develop advanced predictive tools. This study was to propose an artificial intelligence (AI)-based framework to predict F-T resistance (i.e., mass loss rate, MLR; relative dynamic modulus of elasticity, RDME) with high precision and interpretability, addressing a critical deficiency in material design and durability assessment.

A comprehensive database of 537 experimental datasets was constructed, including 14 input variables (i.e., cement content, FA dosage, water-to-binder ratio (W/B), F-T cycles (NFT)). To address the curse of mitigate multicollinearity, a hybrid feature selection strategy combining the Maximum Information Coefficient (MIC) and Lasso regression was implemented. The MIC quantified nonlinear correlations between input variables and target responses (i.e., mass loss rate, MLR, and relative dynamic elastic modulus, RDME), while the Lasso regression performed variable subset selection through L1 regularization. Three machine learning algorithms-Random Forest (RF), Support Vector Regression (SVR), and Extreme Gradient Boosting (XGBoost) were subsequently employed to develop prediction models. Hyperparameter optimization for each algorithm was conducted via the Bayesian optimization with 5-fold cross-validation to prevent overfitting. The model performance was rigorously evaluated using multiple metrics, including coefficient of determination (R²), mean absolute error (MAE), and root mean squared error (RMSE). Furthermore, the SHapley Additive exPlanations (SHAP) values and Partial Dependence Plots (PDP) were utilized to dissect the contribution hierarchy and nonlinear marginal effects of critical parameters on the FAC freeze-thaw performance, thereby enhancing the model interpretability.

The XGBoost-T model (with feature selection) achieves the a maximum accuracy for MLR (R²=0.9224) and RDME (R²=0.9127), outperforming XGBoost model by 9.5% and 4.7%, respectively. The feature selection reduces a mean absolute error (MAE) by 26.1% (MLR) and 62.6% (RDME), proving its necessity. The study also analyzes the contributions and nonlinear effects of key parameters by the Shapley Additive Explanations (SHAP) and Partial Dependence Plots (PDP). The findings indicate that cement content and total binder content increase can reduce MLR and improve RDME, while water-to-binder ratio and freeze-thaw cycles have the opposite effects. The PDP analysis provides recommendations for optimizing FAC mix designs in cold regions, such as a water content range of 130-180 kg/m³ and a sand content range of 550-700 kg/m³ for optimal RDME.

This research introduced an AI-driven framework for predicting the freeze-thaw resistance of FAC, integrating feature engineering, intelligent optimization, and explainable machine learning. The framework could overcome the limitations of conventional empirical models via leveraging data-driven insights and high-dimensional nonlinear modeling. The XGBoost-based prediction model demonstrated state-of-the-art accuracy and robustness, while the PDP-derived parameter sensitivity analysis offered actionable strategies for enhancing FAC durability in cold environments. This study could contribute to the theoretical advancement of FAC durability design and underscore the transformative potential of artificial intelligence in civil engineering materials research. Future work could focus on expanding the database to include emerging cementitious materials and integrating multi-physics simulations for holistic durability assessments.

Research on durability degradation under multiple deterioration mechanisms has made remarkable progress, yet several breakthroughs are needed to achieve reliable long-term predictions. First, advancing the coupling of multi-mechanism models is essential. The mutual interactions between crack propagation and ionic transport, as well as pore-structure evolution under multiple deterioration modes, requires future numerical models that can dynamically update geometry and transport properties over time. More comprehensive physical representations of environmental effects such as temperature, humidity, salt concentration, freeze-thaw intensity will further enhance predictive accuracy. Second, there is an urgent need to improve the geometric dimensionality and computational efficiency of multi-scale, multi-physics frameworks. High-resolution three-dimensional modelling is still limited for multi-ion systems due to nonlinear chemical reactions and the complexity of heterogeneous microstructures. Future work should focus on algorithm optimization, parallel computing, and innovative discretization schemes to enable large-scale simulations with realistic material characteristics. Third, the scope of durability research must expand across both spatial and temporal scales. Linking micro-level deterioration mechanisms with macro-scale structural performance remains a key challenge, particularly for reinforced concrete exposed to multiple aggressive agents. Early-age behavior, interfacial transition zone development, and long-term interaction between mechanical and chemical processes should also be considered into lifecycle-oriented predictive frameworks. Finally, as sustainable and alternative binders such as alkali-activated materials and high-performance composite cements become more prevalent, existing models must be adapted or reconceptualized to accommodate their unique microstructure, chemical composition, and transport properties. Understanding multi-deterioration behavior in these emerging materials will be essential for their safe and widespread engineering application. Overall, progress in numerical modelling, strengthened by data-driven techniques, multi-scale experiments, and advanced characterization methods, is expected to transform our ability to predict durability degradation under complex service conditions. These advances will ultimately support the development of more resilient, sustainable, and long-lasting concrete structures.