[Objective] Vegetation concrete in alpine regions is prone to structural loosening and mechanical performance degradation after freeze-thaw cycles, which in turn limits the effectiveness of slope ecological restoration, while the coupled effects of soil type and biochar content on the freeze-thaw characteristics of vegetation concrete under unidirectional freeze-thaw conditions remain insufficiently understood. To address the above issues, this study investigates the effects of soil type and biochar content on the freeze-thaw characteristics of vegetation concrete, reveals the underlying mechanisms, and provides theoretical support for the optimization design of frost-resistant mix proportions in alpine regions. [Methods] Sandy soil and cohesive soil collected from Yichang were selected as planting substrates. Vegetation concrete specimens using sandy soil (VC-SS) and cohesive soil (VC-CS) were fabricated, respectively. Unidirectional freeze-thaw tests were conducted. The temperature field changes at different depths of the specimens were monitored in real time, and frost heave deformation data were collected using displacement sensors. The layered water content before and after freeze-thaw cycles was determined using the oven-drying method. The effects of soil type and biochar content on freezing temperature, frost heave amount, and water migration patterns of vegetation concrete were systematically analyzed, and the mechanisms were interpreted from the perspectives of thermal conduction, pore structure, and water transport. [Results] 1) Soil type had a significant effect on the freeze-thaw characteristics of vegetation concrete. Under the same biochar content, the freeze-thaw resistance of VC-SS was significantly better than that of VC-CS. The freezing center temperature of VC-SS was 0.2 ℃-1.8 ℃ lower than that of VC-CS, the maximum frost heave amount reduced by 5.6-7.0 mm, the water migration amount decreased by 0.2%-1.3%, and VC-SS reached the frost heave peak earlier. 2) During the freeze-thaw process, the water content of both types of specimens exhibited an “inverted C-shaped” distribution pattern. In freezing stage, water showed a unidirectional upward migration pattern from bottom to top, with the water content in the deep layer decreasing to 15.2%-19.83% and that in the shallow layer increasing to 20.2%-22.6%. In thawing stage, the water migration pattern shifted to bidirectional migration. The surface layer water content decreased by 0.05%-1.3%, the middle layer increased by 0.02%-1.4%, and the deep layer showed an overall decreasing trend of 0.4%-1%. 3) The effect of biochar content on the freeze-thaw characteristics of vegetation concrete exhibited a nonlinear pattern. With increasing biochar content, the freezing center point temperature, frost heave amount, and water migration amount of VC-SS and VC-CS all showed a trend of first decreasing and then increasing, with 0.5% being the optimal content. [Conclusion] Under unidirectional freeze-thaw conditions, sandy soil with low fine-particle content combined with 0.5% biochar content can significantly improve the freeze-thaw resistance of vegetation concrete and is an optimal scheme for mix proportion design in alpine regions. This mix proportion has relatively high thermal conductivity and low thermal insulation performance. Therefore, plant species with low-temperature germination characteristics should be selected in engineering applications to ensure the effectiveness of slope ecological restoration. The innovation of this study lies in the systematic clarification of the coupled regulatory mechanisms of soil type and biochar content on hydrothermal migration and frost heave deformation of vegetation concrete under unidirectional freeze-thaw action for the first time. This study clarifies the internal mechanisms of freeze-thaw deterioration under different mix proportions, and addresses the insufficient understanding of unidirectional freeze-thaw characteristics of vegetation concrete in alpine regions. The findings provide key theoretical support for frost-resistant design of vegetation concrete in slope ecological restoration of water conservancy and transportation engineering in alpine regions. Future studies can further investigate the evolution of geotechnical properties of vegetation concrete under freeze-thaw cycles to improve the engineering application system.

[Objective] Owing to its abundant availability, aeolian sand can be used in concrete production. To investigate the influence of curing age on the pore structure of aeolian sand concrete, concrete specimens are prepared by replacing river sand with varying amounts of aeolian sand, and the effects of aeolian sand content and curing age on the development of pore structure and compressive strength are explored. [Methods] In the experiments, compressive strength was used as a macroscopic indicator for aeolian sand concrete specimens with different aeolian sand contents at different curing ages. At the microscopic level, nuclear magnetic resonance (NMR) was employed to analyze the internal pore structure, including T₂ spectrum, porosity, free fluid saturation (MFFI), and bound fluid saturation (BVI). Scanning electron microscopy (SEM) was used to observe the internal microstructure and monitor structural changes in the concrete. [Results] Results showed that the internal porosity of concrete gradually decreased with increasing curing age, while the compressive strength increased correspondingly. At the same curing age, the compressive strength of concrete initially increased and then decreased as aeolian sand content increased. The optimal improvement occurred at 25% aeolian sand replacement. At 28 d age, the compressive strength of the concrete with 25% aeolian sand replacement was 1.02, 1.08, 1.11, and 1.19 times that of ASC-0, ASC-50, ASC-75, and ASC-100, respectively. SEM observations showed that microcracks and pores gradually decreased with curing age, and ASC-25 exhibited superior compactness among all mixtures. The T₂ spectrum displayed three to four peaks, with the first peak as the dominant component. As the curing age increased, the proportion of the first peak area gradually increased in all groups. At 14 d age, the first peak proportions of ASC-0, ASC-25, ASC-50, ASC-75, and ASC-100 were 89.7%, 92.68%, 88.74%, 86.66%, and 86.13%, respectively. The proportion of harmless pores gradually increased in each group. For ASC-25, the harmless pore proportion was 43%, 48%, 60%, 63%, and 66% at 7, 14, 28, 56 d, and 84 d, respectively. The internal pore structure of the concrete gradually became denser. For ASC-25, the bound fluid saturation values were 91.51%, 92.72%, 94.53%, 95.21% and 96.37% at 7, 14, 28, 56 d, and 84 d, respectively, with corresponding porosities of 3.00%, 1.65%, 1.25%, 1.22%, and 1.04%, respectively. [Conclusion] The gray correlation analysis indicates that bound fluid saturation and the proportion of harmless pores are strongly correlated with compressive strength, with correlation coefficients exceeding 0.8. The compressive strength predicted by the GM(1,3) model closely matches the experimental results, with a maximum residual of 0.44 MPa, and the relative errors are all within 5%. The established GM(1,3) model can effectively predict the compressive strength of aeolian sand concrete at different curing ages, providing a reference for practical engineering applications.

