With the ongoing expansion of global housing and infrastructure, cement as an essential component of concrete is projected to reach a global demand of 468 million tonnes by 2050, contributing approximately 6%-7% of total CO₂ emissions. Reducing carbon emissions in cement and concrete production is thus critical for achieving carbon neutrality goals. In the central and western regions of China, a growing demand for large infrastructure projects such as dams, tunnels, and mountain bridges increasingly relies on the use of mass concrete. However, a great heat release from cement hydration with a low thermal conductivity leads to internal heat accumulation and significant temperature differentials. These thermal gradients increase cracking risk and compromise long-term durability. Conventional strategies like low-heat cement, mineral admixtures, and chemical retarders are adopted to mitigate early hydration heat. Nevertheless, these approaches are limited due to high production costs, reduced strength, or insufficient control over heat evolution rate. Recently, carbon capture, utilization, and storage (CCUS) technologies, particularly semi-dry carbonation, show a promising potential for reducing carbon emissions and regulating hydration heat, especially for high-alkaline materials. In this study, semi-dry carbonation pretreatment was applied to Portland cement, forming a surface layer of CaCO₃ that serves as a physical barrier to suppress early hydration. The effect of semi-dry carbonation on the hydration behavior and microstructure was investigated. The results could provide some insights into applying CCUS technology for hydration heat control in cement-based materials.

A P·I 42.5 Portland cement (Fushun Cement Co., Ltd., China) was used. Semi-dry carbonation was performed in a NELD-CA070 carbonation chamber using simulated kiln gas containing 20% CO₂ at a RH of (70% ± 5%) and a temperature of (20 ± 2) ℃. The cement was loosely spread in trays and carbonated for 2, 4 h, or 8 h, respectively, under manually stirring for every 30 min. The resulting samples were designated as C2, C4, and C8, respectively, while the uncarbonated sample was labeled as C0. All the samples were dried at 40 ℃ for 24 h after carbonation.

Cement pastes were prepared at a water-to-cement ratio of 0.4 and cured under standard conditions (i.e., 20 ± 1 ℃, 95 ± 5% RH). For the determination of compressive strength, cube specimens (40 × 40 × 40 mm) were tested at 1, 3, 7 d, and 28 d, respectively. For the microstructural analysis, fresh pastes were cast in sealed vials, demolded after 24 h, immersed in water, and cured until the specified age. Hydration was stopped by solvent exchange with isopropanol, followed by vacuum drying and grinding to a fine powder with the particle size below 80 μm.

The early hydration heat evolution was measured by isothermal calorimetry (TAM Air). The phase composition was determined by isothermal calorimetry, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The pore solution alkalinity was assessed through pH measurement and elemental analysis by inductively coupled plasma optical emission spectrometry (ICP-OES). The pore structure was characterized by mercury intrusion porosimetry (MIP), and the microstructure was determined by scanning electron microscopy (SEM).

The semi-dry carbonation alters the phase composition and microstructure of cement. The TG analysis shows a weakened gypsum decomposition peak and an intensified CaCO₃ decomposition, indicating that gypsum participates in carbonation. The mild carbonation (i.e., 2-4 h) increases CH content due to enhanced pre-hydration. CO₂ sequestration reaches 1.60%, 2.27%, and 3.62% after 2, 4 h, and 8 h, corresponding to 3.62%, 5.16%, and 8.23% CaCO₃ formation, respectively. As carbonation progresses, C₃S, C₂S, C₃A, C₄AF, and gypsum decrease, while calcite and amorphous silica increase, driven by reactions with CO₂ and moisture. The FTIR spectra show strengthened C—O stretching (1410 cm^-1), weakened H₂O peaks (3527 cm^-1), and intensified SO₄^2- bands (1124 cm^-1 and 1197 cm^-1), indicating interactions among gypsum, CO₂, and alkali oxides. The SEM images reveal roughened clinker surfaces caused by the deposition of carbonation products such as calcite and silica gel.

The semi-dry carbonation significantly reduces early compressive strength, with 1-d reductions of 31.2%, 77.5%, and 89.6% after 2, 4 h, and 8 h, respectively. However, this effect diminishes over time, with less than 5.4% strength loss at 28 d. Carbonation extends the induction period and reduces hydration heat, lowering cumulative heat by 67.2% at 10 h. Early hydration degrees and chemically bound water also decrease, corresponding to reduced CH content and early strength. Despite this, hydration products at < 400 ℃ are largely unaffected, having the minimal difference in 28-d strength. Early-age pore solution alkalinity declines significantly. In the C8 group, Na⁺ and K⁺ concentrations at 1 d are reduced by 60% and 40.7%, and the pH value is decreased from 13.52 to 13.18. At 28 d, the pH value becomes 13.5, indicating no risk to reinforcement passivation. At this age, total porosity increases slightly from 21.1% to 22.8%, with a minor rise in capillary and gel pores. The SEM images and MIP analyses confirm similarly dense microstructures across all the samples, indicating that the calcite layer is eventually penetrated and has no adverse long-term effect.

This study showed that emi-dry carbonation could alter the phase composition and microstructure of Portland cement. After 2-h, 4-h, and 8 h-carbonation, CO₂ sequestration increased to 1.60%, 2.27%, and 3.62%, respectively, accompanied by a reduction in clinker minerals and gypsum and formation of amorphous silica and calcite layers on cement particle surfaces. These calcite layers could effectively suppress early hydration by reducing the contact area between water and reactive clinker phases. As a result, cumulative heat release at 3 d was decreased by 14.7%, 35.1%, and 58.1%, respectively, leading to a lower early compressive strength, a pore solution alkalinity, and a looser microstructure. However, the long-term effect was limited. At 28 d, the continuous formation of hydration products penetrated the calcite layer, and hydration degree and product quantity were reduced by less than 7%, and the compressive strength, pH value, and porosity were altered by no more than 5.4%, 0.1, and 8.1%, respectively.

The low alkalinity makes super-sulfate cement (SSC) vulnerable to being carbonated by CO₂. In this paper, we investigated two magnesium slags, i.e., MSⅠ (the proportion of β-C₂S is higher) and MSⅡ (the proportion of γ-C₂S is higher), and analyzed the impact of CO₂ concentration (i.e., 3% and 20%, in volume) on the SSC from the perspectives of hydration and carbonation. The results show that the addition of MSⅠ (with the higher proportion of β-C₂S) to SSC has a negative impact on the hydration and carbonation resistance due to β-C₂S with a stronger hydration activity, suppressing the hydration of SSC itself and lowering the hydration degree of SSC, reducing the amount of hydrates and further deteriorating the strength and structure development of SSC. The addition of MSⅡ (with the higher proportion of γ-C₂S) has the hydration reaction, and maintains the relative content of hydrates after carbonation and compressive strength increase. This is due to the fact that γ-C₂S significantly enhances the carbonization resistance of SSC.

The binder consisted of blast furnace slag (BFS), desulfurization gypsum (DGP), red mud (RM), magnesium slag (MSⅠ (with the higher proportion of β-C₂S) and MSⅡ (with the higher proportion of γ-C₂S), and calcium hydroxide. The BFS substitution levels (i.e., 10% and 20%) by MSⅠ and MSⅡ magnesium slag were investigated. The mineral compositions of used magnesium slags were analyzed by X-ray diffraction (XRD). The prepared pastes were cast in the cubic molds with dimensions of 40 mm × 40 mm × 40 mm. After de-molding, the hardened samples were sealed and cured at 25 ℃. After being cured for 28 d, the carbonation test was conducted at different CO₂ concentrations (i.e., 3% and 20%) and a relative humidity of 70%. At the carbonation age of 7, 14 d, and 28 d, the compressive strength, phase composition, and carbonation depth of SCC with and without carbonation were determined by strength test, X-ray diffraction (XRD), thermogravimetric analysis (TGA) and phenolphthalein color test. The microscopic morphologies of SSC before and after carbonation were also analyzed by scanning electron microscopy (SEM). Besides, the early-hydration performance of fresh SSC pastes with MSⅠ and MSⅡ was evaluated via setting time and isothermal calorimetry tests.

In the hydration stage, the MSⅠ significantly inhibits the hydration rate of SSC. The MSⅠadversely affects the setting time and delays the reaction of SSC. This is because β-C₂S can inhibit the hydration of BFS and reduce the hydration degree of SSC, leading to reduced compressive strength, amounts of hydrates (i.e., ettringite and Calcium aluminum silicate hydrates), and increased carbonation depth after carbonation. The porous and loose microstructures via the SEM images also validate an adverse effect due to the MSⅠ (with the higher proportion of β-C₂S). Also, the addition of MSⅡ does not disrupt the hydration process of SSC and ensure the integrity of hydration phase matrix structure. Meanwhile, a high carbonization activity allows it to react with CO₂ to generate more CaCO₃ and a dense matrix structure, which enhances the carbonation resistance and compressive strength of SSC.

Different types of magnesium slag had different effects on the setting time, reaction rate, and amounts of SSC hydrates. Specifically, the MSⅠ with a higher proportion of β-C₂S and a small amount of Ca(OH)₂ could extend the setting time of SSC, decelerate the hydration rate of the matrix, and markedly reduce the quantity of hydrates. In contrast, the MSⅡ with a high γ-C₂S proportion had a negligible impact on the hydration process of SSC. Furthermore, the XRD patterns and TG results indicated that compared with MSⅡ, the incorporation of MSⅠ hindered the dissolution of gypsum and reduced the generation amount of hydration products, thus decreasing the compressive strength.

Under carbonation environments of different concentrations, the carbonation of calcium silicate in both types of magnesium slags enhanced the compressive strength of magnesium-doped slag-based SSC after carbonation to different extents. Compared with the control group, after 28-d carbonation, SSC doped with MSⅠ exhibited a carbonation depth, which was nearly twice greater than that of the control group (16.3 mm), while the content of hydration products post-carbonation was lower than that of the control group. Conversely, MSⅡ, which was abundant in γ-C₂S, demonstrated a superior protective effect on SSC during the carbonation process. After 28-d carbonation, the carbonation depth was only 6 mm, approximately 20% less than that of the control group. In addition, the content of hydration products post-carbonation remained consistent with the control group, and the strength after carbonation increased significantly.

For the compressive strength of carbonated SSC, the incorporation of slag containing various types of calcium and magnesium silicates could enhance the anti-carbonation performance of magnesium-doped slag SSC to different extents. However, compared to the MSⅡ containing γ-C₂S with a lower hydration activity and the MSⅠ with β-C₂S with a higher hydration activity, the reaction rate of SSC could be reduced. Before the carbonation, only a loose structure with a limited amount of C-A-S-H gel appeared. Conversely, the MSⅡ exhibited a higher proportion of γ-C₂S, which could have a low hydration activity and a high carbonation reactivity. This characteristic minimally affected the reaction rate of SSC, thereby ensuring that SSC doped with the MSⅡ demonstrated an excellent anti-carbonation performance in a carbonation environment. After the carbonation, there was a significant increase in matrix strength, and the microstructure became notably denser.

Red mud (RM) as a highly alkaline solid waste generated during alumina production presents significant environmental challenges due to its massive annual output and stockpiling, including land occupation and pollution. The utilization of RM in combination with other solid wastes or minerals to produce geopolymer cementitious materials becomes one of the effective approaches for its resource recovery. However, Bayer-processed RM exhibits a low Si/Al molar ratio and a poor reactivity, making it difficult to form a stable aluminosilicate polymer structure. Most RM-based geopolymers require mechanical/thermal activation pretreatment or high-temperature curing to enhance early strength. The existing studies typically involve either low RM incorporation ratios or result in a low geopolymer strength at high RM dosages. Based on previous research and the concept of multi-source solid waste synergy, this study was to introduce highly reactive ground granulated blast-furnace slag (GGBS) and calcium-rich carbide slag (CS) into an RM-Class II fly ash (FA) geopolymer system. The influence of GGBS and CS composite dosages on the mechanical properties of the geopolymer was investigated. Furthermore, the microstructure of the geopolymer was analyzed from multiple perspectives to elucidate the synergistic reaction mechanism and strength development under the co-incorporation of GGBS and CS.

In this study, RM, FA, GGBS, and CS were used as raw materials at a RM/FA mass ratio of 1.0/1.0. A mixed alkaline activator solution consisting of NaOH solution (NH) and water glass (WG) was used at an NH/WG mass ratio of 1.0/2.5. The NaOH solution had a molarity of 10 mol/L, and the liquid-to-solid ratio (L/S) was 0.65. Different dosages of GGBS and CS were used as the experimental variables.

The RM, FA, GGBS, and CS were manually mixed for 30 s, followed by blending the solid powder mixture with alkaline activator in a cement paste mixer for 3 min. The resulting slurry was then cast into triple molds with internal dimensions of 70.7 mm × 70.7 mm × 70.7 mm. The specimens were cured at 20 ℃ and 95% relative humidity for 24 h before demolding, after they continued to cure under standard conditions.

The compressive strength tests were conducted at different curing ages of 7, 14, 28, 45 d, and 60 d. The collected fragments taken from the center of the specimens were stored in vials and immersed in anhydrous ethanol to halt further reaction. The chemical composition, microstructure, and mineral phases on the geopolymer's strength were determined by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). Prior to testing, the samples were dried at 60 ℃ for 24 h and ground for subsequent characterization.

All the specimens exhibit a strength development trend with increasing curing age. Among them, the geopolymer RFGC-1 with CS as sole additive shows the lowest strength, reaching only 1.8 MPa at 60 d. In contrast, RFGC-5 with GGBS as sole additive demonstrates the highest strength among the five formulations, achieving 11.6 MPa at 60 d, indicating that GGBS is more effective than CS in enhancing the strength of RM-Class II FA-based geopolymers.

For the samples RFGC-2, RFGC-3, and RFGC-4 incorporated both GGBS and CS, the 60-d compressive strengths are 5.8, 8.8 MPa, and 6.5 MPa, respectively. The strength initially increases and then decreases with increasing GGBS content and decreasing CS content, indicating that the relative proportions of GGBS and CS must be carefully controlled to optimize the geopolymer strength. At a total additive content of ≤12%, the RM-FA geopolymer with GGBS alone exhibits a superior strength, compared to those with CS alone or GGBS/CS combinations.

The microstructural characterization reveasl that, in addition to the original mineral phases (i.e., hematite (Fe₂O₃) and quartz (SiO₂)), the primary cementitious products in the geopolymers include calcium aluminate hydrate (C-A-H), calcium silicate hydrate (C-S-H), calcium aluminosilicate hydrate (C-A-S-H), ettringite (AFt), aluminosilicate oligomers (—Si—O—Al—Si—O—)_n, and hydrotalcite. For RFGC-1 (CS only), the microstructure is relatively loose, with the limited formation of amorphous C-S-H and insufficient conversion into C-A-S-H or (—Si—O—Al—Si—O—)_n, resulting in a low strength. As the GGBS content increases and CS decreases, intermediate products (C-S-H, C-A-H) further react to form C-A-S-H, AFt, and (—Si—O—Al—Si—O—)_n. The gel phases envelop adjacent particles, forming interconnected coatings and enhancing the aluminosilicate and SiO₄ tetrahedral networks, thereby strengthening the geopolymer matrix.

The incorporation of GGBS alone was more conducive to strength development in the RM-Class II FA based geopolymers, compared to CS alone. In the case of co-incorporating GGBS and CS, the primary cementitious products included C-A-H), C-S-H, C-A-S-H monomers, as well as ettringite, aluminosilicate oligomers (—Si—O—Al—Si—O—)_n, and hydrotalcite, which collectively contributed to the strength of the geopolymers. The addition of GGBS and CS to the RM-Class II FA system modulated the contents of Ca, Si, and Al in the composite system, significantly influencing geopolymer strength development. At a higher content of Ca, C-S-H became the dominant product. Conversely, at a higher content of Si or Al, these components first dissolved in the alkaline environment to form gels such as C-S-H, C-A-H, and C-A-S-H. These gels subsequently underwent dissolution and polycondensation under strong alkaline conditions, forming long-chain polymers like Si—O—Al—O and Si—O—Al—Si—O. For overtime, water was expelled, leading to coagulation and polymerization, ultimately resulting in the formation of dense and stable geopolymer structures.

