Foamed cement paste (FCP) is a type of cementitious geotechnical material with a high porosity structure, prepared by mixing cement paste with preformed foam and sometimes incorporating soil materials such as clay, silica fume, fly ash, and sand. Due to its lightweight nature, excellent self-supporting ability, easy construction, and environmental friendliness, FCP has been widely used in embankment fills, bridge abutment backfills, slope reinforcement, and retaining wall applications. However, in saline soil regions, FCP is subjected to chemical attacks, particularly sulphate attack, in addition to environmental factors such as temperature fluctuations and water infiltration. The early stage of sulphate attack plays a critical role in determining the long-term durability of FCP. Unlike conventional concrete, which may experience initial strength gain due to ettringite and gypsum formation, FCP's unique pore structure may result in different degradation patterns. At this stage, the interaction between pore water, sulphate ions, and cement hydration products can lead to either early-stage degradation or partial densification. However, the mechanisms linking early-stage mechanical behavior and pore evolution remain unclear. This study focuses on the mechanical behavior and pore structure evolution of FCP during the early stage of sulphate attack. Specimens were exposed to Na₂SO₄ solutions (0%, 0.5%, 2.5%, 5.0%) for 28 d and 56 d, representing the early-stage exposure period. A combination of uniaxial compression testing, X-ray computed tomography (X-CT), and nuclear magnetic resonance (NMR) analysis was used to evaluate the changes in strength, aquiferous pore size distribution, and aquiferous porosity. The findings provide key insights into the initial degradation mechanisms of FCP, highlighting the competing roles of skeleton softening due to water absorption and pore densification due to sulphate reaction products, which are of great significance for further understanding of the influence of service environment on the engineering performance of FCP.

FCP specimens were prepared using P.C42.5 composite Portland cement, anionic foaming agent LG-2258, and tap water. The water-to-cement ratio (m_w/m_c) was set at 0.45, and the foam volume ratio (V_f/V_w) was controlled at 3.5 to achieve a final wet density of approximately 0.9 g/cm³. The specimens were cast in cubic molds (70.7 mm × 70.7 mm × 70.7 mm), demolded after 24 h, and cured under standard conditions [(20 ± 2) ℃, RH > 95%)] for 28 d. To eliminate the influence of initial moisture content on sulphate attack, all specimens were oven-dried at 60 ℃ to a constant weight before sulphate attacking. A multi-field coupling test device was used to control sulphate attack conditions, maintaining a constant temperature of 20 ℃ and an osmotic pressure of 10 kPa. The specimens were exposed in Na₂SO₄ solutions (0%, 0.5%, 2.5%, 5.0%) for 28 d and 56 d, simulating the early stage of sulphate attack. After attacking, NMR and X-CT analyses were conducted to assess aquiferous pore structure and aquiferous porosity, followed by uniaxial compression testing at a loading rate of 2 mm/min. Each experiment was repeated at least three times to ensure accuracy and repeatability.

Uniaxial compression test results showed that, during the early stage of sulphate attack, FCP specimens initially experienced strength reduction compared to the standard condition. However, partial strength recovery was observed after 56 d, particularly at 2.5% Na₂SO₄ concentration, where the peak strength increased by 19.28% and the residual strength increased by 55.56%. This indicates a transition from initial degradation to partial densification, likely due to the formation of sulphate reaction products. Unlike traditional concrete, which often exhibits a continuous strength increase in the early sulphate attack phase, FCP's unique pore structure resulted in a more complex response. At low sulphate concentrations (0.5%), sulphate attack increased porosity, accelerating early-stage degradation. However, at higher sulphate concentrations (2.5% and 5.0%), reaction products partially filled existing pores, leading to strength recovery. X-CT and NMR analyses revealed that sulphate attack significantly altered the aquiferous pore structure of FCP in the early stage: 1) Macropore frequency (>100 µm) decreased as sulphate concentration increased, indicating pore filling by reaction products. 2) Small pore frequency (1-10 µm) increased, particularly at 2.5% and 5.0% Na₂SO₄, due to the formation of ettringite and gypsum. 3) Total aquiferous porosity of sulphate-attacked specimens was higher than that of water-immersed specimens, suggesting that sulphate ions modified water retention behavior. A strong exponential correlation was observed between aquiferous porosity and mechanical strength. As aquiferous porosity decreased, both peak and residual strength increased, confirming that sulphate-induced microstructural changes significantly influence mechanical performance.

The main conclusion of this work are summarized as following: 1) Sulphate attack in the early stage first weakens FCP, then triggers partial strength recovery. Peak and residual strength initially declined, but after 56 d, peak strength increased by 19.28% and residual strength by 55.56%, particularly at 2.5% Na₂SO₄ concentration. 2) FCP's early-stage sulphate attack behavior differs from traditional concrete. At low sulphate concentrations (0.5%), sulphate attack increased aquiferous porosity, accelerating early-stage degradation. However, at higher concentrations (2.5% and 5.0%), sulphate reaction products partially filled pores, leading to strength recovery. 3) Sulphate attack altered FCP's aquiferous pore structure in the early stage. Macropores (>100 µm) decreased, while small pores (1-10 µm) increased, indicating that sulphate products influenced pore connectivity and densification. 4) A strong exponential relationship was identified between aquiferous porosity and strength. As aquiferous porosity decreased, both peak and residual strength increased, demonstrating that sulphate-induced pore transformations impact mechanical stability. These findings highlight the critical role of the early attack stage in determining the long-term durability of FCP, offering new insights for optimizing its application in sulphate-rich environments.

The long-term strength development of high-sulfur belite-calcium sulfoaluminate (high-sulfur BCSA) cement remains constrained by the low hydration reactivity of β-dicalcium silicate (β-C₂S) and the difficulty in obtaining its high-temperature active polymorphs. Ionic doping, which activates and stabilizes highly cementitious dicalcium silicate/calcium sulfoaluminate (C₄A₃$) crystalline phases while inducing lattice distortion, has been proven as an effective approach to enhance mineral reactivity.

With the depletion of high-grade limestone resources, the utilization of low-grade magnesian limestone and phosphorus-containing solid wastes, such as phosphogypsum and phosphorus slag, for producing high-sulfur BCSA has become an inevitable trend. This leads to the unavoidable coexistence of Mg²⁺/P⁵⁺ in the clinker mineral system. However, existing studies have primarily focused on the doping effects of individual ions, while the synergistic influence and underlying mechanisms of magnesium/phosphorus components in the quaternary clinker system (C₂S-C₄A₃$-C₄AF-CaSO₄) on sintering behavior and performance remain poorly understood.

In this study, MgCO₃ and Ca₃(PO₄)₂ were employed as the sources of MgO and P₂O₅ doping, respectively, to systematically investigate the effects of Mg/P co-doping on the phase composition, crystal structure evolution, and mechanical properties of high-sulfur BCSA clinker, along with the associated mechanisms. The findings not only provide a theoretical foundation for optimizing the strength development of high-sulfur BCSA cement but also establish a technical basis for the large-scale, high-value utilization of Mg/P-bearing industrial solid wastes.

The designed mineral composition of high-sulfur BCSA clinker was as follows (mass fraction): 50 % C₂S, 30% C₄A₃$, 7% C₄AF, and 13% f-CaSO₄. The raw meal samples were prepared using industrial raw materials and analytical-grade chemical reagents with varying Mg/P doping levels. Specifically, MgO was incorporated at 0.6%, 1.0%, 1.5%, 3.0%, and 5.0%, while P₂O₅ was added at 0.1%, 0.4%, 0.7%, 1.0%, and 1.3%. The raw mixes were fired at 1320 ℃ for 60 min to obtain clinker samples.

The effects of Mg/P co-doping on the sintering behavior and properties of high-sulfur BCSA clinker were investigated through X-ray diffraction combined with Rietveld refinement, scanning electron microscopy, and mechanical strength testing.

The Mg/P co-doping exhibits a significant synergistic effect, with an appropriate doping ratio effectively enhancing the content of highly reactive mineral phases, including cubic-calcium sulfoaluminate (c-C₄A₃$) and α'-dicalcium silicate (α'-C₂S). P₂O₅ demonstrates a favorable stabilizing effect on high-activity crystalline phases such as α'-C₂S and c-C₄A₃$, whereas MgO shows an adverse influence, particularly when the P₂O₅ content is low (P₂O₅ ≤ 0.4%). The maximum α'-C₂S formation in clinker is achieved at a doping level of 1% MgO and 1.3% P₂O₅. Moreover, Mg/P co-doping reduces the grain size of C₂S and C₄A₃$ minerals. However, excessive P₂O₅ doping 1.3% leads to noticeable melting corrosion, resulting in decreased crystallinity.

At low doping levels [w(MgO) ≤ 1.5%, w(P₂O₅) ≤ 0.7%], the 3 d compressive strength improves while the 7 d strength remains stable, with all samples showing varying degrees of enhancement in 28 d compressive strength, though excessive MgO doping leads to periclase formation and significant strength reduction. The 3 d and 28 d compressive strengths exhibit linear correlations with the total contents of C₄A₃$ and C₂S, respectively, with appropriate Mg/P doping promoting C₂S formation, particularly at 1.5% MgO and 0.4% P₂O₅, which yields the highest C₂S content and achieves a 28 d strength of 74.5 MPa.

Optimal Mg/P co-doping effectively reduces the decomposition temperature of CaCO₃ while enhancing its decomposition rate, and lowers the formation temperature of intermediate minerals C₅S₂$ and C₂S within the 900-1320 ℃ range, thereby promoting the formation of clinker minerals like C₂S. Furthermore, Mg/P doping demonstrates significant synergistic effects: P⁵⁺ selectively substitutes calcium and aluminum/silicon sites, stabilizing reactive phases such as α'-C₂S and c-C₄A₃$. As P₂O₅ content increases from 0.4% to 1.3%, the concentrations of α'-C₂S and c-C₄A₃$ markedly rise. In contrast, Mg²⁺ competes with P⁵⁺ for calcium sites, leading to reduced α'-C₂S and c-C₄A₃$ content with increasing MgO doping. The highest relative α'-C₂S content (51.5%) is achieved at 1% MgO and 1.3% P₂O₅ doping levels.

The 28 d compressive strength of doped clinker was improvd to varying degrees, initially increasing with MgO content but declining when excessive MgO forms periclase. Similarly, strength first rises then falls with increasing P₂O₅ content. Both 3 d and 28 d compressive strengths exhibit linear positive correlations with the total contents of C₄A₃$ and C₂S, respectively. Maximum C₂S formation and peak 28 d strength (74.5 MPa) occur at 1.5% MgO and 0.4% P₂O₅ doping. Comprehensive analysis of mineral formation, crystal phases, and performance reveals an optimal doping range: w(MgO)<3% and w(P₂O₅) 0.3%-1.3%, which balances clinker sintering efficiency and mechanical properties.

With the continuous development of infrastructure in China, the demand for concrete has increased significantly. As a critical raw material in concrete production, sand has also seen a sustained rise in consumption. It is estimated that global annual sand usage ranges between 40 and 50 billion tons. Due to the non-renewable and non-recyclable nature of river sand, its excessive extraction has led to the rapid depletion of existing reserves. To enhance resource efficiency, the construction industry has gradually implemented a variety of mitigation strategies, such as incorporating recycled coarse aggregates into concrete and employing manufactured sand as a substitute for river sand. The use of manufactured sand not only alleviates the environmental degradation caused by overexploitation of river sand but also offers a practical solution to the growing scarcity of this natural resource. Manufactured sand is produced by mechanically crushing and screening rock particles. Compared with river sand, it is characterized by a rougher surface texture and a higher content of stone powder, which can significantly influence the long-term mechanical performance of concrete. Creep, defined as the time-dependent deformation of concrete under sustained loading, plays a crucial role in determining the structural stability and durability. However, existing creep models for concrete have proven inadequate in accurately predicting the creep behavior of manufactured sand concrete. In recent years, machine learning techniques have been increasingly applied to the prediction of creep behavior in recycled aggregate concrete, ultra-high-performance concrete, and concrete incorporating mineral admixtures, demonstrating excellent fitting accuracy and generalization capabilities. Compared with traditional predictive models, machine learning approaches are better suited to handling complex issues such as multi-factor coupling and nonlinear relationships. However, their limitations are also notable: these models often function as "black boxes," lacking physical interpretability, and are highly dependent on the quality and quantity of input data, making it difficult to uncover the underlying physical mechanisms of creep. To address these challenges, some researchers have introduced explainable algorithms to improve model transparency and robustness Simultaneously, other studies have explored explicit modeling techniques to directly generate predictive formulas. Nonetheless, the resulting expressions are often overly complex and lengthy, limiting their applicability in practical engineering scenarios.

To tackle these challenges, a novel modeling framework that integrates both implicit and explicit techniques is proposed. First, a dataset of manufactured sand concrete creep was constructed. Advanced implicit machine learning models, including the Feature Token Transformer, Physics-Informed Neural Networks (PINNs), TabNet, and XGBoost, were employed to predict the creep behavior of manufactured sand concrete. Feature importance analysis was subsequently conducted based on model outputs to identify the most influential variables. Next, AutoFeat was used to automatically generate and select polynomial features from the identified key variables, thereby constructing a high-order explicit model capable of achieving accurate creep prediction. Furthermore, neural symbolic regression (NSR) was introduced to automatically discover nonlinear relationships among variables through symbolic function composition, yielding concise, physically meaningful analytical expressions.

Existing concrete creep prediction models, when applied to manufactured sand concrete, exhibit substantial errors due to their inability to effectively capture the material-specific characteristics and time-dependent behavior. This highlights an urgent need for the development of more targeted and robust predictive models.

The constructed implicit machine learning models—TabNet, FT-Transformer, PINNs, and XGBoost—demonstrated excellent predictive performance. These models effectively capture the complex and nonlinear relationships between the creep behavior of manufactured sand concrete and various influencing factors.

Explicit prediction models for concrete creep compliance were developed based on two approaches: AutoFeat and Neural Symbolic Regression (NSR). Both methods successfully generated functional expressions that map input parameters to the target output, thereby enhancing model interpretability. The AutoFeat-based model combined SHAP-based feature importance analysis with Pearson correlation analysis during feature selection, ultimately yielding an interpretable model primarily composed of first-order linear terms, complemented by selected second- and third-order interaction terms. In contrast, the NSR method exhibited a superior capacity for capturing nonlinear dependencies. It automatically identified key variables and constructed complex mathematical formulations characterized by nested structures and nonlinear operator combinations, providing deeper insight into the underlying mechanisms governing creep development.

Under comparable levels of predictive accuracy, both AutoFeat and Neural Symbolic Regression are capable of generating explicit expressions with clear mathematical structures, significantly enhancing model interpretability. Despite differences in feature selection strategies and model architectures between implicit and explicit modeling approaches, both demonstrate strong fitting performance with respect to real-world data, thereby validating the feasibility of modeling concrete creep through multiple methodological pathways. The integrated application of both approaches facilitates an optimized balance between predictive accuracy and interpretability.

The distribution characteristics of microcapsules incorporated into matrix can influence the quantity of microcapsules required for embedding in the cementitious matrix, thereby determining the self-healing efficiency. Self-healing systems are essentially passive systems that autonomously initiate the healing process when cracks occur. The efficacy of such systems necessitates both adequate distribution and sufficient bonding of the capsules with the matrix. The recent preparation of microcapsules in self-healing composites presents significant challenges in achieving optimal self-healing effects. One of the key obstacles is the aggregation and uneven distribution of capsules within the matrix, which can significantly impair the healing efficiency of microcapsule self-healing cementitious materials. This nonuniform distribution diminishes the effectiveness of the capsules and adversely affects properties like early mechanical strength. Furthermore, the uneven distribution of capsules in the matrix will lead to the reduction of the number of capsules encountered by cracks. It is therefore essential to ensure the uniform distribution of capsules within the matrix in order to maximise the probability of microcapsule rupture and self-healing capacity.