[Objective] With the rapid development of the iron and steel industry, steel slag, a by-product of iron and steel smelting, has become a key focus for environmental protection and sustainable development. However, its low activity can lead to a reduction in the early strength of concrete. Calcium silicate hydrate (C-S-H) not only shortens or even eliminates the induction period through homogeneous nucleation but also provides an excellent physical filling effect without negatively affecting later strength. This feature gives C-S-H broader application potential and greater prospects compared with traditional early-strength agents. However, C-S-H tends to agglomerate due to its small particle size and large specific surface area, which reduces its accelerating effect. [Methods] In this study, calcium silicate hydrate/polycarboxylate ether (C-S-H/PCE) materials were synthesized, and the effects of different dosages of C-S-H/PCE on the setting time, compressive strength, pore structure, and cement hydration of steel slag cement mortar were systematically investigated. The setting time of the cement mortar was measured following the standard test methods for water consistency, setting time, and stability of cement (GB/T 1346). Following the cement mortar strength test method (GB/T 17671-2021), the compressive strength of cement mortar was measured at 6 h, 8 h, 12 h, 18 h, 1 d, 3 d, 7 d, and 28 d. The porosity of the cement mortar after 1 d, 3 d, and 7 d of curing was determined using an Autopore IV 9520 mercury porosimeter. The thermal stability of the mortar at 3 d, 7 d, and 28 d was analyzed using a NETZSCH STA 2500 thermogravimetric analyzer. The mineral composition of steel slag cement mortar with varying C-S-H/PCE content was qualitatively analyzed using a Shimadzu XRD-6100 X-ray diffractometer. [Results] The adsorption of Ca²⁺ by C-S-H/PCE followed the Langmuir adsorption model. The results indicated that Ca²⁺ adsorption by C-S-H/PCE was monolayer, with a maximum adsorption capacity (Qmax) of 26.19 mg/g. Incorporating an appropriate amount of C-S-H/PCE into steel slag cement mortar effectively accelerated its setting time. The reduction in setting time was proportional to the C-S-H/PCE dosage. Higher C-S-H levels led to shorter times for the cement mortar to reach both initial and final setting. Adding C-S-H/PCE enhanced the compressive strength of steel slag cement mortar, particularly at early stages (within 1 day). Higher C-S-H/PCE content resulted in greater early compressive strength, while the rate of compressive strength increase decreased with curing age. Incorporating an appropriate amount of C-S-H/PCE effectively improved the compactness of steel slag cement mortar and refined its pore structure. This effect was particularly pronounced at early stages of mortar curing, during which C-S-H/PCE significantly reduced porosity. As C-S-H/PCE content increased, the number of macropores decreased significantly, while the proportion of gel pores and mesopores increased, promoting a denser and finer pore structure. [Conclusion] C-S-H/PCE not only stimulates the formation of additional hydration products and accelerates the overall hydration process but also does not alter the types of hydration products, ensuring the stability and controllability of cement properties, and providing a new approach for optimizing steel slag cement mortar performance. This study provides a solid theoretical foundation and technical guidance for the scientific and rational utilization of steel slag in concrete, promoting its practical application and the sustainable development of steel slag resources.

[Objective] This study investigates the fracture performance of ultra-high performance concrete (UHPC) reinforced with steel fibers and steel-polyvinyl alcohol (PVA) hybrid fibers through combined experimental tests and extended finite element method (XFEM) simulations. The objective is to determine an optimal hybridization strategy that enhances fracture resistance and cost efficiency, thereby providing theoretical support and practical guidance for engineering applications. [Methods] Notched beam specimens were tested using the three-point bending method. The program included one control group, five groups with varying steel fiber dosages (0.5-2.5% by volume), and five groups reinforced with hybrid steel-PVA fibers, maintaining a total fiber volume of 2.5% while adjusting PVA replacement ratios from 0 to 1.0. P-CMOD (load-crack mouth opening displacement) curves were used to evaluate flexural strength, initiation toughness, unstable toughness, and fracture energy. Parallel XFEM simulations were developed in ABAQUS, where fracture initiation was governed by maximum principal stress criterion and crack growth was modeled with energy-based softening laws. Experimental and numerical outcomes were compared to assess the predictive accuracy of XFEM. [Results] 1)The addition of fibers transformed the fracture behavior of UHPC from brittle through-crack failure to ductile non-penetrating fracture. Three distinct modes were identified: brittle single-crack, ductile single-crack, and ductile multi-crack. Steel fibers mainly provided bridging and anchorage, delaying unstable crack growth and enhancing energy dissipation, whereas PVA fibers controlled micro-crack initiation and dispersed stresses effectively, often rupturing instead of pulling out. This complementary mechanism revealed a clear division of roles, highlighting a “synergistic hybrid effect” that improved toughness and post-cracking performance. 2)Quantitatively, increasing steel fiber dosage yielded significant improvements. At 2.5% steel fibers, the initiation load, peak load, initiation toughness, unstable toughness, and fracture energy increased by 146.55%, 60.94%, 145.13%, 56.28%, and 45.58%, respectively, compared with specimens containing 1.0% steel fiber. Hybrid reinforcement further optimized performance. At a total fiber content of 2.5%, replacing 20% of steel fibers with PVA (γ=0.2) increased initiation toughness by 6%, while unstable toughness decreased by only 2%, representing the most favorable balance between toughness and economy. In contrast, higher PVA replacement ratios (γ>0.2) reduced flexural strength and fracture energy due to fiber agglomeration and uneven dispersion within the UHPC matrix. 3)Cost analysis further emphasized the advantages of hybridization. Copper-coated steel fibers cost approximately 6.5 RMB/kg, whereas PVA fibers were about twice as expensive. By replacing 20% of steel fibers with PVA at 2.5% total content, material costs were reduced by 11.6% compared with 2.5% steel fiber UHPC, without compromising fracture resistance. This finding underscored the engineering value of hybrid design, particularly for large-scale applications requiring both high durability and economic efficiency. 4)XFEM simulations closely reproduced experimental outcomes. Simulated P-CMOD curves were generally enveloped within the experimental results, and predicted crack paths matched observed failure modes. Average relative errors were 4.21% for peak load, 3.88% for unstable toughness, and 13.62% for initiation toughness, which were within acceptable limits. Moreover, XFEM captured the delayed crack penetration behavior in hybrid fiber specimens, showing how fiber synergy effectively slowed crack growth. This predictive capability demonstrated the suitability of XFEM for analyzing complex hybrid fiber systems, reducing experimental workload while offering mechanistic insights into crack evolution. [Conclusion] Steel-PVA hybridization significantly enhances UHPC fracture behavior and reduces cost, confirming the benefits of a synergistic reinforcement approach. The main conclusions are as follows: 1) Fibers convert UHPC failure from brittle through-crack rupture to ductile failure characterized by irregular, non-penetrating cracks, improving structural integrity and durability. 2) Increasing steel fiber dosage enhances toughness and ductility, with contents above 1.5% yielding substantial improvements in fracture parameters and shifting the load-bearing response from brittle to ductile. 3) A replacement ratio of γ=0.2 is optimal, strengthening crack initiation resistance and sustaining fracture toughness while reducing material costs by 11.6%. Excessive replacement (γ>0.2) negatively affects strength and fracture energy, highlighting the need for balance in hybrid design. 4) XFEM effectively simulates crack initiation, propagation,and post-cracking responses, achieving strong agreement with experiments.The method offers a reliable tool for predicting fracture performance in hybrid UHPC and can support performance-based design with reduced reliance on extensive laboratory testing.

[Objective] To investigate the meso-scale fracture mechanisms and crack evolution of hydraulic concrete under tensile loading, and to address the inadequate representation of initial defects and the interfacial transition zone (ITZ) in conventional meso-models, a four-phase 3D meso-scale model consisting of aggregates, mortar, an ITZ, and initial defects is established. The effects of initial defect ratio, ITZ thickness, and aggregate content on tensile strength, failure modes, and energy dissipation are quantified. The innovation of this study lies in explicitly introducing the initial defects with controllable volume fractions within a 3D meso-scale framework and coupling them with ITZ and aggregate factors to enhance the reproducible representation of experimental responses and failure patterns. [Methods] Uniaxial tensile tests on cylindrical specimens of ordinary-strength hydraulic concrete (C40) were conducted to obtain stress-strain curves and tensile strength for model validation. Numerically, a 3D meso-scale geometry was established based on randomly generated aggregates. Aggregates were modeled using a linear-elastic constitutive law, while mortar and ITZ were modeled with a damage-plasticity constitutive law. Spherical initial defects were randomly embedded in the mortar phase to represent pores and micro-cracks. After validating the model against experimental strength, curve morphology, and failure modes, a parametric study was performed to investigate the effects of initial defect ratio (0-7%), ITZ thickness (including a control without ITZ), and aggregate content (10%-40%) on tensile fracture behavior. [Results] (1) The four-phase 3D model well reproduced the mechanical response and crack propagation pattern throughout the uniaxial tensile process. Simulated tensile strength was generally overestimated when initial defects were not considered. (2) Simulated and experimental tensile strengths and stress-strain curves agreed well with a initial defect ratio of 1%-2%. As the defect ratio increased, tensile strength decreased significantly. Cracks tended to concentrate in the mid-region of the specimen and propagate rapidly, exhibiting more pronounced strength weakening, earlier instability failure, and concurrent deterioration in energy dissipation capacity during fracture. (3) Within the small range examined, variations in ITZ thickness had a relatively limited influence on peak tensile strength. However, omitting the ITZ led to systematic deviations in strength and failure mode, making it difficult to correctly capture crack paths and local damage evolution. (4) An increase in aggregate content further reduced the tensile strength and significantly altered crack propagation paths and patterns. This reflected an important modulating role of changes in the proportion of aggregate/ITZ weakened regions on crack evolution. [Conclusion] The proposed four-phase 3D meso-scale modeling method incorporating initial defects provides balanced accuracy in reproducing both mechanical responses and failure patterns. The study demonstrates that for ordinary-strength hydraulic concrete, properly characterizing an initial defect ratio of 1%-2% is crucial for improving the accuracy of tensile fracture simulation. An increase in the initial defect ratio significantly reduces tensile strength and promotes concentrated crack coalescence in the specimen’s mid-region. The ITZ has a non-negligible controlling effect on crack paths and macroscopic responses, and ignoring it will lead to significant errors. Aggregate content further affects strength and crack patterns by altering the proportion of weakened interfaces. The findings can provide references for the determination of meso-scale parameters for hydraulic concrete, risk assessment of dam cracking, and enhancement of structural safety.