As the second largest global carbon dioxide emission source, the cement industry contributes approximately 8% of total anthropogenic carbon emissions. Under the "dual-carbon" strategic framework, developing novel low-carbon cementitious materials emerges as a research priority in civil engineering. Alkali-activated cementitious materials (AACM) are regarded as a promising alternative to conventional Portland cement due to their advantages of eliminating high-temperature calcination and utilizing industrial solid wastes. However, the existing research predominantly focuses on material performance optimization, while the definition of their "low-carbon" attributes remains limited to raw material substitution, lacking a systematic life cycle carbon emission accounting framework. No scientific evaluation methods or quantitative models are established to assess carbon footprint disparities among different activator systems (i.e., carbonate, hydroxide, and silicate). Furthermore, red mud-a massive solid waste generated by the alumina industry with global annual emissions exceeding 150 million tons exhibits natural compatibility with alkali-activated systems due to its high alkali metal content. Nevertheless, the existing studies predominantly emphasize its reactivity activation, while neglecting the development of integrated low-carbon processing technologies. Establishing systematic carbon accounting methodologies and elucidating the coupling mechanisms among activators, carbon emissions, and mechanical properties become critical scientific challenges for advancing the engineering applications of alkali-activated materials.

In the AACM production framework, the study defined a "cradle-to-gate" system boundary spanning from raw material acquisition to final product formation. This boundary was divided into three distinct phases, i.e., raw material acquisition stage (Am), encompasses processes such as raw material production, mining, and extraction, and raw material transportation stage (At). Accounts for carbon emissions generate during the transportation of all required raw materials. Production stage (Ap) includes operational steps such as drying, homogenization, grinding, storage, and packaging. The investigation specifically employed red mud as an intrinsic alkali source, focusing on elucidating the influence patterns of both red mud and external alkalis (i.e., Na₂CO₃, NaOH, and Na₂O·2SiO₂) on the carbon emissions of AACM. Response surface methodology (RSM) with the Box-Behnken design was employed to optimize mix parameters (i.e., Na₂CO₃ dosage, red mud content, and cement content), conducting multi-factor interaction analysis with dual objectives, i.e., 28-d compressive strength and carbon emissions.

To compare the life cycle carbon emissions of AACM production in the same production line with industrial solid waste (i.e., red mud), granulated blast furnace slag, and ordinary Portland cement clinker as precursors, as well as Na₂CO₃, NaOH, and Na₂O·2SiO₂ as activators and to design low-carbon AACM, the following parameters were established. For the raw material transportation phase, heavy-duty diesel trucks (46-ton payload) with a carbon emission factor of 0.057 kg CO₂/(t·km) were selected, assuming a default transportation distance of 1000 km. For the production phase, the carbon emissions from drying red mud and granulated blast furnace slag were estimated based on electricity consumption of drying equipment and exhaust fans. The field data indicated that drying granulated blast furnace slag with 10%-13% moisture content required approximately 11.5 kW·h per ton of wet material. For high-moisture red mud drying (using a Φ3 m×28 m rotary dryer with 2.45-4.20 r/min rotation speed and 3.5% slope), the energy consumption reached 35.0 kW·h per ton of wet material (data from Shanghai Bai Ao Heng New Materials Co., Ltd., China). Carbon emissions from material batching, grinding, and homogenization processes were estimated using reference power consumption data (i.e., ≈33 kW·h per ton of cement) from a plant of Southwest Cement Co., Ltd., China. The average carbon emission factor of China's regional power grids was adopted as 0.5366 kg CO₂/(kW·h) (2022 national average electricity CO₂ emission factor).

This study proposes a life cycle carbon footprint accounting model for AACM production, reveals the synergistic regulation mechanism of activator types and mix proportions on the material low-carbon characteristics and mechanical performance through response surface methodology-based optimization design and establishes a cradle-to-gate carbon footprint accounting system covering raw material acquisition, transportation, and production stages. The contribution weights of red mud-based AACM precursors and activator types to carbon emissions are quantified, providing a quantitative basis for evaluating low-carbon cementitious materials. As an endogenous alkali source, red mud can effectively reduce the dosage of external activators. Coupled with the low-carbon characteristics of Na₂CO₃ (carbon emission factor: 0.79 t CO₂/t), this significantly reduces the system's carbon emissions. At a red mud content of ≤50% and a cement clinker dosage of ≤10%, the material's carbon emissions can reduce to below 280 kg CO₂ per ton of AACM, validating a feasibility of the synergistic pathway of solid waste recycling-low-carbon process. The low-carbon design based on the RSM demonstrates that in the optimal mix proportion of the red mud-slag system (with 5.54% Na₂CO₃ dosage, 35.56% red mud, 57.22% slag, and 7.22% cement), the cementitious material achieves a 28-d compressive strength of 58.5 MPa and a carbon emission of 228 kg CO₂ per ton of AACM. This system exhibits both high strength and low-carbon characteristics, with performance comparable to Portland cement P·O 42.5R. In this mix proportion, red mud with a high alkali content provides abundant endogenous alkali to the system, reducing reliance on exogenous alkali. The amorphous phase structure of blast furnace slag endows it with a high alkali reactivity, enabling a rapid release of Ca²⁺ and SiO₄^4- under Na₂CO₃ activation to promote gel formation and enhance strength. The hydration of minerals such as C₃S and C₂S in cement generates additional Ca(OH)₂, further elevating the pH value of the system. This accelerates the depolymerization of slag vitreous phases and enhances the compactness of the matrix interfacial transition zone, thereby ensuring the mechanical performance of the AACM.

This study could pioneer the integration of red mud's intrinsic alkali properties with low-carbon activators, proposing a holistic "low-carbon design-carbon accounting-performance optimization" methodology. The framework provided a theoretical support for industrial applications of alkali-activated materials, offering dual benefits, i.e., large-scale red mud consumption in the alumina industry and a technical pathway for achieving the "dual carbon" goals in construction materials.

The sulfate source in calcium sulfoaluminate (CSA) cement is an important factor that determines its hydration characteristics, and the properties of CSA cement can be controlled via adjusting the molar ratio of calcium sulfate to ye'elimite (generally referred to as the "M-value"). Sulphosilicate cement is a novel cementitious material developed via incorporating ternesite (C₅S₂$) and free-calcium sulfate (f-C$) into the mineral composition of CSA cement. The calculation of the M-value must comprehensively account for their contents in the clinker due to the presence of SO₃ in C₅S₂$ and f-C$. This study optimized the conventional calculation formula for the M-value and systematically investigated its influence on the compressive strength and expansion behavior of sulphosilicate cement. In addition, the mechanism underlying the effect of anhydrite dosage on cement hydration was also elucidated through the evolution of hydration products.

In this experiment, sulphosilicate clinker was synthesized with limestone, fly ash, phosphogypsum, and bauxite as raw materials. The raw materials were calcined at 1250 ℃ for 1 h, followed by rapid air quenching to obtain the clinker. Five groups of sulphosilicate cement with M-values of 0, 0.9, 1.8, 2.7, and 3.6 were designed. Mortar specimens were prepared to evaluate compressive strength and expansion rate, while paste specimens were prepared for analyzing hydration product characteristics at specific ages. The hydration heat release was measured by an isothermal calorimeter. The phase compositions and microstructure of the hydration products were analyzed by X-ray diffractometer, thermal behavior analyzer and field-emission environmental scanning electron microscope. The pore structure was performed by a mercury intrusion porosimeter.

Among the various sulphoaluminate cement groups, the M_1.8 specimen demonstrates the maximum compressive strength of 43.9 MPa after 3-d hydration. As hydration progresses, the compressive strength of the M_2.7 and M_3.6 groups shows a significant growth, with the M_3.6 group reaching 56.7 MPa at 14 d. However, the M_3.6 specimen exhibits a notable strength retrogression, which, is decreased by 4.3 MPa, after 28-d hydration. In contrast, the compressive strength of the M_0.9 group increases markedly from 14 d to 28 d, with an improvement of 12.3 MPa.

For specimens with the M-value below 1, the early hydration products at 3 d are dominated by AFm phases with insufficient AFt content, resulting in an initial shrinkage. As hydration progresses, the dissolution of SO₃ from anhydrite and C₅S₂$ promotes AFt formation, leading to a measurable expansion after 7 d. At low M-values, AFt contributes to pore refinement, while the reaction of C₅S₂$ and C₂S in later stages generates C-(A)-S-H gel and stratlingite (C₂ASH₈), refining the microstructure of hydration products and enhancing long-term strength. In contrast, specimens with higher M-values (i.e., 1.8, 2.7, and 3.6) exhibit expansion as early as 3 d. Increased anhydrite content and prolonged hydration amplify the crystallographic stress exerted by AFt crystals on the surrounding matrix, causing coarsening of pores, and elevated total porosity, and resulting in strength retrogression during 14-28 d of curing.

This study was to propose a formula for calculating the gypsum coefficient M-value in sulphosilicate cement, i.e., C_G= 0.13 × M × (Y - A×2.24 - T×1.27)/S_g, establishing the relationship between gypsum dosage and cement performance, while elucidating the influence of M-values on the mechanical properties and hydration mechanisms. The experimental results demonstrated that within the optimal M-range of 0.9-1.8, cement could achieve continuous strength development for all curing ages. Higher M-values (2.7-3.6) enhanced mid-term strength but induced strength regression at 28 d. Increasing M-values effectively suppressed early-age shrinkage and induced progressive expansion, and expansion rates were positively correlated to M-values at all hydration stages. Hydration products primarily consisted of AFt, AH₃, and C-(A)-S-H gel. At M₀ and M_0.9, AFm phases formed during early hydration, followed by C₂ASH₈ in later stages. Conversely, M-values of ≥1.8 accelerated C₄A₃$ hydration but inhibited both C₅S₂$ dissolution and C₂ASH₈ formation. Microstructurally, M_0.9 specimens exhibited pore refinement and reduced porosity through gel-phase densification, enhancing strength development, whereas M_3.6 samples suffered from coarse pore proliferation due to excessive anhydrite content promoting oversized AFt crystals, ultimately degrading 28-d strength. At the M-value of 0.9-1.8, the system achieved balanced high strength and minimal volumetric deformation, making it suitable for general engineering applications. Elevating the M-value to 1.8-2.7 induced a controlled expansion behavior, enabling targeted use in shrinkage compensation scenarios. However, at the M value of 3.6, delayed strength retrogression and microstructural deterioration could occur, thus establishing 3.6 as a critical upper limit for M-values.

As a type of energy-saving and environmental protection building system, steel structure building is widely used in engineering construction in recent years, and its fire protection measures are particularly important. At present, the commonly used fire retardant coatings are mainly divided into two categories, i.e., organic intumescent and inorganic non-intumescent coatings. Although organic intumescent coatings exhibit a good fire resistance in the initial stage, their duration of protection is relatively short, and they may release toxic and corrosive gases during combustion, posing certain safety risks. In contrast, inorganic non-intumescent fireproof coatings have advantages such as high thermal resistance, strong durability, good wear resistance, and low cost, making them considered as an ideal fire protection material for steel structures. However, some issues such as insufficient adhesion and poor overall durability seriously limit its widespread application in practical engineering. Magnesium phosphate cement (MPC) is regarded as a fireproof material with an application potential due to its high strength, high-temperature resistance and excellent interfacial bonding performance with steel. However, the conventional raw material for MPC like dead-burned magnesia has a high density and carbon emissions, which is unfavorable for its green and sustainable development. To address the above issues, this study was to select a low-grade natural brucite powder as a complete substitute for dead-burned magnesia to prepare magnesium phosphate cement, and optimize the physical, mechanical, and fire insulation properties of brucite-based magnesium phosphate steel structure fireproof coatings (BMPC) via introducing modified expanded perlite.

The raw materials for the preparation of BMPC included natural brucite powder, ammonium dihydrogen phosphate, analytical grade borax, and expanded perlite. Expanded perlite was divided into two types, i.e., untreated (EP) and water absorption pretreatment (HEP) according to different treatment methods. Under the condition of fixed process parameters (i.e., M/P mass ratio of 2.5, borax content of 15%, water-cement ratio of 0.17, and expanded perlite content of 0-20% of the total mass of brucite powder and ammonium dihydrogen phosphate), the BMPC slurry was prepared.

The dry density and surface drying time of the BMPC coating under natural curing conditions were tested in accordance with the standard GB 14907—2018. The compressive strength and adhesive strength tests were conducted on the specimens after 3-d, 7-d and 28-d curing, respectively. The thermal conductivity of BMPC was determined by a transient plane source method. The fire resistance limit was tested according to the standard GB/T 9978.1—2008. The internal pore structure of BMPC samples was analyzed by an industrial computed tomography system, and its microstructure was analyzed by field emission scanning electron microscopy. The phase change of BMPC coating before and after fire test was analyzed by X-ray diffractometry, and its thermal stability in the range of 30-1000 ℃ was analyzed by thermogravimetric analysis.

The expanded perlite (HEP) after water absorption treatment has a more open pore structure and a higher surface roughness rather than the untreated expanded perlite (EP). In the BMPC system, the HEP has a relatively small effect on the mechanical properties, especially under low density conditions, it can still maintain high compressive strength and adhesive strength. At the HEP content of 20%, the microporous structure inside the material significantly reduces the thermal conductivity and thermal diffusion coefficient. This is mainly since the HEP acts as a low thermal conductivity filler, and the air layer retained in its pores (i.e., thermal conductivity of 0.026 W/(m·K)) effectively blocks heat transfer. Also, the introduction of HEP optimizes the pore structure, reduces the proportion of large pores, makes the heat conduction path more complex, delays the diffusion process of heat into the substrate, and significantly slows down the flame propagation rate and thermal decomposition process. The results of fire resistance test and pore structure show that the porosity of BMPC-HEP sample is higher than that of the control group, and the standard deviation of pore fluctuation increases from 4.0% to 5.1%, indicating that HEP forms a lightweight porous structure at a high temperature, further improving the thermal insulation performance and thermal stability of the material. The addition of HEP has a significant effect on the microstructure of BMPC at a high temperature. Some struvite and unreacted raw materials absorb a large amount of heat during pyrolysis, which plays a role in delaying temperature rise and enhancing thermal buffering, and provides an important support for the thermal insulation protection mechanism of fire retardant coatings.

The expanded perlite after water absorption treatment had little effect on the mechanical properties of BMPC system. When the density reduced from 1062 kg/m³ to 647 kg/m³, the 28-d compressive strength and adhesive strength were only reduced by 26.5% and 17.7%, respectively. At the content of HEP of 20%, the thermal conductivity and thermal diffusivity of BMPC were 49.3% and 44.0% lower than those of BMPC without HEP, respectively. The significant improvement of this thermal performance could be mainly attributed to the fact that HEP itself had low density and low thermal diffusion characteristics, effectively slowing down the heat transfer rate in the process of temperature change, and thereby improving the thermal insulation performance of the material. The fire resistance limit of BMPC-HEP system reached 206 min, which was significantly better than that of the control sample. Its excellent fire resistance was mainly attributed to the synergistic effect of porous structure, low thermal conductivity filler and microstructure stability at a high temperature, indicating that it could have a promising application potential in the field of fire retardant coatings for steel structures.

Conventional modification of high-potassium cement clinker relies on adding SO₃ to reduce K₂O solid solution and boost C₃S content. However, SO₃ causes operational issues and stabilizes low-reactive C₂S-β, compromising strength. In this study, SO₃ was used to stabilize highly reactive C₂S-α'_L as a strength-enhancing phase, while K⁺ could suppress C₃S formation. To harness this "contradictory duality", an optimal C₃S/C₂S-α'_L ratio that could maximize the strength contribution of C₂S-α'_L without significantly sacrificing C₃S was established. This strategy could overcome the limitations of SO₃-based methods and enable an efficient utilization of high-potassium limestone.

Raw meals were formulated from analytical-grade reagents (i.e., CaCO₃, SiO₂, Al₂O₃, Fe₂O₃, K₂CO₃, CaSO₄·2H₂O ≥ 99.0%) with fixed silica modulus (SM=2.2), alumina modulus (IM=1.5), and K₂O content (2%, in mass). The lime saturation factor (KH) was adjusted from 0.84 to 0.92 to modulate C₃S/C₂S ratios. Raw materials were wet ground in a ball mill at 600 r/min for 1.5 h, dried at 105 ℃, pelletized under 100 kN, and then step-calcined (i.e., 950 ℃/1 h → 1450 ℃/30 min). The materials were followed by rapid air-quenching to inhibit C₂S-γ formation. The resulting clinker was dry ground to a fine powder with the sizes of ≤200 mesh, blended with gypsum of 3%, and cast into paste specimens (20 mm×20 mm×20 mm, w/c=0.3) for standard curing at 20 ℃ and 95% RH for 3-d/28-d ages.