In order to avoid agglomeration of microcapsules, a uniformed distribution model for microcapsules inspired by the structural form of a honeybee's hive is designed in self-healing cementitious materials and some embedded microcapsules are dispersed within the honeycomb. This honeycomb model ensures the uniform distribution of microcapsules inside the matrix and prevent the aggregation of microcapsules, thereby achieving a design strategy for the uniform distribution of microcapsules in self-healing cementitious materials.

The probabilistic characteristics of a planar crack that appears inside a composite matrix and intersects with embedded microcapsules is investigated from the viewpoint of geometrical probability. The probability of a randomly occurring planar crack in the matrix when it encounters a spherical or spherocylindrical microcapsule is derived, and an intersection probability formula is obtained. Furthermore, the influences of capsule dimensions and quantities on the intersection probability is examined in capsule-based self-healing composites. In addition, comparative analysis is conducted on microencapsulated composite matrix models with varying shapes. Finally, the results of the theoretical model are validated using Monte Carlo simulation.

The honeycomb-like composite structure is a three-dimensional structure composed of several small cubes (cells), with a certain number of spherical or spherocylindrical microcapsules randomly dispersed within the small cubes, ensuring the uniform distribution of the capsules and avoiding the aggregation of microcapsules. When a random planar crack appears within the composite, the crack will meet some microcapsules and causes the ones to rupture releasing a healing agent to heal the crack, which is necessary for self-healing to work. Three-dimensional probabilistic analytical models are developed for the spherical microcapsules and spherocylindrical microcapsules in order to represent the intersection of a planar crack with randomly dispersed microcapsules, respectively.

The findings indicate that uniformly dispersed microcapsules can increase the probability of capsule rupture and improve the self-healing efficiency. The developed three-dimensional geometric probability model of a planar crack intersecting with randomly distributed microcapsules in the matrix cells is close to a binomial probability distribution. The use of spherocylindrical microcapsules can increase the healing potential of the same volume fraction of spherical microcapsules if the microcapsules are correctly oriented. The theoretical probability values are verified by comparison with the results obtained independently by computer programming simulations.

The proposed theoretical formulas quantitatively express the effect of the size and dosage of different-shaped microcapsules within the matrix on the probability of a planar crack hitting microcapsules and to predict the number of microcapsules that are most likely to intersect. If the volume fraction (or size) of microcapsules is given and a planar crack requires a fixed number of microcapsules to heal, the maximum intersection probability can be calculated from the theoretical equation to determine the optimum size (or volume fraction) of microcapsules to be mixed within the model. The results of this research will help to provide a theoretical basis for the quantitative design of self-healing materials based on microcapsules of different shapes. Furthermore, employing 3D printing technology could be a feasible approach to realize the design strategy for the uniform distribution of microcapsules within self-healing cementitious materials.

The free lime (f-CaO) content in cement clinker is a key indicator for evaluating calcination quality and process stability, directly influencing cement soundness and mechanical performance. Efficient and accurate real-time prediction of f-CaO content is essential for achieving stable cement quality control. In actual production, process parameter data are sampled at approximately 1-second intervals, while f-CaO content is typically measured once per hour, resulting in a severe scarcity of labeled data. This asynchronous sampling and data heterogeneity constrain both the scale and diversity of data available for supervised learning models, thereby reducing prediction accuracy and generalization capability. To address this challenge, this study proposes a method based on a semi-supervised generative adversarial network (SSGAN) to enhance the diversity and representativeness of the training dataset. Through evaluating its impact on the downstream f-CaO prediction task and conducting ablation studies, the research explores the feasibility of applying GANs to address the issue of asynchronous sampling between process parameters and offline measurement data.

This study proposes a data augmentation model based on a semi-supervised generative adversarial network (SSGAN), which generates virtual data resembling the actual data distribution to expand the training dataset and effectively enhance the model's learning capability under conditions of limited labeled data. The model consists of three main components: an encoder, a generator network, and a discriminator network. The encoder first extracts features from a small amount of process parameter data with real labels, which are then used as conditional inputs. The generator learns the latent features of the true label distribution through adversarial training and generates corresponding label values based on unlabeled process parameter data. The discriminator distinguishes between real and generated labels, guiding the generator to continuously optimize its generation strategy. As adversarial training progresses, the generator produces synthetic label data that closely resemble actual production data, while the discriminator continuously improves its understanding of the true data distribution and label characteristics.

When the SSGAN model is trained to approximately 10 000 iterations, both the discriminator loss and the generator loss converge to near zero and remain stable, indicating that a balance has been achieved between the generator and discriminator, and the model has effectively learned the latent distribution features within the process parameters. Further analysis of the distribution of generated label values shows that they exhibit a distribution trend generally similar to that of the real labels. The model demonstrates strong learning and fitting capability within the primary range of label values, and the generated data are reliable. After introducing SSGAN for data augmentation, the MAE, MSE, and RMSE of the downstream GRU (Gate Recurrent Unit) model for f-CaO content prediction decreased from 0.24, 0.08, and 0.29 to 0.19, 0.06, and 0.24, respectively, indicating an improvement in the model's ability to predict outliers and extreme samples. Ablation studies revealed that removing the encoder, the GRU module, or introducing random noise led to significantly higher prediction errors, highlighting the critical role of temporal information and deep features embedded in the process parameters for generating high-quality label data. In particular, the encoder and its internal GRU module are essential for capturing long-range temporal dependencies and dynamic variation patterns.

To address the issue of label scarcity caused by the mismatch in sampling intervals between process parameters and f-CaO content measurements, this study proposes a data augmentation method based on SSGAN. By using an encoder to extract high-dimensional features and a generator to learn the latent distribution of true labels, the approach constructs a more diverse and representative training dataset. Ablation experiments were conducted to evaluate the effectiveness of each key module. The main conclusions are as follows: The introduction of the SSGAN-based data augmentation strategy effectively expanded the training dataset. The distribution of the generated label values closely matched that of the true labels, verifying the reliability of the synthetic data. The application of the SSGAN method significantly improved the accuracy of the downstream f-CaO content prediction task, with MAE on the test set decreasing from 0.24 to 0.19, MSE from 0.08 to 0.06, and RMSE from 0.29 to 0.24. Ablation experiments further revealed that introducing random noise, removing the encoder, or omitting the GRU module led to significantly increased prediction errors. This highlights the critical role of temporal information and deep features embedded in process parameters for generating high-quality label data. In particular, the encoder and its internal GRU module are essential for capturing long-range temporal dependencies and dynamic variation patterns.

Alkali-activated slag (AAS) is a cementitious material prepared by using blast furnace slag as the precursor, triggered by alkaline activators to undergo dissolution, polycondensation, and solidification reactions. Compared to traditional cement-based materials, inherent defects of AAS, such as rapid setting and hardening, significant volumetric shrinkage, and susceptibility to cracking, severely limit its engineering applications. The hydration of magnesium oxide (MgO) generates magnesium hydroxide, with a solid-phase volume expansion of approximately 118%, which effectively compensates for the volumetric shrinkage caused by AAS hydration. However, existing studies are predominantly confined to single-dimensional analyses, such as the impact of MgO reactivity and content, fixed activator parameters, or specific curing ages on isolated performance, lacking systematic investigation into the coupling effects of multiple factors, microstructural evolution, and macro-performance development. This paper systematically investigates the regulatory mechanisms of MgO as a modifier on the hydration kinetics, hardening behavior, and microstructure evolution of AAS.

The precursor used in this experiment was slag with a specific surface area of 426 m²/kg. The alkaline activator was liquid sodium silicate which adjusted to a modulus of 1.5 using sodium hydroxide and deionized water. The sand employed was ISO standard sand. The additives included high-reactivity HR-MgO (reactivity time: ~20 s) and medium-reactivity MR-MgO (reactivity time: 70~90 s). The experimental parameters were set as follows: alkali content (Na₂O as a mass percentage of slag) = 6%, modulus (Ms = SiO₂/Na₂O molar ratio) = 1.5, water-to-solid ratio (including water in the activator and mixing water) = 0.4, and MgO dosage (as a mass percentage of slag) = 0%, 4%, 8%, and 12%. The specimens were molded at room temperature, demolded after resting for 6 h, and then cured under sealed conditions at a temperature of (20 ± 2) ℃ and relative humidity ≥ 95% until the specified curing age. The flowability, setting time, hydration heat, compressive strength, TG, XRD, FTIR, and SEM tests were conducted.

During hydration, MgO consumes partial free water, significantly reducing the fluidity of AAS mortar, with the fluidity loss intensifying as MgO reactivity and dosage increase. Concurrently, MgO incorporation prolongs the setting time of AAS paste, particularly with HR-MgO exhibiting stronger retardation effects. This phenomenon primarily arises from the formation of a passivation layer—composed of Mg(OH)₂ and M-S-H gel generated via MgO hydration—on slag particle surfaces, which inhibits further slag dissolution and thus delays setting. Moderate MgO content enhances the early hydration exothermic rate of AAS, shortening the duration of the induction period, with MR-MgO exhibiting superior enhancement effects compared to HR-MgO. While MgO minimally impacts the peak exothermic rate during the acceleration stage, it significantly increases cumulative heat release over 72 h. For instance, the HR-12 group achieved 178.84 kJ/kg, representing a 13.8% improvement over the control group. This indicates that MgO synergistically promotes hydration by accelerating slag dissolution and ion exchange processes, though excessive dosage diminishes early activation efficiency. MgO incorporation improves the early compressive strength of AAS. The HR-8 group attained a maximum 8 h compressive strength of 51.76 MPa, representing a 38.82% increase compared to the control group. However, strength growth in MgO-doped samples lags behind the control group at later ages, with 28 d compressive strength decreasing as MgO reactivity and dosage increase. MgO incorporation reduces the geometric crystal growth index (n) during the nucleation and growth (NG) stage, shifting hydration from homogeneous to heterogeneous nucleation. This accelerates crystal nucleation/growth rates, enabling earlier transition to the phase boundary reaction (I) stage. During Stage I, the synergistic interaction between MgO's secondary reactions and slag particle phase boundary reaction dominates hydration kinetics, causing actual exothermic curves to deviate upward from fitted curves and resulting in reduced curve fitting accuracy. As hydration progresses, the hydration product layer gradually covers slag particles, inhibiting ion diffusion and thereby reducing reaction rates during the diffusion-controlled (D) stage. Additionally, MgO alters AAS hydration product composition: M-S-H gel (with lower hydration heat than C-A-S-H gel) further decreases the D-stage reaction rate constant, impacting long-term hydration.

MgO markedly reduces the fluidity of fresh paste through free water absorption. During the initial hydration phase, the low solubility of Mg (OH)₂ under high alkalinity enables the formation of a passivation layer on slag particles, effectively delaying slag dissolution and prolonging setting time. As hydration progresses, pH reduction enhances Mg(OH)₂ solubility, releasing Mg²⁺ and OH^- ions that exhibit synergistic effects: 1) Elevated alkalinity accelerates slag dissolution; 2) Mg²⁺ reacts with dissolved [SiO₄]^4- and [Al(OH)₄]^- to generate secondary hydration products (M-S-H gel and hydrotalcite, Ht), substantially improving early compressive strength (8 h strength increased by up to 38.82% with MgO content of 8%). However, excessive Mg²⁺ competitively consumes [SiO₄]^4- and [Al(OH)₄]^- with Ca²⁺, suppressing the crystallinity of C-(A)-S-H gels and reducing their aluminum substitution, ultimately leading to structural loosening and compromised long-term strength development (28 d strength reduction up to 15.99%). Krstulovic-Dabic hydration kinetic modeling demonstrates that MgO incorporation enhances the nucleation and crystal growth (NG) rate but hinders the diffusion-controlled (D) stage by forming a shielding product layer. Notably, the total hydration heat exhibits positive correlation with MgO reactivity and dosage, attributed to continuous formation of magnesium-bearing phases (Mg (OH)₂, Ht, M-S-H). This work provides theoretical insights for optimizing AAS formulations to balance setting regulation, early strength enhancement, and long-term strength sustainability.

Magnesium oxide (MgO) content in Portland cement clinker critically influences clinkering and long-term soundness. Limited MgO(≤2%, mass fraction) optimizes mineral formation by reducing liquid-phase viscosity during sintering, but excessive MgO forms low-reactivity periclase. Delayed hydration of periclase to brucite (Mg(OH)₂) causes destructive expansion. Thermodynamically, Mg²⁺ can react with Al³⁺ from C₃A/C₄AF to form magnesium-aluminum layered double hydroxide (Mg-Al LDH), which offers superior anion binding capacity. However, kinetic competition with formation of AFt(ettringite)/AFm(monosulfate) phases in sulfate-rich systems often suppresses LDH formation. This study investigates the formation pathway of Mg-Al LDH and the role of temperature in MgO-doped C₃A hydration (without sulfate), revealing novel Ca-Mg-Al hydrates(X-phase) and microstructural evolution mechanisms.

MgO was obtained by calcining basic magnesium carbonate (900 ℃, 2 h). C₃A-MgO composites (40 g MgO to 100 g C₃A) were synthesized from CaCO₃/Al₂O₃ mixtures (Ca/Al=3∶2), sintered at 1450 ℃ for 3 h, and air-quenched. XRD-Rietveld analysis quantified phases: 69.70% cubic C₃A, 25.09% periclase, 3.58% C₁₂A₇, and 1.63% free CaO. Hydration suspensions (w/s=10∶1, under N₂ atmosphere) were cured at 20 ℃ and 50 ℃ for 1 h-120 d. Hydration was stopped with isopropanol; solids were filtered, vacuum-dried (40 ℃), and characterized. Phase evolution was analyzed via XRD (Rietveld refinement) and in-situ high-temperature XRD (25-200 ℃). Morphology and elemental distribution were examined using FIB-SEM (Helios 5 UC) and EDS (Oxford UltimMax65). Platy particles were cross-sectioned by FIB etching (500 nm-2 μm depths) for compositional profiling.

At 20 ℃, C₃AH₆ forms rapidly, constituting 50% of solids within 1 h, and C₃A is fully consumed by 3 d. Mg-Al LDH emerges after 6 h, grows gradually to 45% and with approximately 7% periclase residue at 56 d. XRD analysis reveals a novel X-phase at 28 d, characterized by distinct diffraction peaks at 6.70° and 13.38° 2θ. In contrast, at 50 ℃, accelerated dissolution of periclase enables complete hydrolysis of MgO by 28 d. Mg-Al LDH dominates the system after 7 d, reaching 54% at 56 d and replacing C₃AH₆ as the primary phase. The X-phase forms within just 1 h and consistently increases with prolonged curing.

The morphology and composition of Mg-Al LDH exhibit significant temperature dependence. At 20 ℃, LDH crystallizes as well-defined hexagonal sheet with Mg/Al ratios ranging from 2.77 to 3.16 (EDS data). SEM images and EDS confirmed C₃AH₆ dissolution is driven by Mg²⁺, and Al³⁺ originated from C₃AH₆ supports Al³⁺ for LDH growth. At 50 ℃, however, LDH shows irregularly shaped flake with a lower, more homogeneous Mg/Al ratio (2.21 ± 0.11). This decrease of Mg/Al ratio suggests enhanced Al³⁺ availability from C₃AH₆ at elevated temperatures.

The formation pathway of the X-phase involves a three-component reaction: C₃AH₆+Mg₄Al₂(OH)₁₄·3H₂O+CH→X. X-phase early appearance at 50 ℃ (1 h vs. 28 d at 20℃) confirms temperature-dependent kinetics. In situ high-temperature XRD indicates its structural reversibility: heating above 50 ℃ induces peak shifts (e.g., new peaks at 8.82° and 10.64° 2θ), yet cooling restores the original pattern, confirming thermally reversible structure. EDS of X-phase surface identifies composition of CaMgAl(OH)₇. FIB-EDS cross-sectional profiling exposes a linear Ca/Mg ratio within X-phase inner lamellae (slope = -0.46), implying continuous Mg²⁺ substitution for Ca²⁺ during maturation. Under curing at 50 ℃ for 120 d, platy X-phase particles evolve into stacked lamellar up to 20 μm width and 2 μm thickness. These particles encapsulate residual C₃AH₆, demonstrating the role of C₃AH₆ as reactants.