[Objective] This paper aims to investigate the influence of slag content on the resistance to water penetration and pore structure of recycled aggregate concrete(RAC), to reveal the relationship between resistance to water penetration and pore structure, and to provide a reference for its durability research and engineering application. [Methods] The slag content was designed to replace cement at proportions of 0%, 10%, 15%, 20%, 25%, and 30%, respectively. Six groups of C30 RAC with slag were prepared according to these proportions. Tests on water absorption and water penetration height with different slag contents were conducted, as well as microscopic pore structure tests using low-field nuclear magnetic resonance. Based on the results of the water penetration tests and microscopic tests on RAC, the relative permeability coefficient was taken as the reference sequence, and the proportions of different pore types as the comparison sequence, Then,the grey correlation analysis method was used to analyze the relationship between the resistance to water penetration and the pore structure of RAC. [Results] The results showed that with increasing slag content, the water penetration height and relative permeability coefficient of RAC both decreased. Compared with the test group without slag, the water penetration height and relative permeability coefficient of RAC test group with 30% slag content decreased by 26.01% and 61.12%, respectively, indicating that the resistance to water penetration of RAC was significantly enhanced. Based on the experimental data, a relationship model between the relative permeability coefficient of RAC and the slag content was constructed, and the fitting degree of the model reached 98.97%. Furthermore, the results of microscopic pore structure tests using low-field nuclear magnetic resonance showed that the porosity of RAC also decreased with increasing slag content. This indicated that slag could refine and reduce the internal pores of RAC, increase its compactness, and thereby enhance its resistance to water penetration. Grey relational analysis showed that incorporating slag led to significant changes in the pore structure of RAC. Specifically, slag reduced the proportion of intermediate pores and large pores in RAC, leading to a decrease in overall porosity and a more compact microstructure, which in turn enhanced its resistance to water penetration. [Conclusion] Incorporating an appropriate amount of slag can improve the resistance to water penetration of RAC, providing a reference for its engineering application.

[Objective] Natural volcanic ash has environmental advantages in reducing cement consumption and lowering carbon emissions. However, due to its low reactivity, the use of volcanic ash alone often leads to reduced early-age performance of cement-based materials. This study aims to develop a volcanic ash-silica fume composite cementitious system based on low-reactivity volcanic ash from the Sichuan-Xizang region, and to systematically investigate its hydration behavior, microstructural evolution, and mechanical properties, thereby providing theoretical and technical support for the engineering application of volcanic ash. [Methods] Multiple characterization methods were used to systematically analyze the hydration characteristics and microstructure of the composite cementitious system. First, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to qualitatively and quantitatively analyze hydration products at different curing ages to investigate the formation of C-S-H gel and its microstructural characteristics in the volcanic ash-silica fume composite system. Second, thermogravimetric and derivative thermogravimetric analysis (TG-DTG) was conducted to examine the thermal decomposition behavior of the materials, thereby revealing the composite effect of volcanic ash and silica fume from a thermodynamic perspective. To further evaluate the influence of pore structure on strength, mercury intrusion porosimetry (MIP) was used to analyze the pore size distribution of samples at different curing ages. Meanwhile, mechanical performance tests, such as compressive strength tests, were conducted to evaluate the mechanical properties of systems with different mix proportions. [Results] The single volcanic ash-blended system exhibited relatively sluggish early hydration, resulting in a lower strength activity index and mechanical strength. In contrast, the incorporation of silica fume significantly promoted the formation of C-S-H gel, refined the pore size distribution, enhanced the densification of cement-based materials, and consequently improved both the early- and late-age strength of the composite system. At 28 days, the composite system containing 27% volcanic ash and 3% silica fume showed a significant advantage, with its compressive strength approximately 28% higher than that of the single volcanic ash-blended system, and with a markedly increased strength activity index. Microstructural analysis further indicated that the C-S-H gel in the composite system was dense and uniformly distributed, and the pore structure was effectively optimized. [Conclusion] The combined use of low-reactivity volcanic ash and silica fume significantly enhances hydration activity and mechanical performance, particularly strength. The novelty of this study lies in optimizing the combined dosage of volcanic ash and silica fume, which not only improves mechanical performance but also optimizes the microstructure, resulting in a denser structural system. Therefore, the volcanic ash-silica fume composite cementitious system shows strong engineering applicability and provides an important reference for the development of low-carbon and environmentally friendly cement replacement materials.

[Objective] Grouting has been recognized as an effective solution for water inrush treatment in underground engineering. Organic grouting materials such as polyurethane (PU) are strongly adhesive but expensive,whereas inorganic grouting materials such as waterglass (WG) are cheap but have low elasticity. To improve the performance of polyurethane grouting materials,we investigated the influence of waterglass content on the macroscopic and microscopic properties of polyurethane. [Methods] Polyphenyl polymethylene polyisocyanate (PAPI),waterglass,and polyether polyol were used as the basic raw materials,and the inorganic/organic hybrid method was adopted to prepare polyurethane foam grouting material modified by waterglass. The influence of waterglass content by weight (0,20%,40%,60%,80%) on the foaming rate,density,and compressive strength of the modified polyurethane was investigated. A fitting relationship between polyurethane density and elastic modulus was established. Scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were used to measure the influence of waterglass content on the cross-sectional morphology and thermal stability of materials. [Results] (1) Macroscopic property results showed that,with increasing waterglass content,both the foaming rate and compressive strength first increased and then decreased. When the waterglass content was 20%, the foaming rate and compressive strength reached maximum. Under this condition, the foaming rate was 2 684%, and the compressive strength at 7 d was 38.9 MPa. In addition, the heat released by the reaction increased the instability of the cell structure and even caused cell collapse, resulting in decreases in foaming rate and compressive strength. Notably, when the density changed from 1.102 g/cm³ to 0.959 g/cm³, the compressive strength of materials decreased by approximately 70%. Based on the experimental results, the fitting relationship between the density and elastic modulus of the modified polyurethane was obtained as E∝ρ^1.8.(2) The results of scanning electron microscopy showed that, with increasing waterglass content, the smooth polyurethane surface was gradually covered by an amorphous inorganic phase surrounding the spherical cells. The structure of the materials eventually became loose and porous.(3) TGA results showed that the thermal stability of the modified polyurethane was better than that of pure polyurethane. The TG curve showed that when T<80 ℃, the thermal mass loss of the materials was only about 5%, indicating that the initial decomposition temperature of the materials was about 80 ℃. When T >600 ℃, the curve became stable, meaning that the modified polyurethane with waterglass content of 0, 20%, 40%, 60%, and 80% lost approximately 84%, 77%, 72%, 65%, and 53% of their mass, respectively. The curves then tended to stabilize, indicating that the incorporation of waterglass could enhance the thermal stability of polyurethane.(4) DTG (Differential Thermogravimetry) curve showed that the mass loss rate of polyurethane was significantly higher than that of the modified polyurethane. This was attributed to the fact that the inorganic components precipitated in the reaction system were encapsulated on the surface of polyurethane matrix. The presence of Si-O bonds increased the intermolecular forces, resulting in the need for more energy to cause thermal decomposition of the materials. Therefore, the addition of waterglass effectively reduced the thermal decomposition rate of polyurethane.[Conclusion] Polyurethane/waterglass (PU/WG) is an organic-inorganic hybrid material that has the advantages of both polyurethane and waterglass. Analysis of the relationship between waterglass content and the macroscopic and microscopic properties of the modified polyurethane shows that the addition of waterglass helps increase the foaming rate, compressive strength, and thermal stability of polyurethane. These findings offer data support for the performance improvement and system optimization of the modified oil-soluble polyurethane grouting materials.