The chemical composition was analyzed on borate-pressed pellets with a starch binder of 10% by X-ray fluorescence spectrometry (XRF). The phase quantification was determined by X-ray diffraction (XRD)-Rietveld refinement (Bruker D8 Advance, 5°-80°, 40 kV/20 mA) via a software named TOPAS V4. 2 with March-Dollase texture correction for the key phases. The microstructural evolution was characterized by scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) (gold-sputtered polished sections, 10-point EDS averaging) and optical microscopy (NH₄Cl-etched surfaces). The hydration kinetics were monitored by isothermal calorimetry (TAM Air, at 20 ℃ for 72 h), while the thermal decomposition behavior was assessed by thermogravimetric analysis (TGA) (RT→550 ℃ at 10 ℃/min under N₂). The amorphous content in hydration products was quantified using corundum internal standard (20%) after ethanol immersion and vacuum drying.

The results demonstrate that elevating the C₃S/ C₂S-α'_L ratio fundamentally restructures phase equilibria and ion distribution. As the ratio increases from 0.78 to 1.80, K⁺ solid solubility in C₂S is decreased by ~42% (quantified via EDS point analysis), destabilizing the metastable α'_L phase and triggering its transformation to C₂S-β, which is evidenced by α'_L content reducti on from 39.27% to 25.08% and intensified twinning striation. Concurrently, K⁺ migration toward interstitial phases increases the conversion from C₃A-o to C₃A-c, correlating with increasing C₃A content (i.e., from 9.97% to 1.02%) and declining C₄AF (i.e., from 8.94% to 7.39%). Based on the microstructural analysis, enlarged C₃S crystals with prominent etch pits emerge at high ratios due to K⁺-enriched liquid phases suppressing nucleation, while promoting disordered growth. This is compounded by elevated C₃A/C₄AF ratios increasing melt viscosity, hindering ion diffusion. The elemental mapping confirms that K⁺ predominantly partitions into C₂S and interstitial phases, with minimal C₃S incorporation (i.e., <0.3%, in mole).

The mechanical performance exhibits dual-phase characteristics. The 3-d strength is increased by ~18 MPa across the ratio gradient, driven by three synergistic factors, i.e., increased C₃A hydration activity, accelerated reaction kinetics from free CaO, and enhanced C₃S surface defects. The 28-d strength is declined by ~12 MPa, mechanistically linking to hydration restructuring. The result of calorimetry reveals sulfate depletion peaks advancing for 1.7 hours at higher ratios due to a rapid gypsum consumption by abundant C₃A. This shifts hydration products from AFt to Hc phases, which is evidenced by the XRD quantification, where AFt content decreases from 5.44% to 1.19% at 3 d, while Hc increases from 3.94% to 6.14%. At 28 d, Hc dominates (i.e., 7.55% at the highest ratio), with AFt nearly depleted (i.e., 0.27%).

The C-S-H gel morphology transitions from needle-like aggregates (i.e., Ca/Si=1.99 at a low ratio) to flaky networks (i.e., Ca/Si=1.26 at a high ratio). This structural shift reduces K⁺ adsorption capacity. The data establishe a Ca/Si ratio as a critical variable governing gel-K⁺ interactions, resolving prior discrepancies.

As the C₃S / C₂S-α'_L ratio decreased, the solid solubility of K⁺ and Al³⁺ in C₂S declined, whereas K⁺ incorporation in interstitial phases increased. This ultimately reduced the C₂S-α'_L / C₂S-β ratio in clinker, while elevating the C₃A-o/C₃A-c ratio. Decreasing the ratio could significantly enlarge C₃S particle size in high-potassium clinker, resulting in less distinct edges and increased etch pits. Although the fundamental morphology of C₂S remained largely unchanged, its surface became rougher with pronounced twinning striations.

The 3-day compressive strength of high-potassium clinker diminished at a lower C₃S/ C₂S-α'_L ratio, whereas the 28-day strength exhibited an inverse trend. As C₂S-α'_L competitiveness weakened, the peak heat flow and cumulative heat release in hydration curves both intensified. However, the sulfate depletion peak could emerge earlier, accompanied by increased hydrocalumite (Hc) phase content, further amplified when C₃S dominated over C₂S-α'_L.

The C-S-H gel morphology transitioned from aggregated needle-like fibers at low C₃S/ C₂S-α'_L ratios to flake-assembled network structures at high C₃S/ C₂S-α'_L ratios.

The construction industry is a primary contributor to global CO₂ emissions, making the development of novel low-carbon building materials a critical scientific and engineering challenge. A promising strategy involves the valorization of waste concrete fines, which are generated in vast quantities. The thermal activation of these fines at 600-800 ℃ transforms the hydrated cement paste into a recycled cement rich in dicalcium silicate (i.e., β-C₂S), a clinker phase with a high intrinsic carbonation reactivity. This makes thermally activated recycled cement an ideal precursor for carbon-sequestering materials where CO₂ is utilized as a curing agent to enhance mechanical properties. However, its practical application is hindered by a key technical bottleneck, i.e., relatively low reaction kinetics and insufficient mechanical strength development. Therefore, identifying an effective modifier to stimulate its carbonation potential and optimize the final microstructure is of significant urgency. Metakaolin (MK) as a highly reactive pozzolan is a promising candidate due to its known ability to promote hydration and serve as a nucleation site. This study was to systematically investigate the role of MK as a synergistic component to overcome the performance limitations of carbonated recycled cement, providing a scientific basis for the development of high-performance, green building materials.

The raw materials used were a waste concrete powder (WCP) and a commercial metakaolin (MK). A thermally activated recycled cement (TARCP) was produced via first pressing a mixture of WCP and MK (0%, 10%, and 20% by mass, denoted as TARCP, MK10, and MK20, respectively) and then calcining it in a muffle furnace at 800 ℃ for 1 h. After cooling and grinding, the resulting powders were mixed with water at a water-to-binder ratio of 0.55 to prepare paste specimens. The pastes were cast and cured for 24 h before being demolded and subjected to one of two curing regimes for 7 d and 28 d, i.e., (1) water curing (25 ℃, 75% RH) and (2) accelerated carbonation curing (25 ℃, 75% RH, 20% CO₂).

The compressive strength of 20 mm cubic specimens was tested at each age. The phase evolution was comprehensively characterized by X-ray diffraction (XRD) with Rietveld quantitative analysis, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The pore structure distribution was analyzed by low-field nuclear magnetic resonance spectroscopy (NMR). The microstructure and morphology of the reaction products were determined by scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX).

The experimental results demonstrate that the incorporation of an optimal amount of MK significantly enhances the performance of the carbonated recycled cement. The addition of 10% MK (MK10) yields the optimum results, with the 7-d compressive strength under carbonationof 37.7 MPa, which is increased by 119.2%, compared to the carbonated control sample (TARCP). This superior performance is attributed to a combination of physical and chemical effects induced by MK.

The phase analysis reveals that MK accelerates the consumption of the primary reactive phase like β-C₂S. After 7-d carbonation, the β-C₂S content in the sample MK10 is reduced to just 1.2%, compared to 29.8% in the TARCP control. This accelerated reaction leads to a much higher degree of carbonation. The 7-d carbonation degree for the sample MK10 is 50.79%, which is improved by 70%, compared to the sample TARCP (i.e., 29.87%). Furthermore, the MK promotes the transformation of metastable vaterite to the thermodynamically stable calcite. At 7 days, the sample MK10 is overwhelmingly composed of calcite (i.e., 72.7%), with only a negligible amount of vaterite (1.6%), whereas the sample TARCP contains a significant amount of vaterite (i.e., 8.5%). The FTIR analysis indicates the formation of a highly polymerized amorphous silica-alumina gel in the carbonated MK-modified systems, which is evidenced by the shift of the main Si—O vibration band to a higher wavenumber (~1080 cm^-1), indicating a fundamental restructuring of the silicate network.

The microstructural analysis provides the strengthening mechanism. The NMR results show that the addition of 10% MK significantly refines the pore structure. After 7-d carbonation, the proportion of large capillary pores (i.e., >50 nm) in the MK10 sample decreases from 49.88% (in the sample TARCP) to 36.56%, while the proportion of fine gel pores (i.e., <10 nm) nearly increases from 8.98% to 17.81%. This pore refinement, which is driven by the uniform deposition of carbonation products, results in a much denser matrix, which is the primary reason for the observed strength enhancement. However, the effect of MK is non-linear. An excessive dosage of 20% MK (the sample MK20), while further refining the pore structure (the capillary of pores reduces to 29.75% at 7 d), resulting in lower compressive strength than the sample MK10 at all ages. This is accompanied by a slightly lower final degree of carbonation at 28 d (51.72% for the sample MK20 vs. 52.88% for the sample MK10). This indicates that the excessive pore refinement can create a dense, impermeable microstructure that impedes the diffusion of CO₂ gas into the material core, thereby limiting the overall reaction rate and extent in the later stages. Moreover, the anomaly of lower strength despite a denser pore structure in the sample MK20 increases that the intrinsic properties of the solid phases play a critical role. It is hypothesized that the higher Al/Si ratio in the sample MK20 leads to the formation of an amorphous alumina-silica gel with a lower degree of polymerization, possessing the inferior intrinsic mechanical properties, compared to the gel formed at an optimal Al/Si ratio.

This study demonstratesd that incorporating an optimal amount of metakaolin (10% by mass) could be a highly effective strategy for enhancing the carbonation reactivity and mechanical performance of thermally activated recycled cement. The improvements could be attributed to the synergistic effects of MK, i.e., accelerating the consumption of β-C₂S, promoting the formation of stable calcite, and refining the pore structure via creating a denser, more uniform microstructure. However, an excessive dosage of MK (20%) could be detrimental. It led to a further pore densification, this could inhibit a later-stage CO₂ diffusion, limiting the final carbonation degree. Furthermore, the higher Al/Si ratio could negatively affect the intrinsic strength of the amorphous binder phase. These findings could highlight the non-linear effect of metakaolin and provide a scientific basis for optimizing the design of high-performance, carbon-sequestering building materials derived from recycled cement.

Supersulfated alkali-activated binders (SS-AABs) have emerged as a superior alternative to Portland cement due to their extensive utilization of silicon-aluminum solid waste, sulfate solid waste, and alkaline solid waste, as well as their exceptional mechanical qualities and corrosion resistance. The primary precursors of silicon-aluminum components in SS-AABs are predominantly ground granulated blast furnace slag (GGBFS), comprising 70% to 90% of the composition. Identifying alternative industrial solid waste products that serve as superior precursors for silicon and aluminum can yield significant economic and environmental advantages. Bayer red mud (BRM) is a high-alkali, challenging silicon-aluminum by-product generated in the alumina industry. Nonetheless, the crystal structure of unactivated BRM has some challenges in engaging with the alkali-activated process of the gelling system, rendering it unsuitable as a precursor for SS-AABs. The BRM, following thermal activation treatment, can yield a more active amorphous structure, facilitating enhanced dissociation and repolymerization reactions with alkaline solid waste activators, resulting in the formation of more stable hydration products and establishing a high-quality silicon-aluminum component precursor. The rationale of the detection approach for the comprehensive activity of silicon and aluminum BRMP in SS-AABs is insufficient, hence constraining engineering applications. This paper was to examine the evolution of minerals during the thermal activation of BRM, systematically analyze the characteristics of three activity testing methodologies, i.e., chemical dissolution methods (e.g., inductively coupled plasma (ICP) or chemical titration), pozzolanic activity index methods, and microstructural characterization methods (such as X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR)). In addition, this paper was also to investigate the changes in activity of BRM as a silicon-aluminum precursor at varying activation temperatures and propose some insights for assessing the silicon-aluminum activity of thermally activated Bayer red mud precursors (BRMP), tailored for the preparation of SS-AABs.

The activity index of thermally activated red mud was analyzed in accordance with the standard GB/T 1596—2005. A specimen with the sizes of 40 mm × 40 mm × 160 mm was fabricated with thermally activated red mud, cement, standard sand, and water in accordance with the stated proportions. It was then cured under standard conditions (i.e., (20 ± 2) ℃, 96% RH) for 7 d and 28 d, respetively. The compressive strength of the specimens was evaluated at two ages in accordance with the standard GB/T 17671—2021. The activity index was the ratio of the 7-d and 28-d compressive strength of cement mortar specimens incorporating thermally activated red mud to the compressive strength of pure cement mortar specimens at the equivalent age. A specimen SS-AABs with the sizes of 40 mm × 40 mm × 40 mm was prepared with 58% heat-activated BRMP, 25% desulfurization gypsum, and 17% carbide slag at a water-cement ratio of 0.6. The standard curing was conducted at (20 ± 1) ℃ and 96% relative humidity for 3, 7 d, and 28 d, respectively. The compressive strength of the specimens was evaluated at three ages in accordance with the standard GB/T 17671—2021. The microstructure was examined by scanning electron microscopy (SEM), its mineral composition was assessed by X-ray diffraction (XRD), and the alterations in chemical bonds were evaluated by Fourier transform infrared spectroscopy (FTIR). The solubility of Si⁴⁺ and Al³⁺ in thermally activated BRMP at various temperatures was assessed by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The leaching quantity of active Si⁴⁺ was ascertained by a potassium fluosilicate volumetric method, and the leaching quantity of active Al³⁺ was evaluated by ethylenediaminetetraacetic acid (EDTA) complexometric titration method.

The result indicates that the combined analysis of XRD-FTIR can proficiently characterize the mineral composition and alterations in chemical structure of BRMP. The pozzolanic activity index approach yields quantitative data but limits to assessing pozzolanic activity in cement systems and cannot differentiate the contributions of silicon and aluminum. The chemical titration method can quantitatively assess the dissolution rates of silicon and aluminum. However, it lacks a sensitivity to variations in ion concentration, particularly within the activation temperature range of 300-700 ℃, making it challenging to accurately represent changes in activity. Conversely, for precisely quantifying the dissolution concentrations of Si⁴⁺ and Al³⁺, the ICP-AES method can quantitatively indicate the activity changes in BRMP at different activation temperatures and differentiate the dissolution behaviors of silicon and aluminum, thereby offering a more dependable foundation for the optimization of components in SS-AABs. The thermal activation at 800 ℃ significantly enhances the breakdown of silicon and aluminum components, compared to that of unactivated BRMP.

The alteration of the mineral phase in red mud could be constrained by the activation temperature. Katoite decomposed at 400 ℃, chlorite and diaspore decomposed at 500 ℃, and grossular and nepheline decomposed at 800 ℃. The testing methodology for volcanic ash activity possessed specific constraints when examining the activity of BRMP. The conventional activity index approach was constrained by the gypsum concentration (< 5%) in Portland cement (the standard GB 175—2023), making it challenging to precisely assess the contribution of active Al₂O₃ in BRMP. The chemical titration method possessed specific advantages in indicating the mineral activity of BRMP following thermal activation. The quantity of leached active aluminum in BRMP post-thermal activation could be greater than that of active silicon. The analysis by XRD-FTIR indicated that at 800 ℃, silicon-containing minerals could fully activate, whereas aluminum-containing minerals could disintegrate more readily. The chemical titration method could partially indicate the activation impact of BRMP. However, the detected fluctuation in silicon and aluminum leaching was limited, leading to inadequate sensitivity to changes in ion concentration. The dissolution rate of active silicon and aluminum following BRMP activation by the ICP-AES method showed that the thermally activated BRMP had considerable benefits in terms of activity. The leaching quantity of Al³⁺ in BRMP post-thermal activation consistently exceeded that of Si⁴⁺, aligning with the leaching patterns of active SiO₂ and Al₂O₃ determined using chemical titration. At 800 ℃, the activation effect could be markedly intensified, with the leaching concentration of Si⁴⁺ attaining 96.4 mg/L, and the leaching concentration of Al³⁺ reaching 167.5 mg/L. Compared with the unactivated BRMP, the dissolution of silicon and aluminum elements of activated BRMP was greatly improved by thermal activation at 800 ℃.