Elevating curing temperature from 20 ℃ to 50 ℃ accelerates periclase dissolution, enabling Mg-Al LDH to dominate C₃A hydration after 7 d and reducing its Mg/Al ratio from 2.77 to 2.21. Mg²⁺ etches C₃AH₆ surfaces, facilitating LDH growth by the release of Al³⁺. A novel Ca-Mg-Al hydrate (X-phase) forms through reactions of C₃AH₆, Mg-Al LDH, and Ca(OH)₂, exhibiting thermally reversible phase transitions and a composition of this phase surface near CaMgAl(OH)₇. Internal Ca/Mg ratio within X-phase lamellar particles suggests ion exchange during maturation.

Magnesium silicate hydrate (M-S-H) attracts significant attention as an alternative to traditional portland cement due to its unique properties, including reduced energy consumption during preparation. However, its synthesis typically requires MgO from calcined magnesite, which undermines the environmental benefits it provides. This study addresses these limitations by proposing a waste valorization strategy: recycling waste acid to dissolve naturally abundant magnesium silicate minerals (serpentine, olivine, and sepiolite) for M-S-H production. By combining acid waste management with mineral resource utilization, this approach not only avoids the energy-intensive MgO production process but also eases the ecological hazards caused by acid disposal.

The experimental involved three key stages: acid leaching, material preparation, and characterization. First, serpentine, olivine, and sepiolite were subjected to dissolution in a mixed waste acid solution (HCl: 6.20 mol/L, H₂SO₄: 7.77 mol/L) at 90 ℃ for 3 h under continuous mechanical agitation. The solid-to-liquid ratio was maintained at 1∶5 for acid leaching. After leaching, the insoluble residues (mainly amorphous silica) were separated with a centrifuge (Thermo Scientific ST 16R). The obtained residues were washed ultrasonically with deionized water 5 times to remove residual acid. Subsequently, the residue was dried in a vacuum drying oven at 50 ℃ for 24 h. The acid-leached filtrate was added into a 10 mol/L NaOH solution, with real-time pH monitoring until reaching pH 11.5. The obtained suspension was left to settle at room temperature for 24 h. The white flocculent precipitate was collected by vacuum filtration and dried at 100 ℃ for 24 h to obtain Mg(OH)₂-rich powder. The dried residue was mixed with Mg(OH)₂ powder at a ratio of n(SiO₂) ∶ n(MgO)=1.0∶1.4, and blended with water with a water-to-solid ratio of 1 ∶ 2 to form a paste. The paste was sealed in glass tubes and cured in water baths at 20, 50 ℃, and 80 ℃ for 3, 7, 14 d, and 28 d, followed by vacuum drying at 50 ℃ for 24 h prior to characterization. Mortar samples with the same residue-Mg(OH)₂ ratio were tested for compressive strength, fluidity, and drying shrinkage according to Chinese national standards.

The residues contain 89%-96% amorphous SiO₂, and Mg(OH)₂ precipitates exhibit 96% purity. Promoting curing temperature significantly accelerates M-S-H formation. At the same age, the diffraction peak of Mg(OH)₂ is lower in samples cured at higher temperature. TG curves reveal 3 distinct mass loss: 50-350 ℃ (loss of physically bound water, Δw₁), 350-450 ℃ (Mg(OH)₂ dehydroxylation, Δw₂), and 450-750 ℃ (Si-OH dehydroxylation, Δw₃). The Δm₂ content decreases with age and elevated temperatures, indicating accelerated Mg(OH)₂ reaction. A linear relationship between Δm₂ and (Δw₂+Δw₃) is established. A decline is observed in the slopes of fitting line, from 0.96 (20 ℃) to 0.80 (50 ℃) and 0.76 (80 ℃), which indicates that the contribution from unreacted Mg(OH)₂ decreases while the formation of M-S-H is promoted with increasing temperature. The broadening and intensification of the exothermic peak near 850 ℃ (M-S-H recrystallization) in the DSC curve indicate enhanced maturation of M-S-H with increasing curing temperature. ²⁹Si NMR spectra indicate elevated curing temperatures reduced the relative content of Q¹, confirming enhanced polymerization of silicon-oxygen tetrahedra in M-S-H phases, though infinite chain structures were not formed. The diminished Q³_c signal (corresponding to talc-type 2∶1 layered structures) indicated a gradual decline in "transitional" M-S-H (Mg/Si = 0.9-1.1), while increased Q³_a (serpentine-like 1∶1 layers) reflected accelerated alignment of Mg/Si ratios with the raw material's composition (1.4). M-S-H gels form via Mg²⁺/H₃SiO₄^- polymerization into Q²/Q³ networks with minor Q¹, accelerated by elevated temperatures to yield serpentine-like layer structure. The M-S-H mortars demonstrated remarkable early strength (12-18 MPa at 3 d). This was attributed to the high reactivity of amorphous SiO₂ and the rapid dissolution of Mg(OH)₂. After 28 d, compressive strength reached 40 MPa at 80 ℃. The mortar exhibited superior fluidity (157-181 mm flow diameter) without any superplasticizer, enabled by 28%-32% moisture in residues. However, excessive drying shrinkage was observed.

The mixed waste acid effectively dissolved serpentine, olivine, and sepiolite, yielding Mg(OH)₂ (≥96% purity) and SiO₂-rich residues (>89% SiO₂). The polymerization degree of silicon-oxygen tetrahedra in the resulting M-S-H phases increased with rising curing temperature (from 20 ℃ to 80 ℃) and extended curing time (from 3 d to 28 d), leading to the formation of serpentine-like layered structures. The mortar exhibited high early strength (12-18 MPa at 3 d) and reached 40 MPa at 28 d, which is attributed to the high reactivity of silica residues and rapid dissolution of Mg(OH)₂. Although the inherent moisture in the residues and the absence of water consumption by Mg(OH)₂ contributed to excellent fluidity without adding superplasticizers, this combination still causes significant shrinkage.

In recent years, CO₂ curing has attracted increasing attention as it not only enhances the early age strength of cement-based materials but also captures and sequesters CO₂, thus it has been regarded as a new curing method with a broad application prospective for eco-friendly construction materials. Since CO₂ curing generally lasts several hours to several days, there are considerable residual cement clinkers in the cement matrix after CO₂ curing. These un-carbonated cement clinkers will rehydrate once in contact with water. The rehydration products of cement clinkers can recover the pH value of the pore solution, reduce the porosity, and refine the pore size distribution of the cement matrix. Thus, the subsequent hydration of CO₂-cured cementitious materials largely governs the long-term performance of cement-based materials, such as compressive strength and durability. Several researchers have studied the subsequent hydration of CO₂-cured cementitious materials, but their conclusions were contradictory. Some researchers reported that the carbonation products calcite promoted the subsequent hydration of un-carbonated cement clinkers. On the contrary, some researchers reported that CO₂ curing hindered the subsequent hydration of un-carbonated cement clinkers and thus lowered the late compressive strength of cement-based materials. Thus, more studies should be conducted to reveal the subsequent hydration mechanism and strength evolution of early CO₂-cured cement-based materials. In this investigation, the subsequent hydration and compressive strength evolution characteristics of early CO₂-cured cement paste were studied by macro-physical experiments and micro-characterization techniques.

Ordinary Portland cement P·I 42.5 complying with GB8076—2008, and de-ionized water was used to prepare cement paste samples. The water-cement ratios were 0.3, 0.4, and 0.5, respectively. Two groups of cement paste cubes measuring 20 mm were cast. One group of cement paste samples was subjected to 24 h of early CO₂ curing and subsequent standard curing in saturated lime water. The other group of cement paste cubes was subjected to standard curing in saturated lime water, which served as a reference.

After CO₂ curing, the carbonation depths of cement cubes were determined using phenolphthalein indicator, and CO₂ uptake was determined by thermal gravimetric (TG) analysis. The compressive strength of cement paste was measured at the curing ages of 2, 28 d, and 90 d, respectively. The pore solution was extracted with an expression device for pH value measurement. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the phase evolution. The amounts of silicate and aluminate hydrates were assessed using thermal gravimetric and differential thermal gravimetric (TG-DTG) analysis. The microscopic characteristics and pore structure were examined by scanning electron microscopy (SEM) and mercury intrusion porometer (MIP) at the curing ages of 2, 28 d, and 90d, respectively.

Similar to the cement paste subjected to standard curing, the compressive strength of early CO₂-cured cement paste continues increasing as it is cured in lime water after CO₂ curing. Nevertheless, the strength improvement of the latter is lower than the former as the subsequent hydration of the former is less significant. In addition, the strength improvement of early CO₂-cured cement pastes in the lime water decreases with the increase of water-cement ratio. After curing to 28 d in lime water, the pH value of early CO₂-cured cement pastes with water-cement ratios of 0.3, 0.4, and 0.5 recovers to 13.14, 12.41, and 12.33, respectively, which is higher than the threshold of de-passivation.

XRD patterns reveal that the diffraction intensities of tricalcium silicate and dicalcium silicate decrease continuously when the early CO₂-cured cement pastes continue curing to 28 d and 90 d in lime water. However, this decrement is less significant than that of standard-cured cement paste. It indicates that CO₂ curing inhibits the subsequent hydration of un-carbonated cement clinkers. This can be attributed to the fact that the formation of calcium carbonate coating on the surface of cement particles. The FTIR spectra reveal that carbonation products silica gel is reactive towards Ca(OH)₂, and it is completely consumed before the age of 28 d. This is responsible for the absence of Ca(OH)₂ in the XRD patterns of early CO₂-cured cement paste with water-cement ratios of 0.4 and 0.5 at 28 d and 90 d, respectively. TG-DTG results suggest that less silicate and aluminate hydrates are produced in early CO₂-cured cement matrix than in standard-cured cement matrix during subsequent lime water curing, and it decreases with the increase of water-cement ratio as denser calcium carbonate coating on the surface of un-carbonated cement particles.

SEM results show that subsequent lime water curing improves the microstructure of early CO₂-cured cement paste due to precipitation of hydration products, while the improvement is less significant than that of standard-cured cement paste. Subsequent lime water curing decreases the porosity and refines the pore size distribution of early CO₂-cured cement paste. The pore diameter of early CO₂-cured cement paste mainly distributes in the range of 10 nm to 50 nm, both at 28 d and 90 d.

In the course of subsequent lime water curing, the main reactions that occurred in early CO₂-cured cement paste include the hydration of un-carbonated cement particles and the interactions between silica gel, calcium-modified silica and Ca(OH)₂. The subsequent hydration degree of early CO₂-cured cement paste is lower than that of cement paste subjected to standard curing, and it decreases with the increase of water-cement ratio. The compressive strength of CO₂-cured cement pastes continually increases with the prolonging of the subsequent curing period, and the growth rate of late compressive strength decreases with the increase of water-cement ratio.

The widespread use of cement in concrete leads to increased carbon emissions, so the demand for supplementary cementitious materials increases significantly. Limestone powder and steel slag powder are widely used as supplementary cementitious materials in modern concrete. However, for UHPC and self-compacting concrete, an extremely low water/binder (W/B) ratio is on one hand a key factor in achieving ultra-high strength and ultra-low porosity of the materials, on the other hand, also leads to the deterioration of the rheological properties of the fresh paste. Meanwhile, the existing researches focus on the influence of single limestone powder or steel slag powder on the mechanical properties or microstructure of cement-based materials. Little work on the influence of steel slag powder or limestone powder on the rheological properties of composite paste at an extremely low water/binder ratio has been investigated quantitatively. The mechanism of the effect of steel slag powder or limestone powder on the rheological properties of composite paste at extremely low water/binder ratios is still unclear. In this work, the effects of steel slag powder and limestone powder on the rheological properties of composite paste at different low water/binder ratios were analyzed via determining the flow diameter, setting time, marsh cone flow time, rheological parameters, and total organic carbon content.

A composite paste was prepared with P.I 42.5 ordinary Portland cement, steel slag powder, limestone powder, blast furnace slag and silica fume as raw materials in a certain proportion. To achieve the preparation of composite paste with extremely low water/binder ratios, a polycarboxylate superplasticizer with a water/reducing rate of 40% (Jiangsu Subot New Materials Co., Ltd., China) was used. The dosage of polycarboxylate superplasticizer for the composite paste with different water/binder ratios of 0.16 and 0.21 was 2% and 0.8%, respectively.

Composite pastes with different proportions of steel slag powder or limestone powder at water/binder ratios of 0.16 and 0.21 were prepared. The flow diameter (i.e., the larger the flow diameter, the better the fluidity), setting time (i.e., the time when the fluidity is lost) and marsh cone flow time (i.e., the shorter the flow time, the better the fluidity) were determined to analyze the fluidity of the composite pastes. the rheological properties of composite paste at a water/binder ratio of 0.16 for rheological properties tests were determined, and the rheological parameters were obtained by the H-B model. The adsorption performance of the polycarboxylate superplasticizer was analyzed by testing the TOC content.

When the ratio of water/binder is 0.16, both limestone powder and steel slag powder initially increase the flow diameter of the composite paste. However, the fluidity of the composite paste decreases over time, and the reduction is more pronounced with steel slag powder. This is because the nucleation and hydration promotion effect of limestone powder can reduce the loss rate of flow diameter, while the rough particles of steel slag powder increase the internal friction force, resulting in a decrease in the flow diameter of composite paste. Adding limestone powder and steel slag powder both shortens the setting time and marsh cone flow time of the composite paste. However, the steel slag powder addition of 30 % delays the final setting time due to its delaying effect. Also, limestone powder can enhance the thixotropy and reduce the yield stress and plastic viscosity, thereby improving the rheological properties. In contrast, steel slag powder can increase the yield stress and plastic viscosity, thereby weakening the rheological properties and thixotropy. Steel slag powder and limestone powder both can enhance the adsorption effect of polycarboxylate superplasticizer. Steel slag powder has a stronger adsorption effect. The composite paste containing limestone powder has a higher free water content. This is because the rough and porous surface of steel slag itself and the uneven particle shape lead to the physical adsorption of polycarboxylate superplasticizer molecules on the surface of steel slag particles, thereby reducing the effective content of the water reducer dispersion. Increasing the water/binder ratio to 0.21 results in a decrease in the flow diameter of the composite paste. Furthermore, the setting time and marsh cone flow time can prolong due to the reduction in the dosage of polycarboxylate superplasticizer.

The results showed that the fluidity loss rate of composite paste with limestone powder could be lower than that of composite paste with steel slag powder. Compared to steel slag powder, the addition of limestone powder reduced the setting time and marsh cone flow time of the composite paste. The addition of limestone powder could shorten the yield stress and plastic viscosity of the composite paste. Therefore, the composite paste with limestone powder had better rheological properties and stronger thixotropy rather than that with steel slag powder. The addition of limestone powder could improve the rheological properties of the composite paste. Compared to composite paste with limestone powder, a better adsorption effect of polycarboxylate superplasticizer on the surface of the composite binder with steel slag powder could be obtained. The free water content of the composite paste with limestone powder was still higher than that of the composite paste with steel slag powder. The fluidity and rheological properties of the composite paste with limestone powder could be better. The comprehensive analysis indicated that a positive effect of limestone powder on rheological properties of composite paste at an extremely low water/binder ratio could be more dominant than that of steel slag powder.