[Objective] Severe reservoir sedimentation reduces storage capacity and increases global desilting costs. Traditional disposal of dredged sediments (SD), such as landfilling, occupy land resources and pose ecological risks. This study aims to prepare high-strength geopolymers for high-value utilization of solid wastes. [Methods] Sediment from the Zhongxian section of the Three Gorges Reservoir (D₅₀≈67 μm) as the primary raw material. A ternary system is constructed by incorporating ground granulated blast furnace slag (GGBFS) and Class C Grade II fly ash (FA) to overcome alkali activation constraints, including high SiO₂/Al₂O₃ molar ratio of the sediments and the low reactivity of clay minerals. Specimens were cured under standard conditions for unconfined compressive strength (UCS) measurement. XRD, SEM-EDS, and FTIR characterized the mineral composition, microstructure, and surface functional groups of the ternary geopolymers. Additionally, the leaching concentrations of heavy metals (Cr, As, Cd, Co) from the ternary geopolymers were analyzed using the TCLP method with ICP-MS. [Results] In the SD-GGBFS-FA ternary system, the 28-day UCS ranged 51.9-82.9 MPa. The 1-day UCS increased with GGBFS content, indicated diminishing marginal efficiency of GGBFS reinforcement. For GGBFS and SD fixed at 80% and FA at 20%, increasing GGBFS from 0% to 80% produced 1-day UCS increments of 45.8,26.9,22.9,6.4 MPa, respectively, indicating higher alkali activation efficiency when the GGBFS content was below 40%. XRD patterns revealed a typical amorphous characteristic peak in this specimen in the 28°-30° range. SEM of the B2F8S0 specimen revealed the formation of a continuous and dense C-A-S-H gel, indicating that the co-alkali-activated product of GGBFS and SD was C-A-S-H gel, providing primary mechanical support for the early strength development of the material. FA, rich in components such as hematite and mullite, were hardly susceptible to alkali erosion at early ages. Increasing FA content improved stability of strength at later stages (28 d). At FA content of 40% or more, the UCS from 7 d to 28 d remained stable or even increased slightly, contrasting sharply with the significant strength attenuation of the GGBFS-SD system. XRD patterns showed that for B2S0F8 specimen (20% GGBFS + 80% FA) cured for 28 d, the crystalline peak of limestone (CaO) in FA disappeared, and characteristic diffraction peaks of zeolite-type C-A-S-H minerals emerged. FTIR revealed that, after 28 d of curing, the intensities of Si-O-Si stretching and Si-O bending vibration peaks in the B2S0F8 specimen remained stable, indicating greater geopolymer stability in this system than that in the B2S8F0 system. SEM confirmed that the tacharanite generated in the B2S0F8 system filled the pores, improving the compactness of the matrix and maintaining the long-term strength development. TCLP leaching tests showed that the leaching concentrations of Cd and Co from geopolymers were significantly lower than those from raw materials, indicating that Cd and Co could be stabilized by the geopolymer system. However, Cr and As mainly existed as anionic species, and the leaching concentrations of Cr and As from some samples increased after geopolymerization. [Conclusion] High-calcium GGBFS promotes the dissolution of low-activity aluminosilicate components in reservoir sediment clay minerals and forms dense C-A-S-H structures that enhance structural stability. In high-calcium environments, FA undergoes alkali-activated secondary reactions to generate tacharanite, which fills geopolymer gel pores and maintains long-term strength stability of the GGBFS-FA system. B2S6F2 achieves 72.4 MPa at 28 d, and its heavy metal leaching meets Class Ⅰ criteria. The ternary GGBFS-SD-FA system enables the preparation of environmentally safe, high-strength geopolymers, providing a reference for high-value sediment utilization.

[Objective] To facilitate the construction of asphalt concrete core walls in the severe cold regions of Western China, this paper undertakes a systematic investigation into the influence of acidic gravel aggregates on the performance characteristics of hydraulic asphalt concrete. [Methods] The research methodology and evaluation framework were strictly guided by two pivotal Chinese technical standards: Test Code for Hydraulic Asphalt Concrete (DL/T 5362-2018) and the more recent Technical Specification for the Application of Acidic Aggregates in Hydraulic Asphalt Concrete (DL/T 5876-2024). The key performance indicators (mechanical properties, deformation behavior, impermeability, and overall durability) of asphalt concrete incorporating crushed gravel aggregates were evaluated through asphalt concrete water stability tests, direct tensile tests, bending tests, pressure-based impermeability tests for dense-graded asphalt concrete, triaxial compression tests, long-term water immersion stability tests, and long-term freeze-thaw splitting tests. [Results] (1) Complex Composition and Durability Challenge: Gravel aggregates typically exhibited a complex mineralogical composition, encompassing alkaline, neutral, and acidic aggregates, with acidic rock types often being predominant. A primary concern identified was the inherently weak interfacial bonding force between asphalt binder and acidic aggregate surfaces. Under prolonged water immersion, this weak bond facilitated a gradual displacement process where water molecules infiltrated and substituted the asphalt at the aggregate interface, leading to stripping or detachment of the asphalt film from the aggregate surface. This mechanism posed a substantial threat to the long-term durability of the asphalt concrete. Consequently, a thorough durability assessment should be required when considering the application of gravel aggregates-based hydraulic asphalt concrete in critical structures. (2) Enhancement Mechanisms via Cement Filler and Anti-Stripping Agents: The study identified effective methods to mitigate the adhesion issue. Metal ions present in cement, such as Ca²⁺ and Mg²⁺, engaged in chemical bonding with oxygen atoms within the asphalt. In the mixing process, this interaction promoted a more robust and durable bond between the asphalt and the aggregates. Furthermore, the use of anti-stripping agents was found to be highly beneficial. These agents operated through multiple synergistic mechanisms, including chemical bonding with the aggregate surface, modification of the interfacial properties, and the creation of a physical barrier against water intrusion. Collectively, these actions significantly enhanced both the durability and the mechanical performance of asphalt concrete made with acidic aggregates. Incorporating cement filler alone, or using a combination of cement filler and non-amine anti-stripping agents, effectively strengthened the adhesive bond between the gravel aggregates and the asphalt matrix, thereby markedly improving the durability of the resulting acidic gravel aggregate hydraulic asphalt concrete. (3) Mechanical Behavior and Modeling: Asphalt concrete was recognized as a temperature-sensitive material. Analysis of triaxial test data revealed that the relationship between lateral strain and axial strain approximated a linear relationship. Specifically for the gravel aggregate asphalt concrete studied, its strength demonstrated a well-defined and favorable linear increase with rising confining pressure. The material's strength could be effectively characterized using the parameters of a linear strength model, namely the cohesion and the angle of internal friction. (4) Overall Performance Improvement: Incorporating cement filler or adding non-amine anti-stripping agents substantially improved the comprehensive performance profile of acidic gravel aggregate asphalt concrete. These enhancements directly translated to superior mechanical properties, increased resistance to water-induced damage, and extended long-term durability. [Conclusion] The application of acidic gravel aggregates in the construction of asphalt concrete core wall dams is demonstrated to be technically feasible. Key performance parameters evaluated in this study, including the long-term water immersion stability coefficient, the freeze-thaw cycle splitting tensile strength ratio, and the failure tensile strain, provide a robust theoretical foundation and essential technical support for the engineering application of this material.