The appropriate curing methods for 3D-printed cement-based materials have not yet been clearly defined, thus restricting their durability in cold regions. This study was to investigate the effects of natural curing (NC), standard curing (SC), high-temperature curing (T₆₀), saturated Ca(OH)₂ solution curing (SCH), and internal curing withabsorbent polymer (SAP) on the performance of solid waste-based 3D printed mortar (3DFGCM). The objective was to establish quantitative relationships among curing methods, mechanical anisotropy, frost resistance, and pore structure, thereby providing a theoretical foundation for improving the service life of 3D printed structures in cold regions.

The fly ash-slag-cement based mortar (FA : GGBS : OPC=1 : 2 : 7) was prepared at a sand-to-binder ratio of 1 : 1 and a water-to-binder ratio of 0.3. After vibration casting and 3D printing of the slurry, the specimens were naturally cured for 1 day and then divided into five groups for further curing. The 28-d compressive strength and anisotropy were tested in the X, Y, and Z directions. After 28-d curing under respective conditions, the specimens were immersed in water at 15-20 ℃ for 2 d before undergoing freeze-thaw cycles. The mass loss rate, relative dynamic elastic modulus, and strength loss rate were measured. The pore structure was analyzed before and after freeze-thaw cycles to determine parameters such as air content, chord length distribution, and spacing factor. Finally, a damage evolution model based on the Weibull distribution was established to predict service life, and a neural network was used to map the relationship between pore parameters and damage.

This study systematically investigates the effect of curing method (i.e., NC, SC, T₆₀, SCH, and SAP) on the performance of 3D printed fly ash-slag-cement-based mortar (3DFGCM). The comprehensive analysis of mechanical anisotropy, frost resistance, and pore structure, combined with freeze-thaw models and service life prediction models indicates that the 28-d compressive strength and isotropy coefficient of 3D printed specimens are lower than those of cast specimens. Among the curing methods, 3D-SC demonstrates the most significant enhancement in the 28-d compressive strength, while the SAP increases the 28-d compressive strength to 36.8 MPa, which is increased by 41.17%, compared to the NC group, with the lowest anisotropy coefficient (i.e., 0.187). The frost resistance of 3D printed specimens varies significantly. The 3D-NC and 3D-T₆₀ groups both fail after 125 freeze-thaw cycles, the 3D-SAP group exhibits the optimum frost resistance, retaining 61.5% of its dynamic elastic modulus and showing a mass loss of 4.57% after 150 freeze-thaw cycles. Its closed micropores and humidity balance mechanism effectively mitigate frost heave damage. In terms of the pore structure, 3D printed specimens exhibit distinct elliptical pores. The 3D-SCH group shows a limited improvement in cavity and interlayer cold joint defects, with large pores accounting for 75% more than those in the 3D-SAP group. The incorporation of SAP improves process-induced defects and significantly inhibits pore coarsening. With the relative dynamic elastic modulus as an index, the Weibull damage and service life prediction models are applied to assess freeze-thaw damage and service life of 3DFGCM, with R² of > 0.98. The 3D-SAP group performs optimally, with a predicted service life of 15.9 years. Furthermore, based on BP neural networks, the effective correlations are established between pore structure parameters and the Weibull-predicted damage and service life, with R² of > 0.97.

In terms of mechanical anisotropy, the strength of 3D-printed specimens followed a decreasing order of Cast>X>Z>Y. The SAP curing resulted in the lowest anisotropy coefficient (i.e., 0.187), with a 41.2% increase in the strength, compared to the NC group. For the frost resistance, after 150 freeze-thaw cycles, the SAP group retained 61.7% of its dynamic elastic modulus and exhibited only 4.57% mass loss. In contrast, the NC and T₆₀ groups failed after 125 cycles due to pore coarsening. In terms of the pore structure, the initial air content of 3D-printed specimens was 1.5 times higher than that of cast specimens. After freeze-thaw cycles, the total chord length of the NC group was increased by 65.3%. The SAP effectively inhibited pore coarsening, with large pores accounting for only 36%, representing a 75% reduction, compared the NC group. Finally, a damage evolution equation was established based on the Weibull distribution model, predicting a service life of 15.9 years for 3D-SAP in cold northwestern regions. The BP neural networks were employed to predict damage and lifespan based on microscopic pore structure. This study could provide a reference for the design of methods and frost resistance optimization in 3D-printed construction materials.

The introduction of carbon peaking and carbon neutrality targets has made a green, low-carbon transition the prevailing trend in the cement manufacturing industry. Solid-waste valorisation, which involves substituting natural minerals and fossil fuels with suitable solid waste, can effectively reduce emissions from carbonate decomposition and fuel combustion. It is therefore regarded as a key technological pathway for achieving the carbon reduction goals. The existing studies on single dimensions such as energy or material saving, or carbon mitigation in clinker production using solid waste are relatively well-developed. However, valorisation can generate both environmental benefits and drawbacks, there is a clear need for a comprehensive assessment framework based on multiple indicators that can capture trade-offs across impact categories, rather than drawing conclusions from a single metric.

A multidimensional assessment framework was developed based on standard product carbon footprint and life-cycle assessment models. This framework could comprise three primary indicators, i.e., carbon, resources and environment and twelve secondary indicators, enabling the structured and transparent evaluation of benefits as well as potential burden shifting. A consistent system boundary was defined to cover the entire clinker production value chain, including solid waste inputs, raw material production, waste pre-treatment, transportation, clinker production and credits for avoiding conventional disposal methods (landfilling or incineration). Within this boundary, five substitution scenarios involving the use of solid waste were constructed and benchmarked against a mineral-only baseline, i.e., carbide slag (S1), manganese slag (S2), coal gangue (S3), fly ash (S4) and sewage sludge (S5). We then systematically quantified the potential benefits and burdens of solid waste utilisation in terms of carbon emissions, resource consumption and environmental impacts. This could provide a decision-relevant evidence to support the cement industry's transition to a low-carbon economy.

In terms of carbon emissions, all scenarios achieve net reductions, albeit with significant variation in magnitude. Scenario 1 demonstrates the most significant mitigation, reducing emissions by 35.2%, compared to the baseline, resulting in a total reduction of 351.3 kg CO₂eq. By contrast, Scenario 2 involves pre-treatment requiring additional Ca(OH)₂ and anhydrous Na₂SO₄, which increases raw-material production emissions by 9.8 kg CO₂eq relative to the baseline. Scenario 5 results in an increase of 4% in fuel-combustion emissions due to the high moisture content of the waste, leading to the weakest overall mitigation with a net reduction of only 38.9 kg CO₂eq. Overall, these results indicate that although carbon benefits are generally achievable across different types of waste, their magnitude depends on waste composition and the intensity of the pre-treatment. In terms of resources, overall consumption decreases across all scenarios, but there is a trade-off between "material savin" and "energy saving". Scenario 1 achieves the greatest overall saving of resources, with a reduction of 19.4% relative to the baseline. This is primarily due to its high substitution rate and avoidance of waste landfilling. However, its energy-intensive pre-treatment increases fossil resource consumption by 27.2%, offsetting the net energy benefit. Scenario 3 exhibits the most pronounced energy-saving effect, reducing fossil resource consumption by 18.9% via substituting approximately 20% of coal with coal gangue. However, although the substitution rate in Scenario 5 is 20%, a need to add bauxite to satisfy mineralogical requirements offset some of its material-saving potential. These findings highlight that the outcomes of resource use depend on substitution ratios and on the additional inputs required to meet clinker quality constraints. In terms of the environmental impact, the avoided disposal process has a significant effect on the results. When this process is included in the assessment, Scenario 1 demonstrates the most significant reduction in overall environmental impact at 84.7%, compared to the baseline. This is primarily due to the prevention of heavy metal emissions (i.e., Cd, Se and As) linked to landfill. Scenario 2 shows a similar pattern, but improvements in human toxicity indicators are limited because mercury (Hg) emissions associated with producing anhydrous sodium sulphate (i.e., anhydrous Na₂SO₄) offset part of the benefits. If avoided disposal is excluded, Scenario 3 exhibits the optimum overall environmental performance. In the comprehensive evaluation, Scenario 1 consistently ranks first under equal weighting and across all three dual-priority schemes, establishing itself as a core pathway with optimal overall benefits. Scenario 3 is suitable as a priority enhancement option under both "carbon emissions-resource priority" and "resource-environment priority" orientations. Scenario 2 can serve as an alternative under the objective of "carbon emissions-environment priority".

The five solid waste valorisation scenarios for clinker production had different advantages and constraints with regard to carbon mitigation, material conservation and environmental improvement. Scenario 1 delivered the optimum overall performance, but its high energy demand and associated pollutant emissions during pre-treatment could be limiting factors. Scenario 2 achieved considerable carbon reduction and environmental benefits, but it was limited by heavy metal emissions and energy consumption during pre-treatment. Scenario 3 could be characterised as "low risk, steady benefit", offering significant savings in fossil energy without apparent adverse environmental impact. Scenario 4 yielded the modest gains across the board without any negative effects, making it suitable for routine implementation. Scenario 5 showed the greatest advantage in terms of photochemical smog control, but it had the weakest overall performance. To advance the valorisation of solid waste in the cement sector, we could recommend optimising the pre-treatment of high-substitution waste (i.e., carbide and manganese slag) to reduce energy use and air pollutant emissions arising from energy-intensive steps. Large-scale substitution could be encouraged for wastes with a substantial valorisation potential (such as carbide and manganese slag) to further enhance integrated environmental benefits.

The carbon mineralization of solid wastes enables the simultaneous achievement of efficient CO₂ capture and the activation enhancement of basic oxygen furnace (BOF) steel slag. However, the relatively low CO₂ concentration in industrial flue gas, the direct mineralization reaction between CO₂ on reaction of steel slag and the issue of low conversion efficiency should be concerned. This study was to employ a potassium glycinate (KG) solution to absorb and enrich CO₂ from flue gas. The CO₂-loaded KG solution was subsequently utilized to indirectly mineralize and modify BOF steel slag, thereby achieving both mineralization modification of steel slag powder and regeneration-recycling of the KG solution. The kinetic characteristics of the mineralization reaction between the CO₂-loaded KG solution and steel slag were investigated. The phase composition of mineralization products, the carbon sequestration capacity of steel slag, and the evolution of its reactivity were analyzed. Furthermore, a synergistic carbon capture approach integrating steel slag mineralization modification with carbonation curing of cement paste was explored, and its influence on the mechanical strength of cement was evaluated. This study could provide a technical support for CO₂ capture using steel slag and its high-value utilization in cement-based materials.

BOF steel slag with a CaO content of 37% was selected as a raw material. A 1 mol/L potassium glycinate (KG) solution was first carbonated to obtain a CO₂-loaded solution, which was subsequently used to mineralize the steel slag for different durations. The residual CO₂ loading in the mineralization filtrate was determined by acid-base titration. Combined with the measurements of pH value in the filtrate, the kinetic characteristics of steel slag mineralization at different reaction stages were elucidated, thereby identifying the optimal mineralization duration. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were employed to quantify the carbon sequestration capacity and characterize the crystalline phases of both mineralized steel slag and carbonation-cured cement paste at different mineralization durations. Furthermore, the pozzolanic activity after mineralization was evaluated by a strength activity index method, based on a 30% replacement ratio and 28-d compressive strength. In addition, the mineralized steel slag was incorporated as a supplementary cementitious material into cement-based systems and subjected to synergistic carbonation curing in CO₂ atmosphere. The carbon sequestration efficiency and mechanical performance enhancement of mineralized steel slag under dual carbonation conditions were systematically assessed via compressive strength testing and microstructural phase analysis.

This study proposes and validates a complete collaborative process route, confirming that the mineral carbonation modification of steel slag with carbon-rich KG solution enhances its activity. The mineralized steel slag as an admixture can effectively improve the compressive strength of cement-based specimens. Compared with conventional organic amine solutions, the KG solution exhibits a faster CO₂ adsorption rate and a better adsorption efficiency. Also, the mineralization of steel slag with carbon-rich KG solution re-releases the CO₂ adsorption capacity of the KG solution, which, can effectively reduce the cost of indirect carbon mineralization of steel slag with respect to the CO₂ absorption effect in the first stage. The carbon mineralization product of steel slag contains calcite with an aragonite-like structure. The porous morphology of the product enhances its carbon sequestration capacity during the carbonation curing process of cement paste specimens. Furthermore, the metastable aragonite undergoes a dissolution-recrystallization effect during the curing process, effectively improving the early strength of cement-based materials.

This work evaluated the CO₂ capture and steel slag mineralization kinetics using KG solution, as well as the resource utilization effects. The results showed that the reaction rate between steel slag and carbon-rich KG solution could be rapid within the first 120 min, with the CO₂ reaction rate in KG reaching 92.8%. The calcium carbonate produced during the reaction encapsulated the steel slag particles, hindering the mineralization process, leading to a gradual slowdown of the reaction rate and stabilization in subsequent stages. After mineralization, the early activity of the steel slag was significantly enhanced, with a maximum 16.7% increase in 7-d compressive strength at a 10% dosage. Carbonation curing could effectively improve the early compressive strength of the steel slag cement paste. After 12 h of carbonation curing with 30% mineralized steel slag, the 7-d compressive strength was increased by 11.3%. The mineralization of steel slag with carbon-rich KG solution resulted in the formation of calcium carbonate with an aragonite crystal structure. The encapsulated calcium carbonate mineralization product on the steel slag provided nucleation sites for C-S-H gel in the cement hydration process, thereby accelerating the hydration process. Furthermore, the transformation of metastable aragonite into calcite during carbonation curing could enhance the cement matrix, leading to a cementitious strengthening effect. This method could have significant application prospects for the resource utilization of steel slag.

Magnesium silicate hydrate (M-S-H) cementitious material is regarded as a potential alternative to conventional Portland cement due to its excellent sulfate erosion resistance, heavy metal ion solidification performance, and low pH value during hydration. However, the industrialization process of M-S-H cementitious materials is currently restricted due to the high cost and unstable supply of traditional silicon sources that rely on industrial by-products (such as silica fume, fly ash, and slag). Natural layered silicates such as montmorillonite and kaolin have a similar tetrahedral-octahedral stacking unit structure to M-S-H, and can exhibit a good pozzolanic activity after mechanical or thermal activation. Exploring the use of calcined montmorillonite and kaolin as alternative silicon sources to partially or completely replace silica fume broadens the source of raw materials for M-S-H cementitious materials and reduces production costs. This work was to investigate the effects of partial or complete replacement of silica fume by calcined montmorillonite (MT) and calcined kaolin (KL) on the structure and strength of M-S-H cementitious materials, clarifying the action mechanism of aluminum in the two clay minerals on the M-S-H hydration process and layered structure.

Light-burned magnesia (LM) was used as a magnesium source and silica fume (SF) as a reference silicon source. Montmorillonite and kaolin were calcined in a muffle furnace at 750 ℃ for 1 h at a heating rate of 10 ℃/min to obtain calcined montmorillonite and calcined kaolin. The reference sample was prepared via mixing MgO and silica fume at a mass ratio of 4 : 6. On this basis, different mass fractions (i.e., 10%, 30%, 50%, 70%, and 100%) of calcined montmorillonite and kaolin were used to replace silica fume, and the samples were named as MT-10~MT-100 and KL-10~KL-100, respectively according to the replacement ratio. The mortar strength specimens were prepared with the samples above, superplasticizer, tap water, and standard sand, and the compressive strength was tested according to the standard GB/T 17671—1999 "Methods of testing cement- Determination of strength". The paste was prepared at a water-to-solid ratio of 0.5 and 0.7% superplasticizer based on the total mass of solids, cured in sealed glass tubes with liquid paraffin, and terminated hydration with isopropanol at a certain age, then dried in a vacuum oven at 40 ℃ for 48 h. The mineral phase, thermal behavior, functional groups, microstructure, and element distribution of the samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and solid-state ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS NMR).

Calcined kaolin exhibits a great early strength enhancement effect, and its compressive strengths at 3 d and 7 d have a positive correlation with the dosage (i.e., correlation coefficients are 0.996 and 0.905, respectively). At the dosage of calcined kaolin of 70%, the strength at 3 d reaches 39.37 MPa and the strength at 7 d reaches 49.58 MPa, which are 464.0% and 400.3% higher than those of the reference sample, respectively. In contrast, the strength enhancement effect of calcined montmorillonite is weak. The strengths at 3 d and 7 d increase slowly with the increase of montmorillonite dosage. At the montmorillonite dosage of 70%, the strength at 3 d is only 8.94 MPa and the strength at 7 d is 14.28 MPa, which are 36.5% and 53.7% higher than those of the reference sample, respectively. The strength increases slowly in the montmorillonite dosage range of 10%-50%.