Inspired by the prestressing principle of tempered glass, this study proposes a novel strengthening method for mortar that establishes internal compressive stress without the use of steel reinforcement. This is achieved by restraining the expansion of an expansive surface layer through a shrinking substrate. The resulting pre-compressive stress significantly enhances the flexural performance of the mortar. Experimental results demonstrate that the flexural strength of unreinforced prestressed mortar (UPM) increases with the cross-sectional height ratio. When 6% expansive agent is incorporated into the surface layer and the height ratio reaches 4, the flexural strength improves to 11.24 MPa. As the expansion rate of the surface layer increases, the flexural strength of UPM continues to rise. For instance, with a 10% expansive agent, the 28 d flexural strength increases by up to 45%. Furthermore, by employing the casting method with time interval combined with surface roughening and the use of an interfacial adhesive, the flexural strength of UPM can reach 12.33 MPa. The maximum flexural-to-compressive strength ratio approaches 1∶3. These findings highlight the critical role of interfacial bonding quality in enhancing flexural performance—stronger bonding leads to greater strength gains. The existence of internal prestress is further verified through stress release monitoring during specimen cutting, where a maximum released stress of 1.68 MPa is recorded. However, it is important to note that the cutting method only releases a portion of the internal stress. As some prestress remains unreleased, the actual prestress present in UPM is likely higher than the measured value. Therefore, the released stress should not be regarded as the absolute value of the applied prestress. Although a correlation exists between stress relief and flexural strength improvement, the two are not equivalent. In practical applications, UPM should be evaluated and selected based on the overall enhancement in flexural strength rather than the measured released stress alone. Mechanistically, the expansion of the surface layer induces tensile stress in the substrate and compressive stress in the surface layer. When the surface layer is relatively thick, the compressive prestress decreases with depth along the normal direction of the interface. This stress gradient resembles that observed in tempered glass, where compressive stress is lower at the edges than at the center, rendering edge regions structurally more vulnerable. To better understand and optimize the internal stress distribution, stress redistribution, and failure mechanisms under external loading, finite element simulations are recommended. These analyses can offer theoretical insights that support the practical design and application of UPM in engineering practice. Overall, this study introduces a new approach for mortar toughening and strength enhancement, providing a foundation for the development and application of unreinforced prestressing technology in thin-walled structures and structural repair projects.

ECC is characterized by high ductility and high toughness, and the generation of a large number of fine cracks gives the material a significant self-healing ability, thereby improving its safety and durability. In some northern regions of China, the soil pH value exceeds 9 in alkaline environments, sulfate content in marine environments can generally reach 1400-2700 mg/l, structures are typically subjected to dry-wet cycles in areas of water level fluctuations, and water is a necessary condition for the occurrence of self-healing. In order to comprehensively and systematically analyze the self-healing performance of ECC under different environments and preloading conditions, the tensile and flexural properties of ECC after self-healing under three environments (5% Na₂SO₄, saturated Ca(OH)₂, and water) were investigated in this paper, providing a basis for the further application of ECC materials in engineering practice.

Three environments were designed: 1) 5% Na₂SO₄ solution (by mass) under dry-wet cycles; 2) Saturated Ca(OH)₂ solution under dry-wet cycles; 3) Water environment which served as a control group. The preloaded specimens were placed in them individually.

A total of 9 batches of dog-bone specimens under three different environments were prepared. The group preloading to 1% and 2% strain and the intact specimens without preloading were placed in the self-healing environment. After reaching the corresponding age, tensile tests were conducted to obtain stress-strain curves under different conditions, and the recovery of tensile strength and ultimate deflection was analyzed.

Besides, 9 batches of four point bending thin plates under three different environments were prepared, the ultimate strength was obtained from the intact specimens, and 20% and 40% of the value were selected as the preloading levels for bending tests. The preloaded group and intact specimens without preloading were placed in the self-healing environments for the flexural tests to obtain the load-deflection curves, the recovery of properties such as flexural strength, ultimate deflection, and flexural toughness were investigated.

Preloading to 1% strain gained a significant advantage in restoring the tensile strength of specimens at different ages in three different environments. The tensile strength of ECC was improved when subjected to dry-wet cycle in both Na₂SO₄ and Ca(OH)₂ solutions, with a supreior recovery ability observed in the Ca(OH)₂ environment. It is also noted that under a 1% preload, the tensile strength increased by about 36.33% throughout the 90 d aging process.

Reloading after pre-tensioning led to a significant improvement on the tensile ductility of specimens at 0 d, with an increase in ultimate tensile strain of up to 138.4% with the preloading level increasing from 0% to 2%. The water environment had a significant effect on the ductile healing of intact specimens without preloading at long ages, with an increase in ultimate tensile strain of 137.52% with age from 0 d to 90 d, while the Na₂SO₄ and Ca(OH)₂ environments weakened the ductility of cracked ECC specimens, with the largest decreases of 52.78%-65.39% at 2% preloading, and the Na₂SO₄ environment had a better effect on the maintenance of ductility in the specimens. At 90d of self-healing, increasing preloading levels weakened the tensile ductility of the specimens, with the greatest decrease in water.

Under the unsoaked condition, 20% preloading increased the flexural strength of the bent plate, but weakened the flexural ductility. As the age of self-healing increased, the initial flexural strength and ultimate flexural strength of the specimens under 40% preloading were improved, and Na₂SO₄ environment was more effective, with an increase of 28.91% and 40.35%, respectively, compared with that at 0 d. Preloading by 20% significantly increased the ultimate deflection, and the best effect was achieved in water, with an increase of up to 202.83% compared with 0 d. Flexural ductility was positively correlated with multi-seam cracking pattern. Na₂SO₄ and Ca(OH)₂ environments were effective in restoring the flexural strength of the specimens at a short age, and had a positive effect on the healing of microcracks. After 90 d of self-healing, the initial flexural toughness ratio and residual flexural toughness ratio of the preloaded 20% thin plate recovered best. Ca(OH)₂ environment was favorable to enhance the pre-peak flexural toughness, and the water environment was favorable to enhance the post-peak flexural toughness.

The tensile and flexural properties of ECC after self-healing in three environments were investigated. The results showed good tensile and flexural self-healing properties. A 1% preloading level was clearly beneficial for tensile strength recovery at all ages in the three environments. Although Dry-wet cycling in Na₂SO₄ and Ca(OH)₂ enhanced the tensile strength of ECC, it reduced the tensile ductility of preloaded ECC specimens. The greatest decrease in ductility occured at 2% preloading, and the Na₂SO₄ environment was more effective in maintaining it. After 90 d of self-healing, tensile ductility of the specimens decreased with increasing preloading levels. A 40% preloading level had the most significant effect on flexural strength recovery after long-term aging, particularly in the Na₂SO₄ environment. The ultimate deflection recovery of the specimen was most significant at a 20% preloading level, with better performance in water. Furthermore, 20% preloading significantly improved the flexural toughness after 90 d of self-healing.

Calcium sulfate hemihydrate is widely used in the construction and industrial fields, and its application effect largely depends on its dispersibility. Early studies mostly used anionic polymer dispersants to improve its performance. Later, with the application of polycarboxylate superplasticizers (PCE) in the cement and concrete fields, the exploration of its application in half-water gypsum began. Research shows that the charge characteristics and side chain structure of PCE have a significant impact on its adsorption behavior and dispersibility. Introducing functional groups using strong complexation ability of Ca²⁺ can improve its dispersibility. However, the mechanism of the influence of PCE molecules' adsorption behavior and dispersibility in calcium sulfate hemihydrate paste with high SO₄^2- concentration and weak acidity is still unclear. Therefore, three types of PCE containing carboxyl groups, sulfonic acid groups and phosphate ester groups were synthesized. Through various means, the dispersibility and interface interaction mechanism of these PCE in the calcium sulfate hemihydrate system were systematically studied, providing a theoretical basis for the development of high-performance PCE molecules.

Using MAA, SMAS and HEMAP as the main chains, PCEs containing carboxylic acid groups (PCE-C), sulfonic acid groups (PCE-S) and phosphate groups (PCE-P) were synthesized. The synthesized water reducers were subjected to tests such as PCE molecular structure characterization, gypsum paste test, hardened gypsum performance test, simulated gypsum pore solution test, quantum chemical calculation and molecular dynamics simulation.

Through comprehensive analysis of ¹H NMR spectrum and FTIR spectrum, it can be known that carboxylic acid groups, sulfonic acid groups and phosphate groups were successfully introduced into the three PCEs respectively. GPC showed that the conversion rate of the monomers was above 90%. The performance test of gypsum paste showed that the three water-reducing agents all had dispersing effects on gypsum, among which the dispersing effect and retention effect of PCE-S were the best. After introducing Na₂SO₄ (SS), the influence on the other two water-reducing agents was significant, while PCE-S still showed good fluidity. The water-reducing agents prolonged the setting time of gypsum, while the introduction of SS reduced the setting time of gypsum to a certain extent. PCE-S exhibited better crystal growth regulation ability and had a relatively small impact on the strength of the gypsum hardened paste. The introduction of SS led to a slight decrease in the absolute values of the zeta potential of PCE-S and PCE-P, indicating high adsorption stability in the presence of sodium sulfate. In the adsorption capacity test, PCE-S showed a higher saturated adsorption capacity, and it was least disturbed by SS. The introduction of SS significantly increased the particle size of PCE and broadened the particle size distribution, weakening the electrostatic stability between molecules. The pH titration test showed that PCE-S and PCE-P had higher charge density in the gypsum system. The static charge density distribution of gypsum crystal surfaces showed that the sulfonic acid groups induced obvious electron cloud rearrangement and structural reconstruction on the gypsum surface, and through the "cooperative adsorption effect" induced by surface reconstruction, its macroscopic adsorption capacity and dispersion effect could be significantly enhanced.

The type of PCEs adsorption groups significantly affects its dispersion performance in the calcium sulfate hemihydrate system. PCE-S shows the best improvement in fluidity and resistance to sulfate interference, followed by PCE-P and PCE-C, with the poorest performance. The degree of ionization and apparent charge density of PCE in the calcium sulfate hemihydrate environment play an important regulatory role. PCE-S and PCE-P almost completely ionize at the experimental pH, for both having a higher apparent charge density and enhancing electrostatic adsorption with Ca²⁺ and the degree of molecular unfolding, thereby improving its dispersion stability. The sulfonic acid groups achieve enhanced "cooperative adsorption effect" by inducing significant electronic rearrangement and crystal plane reconstruction on the (001) surface of calcium sulfate hemihydrate, thereby improving its performance in macroscopic adsorption capacity and dispersion properties. Additionally, PCE-S possesses a higher affinity for Ca²⁺, evidenced its stronger complexation capacity and superior competitive adsorption against SO₄^2-. Its increased steric hindrance and high surface coverage further lead to enhanced dispersion efficiency.

Silane-impregnation is one of the most used surface waterproofing treatments for concrete structures to improve the durability, but long-term engineering observations find that silane-impregnation will deteriorate during the service life of concrete structures. The deterioration of silane treatment undermines its effectiveness in protecting the concrete structures and improving their durability. However, the deterioration mechanism of silane-impregnated treatment has rarely been studied. To fill this gap, This paper studied the deterioration mechanism of surface hydrophobicity of silane-impregnated cement-based materials through laboratory accelerated tests. The accelerated tests include drying-wetting cycles by pure water and ultraviolet (UV) irradiation. Then, the silane-impregnated materials are characterized by infrared spectroscopy, pyrolysis gas chromatography-mass spectrometry, thermal analysis system, and dynamic water vapor desorption. The aim of this study is to provide a research basis for quantifying the usage life of surface silane impregnation treatment used in engineering.

Laboratory accelerated deterioration tests were conducted to study the deterioration mechanism of surface hydrophobicity of silane-impregnated cement-based materials. The accelerated tests include drying-wetting cycles by pure water and ultraviolet (UV) irradiation. For drying-wetting cycle tests, an experimental design of 4 h wetting time and 8 h drying time was adopted. The irradiance of ultraviolet (UV) irradiation tests is irradiance, and the irradiation cycle is 30 d. After accelerated deterioration tests, the silane-impregnated cementitious materials are characterized by infrared spectroscopy, pyrolysis gas chromatography-mass spectrometry, thermal analysis system, and dynamic water vapor desorption.

Infrared spectroscopy was conducted to study the silane molecular structural changes during accelerated deterioration tests; pyrolysis gas chromatography-mass spectrometry and thermal analysis system was conducted to characterize the silane content in cementitious materials; and dynamic water vapor desorption was conducted to characterize the hydrophobic performance of impregnated cementitious materials.

Under the action of drying-wetting cycles: 1) infrared spectroscopy results of impregnated cementitious materials showed that the relative content of different alkyl groups had not changed, indicating that the molecular structure of silane was stable; 2) Both pyrolysis chromatography-mass spectrometry and thermal analysis system results showed that the silane content in impregnated cementitious materials was decreasing, combining with the analysis of infrared spectroscopy results, silane in impregnated cementitious materials was most likely underwent hydrolysis reaction, and the hydrolyzed silane continuously detached from impregnated particles under the action of drying-wetting cycles; 3) In addition, the water vapor adsorption capacity of cement paste powders, the water sorptivity and contact angle of mortar specimens had all increased.

Under the action of UVB-313 ultraviolet radiation: 1) the silane impregnated on the surface of cement-based material particles underwent chemical bond breakage. Based on the wavelength of the ultraviolet radiation, it is inferred that this likely caused the breaking and recombination of tertiary carbon bonds. Infrared spectroscopy, gas chromatography-mass spectrometry, thermal analysis system, and water vapor adsorption capacity results all indicate this aging process; 2) Due to the direct impact of ultraviolet irradiation on the surface of impregnated materials, the water sorptivity and contact angle of the impregnated mortar specimens had changed significantly. However, it should be noted that this significant aging effect depends on the UV wavelength and irradiation energy of UVB-313 lamps using in the laboratory. In the natural environment, the irradiance of sunlight is much lower than that of UVB-313. Therefore, the accelerated UV testing results are primarily suitable for mechanistic research and cannot not be directly applied to the aging behavior of silane impregnations in natural environment.

The stability of cement-based materials impregnated with high-purity silane (isobutyl triethoxysilane) was confirmed through drying-wetting cycles and ultraviolet irradiation. The stability is attributed to the condensation network between silane monomers and the condensation reaction with the surface of cement-based materials (Si—O—Si bonds); The wavelength distribution of natural light and the barrier effect of cement-based materials make the deterioration caused by ultraviolet irradiation less likely to occur in natural environment. The deterioration caused by hydrolysis of silane should be more related to the effect of environmental water. Based on the study of the drying-wetting cycles results and the mechanism of silane hydrolysis, it can be reasonably inferred that the hydrophobicity degradation of cement-based materials surface (layer) impregnated with silane in natural environments is related to the hydrolysis of silane and the migration of hydrolyzed silane.

For the four different cement pastes used in this study, OPC showed the greatest decrease in water vapor adsorption capacity after silane impregnation, indicating that the silane impregnation effect of OPC is better than that of OFA, OSG, and OFS. Combined with the chemical reaction mechanism of silane impregnation, alkaline conditions can catalyze the impregnation reaction of silane. However, pozzolanic reaction, occurred in the three cementitious materials mixed with fly ash or slag, consumes the alkali reserve of the pore solution. Therefore, the alkalinity of the pore solution of OPC is the highest, resulting in the best silane impregnation effect.

Strain Hardening Cementitious Composite (SHCC) is one kind of engineered materials designed based on the principles of mesoscale and fracture mechanics. It exhibits a tensile strain capacity of up to 3%-8%, which are approximately 300~800 times greater than ordinary concrete. SHCC has been successfully used in the rehabilitation of dams and bridge decks in Japan owing to its excellent mechanical properties. These structures withstand not only static loads but also potential dynamic impact loads during service. To accelerate the application of SHCC in seismic-resistant structures, blast-resistant structures and other critical infrastructure, it is essential to have knowledge of its dynamic mechanical properties and failure mechanisms. Therefore, this study characterized the mechanical behavior and failure modes of SHCC under high strain rate loading using Split Hopkinson Pressure Bar (SHPB) testing, with high-speed digital image correlation (DIC) capturing full-field strain evolution on specimen surfaces during impact tests. X-ray computed tomography (X-CT) technology was used to observe the internal damage of SHCC after high strain rate loading. The influences of strain rate on the dynamic peak stress, energy absorption capacity, dynamic growth factor, full-field dynamic strain distribution, internal damage form and microstructure of SHCC were investigated.