[Objective] With advances in dam construction technology and equipment, faced rockfill dams have emerged as a highly competitive dam type. The development of concrete-faced rockfill dams (CFRDs) in China is progressing toward the goal of “taller dams and larger reservoirs.” For ultra-high CFRDs, a gravity-type concrete high toe wall is incorporated at the upstream heel area of the riverbed, replacing a portion of the lower face slab. This not only reduces the face slab length but also improves maintainability in the lower dam zone. This study aims to further evaluate the rationality and safety of adopting this composite structural scheme for ultra-high CFRDs. [Methods] An ultra-high (250m) CFRD under construction in China was taken as the research subject. Two-dimensional nonlinear finite element analysis was employed to compare the stress and deformation differences between the composite structure with a concrete high toe wall and the conventional structural scheme. A three-dimensional nonlinear finite element model was further applied to examine the mechanical behavior of the composite-structure ultra-high CFRD during staged construction filling and multi-level reservoir impoundment. The rockfill was modeled using Shen Zhujiang’s double-yield-surface elastoplastic constitutive model, while concrete structures—including the face slab, toe slab, and riverbed high toe wall—were simulated with a linear elastic model. Interfaces between rockfill and concrete were modeled using thin-layer elements with low modulus. [Results] The concrete high toe wall significantly restrained the streamwise deformation of the rockfill, reducing the maximum upstream deformation by 23%. Face slab deflection was notably decreased, with a 10% reduction in peak deflection. During staged filling and multi-stage impoundment, the internal stress distribution in the rockfill remained uniform, with low stress levels and no evidence of stress concentration or plastic limit zones. Compressive stresses in the face slab and concrete high toe wall were within the allowable compressive strength of concrete, and all deformations fell within acceptable engineering limits. [Conclusions] (1) The composite “concrete high toe wall-faced rockfill dam” structure, which replaces the lower face slab in the riverbed with a gravity concrete wall, effectively restrains streamwise rockfill deformation, reduces face slab deflection and improves its stress distribution, while also enhancing maintainability at the slab bottom. This scheme provides a new research direction for CFRD construction and merits further promotion and application. (2) Validation and wider adoption of novel dam structural schemes require support from field monitoring data. This study only considered the stress-deformation behavior of the composite-structure CFRD under static water load. In practice, stress and deformation in faced rockfill dams involve complex conditions such as dynamic actions, seepage, and multi-field coupling. The actual performance and feasibility of the composite structural scheme still need to be verified by long-term operational monitoring data from the project.

[Objective] The concrete lining structures of underground air storage caverns in compressed air energy storage (CAES) systems are subjected to the combined effects of periodic air pressure fluctuations and temperature variations during long-term operation. This study aims to systematically reveal the evolution of mechanical properties, the microscopic damage mechanisms, and the macroscopic constitutive behavior of concrete under synchronous temperature-pressure cyclic loading, establish a damage constitutive model that accurately describes its stress-strain response characteristics, and further clarify the synergistic deterioration effect of coupled temperature-pressure cyclic loading. [Methods] A method combining experimental investigation and theoretical modeling was adopted. First, based on the actual operational parameters of CAES caverns, coupled cyclic loading tests were conducted on C50 concrete cylindrical specimens to investigate the individual and combined effects of three key variables: the number of cycles, the upper limit of cyclic temperature, and the upper limit of cyclic stress. After cyclic loading, uniaxial compression tests and SEM observations were performed on the specimens to reveal the changes in macroscopic mechanical properties and the underlying microscopic mechanisms of concrete under such loading. Based on continuum damage mechanics and the equivalent strain principle, and using the Weibull statistical distribution to describe the distribution and evolution of micro-defects within concrete, a damage constitutive model was established. [Results] (1) Temperature-pressure synchronous cyclic loading significantly deteriorated the mechanical properties of concrete. Both the peak compressive strength and the secant elastic modulus of concrete decreased monotonically with the number of cycles, the upper limit of cyclic temperature, and the upper limit of cyclic stress. The most severe performance degradation occurred during the first 30 cycles, and the degradation rate gradually leveled off after 40 cycles. Particularly, the coupled temperature-pressure action produced a significant synergistic deterioration effect, where the combined effect exceeded the sum of individual effects. After 30 synchronous temperature-stress cycles, the peak strength of concrete decreased by approximately 22% compared to the uncycled reference group. In contrast, the same number of single-factor temperature cycles (without stress) or single-factor stress cycles (constant temperature of 25 ℃) resulted in strength reductions of only about 15% and -3%, respectively, with low-level stress cycling exhibiting a slight strengthening effect. This indicated that the synchronous alternating action of temperature and stress was not a simple superposition of independent effects but rather exacerbated the damage accumulation within concrete through mutually reinforcing mechanisms. (2) SEM observations indicated that the difference in thermal expansion coefficients between aggregates and cement mortar was the fundamental cause of thermally induced microcracks. The presence of periodic axial compressive stress generated additional stress at the tips of existing microcracks during the heating/pressurization phase, promoting their further propagation, and induced repeated shearing and friction on crack surfaces during the cooling/depressurization phase, exacerbating the degradation of interfacial bonding. This coupled thermo-mechanical process ultimately led to the accelerated accumulation and interconnection of damage in the interfacial transition zone (ITZ), the weakest link in concrete, which became the dominant mechanism for the sharp degradation of macroscopic mechanical properties. (3) The established damage constitutive model showed good agreement between the theoretically calculated stress-strain curves and the experimental curves under various working conditions. The model accurately reproduced the shape of the ascending branch, the location of the peak point, and the descending branch trend of the concrete stress-strain relationship after different cyclic histories. The damage evolution curves indicated that temperature-pressure synchronous cyclic loading not only significantly increased the initial damage value of concrete but also altered the development of damage during subsequent loading. Compared to reference concrete or concrete subjected only to temperature cycles, concrete that underwent coupled temperature-pressure cycles exhibited a faster development rate of the damage variable before reaching peak stress and a higher degree of damage at the peak stress point. This quantitatively confirmed the dual effects of “acceleration” and “aggravation” of the coupled temperature-pressure loading on the concrete damage process. [Conclusion] Through systematic experimental and theoretical analysis, this study comprehensively explains the performance degradation and damage evolution mechanisms of concrete under the coupled action of synchronous cyclic temperature-pressure loading. It clearly reveals the unique synergistic deterioration effects resulting from the coupling of temperature and stress fields. The established damage constitutive model provides a directly applicable constitutive relationship for nonlinear mechanical analysis and safety assessment of concrete structures in complex service environments such as CAES underground air storage caverns. The results confirm that the coupled effects of temperature-pressure cycles must be considered in the design and durability evaluation of such structures. Extrapolation based solely on single-factor test results may lead to a severe overestimation of the actual service life and safety margin of the structures.