The microscopic analysis shows that calcination at 750 ℃ causes more thorough damage to the crystal structure of kaolin than that of montmorillonite. The XRD diffraction peaks of kaolin disappear completely after calcination, and the layered structure collapses completely, resulting in a higher pozzolanic activity. After calcination, the interlayer spacing of montmorillonite decreases, and some characteristic diffraction peaks are still retained, indicating that its crystal order is not completely destroyed and the pozzolanic activity is lower than that of calcined kaolin. The FTIR and NMR analysis further confirms that the structural hydroxyl groups of calcined kaolin are completely removed, and a large number of active Al and Si sites are exposed. Although montmorillonite also undergoes dehydroxylation and structural damage after calcination, the degree of disorder is lower than that of kaolin.

Aluminum in both calcined montmorillonite and kaolin can enter the M-S-H phase in the early stage of hydration, and form magnesium aluminosilicate hydrate (M-S-A-H) gel via substituting Si⁴⁺ in the silicate tetrahedron or Mg²⁺ in the magnesium-oxygen octahedron, which significantly improves the polymerization degree of the product. The formed M-S-A-H is external hydration products, mainly formed through element diffusion between particles. The analysis by the XRD, FTIR, and DTG shows that Mg-Al layered double hydroxide (Mg-Al LDH) is generated within the first 28 d of hydration in both raw material systems. The enhancement of cementitious material strength by calcined kaolin and montmorillonite is related to the formation of M-S-A-H. The strength improvement effect of calcined kaolin is higher than that of calcined montmorillonite due to the more thorough destruction of the crystal structure, higher degree of network disorder, more active Al and Si sites, and higher content of Al(V).

Calcined kaolin and montmorillonite could be used as alternative silicon sources for M-S-H cementitious materials. Calcined kaolin had a great early strength enhancement effect, and its compressive strength had a positive correlation with the dosage. At the dosage of 70%, the compressive strength at 7 d was increased by more than 400%, compared with the sample prepared with pure SF. The strength enhancement effect of calcined montmorillonite could be weak, and the strength was only increased by 53.7% at the same dosage and age. The difference in strength enhancement effect between calcined kaolin and montmorillonite could be due to the fact that the calcination at 750 ℃ caused more thorough damage to the crystal structure of kaolin than that of montmorillonite, resulting in a higher pozzolanic activity. Aluminum in both calcined montmorillonite and kaolin could enter the M-S-H phase in the early stage to form M-S-A-H gel, significantly improving the polymerization degree of the product. The formed M-S-A-H were external hydration products, and Mg-Al LDH was generated within the first 28 d of hydration.

Cement is a fundamental material for global infrastructure. Global cement production is projected to exceed 4.5 billion tons by 2030. The cement industry contributes about 7.5% of global anthropogenic carbon dioxide (CO₂) emissions. These emissions mainly come from fossil fuel combustion and limestone decomposition during the calcination process. Replacing fossil fuels with concentrated solar energy (CSE) for clinker calcination is a key pathway to achieve carbon neutrality in this sector. The CSE technology can concentrate solar radiation up to 7000 times and generate ultra-high temperatures of above 3000 K. These conditions can meet the thermal requirements for cement clinker production.

The spectral mismatch between cement materials and solar radiation as a critical bottleneck limits the efficiency of solar calcination.. The solar spectrum concentrates its energy in the visible (400-700 nm) and near-infrared (NIR, 700-2500 nm) bands. Most cement raw materials, such as calcium carbonate (CaCO₃), silicon dioxide (SiO₂), and aluminum oxide (Al₂O₃), are wide-bandgap materials. These materials absorb mainly in the ultraviolet region and are nearly transparent to visible and NIR light. This transparency leads to a low solar-to-thermal conversion efficiency. Iron oxide (Fe₂O₃) and iron-bearing ferrite minerals have narrower bandgaps of 2.0-2.2 eV. These minerals offer a stronger light absorption in the solar spectrum. This study was to investigate high-ferrite cement (HFC) clinkers to use the high absorption of the iron phase for better photothermal conversion. The calcination performance in electric furnace and solar furnace was compared and the first-principles calculations were performed. The goal was to reveal the photothermal coupling mechanism that could enable low-temperature and rapid clinker formation.

Three high-ferrite cement raw meals were prepared with analytical-grade reagents. These samples, named HFC-16, HFC-18, and HFC-20, were designed with different mass fractions of tetracalcium aluminoferrite (C₄AF, 16%, 18%, and 20%), respectively. All raw materials were ground and passed through a 200-mesh sieve. The sintering was carried out in a conventional electric elevator furnace and a high-flux solar simulator, respectively. The solar simulator used a xenon lamp array to mimic the solar spectrum (AM1.5). This simulator generated a peak heat flux of > 1800 kW/m² at the focal plane.

For the experiments in the electric furnace, the samples were heated to 800 ℃ and held for 30 min. The samples were then heated at 1000, 1200 ℃, or 1375 ℃ and held for 2 h, respectively. For the experiments in the solar furnace, the samples were rapidly heated to 800 ℃ and held for 1 min. They were then heated to the target temperature and held for 5 min. All the samples were rapidly cooled after firing. The phase composition of each sample was analyzed by X-ray diffraction (XRD). The Rietveld refinement was used for quantitative phase analysis. The optical properties were measured by ultraviolet-visible (UV-Vis) spectrophotometry in the range of 200-2500 nm. The total solar absorptance was calculated by the AM1.5 solar irradiance distribution. The electronic structures were simulated by a software named Vienna Ab initio Simulation Package (VASP). The simulations were based on the Density Functional Theory (DFT) with the GGA-PBE functional and a Hubbard U correction for Fe 3d orbitals.

The UV-Vis spectra of the raw materials show that wide-bandgap oxides (i.e., CaO, SiO₂, Al₂O₃) have a weak absorption in the solar spectrum region. Their absorptance values are all below 20%. Fe₂O₃ is a main absorber among all the raw materials. Its solar absorptance reaches 71.1%. As a result, increasing the Fe₂O₃ content in the raw meal improves the overall light absorption. The solar absorptance of the raw meal increases from 56.6% for HFC-16 to 60.5% for HFC-20. The light absorption of the clinker minerals depends on the calcination temperature. At 1000 ℃, the absorptance decreases slightly. This decrease is due to the decomposition of CaCO₃ into weakly absorbing CaO and the low crystallinity of intermediate phases. At 1200 ℃, the absorptance increases sharply. The iron phase mineral formed at 1200 ℃ shows a solar absorptance of 83.9%. This value is greater than that of the raw Fe₂O₃. For the clinker sample HFC-20, the total light absorptance is 75.6% at 1375 ℃.

The results by the First-principles calculations explain the mechanism behind this enhancement. In the high-temperature ferrite solid solution (C₄AF), Fe³⁺ ions occupy both tetrahedral and octahedral sites. This mixed coordination induces a crystal field splitting and creates diverse intermediate energy levels. The substitution of Al³⁺ for Fe³⁺ also introduces lattice defects. These defects create dense defect states within the bandgap. The density of states (DOS) analysis indicates that Fe 3d orbitals dominate the conduction band minimum. O 2p orbitals dominate the valence band maximum. The strong O 2p → Fe 3d transitions are responsible for the intense broadband absorption in the visible and NIR regions.

The comparison between the two heat sources shows a clear photothermal coupling effect in the solar furnace. The XRD patterns indicate that the solar-calcined samples processed at a thermocouple-measured temperature of only 800 ℃ already contain distinct silicate mineral phases (i.e., tricalcium silicate, C₃S, and dicalcium silicate, C₂S). In the conventional electric furnace, these phases only form at 1200 ℃. This result indicates that the iron phase absorbs a high-intensity photon energy locally and lowers the formation temperature of clinker minerals by approxiamtely 400 ℃. The electric furnace transfers heat slowly through conduction from the surface to the interior. The solar furnace delivers energy directly to the reactive iron-bearing sites. This direct energy delivery greatly accelerates the solid-state reaction kinetics.

At the final sintering temperature of 1375 ℃, the two methods both produce clinkers with the target mineral phases (i.e., C₃S, C₂S, tricalcium aluminate C₃A, and C₄AF). No free CaO appears in either case. This result confirms that the solar furnace can fully sinter cement clinker. The Rietveld refinement of the XRD patterns shows that the solar-calcined clinker exhibits broader diffraction peaks than the electric furnace clinker. The rapid heating rate and the unique photothermal environment in the solar furnace likely caused this broadening. Smaller crystallite sizes or higher lattice defect concentrations are the probable reasons. These findings indicate that solar calcination saves energy and changes the microstructural evolution of the cement minerals.

Based on the UV-Vis absorption testing and XRD analysis of raw materials, raw meals, and samples calcined at different temperatures, the overall light absorption intensity of the raw meal increased significantly as the Fe₂O₃ content in the raw meal increased. Furthermore, the light absorption intensity of the iron phase in the clinker minerals increased with increasing the calcination temperature. At 1200 ℃, its light absorption rate reached 83%. The light absorption intensity of the clinker also increased with the calcination temperature, reaching 75.6% at 1375 ℃. More importantly, the iron phase could trigger a photothermal coupling effect when calcined in a simulated high-concentration solar furnace, significantly lowering the formation temperature of clinker minerals. The results by the XRD Rietveld refinement and hydration calorimetry indicated that, compared with the electric furnace clinker, the simulated concentrated solar calcined clinker (SF-HFC) exhibited diffraction peak broadening characteristics. This could demonstrate that the photothermal effect of the iron phase significantly reduced the calcination temperature of cement clinker and lowered the production energy consumption, providing a material design strategy for high-efficiency and low-carbon cement manufacturing.

In recent years, Portland cement production in China has exceeded 1.8 billion tonnes per year, while CO₂ emissions from limestone decarbonation and fuel combustion remain high (i.e., ≈1.1 billion tonnes), accounting for ~8% of the national total. Replacing limestone with Ca-rich, non-carbonate solid wastes (e.g., recycled concrete powder, RCP) represents a promising pathway toward low-carbon clinker production. The RCP mainly consists of quartz and hardened cement paste. The RCP with higher paste fractions is reported to have lower clinker formation temperatures. Previous studies indicate that Ca(OH)₂, unhydrated clinker particles, and certain minor elements (e.g., S) can facilitate clinkering. C-S-H is a dominant phase in hardened cement paste (>55%). Upon heating (≈500-700 ℃), C-S-H undergoes dehydration and structural depolymerization/rearrangement, potentially generating reactive Ca-Si intermediates that may affect clinkering reactions. However, most previous work treated a hardened cement paste as an integrated entity, and the specific role of C-S-H in governing clinker mineral formation, reaction kinetics, and microstructural evolution remains insufficiently understood.

To decouple the multiphase effects of RCP, this study used synthetic C-S-H to replace quartz, thereby isolating the contribution of C-S-H to clinkering reactions, phase evolution, and kinetics.

C-S-H was hydrothermally synthesized using Ca(OH)₂ and SiO₂ nanoparticles at a CaO/SiO₂ molar ratio of 1.2. Its thermal evolution was preliminarily characterized by X-ray diffraction (XRD). C-S-H lost a long-range order at ~800 ℃ and recrystallized into C₂S and wollastonite (CS) after heating to 900 ℃. At 1000 ℃, the crystallite sizes were ~15 nm for C₂S and ~50 nm for CS (the Williamson-Hall method).

Five raw meals were designed with a silica modulus (SM) of 2.8, an alumina modulus (IM) of 1.8, and a lime saturation factor (KH) of 0.91. The reference mix used analytical-grade SiO₂ together with CaCO₃, Al₂O₃·2SiO₂·2H₂O, and Fe₂O₃. In the remaining mixes, C-S-H progressively replaced SiO₂ (and the corresponding fraction of CaCO₃), with increasing the silicon substitution ratio of C-S-H to SiO₂ from 25% to 100% (on a Si basis). Pellets (Φ 30 mm × 7 mm) were pressed at 40 MPa, dried at 105 ℃, heated in a muffle furnace at 10 ℃/min to selected temperatures, and followed by air quenching.

The phase evolution of clinkers was quantified by X-ray diffraction (XRD) coupled with Rietveld refinement using the ICSD structure models and ZnO as an internal standard. To suppress the dominant endotherm from CaCO₃ decarbonation and enhance clinkering-related thermal signals at a high temperature, CaO was used instead of CaCO₃ in the tests of thermogravimetry-differential scanning calorimetry (TG-DSC), and the derived kinetic parameters were interpreted comparatively as apparent values. The non-isothermal TG-DSC measurements were performed at 5-20 K/min in N₂, and apparent activation energies and kinetic models were evaluated by the Flynn-Wall-Ozawa (FWO) isoconversional method.

Introducing C-S-H into raw meals produces a pronounced exothermic peak associated with C₂S formation at ~870 ℃, consistent with the low-temperature decomposition and recrystallization behavior of C-S-H. When quartz serves as a silicon source, C₂S formation kinetics are well described by a phase-boundary-controlled (R3) model, with an apparent activation energy of 540 kJ/mol. In contrast, when C-S-H serves as a silicon source, the best-fit model shifts to the Avrami-Erofeev (A2), and the higher activation energy (797 kJ/mol) can be attributed to a structural rearrangement coupled with C-S-H recrystallization. The Arrhenius analysis further shows that lnA increases from 54.5 to 84.0; accordingly, the instantaneous reaction rate (dα/dt) at α = 0.5 and T = 900 ℃ increases from 0.43 to 7.60 min^-1, indicating a great kinetic compensation effect likely associated with the disordered/nanostructured nature of C-S-H and the enlarged effective reaction interface.

Consistently, the XRD-detectable onset temperature for C₂S formation decreases from 1000-1100 ℃ (quartz) to 900 ℃ (C-S-H), and the C₂S content is increased by 10-24% at the same sintering temperatures (i.e., 900-1300 ℃). The incorporation of C-S-H also increases the C₃S content by 5-15% at 1350-1450 ℃. Moreover, a high defect density of C-S-H and nanocrystal formation during heating contributes to crystallite refinement, reducing the C₂S crystallite size from 87 nm to 66 nm and the C₃S size from 160 nm to 121 nm.

Mechanistically, C-S-H enhances clinker burnability via (i) structural depolymerization/rearrangement at a lower temperatures, (ii) a higher defect density and early formation of nanocrystalline products that promote C₂S/C₃S nucleation, and (iii) a reduced effective CaO transport requirement because C-S-H (at Ca/Si ratio of 1.2) supplies part of the Ca needed for C₂S formation, thereby shortening the effective CaO diffusion distance.

Using C-S-H as a silicon source could markedly accelerate solid-state reaction kinetics (0.43 min^-1→ 7.60 min^-1), lower the XRD-detectable onset temperature of C₂S formation (1000-1100 ℃→ 900 ℃), and promote the formation of both C₂S and C₃S. In addition, the recrystallization products of C-S-H provided abundant nucleation sites and led to refined silicate crystallites. These findings could offer a mechanistic guidance for clinkering with RCP containing high hardened cement paste (C-S-H-rich) fractions toward low-carbon clinker production.

The predicted models consisting of empirical models, classical models, code-oriented models, theoretical aging models and machine learning algorithms prediction are summarized. The conventional prediction models, such as ACI 209 and CEB-FIP MC90, rely on regression-based equations, but struggle to account for the complexity of recycled aggregate concrete (RAC). The theoretical models like B3/B4 incorporate aging theory, but still require a recalibration for RAC, such as introducing residual mortar coefficients. These models have limitations, including narrow parameter coverage (3-5 factors), reliance on linear assumptions, and poor long-term prediction accuracy (R²~0.7). In contrast, machine learning (ML) models (e.g., XGBoost, CNN, LGBM) achieve significantly a higher performance, with R² > 0.95, compared to 0.6-0.8 for the conventional models. The XGBoost demonstrates the optimum accuracy (i.e., R²=0.989, RMSE=0.117). The ML key advantages include handling nonlinear interactions among 10+ parameters (e.g., the SHAP analysis) and improving stability through CNN's residual encoder. However, some challenges remain, such as high data dependency (requiring 23 300+ samples), overfitting risks, and limited interpretability, due to their "black-box" nature.