This study adopted Ordinary Portland cement 42.5 (GB175—2023, with an average particle size of 27 μm and the particles of 3-30 μm account for about 70%) and Class I fly ash (GB1596—2017, with a particle size less than 10 μm account for more than 80%) as cementitious materials. The quartz sands with a maximum particle size of 0.3 mm were used as fine aggregates. SHCC samples were prepared using PVA fiber produced by Kuraray, Japan. Meanwhile, the polycarboxylate superplasticizer (provided by Nanjing Subote New Materials Co., Ltd.) was added to enhance the workability, ensuring uniform fiber dispersion of the slurry.

After mixing, the slurry was cast into molds, followed by vibration, and demolded after a 24 h curing period under standard laboratory conditions. Specimens were then cured under standard conditions until testing age. Static compression tests were employed using 70.7 mm cube specimens. Moreover, dynamic properties were evaluated using a SHPB system with ϕ48×25 mm cylindrical specimens. During impact loading, high-speed industrial camera was used to captured specimen deformation for subsequent VIC-2D digital image correlation analysis. The damaged SHCC samples after high strain rate loading were scanned using X-ray computed tomography and observed with scanning electron microscope (SEM).

The dynamic compressive stress-strain curve of SHCC presents typical three-stage characteristics under high strain rate loading: linear elastic deformation stage, nonlinear strain hardening stage and damage softening stage. Dynamic compressive stress-strain curves of SHCC exhibit strong dependence on strain rate. At higher strain rates, the dynamic compressive response of SHCC exhibits significant strain hardening, primarily governed by fiber bridging effects and secondarily enhanced by compaction of micro-defects. The dynamic peak stress of SHCC increases significantly with strain rate, demonstrating a pronounced strain rate sensitivity. Simultaneously, the dynamic peak stress of SHCC increases substantially at high strain rates. This enhancement primarily arises from the significantly shorter stress wave propagation time compared to the characteristic relaxation time of internal defects under high strain rate loading. Consequently, stress waves preferentially propagate through high-strength regions (e.g., matrix grains and fiber reinforcements), inducing localized stress concentrations. The energy absorption capacity of SHCC increases with the increase of strain rate. When the strain rate exceeds 180 s^-1, E_d shows an approximate exponential growth trend, reaching 546.12 kJ/m³ at 188 s^-1, with a relative increase of about 106.42%, demonstrating exponential growth behavior and significant strain rate strengthening. This energy absorption surge primarily stems from synergistic multi-scale damage mechanisms. At identical strain rates, the dynamic increase factor for stress demonstrates both a higher mean value and greater variability than that for energy, reflecting its stronger dependence on strain rate effects. This indicates greater strain rate sensitivity in dynamic peak stress than energy absorption capacity, confirming the dynamic peak stress is more susceptible to strain rate.

The main conclusions of this paper are summarized as following. SHCC exhibits strain rate sensitivity of dynamic mechanical properties, with both peak stress and energy absorption capacity increasing significantly under high strain rate loading. The sensitivity of dynamic stress to strain rate is significantly greater than that energy absorption capacity. DIC analysis shows that the vertical expansion of the specimen is substantially more significant than the lateral compression. X-CT analysis reveals significantly denser crack distribution near specimen edges compared to the central region. Moreover, multiple cracks appear at the same crack point, which is an essential reason for the excellent dynamic mechanical properties of SHCC.

The predominant characteristics of high-altitude climates include low air pressure, low humidity, and large diurnal temperature fluctuations. In practical engineering scenarios in high-altitude regions, many pier surface cracks only appear several months after erection, and cyclic thermal stress is identified as the main trigger for such cracking. The thermal stress in concrete structures has been investigated for decades but remains incompletely understood. Structural engineers typically regard concrete as an isotropic material and calculate the thermal stress using code-specified coefficients of thermal expansion (CTEs) along with temperature conditions and constraints. Because the CTE of hardened cement paste is more than twice that of many aggregates, reducing the CTE of coarse aggregates can further exacerbate the thermal deformation incompatibility between the coarse aggregate and mortar matrix. In this paper, a comprehensive thermal-elastic mechanics model for pier concrete was developed to analysis the temperature-induced surface stress.

A series of mechanical and thermophysical tests were conducted on the diorite aggregate, ITZ cement paste, and mortar, and concrete. A test pier was constructed on open ground near the Yarlung Zangbo River at an altitude of 3800 m. The pier had a diameter of 1.8 m and height of 2 m. Temperature sensors were embedded in the cross-section at a height of 1 m, positioned along the south-north and east-west directions. The embedding depths (distances from the pier surface) were 0, 1.5, 3, 4.5, 6, 7.5, 9, 12, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80 cm, and 90 cm.

A model of the bridge pier concrete for surface-level analysis was constructed. The model consists of a concrete unit formed as a sphere-shell-shell composite, including the aggregate, ITZ, and mortar layers, embedded in the surface layer of a bridge pier. Outside this unit, the pier concrete was treated as an isotropic, homogeneous elastic material. The real-time internal temperature fields of high-altitude concrete bridge piers, measured on-site, were incorporated into the model. By applying thermoelastic mechanics theory and finite element solutions for plane strain problems, the three-dimensional thermal stresses on the surface layer of high-altitude bridge piers were analyzed.

During the experimental period, the lowest and highest temperatures on the bridge pier in the high-altitude region were 9.6 ℃ and 42.6 ℃, respectively. These occurred before sunrise and sunset on sunny days, respectively, corresponding to the local maximum temperature gradients during the surface heating and cooling stages, as well as the maximum temperature difference between the surface and center during these stages. The thermal stress on the pier concrete surface was obtained by superimposing the stresses caused by the uneven distribution of the internal temperature field and those caused by the incompatible thermal deformation among the different components in the surface concrete

Before the erection of the upper structures, the absolute values of the tangential and vertical stresses were the same; therefore, only one curve was observed. From 22:00 to 8:00, the pier concrete surface was in tension, whereas from 11:00 to 22:00, the pier concrete surface was in compression. The surface of the pier concrete was subjected to biaxial forces of equal magnitude with a maximum compressive stress of 12.52 MPa and maximum tensile stress of 2.15 MPa, respectively at 18:00 and 8:00. According to the fatigue equation, the concrete was predicted to crack after 21 d of temperature cycling. Moreover, if humidity-induced stress is added on top of this, the tensile stress may approach or even exceed the concrete's tensile strength, thereby posing a significant risk of cracking.

After the erection of upper structures, the tangential and vertical stresses no longer coincide because the upper structures have been erected. The curve of the tangential stress is unchanged, whereas the curve of the vertical stress is translated downwards by 1.57 MPa due to the structural deadweight. Therefore, the maximum tangential compressive stress remained 12.52 MPa, whereas the maximum vertical compressive stress increased to 14.09 MPa. Additionally, the maximum tangential tensile stress was 2.15 MPa, and the maximum vertical tensile stress was 0.58 MPa. According to Appendix C of GB/T 50010 and the fatigue equation, stresses are unlikely to cause cracking of the pier concrete surface.

Although a higher CTE of the coarse aggregate slightly increased the maximum compressive stress, the differences among the three groups of concrete were minimal and could be ignored. Specifically, the maximum compressive stresses on the pier concrete surface were 12.54, 12.45 MPa, and 12.56 MPa when using diorite, limestone, and basalt, respectively. By contrast, a lower CTE of the coarse aggregate results in a greater maximum tensile stress on the pier concrete surface. For example, when using limestone, which has a low CTE, the maximum tensile stress on the pier concrete surface is 2.28 MPa, compared to 2.17 MPa when using diorite and 2.14 MPa when using basalt.

The finite element simulation results indicated that the maximum compressive stress on the pier concrete surface was 11.72 MPa, whereas the maximum tensile stress was 2.10 MPa. These results are approximately consistent with the theoretical calculations. This consistency provides mutual verification.

Surface cracking in pier concrete occurs predominantly before the erection of upper structures. Under sunny conditions, the orthogonal decomposition of the superficial stress revealed that the maximum compressive stress during the day was approximately 12.52 MPa, whereas the maximum tensile stress was approximately 2.15 MPa. This tensile stress approached the tensile strength of the C35 concrete under biaxial tension. The risk of cracking increased significantly when humidity-induced stress was considered. After the erection of upper structures, the maximum tangential tensile stress on the pier surface remained at 2.15 MPa while the maximum vertical stress decreased to 0.58 MPa, both of which are well below the tensile strength of C35 concrete under biaxial tension. Although the use of coarse aggregates with a lower coefficient of thermal expansion reduced the tensile stress induced by temperature gradients, it increased the stress owing to material deformation incompatibility, leading to a slight increase in the maximum tensile stress on the pier concrete surface.

As the depth of resource exploitation in our country increases, power shock disasters occur frequently in underground engineering, which will severely restrict the rapid development of the economy. In the process of strengthening the foundation with fly ash concrete, the strengthened body not only bears static loads but also dynamic loads, forming initial defects such as joints and cracks inside. The development, extension, and penetration of cracks under dynamic loads will further reduce the stability of the strengthened structure. Therefore, it is urgent to deeply study the mechanical properties of fly ash concrete with cracks under dynamic loads.

Dynamic impact tests were carried out with the split Hopkinson pressure bar (SHPB) test system on the samples with different fracture angles. Digital image correlation system (DIC) was used to monitor the deformation of surface strain. Characteristics of the samples with different fracture angles after dynamic impact were compared and analyzed including the dynamic strength, energy dissipation and fractal fracture.

The results show that with the increase of the crack angle, the peak stress-strain presents a typical V-shaped pattern, reaching the minimum value at 30°. With the increase of fracture angle, the initial damage zone also moved from the fracture tip to the middle of the fracture, and then the damage range gradually expanded until the formation of macro failure. The failure modes are tensile failure (0°), shear failure (30°), tension-shear mixed failure (45° and 60°) and collapse failure (90°), respectively. The dissipated energy of the sample firstly increase and then decrease with the increase of the fracture angle, reaching the maximum value at 30°. The process of energy accumulation and transformation in fractured rock mass is characterized by the strain energy density. With the increase of the crack angle of the sample, the fractal value of the sample first increases and then decreases, and reaches a maximum value at 30°, indicating that a higher degree of sample breakage associated with dissipates relatively large energy. Based on fracture mechanics, the analytical solution of stress around the crack is given, and the variation of crack initiation angle, stress distribution and energy dissipation under different crack angles is theoretically explained, which is in good agreement with the experimental measurements.

This paper uses the SHPB and DIC systems to conduct dynamic impact tests on specimens with cracks at different angles, and compares and analyzes the dynamic strength, energy dissipation, and fracture fractal characteristics of specimens with different crack angles after dynamic impact, and gives the theoretical results of the stress around the crack. To provide some theoretical support for the deformation control of cracked concrete mixed with fly ash under dynamic action.

Ultra-High-Performance Concrete (UHPC) demonstrates substantial potential in civil engineering applications owing to its superior mechanical characteristics and enhanced durability. The material's exceptional workability and structural adaptability make it particularly effective for thin-layer strengthening of aging concrete structures. Pumping technology has become prevalent in construction operations due to operational efficiency and site adaptability. Existing investigations have documented performance variations in conventional concrete materials under diverse pumping conditions. These studies indicate material-specific response patterns, with post-pumping mechanical properties exhibiting notable disparities across concrete categories. However, the mechanical behavior evolution of UHPC under pumping conditions remains insufficiently explored, particularly given its distinct material composition compared to conventional concrete. This study conducts a controlled experimental investigation to evaluate the mechanical performance variation of UHPC subjected to three transport-casting conditions: non-transported baseline, screw pump transportation, and piston pump transportation. Through systematic comparison of pre- and post-pumping mechanical parameters, the research aims to clarify the pumping-induced performance modifications in UHPC.

With transport methods (direct casting without transport, screw pump transport, piston pump transport), casting directions (random casting, parallel casting, perpendicular casting; Here parallel casting denotes casting along the long axis of prismatic specimens, while perpendicular casting denotes casting perpendicular to this axis), and steel fiber volume fractions (0%, 1%, 2%) as test parameters, 11 specimen groups were fabricated. Each group comprised 4 prismatic specimens for flexural strength testing and 4 cubic specimens (cut from the prisms) for cubic compressive strength testing. During flexural testing, electrical resistance strain gauges with a 20 mm gauge length were bonded to the tensile face of specimens to quantify longitudinal strain development within the pure bending zone prior to crack initiation. High-precision displacement transducers were vertically aligned at mid-span to quantify flexural deflection, complemented by dual auxiliary sensors longitudinally mounted on opposing sides of the specimen's tensile surface to monitor elongation within the pure bending zone. Following mechanical testing, prismatic specimens were longitudinally divided into four sections. Each exposed surface was digitally imaged using high-resolution photographic equipment. Subsequent computational analysis quantified the angular deviation between steel fiber orientations and corresponding cross-sectional normals through dedicated image processing algorithms.

Screw pump transport casting significantly influenced fiber orientation in specimens. Compared with directly cast specimens, at a 1% fiber content, the fiber orientation angle of screw-pumped parallel-cast specimens decreased by 9.8° across the full section, with the orientation coefficient increasing from 0.454 to 0.607; at 2% fiber content, the orientation angle decreased by 10.6°, and the coefficient increased from 0.461 to 0.637. Which indicates that screw pump transport with parallel casting significantly enhances fiber orientation in the mixture, with similar orientation effects across different fiber contents. Piston pump transport had no significant impact on fiber orientation. Compared to direct casting, at 1% fiber content, the piston-pumped parallel and perpendicular casting methods reduced fiber orientation angles by 1.9° and 0.2°, respectively, and increased the orientation coefficients by 5.7% and 0.4%, indicating no obvious fiber orientation occurred during piston pump transport. A significant wall effect was observed in fiber distribution across specimen sections: fiber orientation coefficients at the edges were higher than those in the middle for all specimens, with notable differences across casting methods. The difference in orientation coefficients between edge and middle sections reached 30%-35% for directly cast specimens; this was reduced to 5%-10% for screw-pumped parallel-cast specimens but expanded to 35%-45% for perpendicular-cast specimens; piston-pumped specimens exhibited edge-middle differences of 15%-20%. Compression tests showed that UHPC compressive strength decreased by 2.6%-8% after transport, though the reduction was not significantly affected by pumping method, casting direction, or fiber content—primarily attributed to disturbance of the UHPC pore structure during pumping. Flexural tests revealed that at 1% and 2% steel fiber contents, screw pump transport with parallel casting increased flexural strength by 19.9% and 32.1%, respectively, compared to non-pumped direct casting. Flexural strength of piston-pumped specimens remained similar to direct casting, with differences within 6%.

Based on the test results, the recommended values for the steel fiber orientation coefficient in UHPC under different transport and casting methods are as follows: 0.5 for UHPC with direct casting or piston pump transport-casting, 0.62 for UHPC with screw pump parallel casting, and 0.38 for UHPC with screw pump perpendicular casting. When screw pumps are employed, material placement direction should align with the structure's principal stress direction. No directional constraints are required for piston pump applications. The flexural strength of UHPC under different transport methods and casting directions was predicted using the proposed fiber orientation coefficients, and the predicted results closedly matched the experimental measurements.

Shanxi Province, as one of the regions with the largest accumulation of coal gangue in China, faces an urgent demand for efficient resource utilisation of bulk solid waste to promote regional sustainable development. The rapidly developing 3D printing construction technology, with its precise control over material distribution and the advantage of forming complex components without molds, provides an innovative path for the large-scale consumption of solid waste. Existing research focuses on the application of coal gangue as fine aggregate substitute in printing mortar, while studies regarding its utilisation as coarse aggregate in 3D printed concrete remain unreported. It is vital to reveal the micro/mesoscale mechanisms governing the printability and fracture mechanics behaviour of such concrete material, in order to develop low-carbon coal gangue-based concrete materials and advance their engineering applications. This study innovatively uses coal gangue as a coarse aggregate and combines it with 3D printing technology to produce a novel 3D printed coal gangue concrete (3DP-CC), offering a data foundation and experimental reference for the resourceful and intelligent application of coal gangue.