[Objective] Autogenous volume deformation is a key factor affecting the cracking resistance of dam concrete; however, its meso-scale mechanism is often neglected in engineering practice. Meso-scale modeling of full-graded dam concrete has long faced challenges of low efficiency and poor mesh quality, particularly in reconciling, within a unified model, the scale disparity between a maximum aggregate size of up to 150 mm and an interfacial transition zone (ITZ) thickness of only about 50 μm, which prevents accurate representation of the true meso-structural characteristics. To address this issue, we propose a random aggregate batch placement method. [Methods] By optimizing the aggregate generation algorithm and introducing an efficient spatial positioning criterion, rapid placement and high-volume simulation of four-graded aggregates were achieved. An adaptively coordinated meshing technique for the interfacial transition zone was combined with local mesh refinement and geometric smoothing at aggregate-mortar interfaces, thereby constructing a two-dimensional meso-scale analytical model of full-graded dam concrete that accurately captured the micrometer-scale interfacial geometry and mechanical properties. Based on this model, the stress distributions and their evolution within the interfacial transition zone and the mortar matrix were systematically investigated under two autogenous volume deformation modes—shrinkage and expansion—and under different constraint conditions, including single-sided and four-sided constraints. [Results] 1) Different autogenous volume deformation modes governed the spatial distribution characteristics of meso-scale stresses. Under autogenous shrinkage, tensile stresses were concentrated in the cement mortar regions adjacent to sharp aggregate corners because the shrinkage of the mortar matrix was restrained by aggregates, with the local stresses reaching 3.6-4.0 MPa, approaching the tensile strength limit of the mortar. In contrast, under micro-expansion deformation, the restriction of expansion by aggregates caused the tensile stress concentration zones to shift to the specimen surface and the corner regions of internal interfacial transition zones, with a significant increase in the stress concentration factor, indicating that the interfaces became the weak links under expansion conditions. 2) Constraint conditions played a critical role in regulating the stress state of concrete. Compared with a single-sided constraint, the four-sided constraint simulating the strong restraint of a dam foundation significantly amplified the adverse effects of shrinkage deformation, with the shrinkage-induced tensile stresses in the cement mortar generally increasing by 0.8-1.2 MPa, and the local tensile stress at interfaces even exceeding 4.0 MPa, resulting in a sharp increase in cracking risk. Under the same constraint conditions, however, the positive effect of micro-expansion deformation was effectively transformed, causing the concrete as a whole to remain in a compressive stress state and thereby significantly enhancing its cracking resistance. 3) Based on the maximum tensile stress criterion and by comprehensively considering the actual tensile strengths of the mortar and the interfacial transition zone, the safe deformation threshold for high crack-resistant concrete was determined. The autogenous volume deformation of full-graded dam concrete should be strictly controlled within the range of -9×10^-6 to 11×10^-6. In addition, sensitivity analysis further revealed the influence patterns of key parameters. When the elastic modulus ratio between mortar and aggregate varied by ±20%, the elastic moduli of the mortar and aggregate had a significant effect on the maximum tensile stress of the specimen. Therefore, in dam concrete design, attention should be paid to the coordination between the elastic moduli of mortar and aggregates to prevent excessive tensile stresses induced by autogenous or external deformation. Meanwhile, deformation control indices should be dynamically adjusted according to specific constraint conditions. [Conclusion] Adopting a deformation control strategy of “low shrinkage or micro-expansion” and precisely limiting the deformation magnitude within the above threshold range is an effective approach to enhancing the cracking resistance of dam concrete from the perspective of material design. The established meso-scale model and the obtained quantitative conclusions provide direct evidence for performance-based crack resistance design of concrete. They also offer clear guidance for concrete mix proportion design, optimization of expansive agent dosage, and crack control in practical engineering, and provide a reliable analytical approach for in-depth investigation of deformation coordination and cracking evolution mechanisms of dam concrete at the meso-scale.

[Objective] To investigate the influence of mainshock-aftershock sequences on seismic performance of hollow gravity dams, the nonlinear seismic response of one overflow dam section in a hollow gravity dam in China is investigated. [Methods] Real mainshock and aftershock records were selected from the PEER (Pacific Earthquake Engineering Research Center) earthquake database. Three mainshock-aftershock sequences, Mammoth Lakes, Chalfant Valley, and Whittier Narrows, were constructed based on aftershock decay characteristics. Taking one overflow dam section of a hollow gravity dam in China as the research object, the 3D finite element model of the dam-foundation system of the overflow dam section was built. The concrete damaged plasticity model was adopted to simulate the nonlinear characteristics of the concrete material of the dam body. The massless foundation was used to simulate the dynamic interaction between the structure and the foundation. The hydrodynamic pressure of the reservoir was simulated by the added mass method. The influence of the mainshock-aftershock sequences on the nonlinear seismic response of the hollow gravity dam was analyzed based on the development of cumulative macroscopic failure areas, the residual deformation of the dam crest, and the cumulative damage dissipation energy. [Results] (1) The failure area of the hollow gravity dam caused by aftershocks usually continued to expand along the failure areas caused by the mainshocks. The aftershocks significantly increased the range and depth of the failure area caused by the mainshocks. In some areas, such as the dam heel and the reverse arc area, when the damage caused by the mainshock was relatively deep, aftershocks could directly lead to a through-going failure of the dam body. (2) Aftershocks could cause a significant increase in the residual deformation at the dam crest and the damage dissipation energy of the dam body. The residual deformations caused by the three mainshocks were 2.13, 2.83,2.12 cm, respectively. When the aftershock coefficient was 0.852 6, the proportion of residual deformation caused by aftershocks was 24.2%, 24.1%, and 26.4%, respectively. The damage dissipation energy caused by the three mainshocks was 131, 121,108 kJ, respectively. When the aftershock coefficient was 0.852 6, the proportion of the dam damage dissipation energy caused by the three aftershocks was 38.8%, 49.6%, and 47.1%, respectively. (3) When the aftershock coefficient was small, it also significantly increased the distribution of failure areas in the dam body. In some regions, non-through failure caused by the mainshock could further develop into through-going failure under aftershocks. When the aftershock coefficient was 0.6, the proportion of residual deformation at the dam crest caused by three aftershocks was 18.4%, 18.2%, and 19.4%, respectively. The proportion of the damage dissipation energy caused by the three aftershocks was 29.9%, 40.1%, and 37.5%, respectively. [Conclusion] The innovation of this paper lies primarily in revealing the influence of mainshock-aftershock sequences on the nonlinear seismic response of the hollow gravity dam. The results indicate that the failure areas caused by aftershocks usually continue to expand along the failure areas caused by the mainshocks. The aftershocks may significantly increase the range and depth of the failure areas caused by the mainshocks. The aftershocks may cause a significant increase in the residual deformation at the dam crest and the damage dissipation energy of the dam body. In the seismic safety evaluation of the hollow gravity dam exposed to earthquake hazards, the influence of aftershocks is non-negligible.