This review highlights the complexities of creep in RAC and proposes key strategies for optimization and innovation. Material optimization can be achieved via controlling residual mortar (<20%), incorporating supplementary cementitious materials (SCMs) like 30% GGBS, and adopting CO₂ curing to mitigate creep. Multiscale modeling, combining molecular dynamics with FEM, can simulate interfacial transition zone (ITZ) effects. Machine learning (ML) advancements should focus on expanding databases to include load history, environmental cycles, and admixtures, while hybrid models (e.g., physics-informed neural networks) can enhance both accuracy and interpretability. Sustainable solutions like bio-chemical ITZ self-healing and digital twins for lifecycle prediction offer promising future directions. Overall, integrating ML-enhanced, data-driven approaches enable safer, large-scale RAC applications, supporting greener construction practices.

Utilizing geopolymers to stabilize silty soil and applying it in engineering construction represents a highly promising approach for resource utilization. This method enables efficient and sustainable use of silty soil and promotes large-scale consumption of industrial solid waste in China, demonstrating exceptional environmental, economic, and social benefits. Geopolymer-stabilized silty soil exhibits excellent mechanical properties, with improvements in compressive strength, shear strength, and bearing capacity, compared to conventional inorganic stabilized soil. In addition, it also shows significant advantages in key durability aspects such as water stability and freeze-thaw resistance. This performance enhancement is a macroscopic reflection of its unique microstructure and chemical bonding capabilities.

Although the existing experimental research on geopolymer-stabilized silty soil has made significant progress in macroscopic mechanical properties and microstructural characteristics, the in-depth analysis of its microscopic stabilization mechanisms and systematic studies on the influence of organic matter content on stabilization effects still require a further attention. Moreover, research on stabilized silty soil predominantly focuses on single-component geopolymer systems, while studies on binary or multicomponent geopolymer-stabilized soil remain insufficient. The application of geopolymers in soil stabilization still faces severe challenges, such as the lack of unified testing methods, performance evaluation standards, and engineering design specifications. Future research should focus on investigating the complex interfacial reactions and bonding mechanisms between geopolymers and silty soil particles, actively expanding exploration of silty soil stabilization under multicomponent geopolymer systems, and accelerating the establishment of design and construction specifications for geopolymer-stabilized soil in various engineering applications. Building a full-lifecycle standardized system can provide a technical assurance for the large-scale promotion and application of this technology.

Lightweight aggregate concrete (LWC) offers some advantages like low density, thermal insulation, heat resistance, fire resistance, and excellent seismic performance. The incorporation of hybrid fibers further enhances its toughness, durability, and crack resistance, resulting in a composite material known as hybrid fiber reinforced-lightweight aggregate concrete (HFR-LWC). In protective engineering, deformations induce a "membrane effect" at the end restraints of structures, altering their ultimate resistance and failure modes. Conventional load-bearing components are often simplified as simply supported or fixed in mechanical analyses, neglecting the influence of the membrane effect. This leads to conservative designs for ultimate bearing capacity, preventing the full utilization of component strength. Therefore, investigating the membrane effect's contribution to improving component bearing capacity is of significant importance. This paper was to design and fabricate a set of support restraint devices for horizontal stiffness and rotational stiffness to simulate the membrane effect in beam structures. Four-point bending tests were conducted on ten HFR-LWC beams under static loading. The contributions of restraint conditions, reinforcement ratios, and the presence of fibers to the ultimate bearing capacity of HFR-LWC beams were analyzed. The variation patterns of mid-span strain, crack distribution, and ultimate bearing capacity of the HFR-LWC beams were examined. In addition, the influences of horizontal stiffness and rotational stiffness on the membrane effect were also quantitatively analyzed.

Based on the Technical Specification for Lightweight Aggregate Concrete (JGJ/T 12—2019), this study developed a hybrid fiber reinforced lightweight aggregate concrete (HFR-LWC) with high strength, low density, and good workability through multiple mix proportion trials. Ten concrete beams with the dimensions of 200 mm×100 mm×1500 mm were cast, featuring a symmetrical reinforcement with A6 deformed longitudinal bars and A6@100 plain stirrups at different reinforcement ratios (i.e., ρ of 0.42% and ρ of 0.28%). After 28-d curing under identical conditions, the compressive strength and elastic modulus of concrete specimens were tested according to the standard GB/T 50081—2019 by a universal testing machine. A specialized support restraint device was designed and fabricated to account for both horizontal and rotational stiffness. The experimental program included three simply supported beams and seven boundary-restrained beams tested under four-point static bending. Five displacement transducers were evenly spaced along the beam span to measure deformation patterns under various load levels and record the deflection at ultimate load. Five strain gauges were installed at mid-span to monitor neutral axis variations and strain distribution under different support conditions and deflection levels. Additional strain gauges were placed at beam ends and mid-span to analyze strain variations caused under different restraint conditions. Through the quantitative analysis of experimental data, this research systematically investigated the influences of support constraint, reinforcement ratio and fiber incorporation on the membrane effects. The findings could elucidate the mechanisms by which these factors affected the ultimate load-bearing capacity of HFR-LWC beams, providing valuable insights for the structural design.

Based on the experimental conditions, the test beams are grouped to investigate the influences of restraint condition, reinforcement ratio and fiber incorporation on the load-bearing capacity. The analysis of test groups under different restraint conditions reveals that the presence of tie rods significantly enhances the ultimate load-bearing capacity of the beams, and variations in both the number and position of tie rods have substantial effects on this capacity. The comparative analysis of test groups at different reinforcement ratios demonstrates that for membrane effects within a certain range of reinforcement ratios, there exists an inverse relationship between reinforcement ratio and the increase in load-bearing capacity. Specifically, a lower reinforcement ratio results in more pronounced improvements in ultimate load-bearing capacity under end restraints. The examination of test groups with and without fiber incorporation shows that fiber materials effectively participate in the crack propagation process and improve the deformation performance of the members. The analysis of strain and crack development in the test beams indicates that the mechanism by which membrane effects affect a load-bearing capacity primarily involves restricting concrete cracking and crack propagation, thereby increasing ultimate capacity, while simultaneously reducing structural deformation performance. The relationship between tie rod force and deflection shows a synchronous variation with beam load-bearing capacity. In the initial deformation stage, these two parameters exhibit the similar results. However, the tie rod force surpasses the peak load-bearing capacity of the beam, with its maximum value occurring later than that of the beam's capacity as the deflection increases.

Unlike conventional calculation methods that could simplify supports, the influence of end restraints on the ultimate bearing capacity of reinforced concrete members could reflects actual engineering conditions. The membrane effect increased the ultimate bearing capacity of beams by approximately 1.5 times to 5.5 times and effectively controlled the crack propagation. Compared to the influence of horizontal stiffness on the ultimate bearing capacity, the membrane effect showed a greater sensitivity to variations in rotational stiffness. The hybrid fibers could significantly improve the cracking performance of concrete, resulting in a more uniform crack distribution, which could facilitate the development of membrane effects.

Conventional cement-based materials exhibit excellent compressive performance, but are inherently brittle, with inadequate tensile, flexural and deformation capacities, which readily induce cracking and impair the durability and long-term service performance of structures. At present, incorporating organic polymers or inorganic nanomaterials is proven as one of the effective strategies to enhance the mechanical properties of cement-based materials. Organic polymers are widely adopted in cement-based materials for their superior plasticity and toughness. They can adsorb onto the surface of cement particles via electrostatic interaction or hydrogen bonding, changing the hydration process of cement and optimizing the distribution and microstructure of hydration products. In addition, inorganic nanomaterials (e.g., nano-silica, carbon nanotubes and graphene oxide) can also improve the mechanical properties of cement-based materials due to their high specific surface area and reinforcement effect. However, these modification methods have some limitations. For most polymers, their effect on the flexural and compressive strengths of cement-based materials has a negative correlation, and improving the flexural strength can result in a significant reduction in the compressive strength. Furthermore, inorganic nanomaterials suffer from a poor dispersion in cement matrix, which easily causes agglomeration and uneven distribution. In addition, inorganic nanomaterials mainly improve the early compressive performance of cement-based materials, while their enhancement effect on flexural strength is relatively limited. Using organic/inorganic nanocomposites to optimize the microstructures of cement hydration products at the micro/nanometre scale is thus a promising method for improving the mechanical properties of cement-based materials. In this work, α-lipoic acid (TA), a natural bioactive molecule with a five-membered ring disulfide bond, was innovatively introduced and self-polymerized at a high temperature to prepare a dynamic polylipoic acid (PTA) network. Selective esterification modification of reactive hydroxyl groups on microcrystalline cellulose (MCC) molecular chains with PTA could improve MCC dispersibility and endow it with organic polymer characteristics. The prepared PTA-MCC hybrid material exhibited excellent mechanical enhancement in the cement matrix, which can significantly improve the flexural strength of cement-based materials at a low dosage without compromising compressive strength, thus effectively breaking through the technical bottleneck of synergistically enhancing strength and toughness of cement-based materials.

For the synthesis of polylipoic acid fiber hybrid material (MCPTS), TA powder was weighed according to a selected ratio, reacted in an oil bath at 150 ℃ for 1 h and cooled naturally to obtain linear PTA. The activated MCC and linear PTA were added to DMF solvent in a proportion, fully stirred to form a uniform system, and reacted in an oil bath at 120 ℃ for 6 h. After the reaction, the product was centrifuged and dried by rotary evaporation to obtain the final product of MCPTS. The products were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM), confirming that the activated MCC and linear PTA could be fully combined, and the target hybrid material was prepared.

The results of mechanical properties of cement paste show that the MCPTS synthesized can effectively improve the comprehensive mechanical properties of cement-based materials, especially a small increase in its compressive strength, which greatly improves the flexural strength of cement-based materials. Under the premise of ensuring that its compressive strength does not decrease, the flexural strength of cement-based materials is synergistically improved. The XRD patterns of cement-based materials at 3-d and 28-d curing ages reveal that the MCPTS-added group promotes cement hydration to a certain extent. Some cement hydration products like calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) gel are formed in the system as the hydration age increases. The SEM images demonstrate that the cement paste has a much denser microstructure with a large number of clustered C-S-H hydration products after the incorporation of MCPTS. The results of cement hydration exothermic tests indicate that cement pastes with MCPTS show a significant hydration acceleration effect, with the occurrence time of their main hydration exothermic peaks advanced to varying degrees and the peak hydration rate increases remarkably.

In this paper, a type of MCPTS hybrid material with dynamic hybrid of PTA and MCC was prepared for a synergistic enhancement of organic-inorganic hybrid. It has reversible covalent bond and non-covalent bond dynamic network structure. The optimal performance was achieved at a dosage of 1.0‰ MCPTS-1. The flexural strength at 3, 7 d and 28 d was increased by 8.56%, 25.91% and 22.68%, respectively, and the compressive strength at 7 d and 28 d was enhanced by 11.80% and 6.45%, respectively. The XRD patterns, SEM images and hydration exothermic analysis indicated that MCPTS could accelerate cement hydration, increase heat release, and promote the formation of a denser hydration product structure, thereby improving mechanical properties. The underlying mechanism was mainly as follows, i.e., the carboxylic acid ion network formed by PTA hydrolysis could chelate Ca²⁺ and optimize the structures of CH crystals and C-S-H gels. MCC dispersed stress concentration through the fiber bridging effect. The synergistic effect of the two components realized the performance enhancement of cement-based materials.

Glass fiber reinforced cement is utilized in construction field. Interfacial modification with coupling agents and functional polymers represents a critical strategy to enhance interfacial bonding. However, molecular mechanisms remain insufficient. In this study, molecular dynamics were used to evaluate interfacial responses of C-S-H/glass fiber modified with KH-570 and poly (vinylpyrrolidone-co-acrylic acid) (PVP-co-PAA). The results demonstrated that KH-570 induced marginal increase, while PVP-co-PAA led to remarkable enhancement. The analyses of coordination number, bond lifetime decay decoded interfacial bonding networks before and after modification. This study could reveal the distinct failure pathways and molecular mechanisms of interfacial bonding.

All the MD simulations were performed based on a package named LAMMPS. Firstly, all the systems were relaxed under NVT ensemble last for 5 ns, with timestep 1 fs and 300 K. Temperature control was achieved by Nose-Hoover thermostat and damping parameter 0.1 ps. The bottom Ca layers of C-S-H were fixed throughout the simulation to prevent model rotation. An additional 2 ns relaxation was conducted and trajectories were recorded every 1 ps. During the shear simulations, spring forces were applied on atoms located at the top of SiO₂ slab.

The molecular dynamics (MD) simulations are employed to evaluate shear mechanical properties of C-S-H/SiO₂, C-S-H/KH-570 and C-S-H/PVP-co-PAA interfaces. The molecular mechanisms of interfacial modification are thoroughly revealed via analyzing the types and dynamic behaviors of interfacial bonding networks. During dynamic failure, PVP-co-PAA exhibits a distinct behavior compared with rigid SiO₂. Flexible polymer networks undergo conformational changes, leading to an increased shear displacement. The density analyses of bonding networks indicates that interfacial bonding networks can reorganize from O_SiO2 —Ca—O_C-S-H into O_KH-570/O_PVP-co-PAA—Ca—O_C-S-H. Moreover, interfacial failure is decided by ionic bonding of O_SiO2 /O_KH-570/O_PVP-co-PAA—Ca. The coordination number follows an increasing order of O_SiO2 <O_KH-570<O_PVP-co-PAA, explaining the differences of interfacial mechanical strength.

The MD simulation revealed that PVP-co-PAA modification showed the most enhancement, with an improvement of 302.7%. The coordination number followed an increasing order of O_SiO2 <O_KH-570<O_PVP-co-PAA. The time correlation function (TCF) also revealed that interfacial stability was enhanced after modification. Based on the coordination number and the TCF results, the differences of interfacial mechanical strength were explained. These findings could provide fundamental insights to guide the functional design of fiber-cement interfaces.

The construction of transportation infrastructure in plateau regions, such as the Sichuan-Tibet Railway and the Xinjiang-Tibet Railway, China, is progressing rapidly under the national development strategies. The annual minimum temperature in Tibet can reach -45 ℃, and the number of days with alternating positive and negative temperatures accounts for more than half of the year, forming an extreme low-temperature, high-frequency freeze-thaw environment. Furthermore, structures such as bridges, culverts, and tunnels in plateau areas are often subjected to erosion from de-icing salts or chlorides in the soil. The combined action of freeze-thaw and chloride attack poses a serious threat to the durability of concrete structures. Fiber-reinforced polymer composites, such as carbon fiber reinforced polymer (CFRP), basalt fiber reinforced polymer (BFRP), and glass fiber reinforced polymer (GFRP) bars, are widely used in corrosive environments like coastal and saline areas due to their high strength and corrosion resistance. The bond performance of the steel/FRP bar-concrete interface significantly affects the mechanical behavior of concrete structures. However, the existing research mostly focuses on the room-temperature or conventional freeze-thaw environments, and the understanding of the degradation mechanism of the bar-concrete bond performance under the combined action of low-temperature freeze-thaw and chloride attack specific to plateau regions remains insufficient. It is thus necessary to conduct a targeted research on the bond performance of the steel/FRP bar-concrete interface under low-temperature freeze-thaw and chloride attack environments in the Tibetan plateau.

A total of 144 pull-out specimens were fabricated. The test parameters included bar type (i.e., steel, BFRP, CFRP, and GFRP bars), bar diameter (i.e., 10, 16 mm, and 20 mm), and the number of freeze-thaw cycles (i.e., 0, 50, 100, and 150). After 28 d standard curing, the specimens were first subjected to a 4 d water saturation pretreatment to ensure that they reached a saturated state before the freeze-thaw cycle test. A programmable high-low temperature test chamber was used to simulate a freeze-thaw cycling process. Each freeze-thaw cycle lasted for 260 min (i.e., consisting of 180 min of freezing and 80 min of thawing). After every 5 freeze-thaw cycles, the specimens were placed at 20 ℃ and sprayed with a NaCl solution to maintain a stable moisture content. Following the designated freeze-thaw cycles, the specimens were immersed in a 5% NaCl solution for 15 d to accelerate chloride ingress. Finally, the central pull-out tests were conducted by a 300 kN MTS testing machine at a loading rate of 0.5 mm/min, with the free-end slip measured by a displacement transducer at a data acquisition frequency of 10 Hz.