In the experiment, 3DP-CC specimens were prepared with coal gangue contents of 0%, 10%, 20%, and 40% by mass. Firstly, the printing properties were evaluated, including flowability, buildability, extrudability, and printable time. Flowability was measured using a concrete slump cone in accordance with the JG/T 248 standard. Buildability was assessed through cylindrical slump and layer stacking tests. Extrudability was determined by evaluating the continuity and uniformity of the extruded filaments, with measurements of their width and thickness. Printable time was recorded as the duration over which the mixture could be extruded smoothly and consistently. Secondly, the internal microstructures of the 3DP-CC were examined using high-resolution X-ray CT scanning technology, allowing for the analysis of pore and aggregate characteristics. Finally, uniaxial compression and three-point bending tests were performed to determine the compressive and flexural strengths. The anisotropy index λ was introduced to quantify the directional variation in mechanical strength, followed by an analysis of the underlying mechanisms by which printing direction and coal gangue content influence mechanical properties. Failure modes were examined to highlight the critical roles of weak interlayer and interstrip interfaces, thereby providing further insight into the microscopic mechanisms governing the mechanical anisotropy of 3DP-CC.

The test results indicate that both the compressive and flexural strengths of 3DP-CC exhibit a trend of first increasing and then decreasing with rising coal gangue content, accompanied by notable mechanical anisotropy. At coal gangue contents of 10% and 20%, strength improved significantly while anisotropy was reduced. In contrast, a content of 40% led to a decrease in strength and an increase in anisotropy. These findings suggest that an appropriate amount of coal gangue coarse aggregate can provide skeletal support, thereby enhancing the mechanical performance of concrete and mitigating anisotropic effects. However, when the coal gangue content is too high, the low strength and high brittleness of the gangue itself, combined with an increase in medium and large pores, result in diminished overall strength and greater anisotropy.

Coal gangue content also has a pronounced effect on the pore structure of 3DP-CC, with total porosity initially decreasing and then increasing as the gangue content rises. At coal gangue contents of 10% and 20%, the total porosity of the specimens is 1.8% and 2.1%, respectively—both lower than the 3.9% observed in the control 3DPM. However, the total porosity increases to 3.4% in the 3DP-CC-40 specimens. From a microstructural perspective, the morphology of coal gangue aggregates plays a significant role in this change. The aggregates exhibit rough surfaces and angular shapes, which lead to loose packing and reduced efficiency in filling medium and large pores. Consequently, the number of pores with equivalent diameters greater than 1 mm increases in 3DP-CC-40 specimens. Furthermore, the water absorption capacity of coal gangue affects the internal pore structure of the concrete. As the aggregates absorb water, the moisture content in the surrounding matrix decreases, resulting in increased matrix porosity.

In terms of failure modes, the compressive behaviour of 3DP-CC is strongly influenced by the loading direction. When loading is applied on the x-face, the failure pattern on the y-side face shows cracks initiating at both edges, propagating inward, and then extending vertically downward. These cracks often penetrate the interlayer interfaces, leading to a relatively brittle failure mode. In contrast, loading on the y-face or z-face produces a distinctive "π"-shaped crack pattern on the x-face, accompanied by spalling damage on both sides, indicating that the weak interlayer and interstrip interfaces play a critical role in crack development. The failure mode under three-point bending is also dependent on the loading direction. When the x-face serves as the loading surface, all specimens—regardless of coal gangue content—exhibit through-thickness, straight cracks and undergo brittle fracture into two parts. When the y-face is loaded, through-thickness cracks are absent in specimens with 0% coal gangue but present in those with higher contents. For loading on the z-face, only specimens with 40% coal gangue exhibit through-thickness cracks, while others show no visible cracking on the loading surface. These observations suggest that specimens loaded on the x-face exhibit greater brittleness, with cracks more readily propagating along interstrip interfaces.

The coal gangue content significantly influences the performance of 3D printed concrete (3DP-CC). Optimal mechanical properties—both compressive and flexural strength—are achieved at a coal gangue content of 10~20%, attributed to the skeletal support provided by the gangue particles and the associated reduction in porosity. However, at a content of 40%, both strengths significantly decrease due to the inherent weakness and brittleness of coal gangue, its tendency to induce cracking, and the increase in macroporosity. The pore structure exhibits a non-linear response to coal gangue content: total porosity decreases when the content is below 20%, but rises sharply at 40% as a result of insufficient fine aggregate filling, leading to the formation of numerous large pores. 3D printing introduces directional porosity, with the upper layers exhibiting slightly higher porosity. In addition, pronounced mechanical anisotropy is observed, as quantified by the anisotropy index. Increasing the coal gangue content tends to reduce the anisotropy of compressive strength. In contrast, the anisotropy of flexural strength exhibits a non-monotonic trend: it initially decreases and then increases, reaching a maximum at 40% gangue content. Failure is predominantly governed by weak interlayer and interstrip interfaces, along which cracks consistently propagate under both compressive and flexural loading. Coal gangue aggregates influence crack trajectories—cracks either bypass or penetrate the aggregates depending on local strength fluctuations. These findings offer valuable insights for the design of sustainable 3D printed concrete, emphasizing the importance of optimizing coal gangue content and microstructures to balance strength, anisotropy, and resource utilisation.

In recent years, the combination of construction 3D printing technology and ultra-high-performance concrete (3DP-UHPC) leverages the rapid construction and controllable shaping advantages of 3D printing along with the ultra-high strength and high toughness advantages of UHPC. Existing research on 3DP-UHPC, constrained by current printing technologies, has primarily focused on shorter steel fibres (6 mm and 10 mm), whereas the steel fibre length commonly used in engineering UHPC is 13 mm. Additionally, research on the micro- and meso-structural characteristics of 3DP-UHPC and the resulting anisotropic mechanical properties and damage-fracture mechanisms is relatively scarce. Therefore, revealing the multiscale effects of anisotropic compressive mechanical behaviour in 3DP-UHPC is of vital importance. This study employs 13 mm steel fibres to investigate the influence of different fibre volume fractions (0%, 0.5%, 1.0%, 2.0%) on the compressive performance of 3DP-UHPC. By combining digital image correlation technology (DIC) and X-ray computed tomography (CT) methods, we aim to clarify the coupling mechanism between fibre orientation and interlayer/interstrip interfacial defects, providing critical insights for material optimisation and engineering deployment of 3DP-UHPC.

In the experiment, 3DP-UHPC specimens with steel fibre volume fractions of 0%, 0.5%, 1.0%, and 2.0% were prepared. First, a particle packing-based mix design was carried out. By testing the particle size distribution of the raw materials and applying the modified Andreasen-Andersen model, the optimal packing proportions of the mortar matrix components were determined. Subsequently, in accordance with the GB/T 2419—2005 standard, a flow table test was conducted using an electric jolting apparatus to evaluate the printability of mixtures with varying fibre volume fractions. Furthermore, high-resolution X-CT was employed to acquire the internal structure of the specimens, from which microstructural parameters such as pore morphology and steel fibre orientation were extracted. Finally, uniaxial compression tests were performed using DIC to measure the compressive strength and to capture, in real time, the processes of crack initiation, propagation, and coalescence, thereby revealing the evolution characteristics of the failure modes. In this process, the synergistic effects between interlayer interfacial defects and fibre orientation were further analysed, and their coupling mechanisms during damage evolution were investigated, thereby providing deeper insights into the microstructural mechanisms governing the anisotropic mechanical behaviour of 3DP-UHPC.

The test results indicate that in the 3DP-UHPC specimens, a total of 37 irregularly shaped large pores were observed at interlayer and interstrip interfaces, with equivalent diameters ranging from 2000 μm to 3200 μm. These accounted for only 0.174% of the total pore volume, while the remaining 92.195% consisted of nearly spherical or ellipsoidal pores, resembling the typical pore morphology of cast UHPC. Notably, the maximum (3200 μm) and average (264 μm) equivalent pore diameters in the printed specimens were both smaller than those reported for cast UHPC (4500 μm and 700 μm), suggesting that the extrusion process during printing effectively compacts the material and facilitates air removal. Therefore, the total porosity of 3DP-UHPC (2.24%) was lower than that of cast UHPC (3.45%-5.90%).

Steel fibres within 3DP-UHPC exhibit a clear tendency to align along the printing direction (x-axis), with more pronounced orientation in the x-z plane than in the x-y plane. Specifically, 65.2% of fibres were aligned at θ = 75°-90° in the x-z plane, compared to 32.8% at φ = 75°-90° in the x-y plane. This anisotropic distribution stems from the filament geometry, where the height (17 mm) is smaller than the width (36 mm), creating spatial constraints that favour alignment in the vertical cross-section. In total, 3,324 fibres (52%) were oriented within the 60°-90° range, reflecting a strong directional preference induced by the layer-by-layer deposition process.

The compressive strength of 3DP-UHPC exhibits directional anisotropy, influenced by layer interfaces, fibre volume fractions, and fibre orientation. At fibre volume fractions of 0.5%, 1.0%, and 2.0%, compressive strength consistently followed the order x < y <z, with the weakest-to-strongest directional differences being 2.95%, 7.82%, and 19.50%, respectively. These values are all below 20% and notably lower than those reported in earlier studies. The achieved reduction in anisotropy is attributed to the optimisation of printing parameters in this study, particularly adjustments to printing speed and path, which improved inter-filament contact, enhanced pore filling, and mitigated the adverse effects of interfacial defects.

3DP-UHPC outperforms cast UHPC in compressive strength across all principal directions (x, y, and z) at fibre volume fractions of 0%, 0.5%, and 1.0%. This advantage is due in part to the absence of fibre sedimentation, water bleeding, and entrapped air defects that may occur during vibration-based casting. The extrusion-based fabrication in 3D printing mitigates these issues, resulting in improved uniformity and mechanical properties. At a fibre volume fraction of 2.0%, the printed specimens exceeded cast specimens in compressive strength along the z-axis by 10%, while showing slightly lower values in the x- and y-directions.

Increasing the steel fibre volume fraction enhances the compressive strength of both 3DP and cast UHPC. For the cast specimens, an increase in fibre content from 0% to 2.0% led to a 49.9% improvement in compressive strength. In the 3DP-UHPC specimens, the same increase led to strength enhancements of 16.8%, 11.0%, and 37.7% in the x-, y-, and z-directions, respectively, indicating that the reinforcing effect of fibres is directionally dependent. Even at a modest fibre volume fraction of 0.5%, compressive strength stabilised around 133 MPa across all directions, indicating a beneficial role in mechanical consistency. In contrast, fibre-free specimens showed brittle failure dominated by interfacial defects, resulting in greater variability in strength measurements.

The 3D printing process significantly influences the microstructural characteristics and mechanical performance of 3DP-UHPC. Although certain interlayer and interstrip voids are inevitably introduced during the 3D printing process, the extrusion-induced "degassing" effect helps lower the overall porosity, resulting in a denser microstructure in 3DP-UHPC compared to conventionally cast UHPC. Under the combined effects of nozzle extrusion and printing path guidance, steel fibres tend to align along the printing direction, in contrast to the random distribution observed in cast specimens. This directional alignment improves the crack-bridging efficiency of fibres. The compressive failure behaviour exhibits pronounced anisotropy: under loading in the x-direction, cracks preferentially propagate along interlayer interfaces, while in the z-direction, fibre alignment enhances bridging capacity and delays crack growth. The coupled effects of fibre orientation and interfacial quality are primary contributors to this anisotropy. Enhancing the extrusion-interlocking mechanism between adjacent filaments can improve interfacial densification, thereby mitigating mechanical anisotropy. The steel fibre volume fraction also plays a critical role in both strength and stability: increasing the fibre content significantly enhances compressive strength, particularly under loading in the z-direction; in contrast, lower fibre volume fractions, although less effective in improving strength, contribute to greater performance uniformity and structural consistency. This study further elucidates the multiscale mechanisms through which 13 mm steel fibres influence compressive anisotropy in 3DP-UHPC. It identifies key microstructural features, such as pore distribution and fibre orientation, and preliminarily establishes their correlation with macroscopic mechanical behaviour, providing valuable insights for optimising the structural performance of 3DP-UHPC.

Polypropylene (PP) separators are widely used in lithium-ion battery (LIBs) separators due to their excellent mechanical properties, high resistance of electrolyte corrosion, superior ionic permeability, easy processability, and low cost. However, PP separators have poor affinity with electrolytes and lack the function of inhibiting lithium dendrite growth, which limits the performance of LIBs. In this paper, polypropylene/polydopamine (PP-PDA_x) composite separator was rapidly prepared by solution polymerization under the catalytic action of sodium periodate (SP), and its properties were systematically studied. The results show that when the modification time is 4 h and the deposition amount of PDA nanoparticles is 0.075 mg·cm^-2, that is, the PP-PDA₄ separator has the maximum electrolyte absorption rate of 324%, the contact angle drops to 9°, and it also exhibits the best electrochemical performance with an ionic conductivity of 0.51 mS·cm^-1. When a LiFePO₄/PP-PDA₄ separator/Li coin-type battery is assembled, the discharge capacity can still remain at 113.9 mAh·g^-1 after 100 cycles at 1C. In addition, the PP-PDA₄ separator shows a remarkable ability to inhibit lithium dendrite growth.

1.2 g of Tris was dissolved in deionized water, and adding hydrochloric acid solution to adjust pH to 8.5 dissolved a certain amount of dopamine hydrochloride in the prepared Tris buffer solution. The polypropylene separator was rolled into a column and immersed in the solution for 5 min to make the surface of the polypropylene septum was evenly wetted on separator. Added Sodium periodate to the solution and fully steep for 2, 4 h, and 6 h. After that, PP-PDA_x separators were be obtained after drying.

In this paper, the optimal sample is PP-PDA₄ that the polydopamine deposition is 0.075 mg·cm^-2, PP separator as a control group. PP-PDA₄ electrolyte contact angle decreased 82% to 9°, the tensile strength increased from 101 MPa to 110 MPa. And tested the separators electrochemical performance, the ionic conductivity increased from 0.26 mS·cm^-1 to 0.51 mS·cm^-1. Introduced positive and negative electrode materials to assemble the full cell, and the discharge specific capacity of PP-PDA₄ was the highest, which was up to 128.2 mAh·g^-1, and tested the rate performance of the full cell in the range of voltage from 2.5 V to 4.2 V, and the current density from 0.2 C to 2C. the discharge capacity of PP-PDA₄ is increased by 9%, 17%, 30%, and 9% compared with PP at 0.5, 1, 2 C, and 0.2 C. Finally, assembled Li/separator/Li batteries to evaluated the ability of PP-PDA₄ to inhibit lithium dendrites, and PP-PDA₄ could work stably for 400 h and maintain at a lower voltage polarization. Meanwhile, the SEM photographs of the lithium film after cycling showed that the lithium deposition was more uniform.

In summary, PP-PDA_x separator were synthesized rapidly via solution polymerization catalyzed by SP. This strategy exploits the intrinsic properties of PDA to enhance the affinity between the separator and the electrolyte, thereby markedly improving electrochemical performance and concurrently inhibiting lithium dendrite formation. Under the catalytic effect of SP, the optimum modification time is only 4 h. This treatment not only improves wettability but also significantly boosts electrochemical performance, without compromising the separator's integrity.