[Objective] The adhesion problem between acidic aggregates and asphalt is a key issue in the application of acidic aggregates in hydraulic asphalt concrete. For hydraulic structures, the mix design of hydraulic asphalt concrete incorporating acidic aggregates modified with anti-stripping agents is not entirely the same as that used in road engineering. This study aims to investigate, under a constant gradation index, the individual effects of the proportion of coarse and fine aggregates, as well as their interaction effects with asphalt-aggregate ratio and filler content, on the performance of anti-stripping-agent-modified hydraulic asphalt concrete with granite aggregates, and to optimize the mix proportion design of granite aggregate hydraulic asphalt concrete. [Methods] A Box-Behnken response surface methodology was employed for experimental design. Air voids, flow value, and stability were selected as response variables to establish response surface regression models. The effects of asphalt-aggregate ratio, filler content, and sand ratio, as well as their single and interaction effects on the response values, were analyzed to determine the optimal mix proportion. Meanwhile, water stability tests were conducted to verify the performance. [Results] (1) The mix proportion design of granite hydraulic asphalt concrete was optimized using response surface methodology, and the established quadratic regression equations were able to well describe the relationships among sand ratio, asphalt-aggregate ratio, filler content, and air voids, stability, and flow value. Analysis of variance and significance tests indicated that the model was effective and reliable, with high credibility. (2) Response surface analysis showed that the order of influence on the air voids of granite hydraulic asphalt concrete was asphalt-aggregate ratio>sand ratio>filler content; the order of influence on stability was sand ratio > asphalt-aggregate ratio > filler content; and the order of influence on flow value was sand ratio > asphalt-aggregate ratio > filler content. (3) The three-dimensional distribution of the water stability coefficient indicated that a relatively high water stability coefficient was obtained in the region where the sand ratio ≤ 36%, filler content ≤13.5%, and asphalt-aggregate ratio ≥6.8%. (4) The optimized mix design parameters of granite hydraulic asphalt concrete were a sand ratio of 33%, an asphalt-aggregate ratio of 7.1%, and a filler content of 12.5%. [Conclusion] The experimental results confirm that this mix proportion exhibits good performance and can provide a reference for the optimization design of granite hydraulic asphalt concrete mix proportions in practical engineering.

[Objective] This study addresses the numerical modeling of nonlinear steady-state heat conduction processes where the thermal conductivity varies with temperature. The governing equation for such problems is a second-order quasi-linear partial differential equation, whose nonlinear nature makes analytical solutions extremely challenging, thus necessitating efficient numerical approaches. This work employs the Numerical Manifold Method (NMM) based on quadrilateral mesh covers to analyze and solve two-dimensional nonlinear steady-state heat conduction problems. [Methods] Within the NMM over traditional methods framework, a discrete formulation suitable for nonlinear steady-state heat conduction was established by incorporating three typical boundary conditions: Dirichlet, Neumann, and Robin. The classical Newton-Raphson iterative algorithm was adopted to solve the resulting nonlinear system of equations. A complete numerical solution procedure was implemented on the MATLAB platform to ensure algorithm stability and computational efficiency. To systematically verify the accuracy and robustness of the proposed NMM in handling nonlinear heat conduction, a series of representative numerical examples were designed and conducted. These examples covered various scenarios, including continuous homogeneous materials, discontinuous media containing circular holes, and heterogeneous materials. The simulation results were compared against analytical solutions, existing literature data, or Finite Element Method (FEM) solutions. [Results] 1) Compared to the traditional Finite Element Method (FEM), NMM demonstrates significant theoretical and practical advantages when simulating problem domains with complex geometries or internal discontinuities. This advantage primarily stems from its distinctive numerical characteristics: in NMM, the interpolation subdomains are independent of the subdomains used for numerical integration, whereas in FEM they coincide entirely on the same mesh. Furthermore, FEM is prone to mesh distortion when handling complex boundaries, which can degrade accuracy and impair computational efficiency. Leveraging its physical cover system, NMM can accurately describe complex geometric boundaries. At material interfaces, the different heat conduction behaviors across materials are naturally captured through physical covers and local functions without introducing additional interface conditions, thereby simplifying the computational process and enhancing efficiency. 2) The proposed NMM not only achieves high accuracy in temperature field and heat flux distribution across all examples but also exhibits excellent stability and convergence when dealing with discontinuous interfaces and complex geometries, fully validating the method’s effectiveness and reliability for nonlinear steady-state heat conduction problems. [Conclusion] This study successfully applies NMM to solve two-dimensional nonlinear steady-state heat conduction problems. Through comprehensive comparative analysis and numerical validation, the unique advantages of this method in handling complex engineering thermal problems are highlighted. It provides a novel solution for the numerical simulation of nonlinear heat conduction problems and extends the application scope of NMM in the field of computational thermal physics.

[Objective] Current micromechanical models pay limited attention to parameter uncertainty and interactions, which makes it difficult for their response results to reflect the dispersed nature of the properties of cement-based materials. Therefore, it is necessary to explore an analysis method that can simultaneously capture the effects of multiple parameters and their interactions on the responses of micromechanical models of cement-based materials. [Methods] To address the discrete distribution of response results in existing micromechanical models and to identify and control the influencing factors causing this phenomenon, a multi-scale micromechanical model of cement-based materials was constructed in this study. Cement-based materials were divided into four scales: calcium silicate hydrate gel, cement paste, cement mortar, and concrete. Considering the mineral composition of cement phases, aggregates, and the ITZ, a multi-scale micromechanical model capable of accounting for the randomness of input parameters was proposed. Meanwhile, probabilistic methods were applied to the constructed micromechanical model, and global sensitivity analysis was employed to quantify the effects of input parameter uncertainty on the elastic modulus of cement-based materials. [Results] The results showed that the proposed model exhibited good applicability in simulating the relationship between elastic modulus and hydration degree of cement-based materials across multiple scales and showed good agreement with experimental results. The discreteness of the model response results mainly originated from the cross-scale propagation of input parameter uncertainty, indicating that uncertainty at the concrete scale incorporated the uncertainties of input parameters at the mortar and cement paste scales. The total-order sensitivity indices, ranked from largest to smallest, were the elastic modulus of sand and coarse aggregates, the volume fraction of sand and coarse aggregates, the elastic modulus of hydration products, the volume fraction of cement clinker, and the elastic modulus of cement clinker. To identify the dominant sources of uncertainty within the model framework, particular attention should be paid to the elastic modulus of sand and coarse aggregates, whereas the volume fraction and elastic modulus of cement clinker can be regarded as insensitive factors. [Conclusion] Screening the number of input parameters has important practical significance for reducing computational complexity and improving the efficiency of model response analysis.

[Objective] This study aims to reveal the exothermic properties and hydration mechanism of low-heat Portland cement (LHC) (hereafter referred to as low-heat cement) mixed with fly ash, the composition of pore solution during hydration, and the evolution mechanism of hydration products. [Methods] Long-term exothermic tests were conducted on the cementitious materials, and a quantitative relationship between heat release and hydration progress was established, from which the hydration state was obtained. Based on the theory of Gibbs free energy minimization using GEMS software, thermodynamic calculations were performed to construct a cement hydration model, whose validity was verified through comparative analysis between simulated and measured Ca(OH)₂ contents in neat paste. [Results] (1) The exotherm from low-heat cement hydration was mainly concentrated within 28 days. Fly ash significantly reduced the hydration heat of low-heat cement: a 20%-50% replacement ratio resulting in a reduction of 14.6%-32.7% in total exotherm after 720 days of hydration. Predictive models for exotherm and degree of hydration of the cementitious system were established. At hydration stabilization, the degree of hydration of low-heat cement reached 86.0%, and the reaction degree of fly ash was 53.3%. (2) The Ca(OH)₂ content calculated by the cement hydration model showed small deviation from measured values, indicating that the developed model adequately characterized the hydration process of low-heat cement. The cement was dominated by dicalcium silicate (C₂S). Compared with ordinary and medium-heat Portland cements, it produced more calcium silicate hydrate (C-S-H) gel and less Ca(OH)₂, which explained its higher later-age strength. (3) Different types of C-S-H were found to exhibit distinct saturation indices, ranked from highest to lowest as C_1.5S_0.67H_2.5, C_0.83S_0.67H_1.83, C_1.33SH_2.17, and C_0.67SH_1.5 (the last being unstable). All types showed linear positive correlations with the saturation index of Ca(OH)_2, as well as with OH^- and silicon ion concentrations, and a negative correlation with calcium ion concentration, with the relevant relationships established. (4) After fly ash incorporation (0-25%), the ettringite (AFt) content in the cementitious system gradually decreased to zero, while the monosulfate (AFm) content continuously increased. (5) At low fly ash dosages (0-20%), Si phases in fly ash first reacted with Ca(OH)₂ to form C-S-H with a higher Ca/Si ratio, increasing the average from 1.61 to 1.63. At higher dosages (20%-50%), C-S-H content decreased by 16.5%, and both Ca/Si ratio and pH declined markedly, primarily due to exhaustion of CH, reduced Ca phase, and increased Si and Al phases. At very high dosages (65%-80%), severe deficiency of Ca phase along with excess Si, Al, and Fe phases and lower pH caused extensive dissolution of C-S-H and a reduction in its Ca/Si ratio. (6) A relationship among pH, solution composition, and product saturation indices was established. At pH=12.86, the saturation indices of the above products were highly similar. When pH≤12.86, significant changes in the stable states of the products occurred. [Conclusion] Incorporation of fly ash further reduces the exotherm of low-heat cement, and heat release is significantly positively correlated with hydration progress. At low dosages, both the content and properties of C-S-H gel are improved, which benefits macroscopic mechanical performance and durability. pH 12.86 appears to be the pH inflection point for the stable state of hydration products, which should be considered when designing low-heat cement concrete with high fly ash dosages.