The failure modes of the specimens are primarily governed by bar type and diameter, with freeze-thaw cycles showing negligible influence. All the steel bar specimens exhibit a pull-out failure accompanied by longitudinal cracks, with crack width increasing alongside bar diameter. The GFRP bar specimens also fail due to pull-out without concrete cracking. In contrast, for the BFRP and CFRP bars, the failure mode transitions from pull-out to splitting failure when the diameter increases from 10 mm to 16 mm. The bond strength decreases with increasing bar diameter for all bar types. When the diameter increases from 10 mm to 20 mm, the bond strength reduction for steel bars (i.e., 51.7%-56.3%) is significantly greater than that for FRP bars (i.e., below 18.2%). After 150 freeze-thaw cycles, the bond strength of steel bars is decreased by more than 31.4%, while the reduction for BFRP bars does not exceed 22.1%. The degradation mechanisms differ between steel and FRP bars. For steel bars, the bond degradation is mainly attributed to concrete deterioration and steel corrosion induced by the coupled environment. For FRP bars, in addition to concrete deterioration, the degradation involves resin matrix damage from freeze-thaw cycles and the formation of interfacial gaps due to the mismatch in thermal expansion coefficients between FRP and concrete. For peak slip, steel bar specimens exhibit the values between 1.6 mm and 2.5 mm, which are considerably lower than those of FRP bar specimens (i.e., ranging from 2.7 mm to 4.1 mm). As the diameter increases from 10 mm to 20 mm, the peak slip is increased by 5.0%-13.3% for all the specimens. Conversely, as freeze-thaw cycles increase from 0 to 150, the peak slip is decreased by 13.5%-23.8%. Note that no significant interaction effect between bar diameter and freeze-thaw cycles on peak slip occurs. Based on the experimental data, the prediction models for bond strength and peak slip are proposed through normalization, incorporating the effects of freeze-thaw cycles and bar diameter. The data by the models are in reasonable agreement with the experimental results, with coefficients of determination (R²) of exceeding 0.86.

This study investigated the bond performance of steel and FRP bars with concrete under low-temperature freeze-thaw and chloride attack. The failure mode was primarily determined by bar type and diameter. The bond strength decreased with increasing diameter, and steel bars had a more pronounced reduction than FRP bars. The freeze-thaw cycles significantly reduced the bond strength for all bar types, the underlying degradation mechanisms differed between steel and FRP bars. The peak slip increased with diameter but decreased with more freeze-thaw cycles, with no significant interaction between these two factors. The proposed prediction models, which could account for freeze-thaw cycles and normalized bar diameter, effectively captured the degradation of bond strength and peak slip, providing a valuable tool for durability design and assessment of concrete structures in harsh plateau environments.

Functional fillers, such as steel fibers, graphene, and carbon nanotubes, introduce a large number of free electrons into UHPC, becoming a conductive material "gene" of UHPC for the establishment of the intrinsic conductive path. The material "gene" simultaneously endows UHPC with excellent mechanical, durability, and sensing capabilities, facilitating in-situ monitoring of the impacts of loads and environment on infrastructure. This provides a crucial data support for infrastructure operation and maintenance, paving a way for infrastructure "digital-intelligence". SSUHPC as a high-performance/intelligent/multifunctional material with digital-intelligent characteristics is expected to transcend the boundaries between functional and structural materials, achieving structural-functional integration and functional diversification, thereby driving a paradigm shift in the field of civil engineering.

Tricalcium silicate (C₃S) is one of Portland cement clinker minerals whose proportion is over 50%. It hydrates quickly and releases lots of heat in early hydration period. The reaction rate and degree of C₃S in early hydration period are the chelloef constituent to determine the exothermal and mechanical performance of Portland cement; as well as the cracking risk of concrete. One of concrete additive, temperature rising inhibitor, was created to suppress the heat release of binder and decrease its exothermic rate. Cyclodextrin (CD) is the main functional composition of temperature rising inhibitor. It is studied that the effect of CD on the early hydration process of C₃S. This work is helpful to understand the action mechanism of temperature rising inhibitor.

Pure C₃S was calcined, chemical pure CD was used. C₃S paste with w/c= 0.5 was prepared. The CD dosage in paste is 0%, 0.05%, 0.10% and 0.20% separately. Dilute C₃S solution with the solid content of 10^-4 and containing 0, 2 mmol/L and 4 mmol/L CD solution was prepared to measure the dissolving rate of C₃S in the induction period of hydration. Pore solution of C₃S was obtained using centrifugal extraction for fresh paste or press method for hardened C₃S paste. The hydration exothermal curves of C₃S pastes containing different dosages of CD were measured using isothermal calorimetry. The time-depended concentration variation of Ca and Si in dilute C₃S solution and pore solution was measured by ICP-AES. SEM was used to investigate the formation of hydrate nucleus on the polished surface of C₃S particles etched by CD solution or pure water. The ion activity product of pore solution relative to C₃S was calculated.

There is not obvious induction period during the hydration of pure C₃S. The induction period of C₃S hydration prolongs with the increase of CD dosage in paste. The accumulate heat release of C₃S is proportional with its hydration degree. The hydration degree of C₃S paste containing CD is higher than blank one at the point of maximal exothermic rate. The accumulate heat release of C₃S paste with or without CD is almost same at the end of induction period. The addition of CD decreases the early hydration speed, but does not negatively influence the final hydration degree. CD inhibits momentarily the dissolution of C₃S in early hydration period, but does not affects the hydration of C₃S in long period. The Ca and Si concentration in pore solution increases with the increase of CD dosage in induction period of hydration. The ion activity product of pore solution relative to C₃S is changed when Cd is added into C₃S paste. CD is adsorbed at the etch pits on the surface of C₃S particles to increase the barrier ΔG_crit of the etch pit expansion model and decrease the growing speed of etch pit. There are lots of etch pit on the polished surface of C₃S particle after 1 h immersion in pure water. There are few etch pit on the polished surface of C₃S particle after 1 h immersion in 1 mmol/L CD solution.

CD prolongs the induction stage and the main reaction stage of hydration, decreases the reaction rate of C₃S. This effect is very intense when the dosage of CD increases to 0.2%. The hydration degree of C₃S paste increases in some extent at the peak reaction rate due to the prolongation of the main reaction period of C₃S paste containing CD. The ultimately hydration heat release of C₃S paste containing CD does not decrease due to the delay of hydration. CD is absorbed on the etch pit of C₃S particles in the initial hydration stage to elevate the energy barrier ΔG_crit of the etch pit expansion model. It decreases the dissolving rate of C₃S and prolongs the induction period of hydration. In the induction period the exist of CD results in an increase of Ca and Si concentration in pore solution and higher degree of supersaturation relative to Ca(OH)₂ and C-S-H. The formation of C-S-H nucleus is inhibited to hinder the acceleration of C₃S hydration

Conventional concrete suffers from its inherently low tensile strength. To address this drawback, researchers propose steel fiber reinforced concrete (SFRC) as a novel material that significantly improves tensile properties and exhibits outstanding performance in cracking resistance, toughness, and durability. However, the incorporation of steel fibers makes the mechanical behavior of SFRC more complex. To ensure its safe application in structural engineering, an accurate prediction of its mechanical properties is essential.

Artificial intelligence technologies are widely applied to accurately evaluate the mechanical properties of concrete. However, the existing machine learning models for SFRC are typically trained on a limited number of samples (i.e., fewer than 300), which restricts their generalization capability. Moreover, these models often lack interpretability, undermining their credibility and limiting their practical application in engineering projects.

This study was to establish a database for the compressive and splitting tensile strengths of SFRC, develop corresponding machine learning prediction models, and evaluate their predictive performance. Furthermore, the interpretability of the models was analyzed. The findings of this study could offer some insights into the development of high-precision machine learning models for SFRC and their interpretability analysis, thereby promoting their application in engineering practices.

In this study, a total of 636 experimental data points on SFRC were collected from 24 independent studies to establish a comprehensive database, including 419 data points for compressive strength and 217 for splitting tensile strength. Each data entry contains a mix-related information such as the water-to-binder ratio, sand ratio, aggregate-to-binder ratio, maximum size of coarse aggregate, fiber shape factor, fiber aspect ratio, fiber volume fraction, and the corresponding compressive or splitting tensile strength.

Based on this database, five machine learning models (i.e., Random Forest (RF), Gradient Boosting Regression Tree (GBRT), Extreme Gradient Boosting (XGB), Light Gradient Boosting Machine (LGBM) and Bayesian Neural Network (BNN)) were proposed. The predictive performance of these models was evaluated using the coefficient of determination (R²), mean absolute percentage error (MAPE), and root mean square error (RMSE), in order to identify the most effective prediction model.

For the selected model, the SHAP (SHapley Additive exPlanations) method was employed to analyze the trends and contribution of each input parameter on the compressive and splitting tensile strengths of SFRC. In addition, the Partial Dependence Plot (PDP) and Individual Conditional Expectation (ICE) methods were also used to quantitatively investigate the variation patterns of predicted values with respect to individual input parameters.

All the models proposed demonstrate a great predictive performance. The R² for the test set ranges from 0.84 to 0.90, while for the training set it ranges from 0.80 to 0.90. A small difference between them (i.e., both being > 0.75) indicates a good predictive accuracy and a generalization ability. For the overall performance across R², MAPE and RMSE metrics, the LGBM model shows the optimum prediction performance.

The SHAP analysis reveals that the compressive and splitting tensile strengths of SFRC decrease with increasing water-to-binder ratio and aggregate-to-binder ratio, while the strengths both increase at a higher sand ratio and a greater fiber reinforcement factor. In addition, as the maximum size of coarse aggregate increases, the compressive strength decreases, whereas the splitting tensile strength increaseds. These results are consistent with the practical observations. Based on the mean absolute SHAP values, the input parameters influencing the compressive strength in a descending order are water-to-cement ratio, sand ratio, fiber reinforcement factor, aggregate-to-cement ratio, and maximum coarse aggregate size. For splitting tensile strength, the order is fiber reinforcement factor, sand ratio, water-to-binder ratio, aggregate-to-binder ratio, and maximum coarse aggregate size.

The influence and parameter importance rankings obtained through the PDP and ICE analysis are in a reasonable agreement with those derived from SHAP, further validating the interpretability and reliability of the model.

A large-scale database containing 636 data entries was established for the compressive and splitting tensile strengths of SFRC. Based on this database, five machine learning prediction models (i.e., RF, GBRT, XGB, LGBM and BNN) were proposed. All the models exhibited good predictive performance and generalization ability, significantly outperforming the existing prediction methods. Among them, the LGBM model showed the optimum overall performance. The SHAP, PDP and ICE techniques were employed to analyze the influence and importance of each input parameter on the compressive and splitting tensile strengths. Based on the variable contribution analysis, the main factors affecting the compressive strength were the water-to-cement ratio, sand ratio, fiber reinforcement factor, aggregate-to-cement ratio, and maximum coarse aggregate size. In contrast, the key determinants of splitting tensile strength were the fiber reinforcement coefficient, sand ratio and water-to-cement ratio.

With the continuous expansion of infrastructure construction in alpine regions, a demand for concrete construction under sub-zero temperatures increases significantly. However, conventional low-temperature construction methods suffer from high energy consumption and complex operational processes. Although sulfoaluminate cement (SAC) exhibits excellent low-temperature adaptability, and calcium chloride (as an antifreeze agent) can effectively lower the freezing point and maintain a liquid-phase environment, the hydration kinetics of the simple SAC-calcium chloride composite system under fully cold construction conditions (≤-15 ℃) are severely inhibited at sub-zero temperatures, leading to an insufficient early-age strength development, which fails to meet the rapid construction requirements of engineering projects. Consequently, there is an urgent need to optimize this composite system to fully exploit its rapid hardening and early-strength characteristics under fully cold conditions at -15 ℃.

Anhydrite as a key component of SAC critically regulates the composition of hydration products, microstructure, and macro-scale performance of the cementitious system. However, the existing studies on the effect of anhydrite dosage primarily focus on ambient temperatures or relatively mild sub-zero conditions (e.g., >-5 ℃). Systematic investigations into how anhydrite dosage governs the hydration product evolution, microstructural formation, and mechanical-deformation properties (particularly early-age performance) of the SAC-calcium chloride composite system under extreme low-temperature conditions (-15 ℃) and fully cold construction processes remain largely unexplored.

This study was to systematically reveal the critical regulatory effects and intrinsic mechanisms of anhydrite dosage on the low-temperature hydration and hardening behavior of the SAC-calcium chloride composite system under fully cold construction conditions at -15 ℃, simulating the harsh environments encountered in real-world engineering. The findings could be expected to expand the application potential of SAC in extreme environments, particularly in remote regions lacking thermal energy supply.

Under fully cold construction conditions (chilled materials, mixing, and curing) at -15 ℃, a composite system was designed with SAC clinker and anhydrite as binders and a 20% calcium chloride solution as a mixing medium. Anhydrite dosage gradients were set at 0%, 5%, 10%, 15%, 20%, 25%, and 30%, respectively.

The mortar specimens (40 mm × 40 mm × 160 mm) were prepared at a water-to-binder ratio of 0.37 based on the standard GB/T 17671—2021 Test Method for Cement Mortar Strength (ISO Method) for strength evaluation. The shrinkage specimens (40 mm × 40 mm × 160 mm) were prepared based on the standard JGJ/T70—2009 for Test Methods of Basic Properties of Construction Mortar.

Cement paste specimens were formulated at a water-to-binder ratio of 0.32. The calcium chloride solution and cement were sequentially poured into a mixer, stirred at a low speed for 120 s, paused for 15 s, and then stirred at a high speed for 120 s. The resultant mixture was cast into molds and compacted by a vibrating table for 60 cycles. All the specimens were cured on racks under uncovered conditions at -15 ℃.

After reaching specified curing ages, the mortar specimens underwent macroscopic tests. Cement paste specimens were crushed on-site, submerged in anhydrous ethanol to terminate hydration at room temperature for ≥7 d, and then vacuum-dried at 40 ℃ and -0.08 MPa for ≥24 h. The dried samples were subjected to the microstructural analyses to investigate the phase composition and hydration mechanisms.

Under fully cold construction conditions at -15 ℃, the incorporation of 10% anhydrite dosage enables the SAC-calcium chloride composite system to achieve the 1-d and 3-d compressive strengths of 20.1 MPa and 38.0 MPa, respectively, representing approximately 100% enhancement, compared to the pure clinker group. At an anhydrite dosage of 15%, the peak hydration temperature increase reaches 18.9 ℃ (33.9 ℃ relative to ambient temperature), effectively addressing the challenges of delayed early hydration kinetics and insufficient strength in sub-zero environments.

Anhydrite significantly suppresses a shrinkage via promoting the formation of expansive AFt phases. At the dosage of 5%, the 3 d shrinkage rate reduces to 0.100% (i.e., 47.4% reduction), while at the dosage of 25%, the 28 d shrinkage rate decreases to 0.237% (i.e., 35.1% reduction), offering a graded solution for a low-temperature shrinkage control.

The microstructural analyses reveals that anhydrite regulates the types and morphology of hydration products. In the 28 d hydrated pure SAC clinker, unstable petal-like Friedel's salt crystals occur. Anhydrite optimizes the Cl/S ratio of the system, promoting AFt formation, while suppressing Friedel's salt generation. At the optimal dosage of 10%, needle/rod-shaped AFt crystals form a dense interlocking network, which is pivotal for early-age strength enhancement. However, the excessive dosage (>10%) leads to unreacted anhydrite enrichment and short rod-shaped AFt aggregates with localized bridging, resulting in the strength regression at later ages.

Under fully cold construction conditions at -15 ℃, anhydrite significantly enhanced the hydration activity and early-age strength of the SAC-calcium chloride composite system. An appropriate anhydrite dosage of 10% promoted the hydration process of calcium sulfoaluminate in the system and accelerated the nucleation and growth rate of AFt, thereby forming a denser microstructure of early hydration products. However, the excessive anhydrite disrupted the sulfate phase equilibrium in the reaction system, inhibited the continuous formation of AFt, and induced anhydrite enrichment, ultimately degrading the mechanical properties due to structural loosening.