Rechargeable lithium-ion batteries (LIBs) are critical energy storage devices widely used in smartphones, electric vehicles, and hybrid vehicles due to their high efficiency and versatility. Spinel LiMn₂O₄ stands out as a promising cathode material owing to its eco-friendliness, safety, and cost-effectiveness. However, its practical application is limited by rapid capacity fading during cycling, especially under high current densities or elevated temperatures (55 ℃), caused by Jahn-Teller (J-T) distortion, Mn³⁺ dissolution, and oxygen defects. To address these issues, doping LiMn₂O₄ with elements like Al³⁺, B³⁺, Ni²⁺, or Mg²⁺ is a common strategy to stabilize Mn's average oxidation state (+3.5) and suppress J-T distortion. Concurrently, crystal facet engineering optimizes performance: truncated octahedral particles with dominant (111) facets (low surface energy, dense Mn packing) minimize Mn dissolution, while (100)/(110) facets enhance Li⁺ mobility for improved rate capability. Combining doping and facet control has emerged as a key research focus. For instance, B-doped LiMn_1.96B_0.06O₄ exhibited 110.3 mA·h·g^-1 at 5.0 C with 51.8% capacity retention after 2000 cycles, outperforming undoped LiMn₂O₄. Ni—B co-doping further refined morphology and boosted retention to 70.6%. Similarly, Mg—Al co-doped LiMg_0.03Al_0.10Mn_1.87O₄ retained 76.2% capacity after 1000 cycles at 10.0 C, while Zn—Al doping achieved 70%-75% retention over 2000 cycles. These studies highlight how doping regulates crystal growth, favoring truncated octahedral morphologies with balanced stability and kinetics. Building on this, our work synthesizes LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) via solid-state combustion to study Al doping's impact. Results reveal that Al content modulates facet growth: low doping levels yield two-facet (111/100) truncated octahedrons, while higher doping introduces (110) facets, ultimately leading to quasi-spherical particles. This controlled facet producing strategy is designed to optimize structural stability and electrochemical performance for advanced LIBs.

LiB_0.08Mn_1.92O₄ and LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) were synthesized by stoichiometric mixing of Li₂CO₃, H₃BO₃, MnCO₃, and Al(OH)(CH₃COO)₂ (total 30 g) with 1.5 g citric acid as a combustion agent. The mixture was ball-milled in ethanol for 10 h, dried, and calcined at 500 ℃ for 1 h. After grinding, it was sintered at 650 ℃ for 6 h. Products were labeled as B_0.08 and B_0.08Al_y. Cathode slurry was prepared by blending active material, carbon black, and PVDF (8∶1∶1) with NMP solvent, followed by 30 min ball milling. The slurry was coated onto Al foil, dried at 80 ℃ for 4 h, and cut into 14 mm discs (active material loading: about 1.0 mg/cm²). CR2032 coin cells were assembled in an Ar-filled glovebox using Li metal anode, LiPF₆ electrolyte (1.0 M in EC/DMC/EMC), and Celgard 2320 separator. Material phases were analyzed via XRD (Cu K_α, 10°-80°). Morphology and microstructure were examined by FE-SEM and TEM. Surface composition and valence states were determined via XPS. Electrochemical tests (Land CT2001A) included cycling (25/55 ℃, 3.0-4.5 V) and rate performance. CV and EIS were conducted using a CHI604D workstation.

XRD analysis confirmed that B_0.08 and B—Al co-doped samples crystallized in the spinel LiMn₂O₄ structure (). B—Al co-doping enhanced crystallinity, with diffraction peaks intensifying until y(Al)≥0.06, beyond which crystallinity declined. A lattice contraction was observed via (400) peak shifts, attributed to smaller B³⁺/Al³⁺(0.0400/0.0535 nm) replacing Mn³⁺(0.0645 nm) and shorter B/Al—O bonds, suppressing J-T distortion. SEM revealed that B—Al co-doping refined particle uniformity and morphology: B_0.08Al_0.02 exhibited truncated octahedral single-crystals with high (111) and minor (100)/(110) facets, while higher Al content (y≥0.04) led to spherical polyhedrons due to reduced facet growth rates. TEM/SAED confirmed improved crystallinity for B_0.08Al_0.02, with distinct lattice fringes (0.476 nm for (111)) and ordered diffraction patterns. XPS verified B³⁺/Al³⁺ incorporation and a higher Mn⁴⁺ ratio (B_0.08Al_0.02∶ Mn³⁺/Mn⁴⁺=47.29%/52.71%), stabilizing the Mn—O framework. Electrochemical tests demonstrated superior performance for B_0.08Al_0.02∶ At 5.0 C, it retained 83.9% capacity after 500 cycles (vs. 71.3% for B_0.08) due to suppressed Mn dissolution and enhanced Li⁺ transport from truncated octahedral morphology. High-rate testing (15.0 C, 20.0 C) showed B_0.08Al_0.02 retained 77.2% capacity after 500 cycles, outperforming B_0.08 (58.2%). Elevated-temperature (55 ℃) cycling revealed B_0.08Al_0.02 maintained 70% capacity retention (200 cycles), attributed to reduced side reactions and structural stability. CV and EIS confirmed faster Li⁺ diffusion (D_Li⁺: 3.14×10^-11 cm²·s^-1) and lower charge-transfer resistance for B_0.08Al_0.02. Kinetic analysis showed a lower activation energy (24.76 kJ/mol vs. 28.53 kJ/mol for B_0.08), indicating an enhanced reaction kinetics.

B—Al co-doped LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) cathode materials were synthesized via solid-phase combustion. B—Al co-doping enhanced crystal growth and facet evolution: particles transitioned from truncated octahedrons with two facets (111, 100) to those incorporating (111), (100), and (110) facets as Al content increased. At y(Al)≥0.04, spherical-like polyhedrons dominated. The optimized LiB_0.08Al_0.02Mn_1.90O₄ (B_0.08Al_0.02) exhibited superior crystallinity and well-defined truncated octahedral single-crystals. B_0.08Al_0.02 demonstrated exceptional rate capability and cycling stability. At 1.0 C, 5.0 C, 10.0 C, 15.0 C, 20.0 C, and 55 ℃/5.0 C, initial discharge capacities reached 114.7, 102.8, 86.3, 82.6, 75.3, and 107.4 mA·h·g^-1, respectively. After 500 cycles, capacity retention rates were 83.9% (5.0 C), 81.9% (10 C), 90.3% (15.0 C), 77.2% (20.0 C), and 62.0% (55 ℃/5.0 C), outperforming other samples. CV and EIS analyses demonstrated superior electrochemical properties for the B_0.08Al_0.02, including a higher Li⁺ diffusion coefficient (3.14×10^-11 cm²·s^-1), a lower apparent activation energy (24.76 kJ/mol), and a reduced charge-transfer resistance (114.2 Ω). These results indicate enhanced Li⁺ transport kinetics and improved structural stability.

In the field of vibration and noise reduction, phononic crystals and their ability to regulate elastic wave propagation have attracted significant attention. Leveraging the bandgap characteristics of phononic crystals, researchers can precisely control elastic wave transmission within specific frequency ranges. To this end, numerous scholars have dedicated efforts to designing and optimizing novel phononic crystals while investigating their bandgap formation mechanisms. Biomimetic design, which innovates engineering solutions by emulating forms, structures, and functions from nature or existing systems, has emerged as a vital source of inspiration for developing advanced phononic crystals in recent years. Furthermore, the acoustic black hole (ABH) structure, an engineered design that leverages wave manipulation characteristics to achieve energy focusing and absorption of acoustic waves, has been demonstrated to effectively optimize phononic crystal configurations, thereby significantly enhancing their bandgap properties. Aiming to achieve broad bandgap implementation in phononic crystal design, this study integrates biomimetic concepts with the ABH structures and proposes a flying saucer shaped phononic crystal configuration based on local resonance theory.

Currently, the bandgaps of phononic crystals are primarily determined through plane wave expansion (PWE), finite-difference time-domain (FDTD), lumped mass methods, and finite element method (FEM). The FEM demonstrates distinct advantages in calculating unit cell bandgaps and has been widely adopted. The fundamental principle of bandgap computation using FEM involves three key stages: First, the unit cell analytical model is discretized into finite elements, where the continuous field within each element is characterized by nodal forces and displacements. Subsequently, finite element equations are established based on equilibrium conditions, incorporating Bloch-Floquet periodic boundary conditions to derive an eigenvalue equation containing wavevector-frequency relationships. Ultimately, solving this eigenvalue equation yields the dispersion relations of the equivalent model. This study employs the COMSOL Multiphysics finite element analysis platform to solve the eigenvalue problem, thereby obtaining the band structure of the phononic crystal while simultaneously acquiring its vibration modes.

The flying saucer shaped phononic crystal proposed in this study exhibits pronounced bandgap characteristics, generating a local resonance bandgap within the 0-1000 Hz frequency range. Specifically, the bandgap spans from 294 Hz to 944 Hz with a width of 650 Hz, exhibiting a relative coverage ratio of 65% over the target frequency spectrum.

The parametric analysis reveals significant geometric sensitivity of bandgap characteristics: 1) With the rectangular width h₁ increasing from 2 mm to 4 mm, the bandgap starting frequency decreases from 352 Hz to 280 Hz while the cut-off frequency elevates from 845 Hz to 961 Hz, resulting in a 38.1% bandwidth enhancement from 493 Hz to 681 Hz; 2) As the large trapezoidal height a₂ grows from 0.6 mm to 1.4 mm, the bandgap shifts downward from 402-1312 Hz to 248-734 Hz, accompanied by a 46.6% bandwidth reduction from 910 Hz to 486 Hz. 3) When the small trapezoidal height a₃ escalates from 0.3 mm to 1.2 mm, the initial frequency increases by 13.2% (288 Hz to 326 Hz) whereas the cut-off frequency drops by 33.3% (1170 Hz to 780 Hz), collectively causing a 48.5% bandwidth decrease from 882 Hz to 454 Hz.

The bandgap characteristics of the flying saucer shaped phononic crystal are determined by the vibration modes of its constituent structures. Specifically, the bandgap starting frequency is primarily governed by the translational vibration of the resonator, while the cut-off frequency results from the reverse-phase vibrations on both sides of the edge structure.

When the excitation frequency falls within the bandgap range, the flying saucer shaped phononic crystal effectively localizes vibrational energy. Most energy is confined within the internal structure, particularly at the resonator and edge regions, with minimal energy transmitted to the output terminal. This process achieves strong vibration suppression, thereby fulfilling the objectives of vibration reduction and noise control.

The bandgap characteristics of the flying saucer shaped phononic crystal, especially the bandgap position and width, are predominantly influenced by the equivalent mass of the resonator, the equivalent mass of the edge structure, and the equivalent stiffness of the soft coating. Geometric parameter optimization enables precise bandgap tuning: increasing the small trapezoidal height elevates the bandgap starting frequency; enlarging the rectangular width adjusts the cutoff frequency; and modifying the height of the large trapezoid alters the bandgap width.

The flying saucer shaped phononic crystal proposed in this study exhibits bandgap characteristics. By configuring the flying saucer shaped phononic crystal into composite acoustic absorption panels through periodic array arrangements, this design fully exploits its low-frequency local resonance bandgap characteristics spanning 650 Hz, providing innovative solutions for multi-domain vibration and noise control. In automotive engineering, the panels can be embedded within door cavities, roof structures, or chassis interlayers to suppress road-induced tire noise (200-500 Hz) and engine resonance harmonics. For rail transit applications, integrating the panels into carriage wallboards, bogie systems, or track underlayments enables simultaneous attenuation of wheel-rail friction noise (500-800 Hz) and carbody structural vibrations (300-600 Hz). This innovative approach offers a mechanism-driven yet engineering-feasible solution for industrial noise and vibration mitigation, demonstrating significant practical potential.

Among the various approaches to realize white light-emitting diodes (WLEDs), phosphor-converted white LEDs (pc-WLEDs) have become the predominant technology owing to their superior color stability, uniform spectral distribution, straight forward fabrication process, and cost-effectiveness. Among the phosphors utilized in WLEDs, red phosphors play an indispensable role in numerous methods for obtaining high-quality white LEDs. They contribute to enhancing the color rendering index and lowering the correlated color temperature, thereby facilitating the development of healthier lighting sources. Therefore, the development of highly efficient red phosphors suitable for blue or near-ultraviolet excitation continues to be a research focus in this field. The 4f electron configuration of Eu³⁺ gives rise to a rich variety of metastable energy levels under the influence of the crystal field. The distinctive ⁵D₀→⁷F_J (J=1, 2, 3, 4) electronic transitions result in high-purity red light emission with the main peak positioned between 610 nm and 620 nm, rendering Eu³⁺ an exceptional red-emitting ion. Borate materials have attracted extensive research interest as promising luminescent hosts owing to their advantages, including low synthesis temperatures, broad ultraviolet-visible transparency ranges, and high luminescence efficiencies. Ba₃Sr₄B₃O₉F₅ exhibits a non-centrosymmetric structure belonging to the hexagonal crystal system with space group P6₃mc. This structure features one non-centrosymmetric Ba²⁺ site and two distinct non-centrosymmetric Sr²⁺ sites. Doping Eu³⁺ into the Ba₃Sr₄B₃O₉F₅ matrix is expected to yield highly efficient red-emitting phosphors. In this study, a series of Ba₃Sr₄B₃O₉F₅∶xEu³⁺ red-emitting phosphors were successfully synthesized. Systematic investigations were carried out on their phase structures, morphologies, luminescence properties, and thermal stabilities. Density functional theory (DFT) calculations were performed to determine the preferred lattice sites occupied by Eu³⁺ after doping. Additionally, the concentration quenching mechanism of Eu³⁺ was thoroughly analyzed.

Ba₃Sr₄B₃O₉F₅∶xEu³⁺ (0 ≤ x ≤ 0.42) fluorescent ceramics were synthesized in vacuum via high-temperature solid-state reaction. High purity SrCO₃、BaCO₃、BaF₂、H₃BO₃ and Eu₂O₃ powders were used as starting materials. These materials were weighed in stoichiometric ratios, and ground to obtain a powder mixture of finer granularity. The powder mixture was placed into the crucible, and heated at 400 ℃ for 2 h. After being cooled down to room temperature, the resulting item was grounded manually and transferred in sequence to another furnace to be annealed at 830 ℃ for 10 h in air to produce the final product.

Based on DFT calculations, it was found that when Eu³⁺ occupied the Ba²⁺ lattice sites in the Ba₃Sr₄B₃O₉F₅ crystal, the total energy E_d of the doping system reached its minimum value. Therefore, it is reasonable to speculate that upon doping into the Ba₃Sr₄B₃O₉F₅ lattice, Eu³⁺ preferentially occupy the Ba²⁺ lattice sites. The excitation spectrum of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ powder comprises a broad band peak and a set of sharp line-shaped peaks. The broad band peak in the range of 210-335 nm can be ascribed to the charge-transfer band (CTB) peak, which arises from the energy transfer between O₂^- and Eu³⁺. A series of sharp excitation peaks within the wavelength range of 300-500 nm originate from the 4f-4f transitions of Eu³⁺, with the strongest peak located at 394 nm. This is well-matched with the near-ultraviolet (NUV) chips currently employed in WLEDs. The emission spectrum of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ powder exhibits five distinct emission peaks located at 580.3, 593, 613, 653.8 nm, and 703.2 nm, which are attributed to the characteristic transitions of Eu³⁺ from the ⁵D₀ excited state to the ⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃, and ⁷F₄ ground states, respectively. Among these, the intensity of the electric dipole transition (⁵D₀→⁷F₂) emission peak at 613 nm is significantly higher than that of the magnetic dipole transition (⁵D₀→⁷F₁) emission peak at 593 nm. The Eu³⁺ ions are situated at the non-centrosymmetric lattice sites within the matrix crystal structure, predominantly exhibiting allowed electric dipole transitions from ⁵D₀ to ⁷F₂. The emission peaks are primarily centered around 613 nm, corresponding to red luminescence.

When the doping concentration of Eu³⁺ (denoted as x) is below 0.38, the luminescence intensity of the phosphor exhibits a gradual increase with rising concentrations of Eu³⁺. Conversely, when x exceeds 0.38, there is a progressive decline in luminescence intensity as the doping concentration continues to rise. The fluorescence intensity reaches its maximum when the Eu³⁺ doping concentration x = 0.38. This concentration quenching phenomenon is attributed to the quadrupole-quadrupole interactions between the doped Eu³⁺ ions at high doping concentrations. Thermal stability is a key factor in evaluating luminescent materials. At the operating temperature of LED devices (150 ℃), the integrated emission intensity of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ phosphor remains above 70% of its initial value. Moreover, at 497 K, the emission intensity is retained at 49.3% of that at 297 K, indicating excellent luminescence thermal stability.