[Objective] This study aims to optimize the coupling conditions for CO₂ reinforcement of recycled coarse aggregate (RCA) by investigating the interactions of three key factors—CO₂ concentration, carbonation temperature, and relative humidity—using response surface methodology (RSM). The innovation lies in using a systematic RSM-based approach to model and optimize the carbonation process, overcoming the limitations of traditional methods by capturing complex inter-factor interactions. This provides a more efficient and reliable framework for enhancing RCA performance in sustainable construction applications. [Methods] A Box-Behnken design using Design-Expert software was applied, encompassing 17 sets of carbonation tests to evaluate the effects of CO₂ concentration (20%-60%), carbonation temperature (20-60℃), and relative humidity (35%-65%). RCA was derived from waste concrete blocks in Baotou, China, and characterized in accordance with GB/T 25177-2010, with particle sizes between 4.75 and 31.5 mm. The measured responses included crush value (indicator of mechanical strength), water absorption (indicator of porosity), and apparent density (indicator of compactness). Carbonation experiments were performed in a controlled environment, and the obtained data were utilized to develop quadratic regression models using RSM. Analysis of variance (ANOVA) was conducted to assess the significance, reliability, and interactions of the models, using evaluation criteria including F-statistic, p-value, coefficient of determination (R²), adjusted R², predicted R², coefficient of variation (C_V), and signal-to-noise ratio (Adeq Precision). Optimization was performed using the numerical module of Design-Expert to determine the optimal carbonation conditions, which were validated experimentally to confirm model accuracy. [Results] The interaction between carbonation temperature and relative humidity had the strongest effect (p>0.05 for BC interaction), followed by the CO₂ concentration-temperature (AB) and CO₂ concentration-relative humidity (AC) interactions. The CO₂ concentration-temperature (AB) interaction was the most significant, resulting in a parabolic response. Water absorption initially decreased with increasing CO₂ concentration and temperature, but increased under extreme conditions due to reduced CO₂ diffusion and calcium ion dissolution. The CO₂ concentration-relative humidity (AC) interaction was the most significant, making apparent density peak under moderate conditions (e.g., 42% CO₂ concentration and 44 ℃) and decline at extremes due to moisture-induced calcium loss or CO₂ saturation. The optimization process determined the optimal carbonation conditions as 38% CO₂ concentration, 41 ℃ carbonation temperature, and 49% relative humidity. Under these conditions, the predicted values were 14.3% for crush value, 3.80% for water absorption, and 2 700 kg/m³ for apparent density. Experimental validation produced measured values of 14.6% (crush value), 3.85% (water absorption), and 2 702 kg/m³ (apparent density), with relative errors of 2.1%, 1.3%, and 0.1%, respectively. All relative errors were below 5%, confirming model accuracy. Compared with untreated RCA, the optimized carbonation treatment reduced crush value by 18.0%, decreased water absorption by 20.5%, and increased apparent density by 0.9%, demonstrating practical effectiveness. Response surface diagrams and contour plots illustrated these interactions. For example, the temperature-relative humidity interaction for crush value showed a steep elliptical contour, while the CO₂ concentration-relative humidity interaction for apparent density presented a flat parabolic surface. These results highlighted the innovation of applying RSM to decipher complex multi-factor couplings, which previous studies did not fully address. [Conclusion] This study successfully develops and validates RSM-based regression models for optimizing the CO₂ reinforcement of RCA, with high reliability and precision confirmed by statistical indicators and experimental validation. The optimal conditions effectively improve RCA properties and provide a sustainable solution for waste concrete recycling and carbon emission reduction. The developed model offers a reliable reference for industrial applications, facilitating the adoption of CO₂ -modified RCA in concrete production. Future research can apply this approach to other aggregate types or larger-scale scenarios, further advancing a circular economy in the construction industry.

[Objective] This study aims to optimize the two-dimensional adaptive analysis strategy of the independent cover-based manifold method, focusing on addressing its deficiencies in error control and mesh distribution, thereby significantly enhancing computational efficiency and engineering practicality. [Methods] Based on the arbitrarily shaped and connected cover meshes of the independent cover-based manifold method, a “split-one-into-two” mesh splitting algorithm was employed for arbitrary refinement, and the degree of continuity of physical field derivatives was adopted as the error control indicator, forming an adaptive analysis strategy. An optimization scheme was proposed. 1) Adopting an absolute error indicator to replace the relative error indicator: the original relative error indicator tended to cause over-refinement in regions of minor stress and was overly sensitive in concave corner singularity regions. Using the absolute error indicator not only simplified the error judgment logic but also permitted larger error thresholds to be set near singular points such as concave corners, thereby effectively avoiding over-refinement. 2) Introducing a local mesh pre-partitioning and short strip elimination strategy: to address the issue of excessively high mesh density and irregular distribution in concave corner regions, a local pre-partitioning strategy was proposed, which pre-set the initial mesh in these regions by inwardly offsetting and reversely extending the edges of the concave corner. Simultaneously, an adjacent point merging algorithm was introduced during the mesh splitting process, which avoided the generation of extremely short connection strips and improved the conditioning of the system equations. [Results] Verification through two typical hydraulic structure examples, the square-hole and the gravity-dam model, demonstrated that the optimized scheme achieved a breakthrough improvement in computational efficiency. For the square-hole example, the original adaptive strategy generated 310 covers, corresponding to 6 520 degrees of freedom (DOFs). Under the same accuracy objective, the optimized scheme required only 59 covers and 933 DOFs. This represented a reduction of approximately 81% in the number of meshes and approximately 86% in DOFs. For the gravity-dam example, the original strategy generated 228 covers and 4 810 DOFs, whereas the optimized scheme required only 106 covers and 2 354 DOFs, achieving significant results of over 53% reduction in the number of meshes and 51% reduction in DOFs. The most notable achievement of the optimized scheme was in the effective suppression of mesh over-refinement near concave corner singularity regions. The calculation results demonstrated that the new strategy could generate more reasonable meshes, while ensuring computational accuracy, it substantially reduced the computational scale, and greatly enhanced the computational efficiency. [Conclusion] The proposed optimization strategy significantly enhances the efficiency of adaptive analysis while maintaining high accuracy. Through absolute error control and local mesh pre-partitioning, it effectively solves the problems of mesh over-refinement and unreasonable distribution near concave corner singularities, laying a foundation for subsequent three-dimensional adaptive analysis and engineering applications. Future research includes: criteria for selecting error thresholds and the highest order of cover series; further automating the local mesh pre-partitioning process to enable it to handle more complex geometries, ultimately achieving the goal of efficient and fully automatic simulation analysis for hydraulic structures.