At early ages, the anhydrite dosage of 5% approached the optimal sulfate balance required for hydration, effectively compensating for plastic shrinkage and chemical shrinkage. In contrast, the dosage of 25% provided sufficient SO₄^2- ions during later hydration stages, ensuring sustained AFt formation and suppressing drying shrinkage over time.

Anhydrite optimized the Cl/S molar ratio in the system, facilitating the formation of a dense network of needle-bar Aft, while inhibiting the formation of the unstable Friedel's salt phase. The excessive anhydrite, however, led to interconnected short-rod AFt and gypsum enrichment, further compromising the structural integrity.

Vanadium-based phosphate composite (VPC) can be used as a highly promising candidate for sodium-ion battery cathode materials due to its excellent structure stability and flexible voltage adjustability. However, some inherent limitations such as low intrinsic conductivity and low ion diffusion, lead to an inferior rate capability and an insufficient cycle stability in VPC materials. Efforts are dedicated to the improvement of cathode materials, such as bulk structure regulation and surface coating modification. Among them, carbon coating is usually applied to modify the inherently low conductivity of polyanion-type cathodes. The graphitization degree of pyrolytic carbon is restricted due to the limitation of VPC sintering temperature. This restriction results in a marginal enhancement of the electronic conductivity in carbon-coated VPC. To compensate for this deficiency, highly conductive carbon materials such as GO and CNTs are often incorporated into the fabrication process of VPC materials. Nevertheless, the utilization of these carbon materials can lead to a significant increase in production costs. It is thus crucial for improving the conductivity of VPC composite and simultaneously regulating the local electronic structure of vanadium ions to develop carbon materials with high cost-effectiveness and conductivity.

A certain amount of sodium citrate (Na₃C₆H₅O₇) was subjected to a sintering treatment in N₂ atmosphere at 800 ℃ for 2 h with a heating speed of 5 ℃‧min^-1. After natural cooling, the resultant black powder was then immersed in a dilute solution of HCl (2 mol·L^-1) for 24 h to remove all sodium-containing compounds. Subsequently, graphene-like nanosheets (marked as GNS) as the product were collected via filtering, repeated rinsing with ultrapure water and ethanol, and finally drying at 80 ℃ for overnight.

Vanadium pentoxide (V₂O₅, 1 mmol) and oxalic acid dihydrate (H₂C₂O₄·2H₂O, 2 mmol) were dissolved in 30 mL ultrapure water. After being stirred at 60 ℃ for 30 min, the initially orange turbid solution gradually transformed into a clear green solution. Subsequently, ammonium dihydrogen phosphate (NH₄H₂PO₄, 2 mmol) and sodium fluoride (NaF, 3 mmol) were added into the green solution above. Meanwhile, different specific quantities (i.e., 0.025, 0.033 g and 0.041 g) of GNS powder were individually dispersed into 10 mL ethyl alcohol under ultrasonic agitation for 5 min to obtain a uniform dispersion solution. The green solution was then added to the dispersion solution above. The mixture solution was subjected to continuous stirring at 60 ℃ until the gel was formed. Afterwards, the obtained gel was dried in an oven at 80 ℃ for overnight, and then ground into a fine powder. Finally, the resultant powder was first sintered at 350 ℃ for 5 h and subsequently in N₂ atmosphere at 600 ℃ for 8 h with a heating rate of 3 ℃·min^-1. The vanadium-base phosphate composites prepared with 0.025, 0.033 g and 0.041 g GNS powder were denoted as VPC-GN6, VPC-GN8 and VPC-GN10, respectively. For comparison purpose, a pristine VPC without GNS was prepared by the same method as for GNS modified VPC samples and labeled as P-VPC.

A novel biphasic-VPC composite (Bis-VPC), composed of high-capacity Na₃V₂(PO₄)₂F₃ (NVPF) and high-stability Na₃V₂(PO₄)₃ (NVP), is effectively prepared by a facile sol-gel assisted solid-state method, and facilitated by the incorporation of GNS. The results obtained from the XRD, XPS and Rietveld refinement demonstrate that the incorporation of an appropriate amount of GNS effectively modulates the content of the NVP phase in VPC composites, while simultaneously broadening the transport channels of Na⁺. Furthermore, The results of Raman spectra, SEM/TEM images, and EIS measurements indicate that GNS can suppress the uncontrolled growth and agglomeration of particles, and facilitate the construction of an efficient ion/electron conductive network. As a result, the electronic conductivity of the optimized sample is significantly enhanced by several orders of magnitude from the 1.25×10^-9 S·cm^-1 to 1.06×10^-4 S·cm^-1. Simultaneously, the VPC-GN8 shows a D_Na+ value of 1.20×10^-12 cm²·s^-1, surpassing that of the P-VPC (5.43×10^-13 cm²·s^-1). The optimized sample also exhibits a high reversible specific capacity of 118.7 mAh·g^-1 at 0.2 C and maintains an excellent capacity retention of 90.4% after 5000 cycles at 20 C. These results indicate that the optimization of phase components through incorporating an appropriate amount of GNS and the construction of highly conductive networks can expand Na⁺ migration channels, facilitate ion and electron transport, and enhance electronic conductivity, thereby improving the durability under high-rate conditions.

In this study, the incorporation of GNS could effectively mitigate the fluorine loss during the synthesis process, thereby facilitating the increase of NVPF phase and improvement of the ionic conductivity of Bis-VPC material. The introduction of GNS constructed an interconnected three-dimensional porous architecture within the matrix of Bis-VPC particles, effectively mitigating the uncontrolled growth and agglomeration of particles and accelerating the transport of both ions and electrons. In addition, the generated porous structure also increased the interface between Bis-VPC particles and electrolyte. Consequently, the modified sample exhibited improved reversible capacity, exceptional rate performance and ultra-long cycle life. This novel modification strategy, which enhanced ion and electronic conductivity through graphene like nanosheets, could accelerate the application of high-energy-density vanadium-based phosphate cathode materials in large-scale energy storage for SIBs.

The rapid development and practical deployment of hyperspectral detection technology render conventional camouflage materials increasingly inadequate for covert protection in complex forest environments, primarily due to their insufficient spectral matching and limited functionality. A critical challenge in forest camouflage lies in the accurate replication of four characteristic spectral signatures of natural vegetation, i.e., the green peak, the red edge, the near-infrared plateau, and the near-infrared water absorption peak. These features form the unique spectral "fingerprint" of vegetation, which serves as a fundamental basis for evading multi-band, high-precision detection. Among them, the green peak governs a visual concealment within the visible region, while the near-infrared water absorption peak acts as a decisive marker for distinguishing vegetation from artificial materials, which are essential for effective camouflage. However, the existing forest camouflage paints and simulation materials are constrained by inadequate structural design and limited functional integration. Most commercially available products can only partially mimic certain spectral traits of vegetation and commonly exhibit deficiencies in core performance. Note that it remains particularly difficult to simultaneously and accurately match both the green peak of healthy foliage and the near-infrared water absorption peak. This shortfall makes such materials susceptible to rapid identification under hyperspectral imaging, thereby severely limiting the practical effectiveness of the existing forest camouflage technologies. Consequently, there is an urgent need for the research and development of novel camouflage materials capable of comprehensive spectral simulation.

Six types of sulfate intercalated Mg/Cr layered double hydroxides (LDHs) were synthesized by a coprecipitation method. The crystalline structure of the as-prepared materials was examined by X-ray diffraction (XRD). The molecular structures and characteristic absorption features were determined by Fourier-transform infrared spectroscopy (FT-IR). The thermal decomposition behavior and mass loss profiles were analyzed by thermogravimetry-differential thermal analysis (TG-DTA). The surface morphology and particle size distribution were characterized by scanning electron microscopy (SEM). These characterization techniques provided a comprehensive structural and physicochemical profile of the synthesized LDHs. Subsequently, to meet the requirement of reproducing the green spectral peak in jungle camouflage, a functional camouflage coating was fabricated via blending the optimized LDHs material with chromium-based green pigment. This formulation enhanced the green-peak characteristics in the visible region and preserved the pronounced water-absorption peak in the near-infrared range. The spectral performance of the coating was evaluated by visible-near-infrared spectroscopy (Vis-NIR), and its thermal camouflage capability was assessed by infrared thermographic imaging.

The results systematically indicate that a series of chromium-containing hydrotalcite materials, prepared by a controlled synthesis process, exhibit typical crystalline structural characteristics of layered double hydroxides (LDHs). The XRD patterns reveal sharp and symmetric diffraction peaks, with the characteristic peaks corresponding to the layered structure displaying a high intensity and a regular shape, indicating a good crystallinity and a well-ordered interlayer arrangement. Among the series of chromium-containing hydrotalcites, sulfate-intercalated magnesium chromium hydrotalcite (MgCr-SO₄^2--LDHs) demonstrates unique structural advantages, i.e., its interlayer spacing is significantly larger than that of other intercalated types of magnesium chromium hydrotalcites. Simultaneously, The TG-DTA thermal analysis indicates that this material exhibits the maximum mass loss of 26.38% at 25-400 ℃. This is directly related to the content of water molecules stored within the interlayers. The spatial configuration and charge distribution characteristics of the sulfate intercalation ions provide a sufficient space for the stable adsorption and storage of water molecules. As a result, MgCr-SO₄^2--LDHs possesses the most abundant interlayer water content among the prepared samples. This rich interlayer water constitutes the core structural basis for simulating the near-infrared water absorption peak of vegetation. The visible to near-infrared spectra (400-2500 nm) further reveal the optical properties of the Mg/Cr hydrotalcite. In the visible region, the material shows a distinct characteristic absorption peak at 490 nm (blue region), attributed to its intrinsic electronic transition behavior. In the near-infrared region, intense and sharp water absorption peaks clearly occur at approximately 1400 nm and 1900 nm, showing a high similarity to the near-infrared water absorption features of natural vegetation leaves. Sulfate-intercalated magnesium chromium hydrotalcite (MgCr-SO₄^2--LDHs) is modified with chromium(III) oxide (Cr₂O₃) by an in-situ coating process via leveraging these characteristics. The near-infrared water absorption properties imparted by the abundant interlayer water in MgCr-SO₄^2--LDHs were combined with the green optical characteristics of Cr₂O₃. This complementary interaction successfully offset the influence of the intrinsic blue absorption peak at 490 nm from the magnesium chromium hydrotalcite. Ultimately, a jungle camouflage coating simulation material is prepared, having both green visual characteristics and distinct near-infrared water absorption peaks. This provides a key material foundation for subsequent high-similarity spectral matching with natural vegetation.

The camouflage coating fabricated with an optimal MgCr-SO₄^2--LDHs@Cr₂O₃ : MgCr-SO₄^2--LDHs ratio of 3 : 7 could enable a precise spectral matching with natural vegetation leaves. It accurately replicated both the green peak and the near-infrared water absorption peak, achieving a spectral similarity of up to 95.80%. Furthermore, the results of thermal imaging tests revealed no significant difference between the coating and natural vegetation. This study on dual-functional materials that simulated color and the near-infrared water absorption peak could thus offer a promising direction for the development of advanced vegetation camouflage technologies.

The review outlines future research directions, such as the development of large-area material growth and transfer techniques, the design of novel device structures, and the integration of multi-functional systems. 2D materials are expected to play a crucial role in the advancement of neuromorphic computing through interdisciplinary collaboration and continuous innovation, paving a way for the development of artificial intelligence, the Internet of Things, and big data technologies.

This review provides the research progress on a comprehensive, first-principles-based examination in elemental-doped graphene as electrode materials for sodium-ion batteries. It examines the governing mechanisms behind the physical properties and electrochemical performance of doped graphene anode materials. The microscopic physicochemical nature of doped graphene's characteristics is revealed via the detailed atomic-scale simulations and analysis.

The first-principles calculations reveal the distinct mechanisms by which different main-group element dopants affect the physical properties of graphene. Nitrogen (N) and boron (B) doping significantly enhance a sodium-ion adsorption capability and a diffusion performance via modifying the electronic structure of graphene, while maintaining a good structural integrity, thus exhibiting excellent overall performance. Sulfur (S) doping optimizes diffusion kinetics via expanding the interlayer spacing, though its adsorption capability and cycling stability require improvement. Fluorine (F) doping enhances adsorption capability via charge transfer mechanisms but suffers from a lower structural stability. Silicon (Si) doping increases the open-circuit voltage, however, its volumetric effect leads to degraded diffusion performance and structural stability.

The electrochemical performance analysis reveals that nitrogen-doped graphene exhibits a balanced performance in terms of specific capacity and capacity retention rate, making it suitable as a high-performance anode material for sodium-ion batteries. Boron-doped graphene achieves the maximum discharge specific capacity, however, its lower capacity retention rate limits its use to scenarios prioritizing a high energy density. Sulfur-doped graphene demonstrates the optimum cycling stability, but its relatively low specific capacity makes it suitable for applications demanding exceptionally long cycle life. Fluorine-doped graphene and silicon-doped graphene require a further performance optimization. Specifically, fluorine-doped graphene suffers from a low specific capacity and a poor capacity retention, limiting its practical viability. Silicon-doped graphene experiences degraded diffusion performance and structural stability due to its volumetric effect, necessitating a further investigation into doping concentration and structural design.

Based on the first-principles calculations, this review delves into the interactions between doped graphene and the electrolyte, predicting the composition and structure of the solid electrolyte interphase (SEI) film. Some strategies are explored to engineer a more stable SEI layer via simulating the SEI formation process, thereby suppressing the electrolyte decomposition and enhancing the cycling stability and safety of batteries.

Based on the research on doped graphene anode materials, novel doping elements and co-doping strategies can be explored to develop sodium-ion battery anodes with an enhanced adsorption capacity, a reduced diffusion energy barriers, and a superior structural stability. Leveraging the first-principles calculations to predict electrochemical performance of emerging dopants, we optimize doping concentrations and atomic configurations to improve electronic conductivity and ion diffusivity, thus propelling the performance enhancement of sodium-ion batteries.

The research on elemental doping of graphene anode materials for sodium-ion batteries synergistically integrates big data analytics, leveraging extensive experimental and computational datasets to identify and predict dopant effects on the material performance. This data-driven approach enables an accelerated translation from laboratory prototypes to industrial-scale production, reducing manufacturing costs and expediting the commercialization of sodium-ion battery technologies. Consequently, it establishes a robust support framework for developing eco-friendly energy materials and their sustainable implementation.

HSSEs emerge as a highly promising material for next-generation all-solid-state lithium batteries. Their unique combination of high ionic conductivity, wide electrochemical window, and good mechanical properties makes them key materials for achieving high energy density and safe energy storage. Significant progress is made in understanding the classification of crystalline and amorphous halide materials, elucidating their ion transport mechanisms, and developing diverse synthesis and modification strategies to tailor their properties.

Despite this significant progress, several challenges are addressed to fully realize their potential. 1) Improving the ionic conductivity of fluoride-based electrolytes: Although fluorides offer the advantage of high-voltage stability, their ionic conductivity remains rather low. Future research must focus on overcoming this bottleneck through novel structural design, doping strategies, or the development of amorphous fluoride phases; 2) Enhancing electrochemical stability against lithium metal anodes: When halide solid-state electrolytes come into contact with lithium metal anodes, their metal cations are susceptible to reduction, leading to interfacial instability and capacity decay. Therefore, the rational design of stable artificial solid-electrolyte interphase layers that can both protect the electrolyte from reduction and ensure smooth lithium-ion transport is crucial; 3) Deepening the understanding of amorphous transport mechanisms: Although amorphous halides exhibit a superior conductivity, the fundamental principles governing ion transport in these disordered structures are not yet fully understood. Advanced characterization techniques and theoretical simulations are needed to elucidate the roles of dynamic structural relaxation and local chemical environments; and 4) Developing scalable and sustainable synthesis methods: Moving beyond laboratory-scale synthesis to develop cost-effective, environmentally friendly, and scalable production methods is key to commercialization. This includes exploring and optimizing water-based or solvent-based synthesis routes for a wider range of materials. The focused research will pave a way for the practical application of halide solid-state electrolytes. Their successful integration into all-solid-state lithium batteries holds a potential to revolutionize energy storage, providing safer, more powerful, and longer-lasting batteries for a wide range of applications, from consumer electronics to electric vehicles and grid-scale energy storage.