A series of red phosphors Ba₃Sr₄B₃O₉F₅∶xEu³⁺ were successfully synthesized using the traditional high-temperature solid-state reaction method. The Ba₃Sr₄B₃O₉F₅∶xEu³⁺ phosphor exhibits a non-centrosymmetric hexagonal crystal structure. Under 394 nm excitation, the Ba₃Sr₄B₃O₉F₅∶xEu³⁺ sample predominantly emits red luminescence centered at 613 nm. The optimal doping concentration of Eu³⁺ ions is determined to be 0.38. Through DFT theory calculations and R/O ratio analysis, it is proved that Eu³⁺ ions will preferentially occupy the Ba²⁺ lattice positions in the Ba₃Sr₄B₃O₉F₅ matrix and the surrounding environment exhibits high non-centrosymmetry. The main mechanism of nonradiative energy transfer in Ba₃Sr₄B₃O₉F₅∶xEu³⁺ phosphor is quadrupole-quadrupole interaction, which also explains the exchange interaction is the main mechanism of the concentration quenching process. The color purity of the Ba₃Sr₄B₃O₉F₅∶xEu³⁺ phosphor reaches 93.9%, and under the operating temperature of LED devices, the integral emission intensity of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ remains above 70% of initial value at 297 K. In summary, Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ phosphor can serve as a promising red-emitting fluorescent material for application in white LED technology.

Reaction-bonded Si₃N₄-SiC refractories are preferred thermally conductive materials for waste incinerators. However, their high apparent porosity limits their high-temperature wear resistance and resistance to water vapor oxidation. Promoting the growth of Si₃N₄ whiskers is an effective approach to improve the densification of Si₃N₄-SiC refractories. Related researches have mainly focused on catalytic nitridation to facilitate the direct nitridation reactions, while there are few studies on enhancing the partial pressure of SiO and promoting the indirect nitridation reactions. In addition to ultrafine silicon powders, diborides are new additives that can enhance the partial pressure of SiO and promote the densification of Si₃N₄-SiC refractories. Nevertheless, the relationship between their microstructure evolution and densification effects remains unclear. In this work, the phase composition and microstructure evolution of TiB₂ and AlB₂ micro-powders during the preparation process of Si₃N₄-SiC refractories were investigated, as well as their effects on the bulk density, apparent porosity, phase composition and microstructure of the refractories. This study is expected to enrich the nitridation mechanism system of silicon powders, and provide a theoretical basis for selecting densification additives for Si₃N₄-SiC refractories.

To investigate the oxidation and nitridation reactions of TiB₂ and AlB₂ micro-powders during the preparation process of Si₃N₄-SiC refractories, thermodynamic analysis of the Si-TiB₂-AlB₂-N₂-O₂ system was performed using HSC Chemistry software, and TG-DSC analysis in high purity N₂ atmosphere was conducted with a synchronous thermal analyzer. The phase composition and microstructure evolution of TiB₂ and AlB₂ micro-powders heat-treated at 1400 ℃ in high purity N₂ atmosphere were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Moreover, quantitative analysis of the phase composition was implemented via the K-value method. Reaction-bonded Si₃N₄-SiC refractories were prepared with TiB₂ and AlB₂ micro-powders as additives. The bulk density and apparent porosity of refractory samples were determined via the Archimedes drainage method. A universal testing machine was used to measure the compressive strength of the refractory samples at room temperature. XRD, SEM, EDS, and TEM were employed to analyze the phase composition and microstructure evolution of Si₃N₄-SiC refractories, with particular attention on the morphology, quantity, and distribution of Si₃N₄ whiskers. Based on the above mentioned analyses, the relationship between the microstructure evolution and densification effects of diboride additives was established.

Thermodynamic analysis results show that the oxidation and nitridation activities of AlB₂ are slightly higher than those of TiB₂ and significantly higher than those of Si. According to TG-DSC analyses, the initial temperature of the nitridation reaction for TiB₂ is 150 ℃ higher than that of AlB₂, which is consistent with the thermodynamic analysis results. After heat treatment at 1400 ℃ in high purity N₂ atmosphere, over 60% of TiB₂ micro-powders remains unreacted and a glass phase surrounded structure can be observed. In contrast, the content of unreacted AlB₂ content is below 10%, and few glassy phases are observed. That's because B₂O₃ glassy phase has reacted with Al₂O₃ to form a new solid phase of Al₄B₂O₉. With an appropriate addtion, both TiB₂ and AlB₂ micro-powders improve the densification of Si₃N₄-SiC refractories, reduce the content of Si₂N₂O phase, and lower the ratio of α/β-Si₃N₄. The optimal addition amount for both additives is 0.6%. Under the same addition amount, TiB₂ exhibits a superior densification effect. When the TiB₂ addition amount is 0.6%, a large quantity of β-Si₃N₄ whiskers are generated, with an average diameter of approximately 200 nm and an aspect ratio exceeding 50. These whiskers entangle with each other to form a three-dimensional network structure, and their growth follows the gas-solid mechanism.

The microstructure evolution of diboride additives modulates the SiO partial pressure and Si₃N₄ whiskers growth during silicon powder nitridation, which in turn affects the densification of Si₃N₄-SiC refractories. For instance, the oxidation products of TiB₂ do not undergo further chemical reactions but instead form a protective glassy phase. This phase protects most TiB₂ from consumption until the nitridation reaction stage, thereby effectively reducing the partial pressure of O₂. Since the partial pressure of SiO in the SiO-N₂-O₂ system increases as the partial pressure of O₂ decreases, TiB₂ micro-powders are effective to enhance the partial pressure of SiO and promote the indirect nitridstion reactions, producing a large number of β-Si₃N₄ whiskers. Therefore, they are highly suitable as densification additives for Si₃N₄-SiC refractories.

CO₂ generated from coal combustion is the main cause of the greenhouse effect, and it is urgent to reduce its CO₂ emissions. CO₂ capture is the key to Carbon Capture, Utilization and Storage (CCUS) technology. Silicon-based materials are commonly used as carriers for adsorbents, but their CO₂ adsorption capacity is weak, and chemical reagents need to be supported. At present, there is little research on the stability of different diamine adsorbents and adsorbents in the presence of impurity gases. Therefore, it is necessary to prepare silicon-based amine adsorbents and explore their adsorption mechanism and the influence of various factors. In this paper, a variety of organic amines are loaded onto silicon-based materials, aiming to prepare various silicon-based amine adsorbents. The influence mechanism of adsorption temperature, amine types, and gas components (SO₂, NO_x, H₂O) on CO₂ adsorption are carefully investigated.

Using P123 as the template agent and Na₂SiO₃ as the silicon source, the silicon-based mesoporous material SM was prepared. Then, tetraethylenepentamine (TEPA), polyethyleneimine (PEI) and 2-amino-2-methyl-1-propanol (AMP) were supported on SM to obtain the silicon-based amine adsorbents.

CO₂ adsorption mechanism of the silicon-based amine adsorbents was explored through pore structure analysis, infrared spectroscopy analysis and thermogravimetric analysis. CO₂ adsorption experiments of the silicon-based amine adsorbents were carried out by using a fixed-bed analyzer and a thermogravimetric analyzer.

Pore structure analysis shows that due to the filling of organic amines, the porosity decreases and the average pore diameter increases. Infrared analysis indicates that the modification of organic amines does not damage the silicon-based carrier skeleton. Some Si—OH groups form bonds with organic amines, and new characteristic peaks appear after loading, which proves the successful loading of organic amines. Thermogravimetric analysis reveals that the weight loss of the adsorbent occurs in two stages, and the silicon-based amine adsorbent exhibits good thermal stability. In terms of adsorption performance, the modification of organic amines enhances the CO₂ adsorption capacity of SM. Modification with diamines is more effective, as CO₂ reacts with amino groups to form specific products. Adsorption kinetics studies find that the adsorption of CO₂ by SM is mainly physical adsorption, and the adsorption of the three diamine adsorbents is jointly affected by physical and chemical adsorption. Regarding the cyclic performance, after 10 cycles of 30PEI-30AMP/SM, the CO₂ adsorption capacity can reach 95% of the initial capacity, and the synergistic effect of the mixed amines improves the regenerability of the adsorbent. As for the influence of gas components, SO₂ reduces the CO₂ adsorption capacity, water vapor enhances it, and NO has a relatively minor effect.

The main conclusions of this work are summarized as follows: 1) A silicon-based mesoporous material was prepared by using P123 as the template and Na₂SiO₃ as the silicon source. By impregnating with TEPA, PEI and AMP, nine monoamine adsorbents and three diamine adsorbents were synthesized. Among them, the CO₂ adsorption performance of the silicon-based diamine adsorbents is better. 2) The macromolecular amine PEI can significantly occupy the pore space, causing a significant decrease in the specific surface area and pore volume, and a relative increase in pore diameter. The small molecular amines like TEPA and AMP can maintain a higher porosity and specific surface area while filling the pores. The combination of diamines can effectively utilize the characteristics of different amines, and significantly improve the pore structure through intermolecular interactions and spatial arrangements. 3) TEPA and PEI contain a large number of primary amine groups, which can react with CO₂. However, their high viscosity limits their dispersion in the carrier. AMP, a steric hindrance amine with hydroxyl groups, forms an additional network structure, improving the thermal stability of the adsorbent. Different types of amines in the diamine adsorbents can cooperate with each other to enhance their CO₂ adsorption performance. 4) SO₂ in coal-fired flue gas will compete with CO₂ for the limited adsorption sites in the adsorbent, preferentially reacting chemically with the amine groups on the surface of the adsorbent, reducing the adsorption sites for CO₂. Moreover, the generated sulfates will also block the pores, increasing the diffusion resistance of CO₂ and resulting in a decrease in the CO₂ adsorption performance of the adsorbent. An appropriate amount of water vapor can improve the utilization efficiency of amino groups and enhance the CO₂ adsorption performance of the adsorbent.

During the resource utilization of solid waste, the neglect of scientific analysis on the composition of iron sulfide-containing aggregates and their reactivity in concrete has led to severe concrete deterioration caused by such aggregates. Through investigating the research progress, the following conclusions are drawn: 1) Concrete failures induced by iron sulfide-containing aggregates have occurred extensively worldwide. Initially, this failure mode was identified as oxidation-induced expansion in geological engineering and foundation backfills. With increasing building service life and recent growth in tailing aggregate usage, European and American countries have widely reported adverse effects of such damage. Although relevant reports emerged later in China, the problems have rapidly manifested due to shorter latency periods. 2) The research of degradation mechanisms of iron sulfide-bearing aggregates in concrete has shifted from initial attention to single factors like pyrite and sulfate attack to comprehensive impacts on concrete performance. Comparatively, studies on pyrrhotite-bearing aggregates remain scarce. The degradation process caused by iron sulfides includes water-absorbing expansion of iron oxidation products, erosion from released sulfate during oxidation, and steel corrosion. 3) Significant variations exist in the reactivity of different iron sulfide-containing aggregates within concrete, leading to inconsistent sulfur content limits in regional standards across countries. Current preliminary identification methods for such aggregates include visual inspection, petrographic analysis, elemental testing, and indirect evaluation approaches. However, laboratory comprehensive assessments remain essential for accurately evaluating their suitability in concrete applications.

The existing research are conducted in the related fields. However, due to the extensive and complex nature of iron sulfides destruction, the field still faces other challenges.:1) Concrete engineering failures caused by iron sulfide-bearing aggregates have already occurred in China. It is crucial to enhance attention to such issues, promptly develop effective prevention and treatment measures, and minimize losses. Concurrently, technical training for Chinese engineers should be strengthened to improve awareness of iron sulfide-bearing aggregates, effectively preventing their misuse while ensuring rational resource utilization of other iron tailing aggregates.2) Current research on iron sulfide deterioration mechanisms in concrete mainly focuses on pyrite. However, pyrrhotite in aggregates exhibits higher reactivity, causing faster and more significant engineering damage. Therefore, urgent research is needed on the deterioration mechanisms of pyrrhotite-bearing aggregates in concrete. 3) Different iron sulfide-bearing aggregates exhibit distinct reaction characteristics, showing varied deterioration manifestations, development speeds, and impact ranges in concrete. Thus, sulfur content limits should be established according to different iron sulfide types based on China's actual aggregate characteristics, while accelerating the development of comprehensive evaluation methods adapted to domestic engineering conditions to propose effective preventive measures in standards.

Nanotechnology plays a crucial role in advancing the development of low-temperature RSOCs. Although significant breakthroughs have been made in developing highly active and stable air electrode materials through nanotechnology, the catalytic activity and stability of air electrode materials at lower temperatures (400-500 ℃) remain key challenges. Related nanotechnology needs to be further explored, mainly in the following aspects: a) How to controllably prepare two-phase or multi-phase materials with specific stoichiometric ratios and accurately determine the phase structure of each phase; b) Further investigation into the water-induced exsolution and performance enhancement mechanism; c) How to more accurately regulate the distribution, size, loading, and infiltration efficiency of nanoparticles in the infiltration method; d) Further exploration of material combinations using ALD technology to enhance performance and stability; e) Development of novel air electrode structures that facilitate gas and ion transport; f) Artificial intelligence-assisted screening of highly active air electrode materials and optimization of manufacturing process.

Natural sugars as templating agents show unique advantages in the synthesis of hierarchical ZSM-5 zeolite, which provide a promising alternative to the traditional templating method due to their green, cost-effective, pore-size-adjustable, and mild template-removal conditions. In addition, the differences between the molecular structures of different sugar templates exhibit unique regulatory properties in the synthesis of hierarchical ZSM-5 zeolite. Specifically, glucose and sucrose induce linear stacking of nanocrystals through hydroxyl coordination, and form a uniform intergranular mesoporous structure by combining with hydrothermal or two-step crystallization, which can effectively enhance the dispersion of loaded metals. But their ability to regulate the pore channels is weak. Other templating agents are needed to synergistically regulate pore structure. For example, polysaccharide templates, such as cellulose and starch, have branched chain structure. By using hydrothermal method or seed crystallization method, they can construct micro-medium-macroporous multilevel pore structure, and realize synergistic distribution of inter-crystalline mesopores and intra-crystalline mesopores. Polysaccharides and sodium alginate, serving as functional sugars, promote the formation of uniform and dendritic mesopores through electrostatic adsorption and carboxyl (—COO—)Al³⁺ chelation, respectively. They also enhance the density of acidic sites (Lewis) on the surface and regulate the morphology of molecular sieves. These modifications tailor the material for high-value-added applications, such as esterification of oleic acid with high acidity, the diffusion-controlled adsorption of macromolecules, and biomass conversion. By improving mass transfer efficiency and accessibility of acidic sites, the sugar templates can significantly enhance the performance of biomass conversion, promote the transformation of biomass from "waste" to "carbon neutral core carrier", and provide a sustainable solution to the goal of "dual carbon".

Although the natural sugar templates in hierarchical ZSM-5 zeolite to make significant progress, the field of research is still facing numerous challenges. Firstly, some of the sugar templates still suffer from complex pretreatment, wide mesopore size distribution, and high mesopore volume, leading to the decrease of crystallinity for synthesis of hierarchical ZSM-5 zeolite. Secondly, the existing synthesis techniques are mainly limited to hydrothermal methods, and other methods such as dry gel conversion and two-step crystallization are hindered in industrialization due to the complexity of the process and cost factors; finally, it is difficult to precisely regulate the pore structure and acidic sites of the molecular sieves, and the constitutive relationship between the templating agent dosage and the material properties has not been fully clarified. Therefore, exploring the controlled depolymerization of polysaccharides and multi-template compounding strategy to break through the bottleneck of crystallinity and mesopore homogeneity, developing more universal synthesis methods, exploring the mechanism of pore-acid synergistic regulation, and in-depth analysis of the mechanism of templating agents are the main directions of the future research, so that we can push forward the practical application of sugar templating method in the large-scale preparation of hierarchical zeolite.