Xusheng Wan

The soil above the water table in the shallow layers of the Earth is typically unsaturated, and understanding the complicated multiphase contact modes and interactions in unsaturated is crucial for accurately simulating multi‐physical field coupling and predicting foundation deformation in frozen soil region. In this study, a theoretical model for unfrozen water content in unsaturated soils by integrating the generalized interface premelting theory with Gibbs free energy minimization. The model resolves the phase transition processes at the interfaces of air‐water‐ice‐soil by determining air‐phase distribution within pore spaces and incorporating interfacial melting effects at air‐ice and soil‐ice boundaries. Additionally, we incorporated the concept of effective water content to improve calculation accuracy, and the model was validated using experimental data. The results indicate that the equilibrium contact angle between air and soil particles is primarily influenced by air saturation and is independent of soil pore size. As air saturation increases, the equilibrium contact angle and the radius of the ice‐water curvature interface enlarges, the melting effect between soil particles and ice crystals diminishes, and the unfrozen water content decreases. In addition, soils with smaller pore sizes and lower saturation exhibit higher surface free energy, which inhibits ice nucleation. Consequently, the soil freezing characteristic curve (SFCC) of silty clay demonstrates significant sensitivity to air saturation, unlike coarse‐grained soils (e.g., silt, sand), where air saturation minimally influences the SFCC. These results advance predictive frameworks for hydrothermal behavior in unsaturated frozen soils, particularly in fine‐grained geomaterials.

Many engineering problems are caused by the destruction of cement-based materials through the combination of salt mixtures and freezing/thawing. To improve the performance of cement-based materials under harsh environmental conditions, mortar was selected as a research object, and its mechanical and phase transformation characteristics were investigated by adding nano-CaCO₃, silica fume, epoxy resin, and mixtures of these components. Laboratory tests were conducted on salt mixtures and freeze/thaw cycles, and the compressive strength and mass loss rate of the mortar recorded before and after freezing/thawing were compared. Subsequently, the effects of the type and amount of additive on the compressive strength and mass loss rate of the mortar were analyzed. The microstructural characteristics of the modified mortar were investigated using scanning electron microscopy, and the mechanisms of the physical and mechanical properties of the mortar modified by additives were revealed. Furthermore, the phase transition properties of water and salt in the modified mortar were studied using differential scanning calorimetry, and the crystallization amount, unfrozen water content, supersaturation ratio, and crystallization pressure in the mortar were calculated on the basis of heat balance and mass balance. The results indicate that during the cooling process, the solution in the internal pores of the mortar undergoes a crystallization phase transition, and the maximum crystallization pressure can reach a value of 30.1 MPa. The additive forms a compact structure inside the mortar to prevent harmful ions from infiltrating the mortar, thereby mitigating the damage caused to the mortar. The apparent deterioration reduced and the compressive strength of the modified mortar improved after several salt and freeze/thaw cycles, and an improvement rate of 15% for the epoxy resin was the most prominent result. The variation in temperature in this study was defined to model cold regions in northwest China and there are similar saline soils in cold regions in other parts of the world, such as the Mediterranean Basin, California, Southeast Asia, Arctic coast, and Central Siberia (Hivon and Sego 1993; Serrano and Gaxiola 1994; Bohren and Albrecht 1998; Shaterian et al. 2005). Therefore, cement-based materials worldwide suffer damage from salt erosion and freeze/thaw cycles. Although there are differences in temperature and salt concentration in different regions of the world, this research has practical reference value for improving the quality of mortar. For the additives considered in this study, all mass loss ratios of the improved mortar significantly reduced. In soil containing sodium chloride, a mixture of silica fume and epoxy resin is suggested to improve the durability of mortar based on reduced crystallization inside pores. In soil containing sodium sulfate, adding a mixture of na-CaCO₃ and epoxy resin can improve the physical and mechanical properties of mortar by reducing salt and ice crystallization in pores. Although the improvement of mortar under the action of salt and freeze/thaw cycles was investigated in this study and the internal mechanisms of mass loss rate and compressive strength enhancement were discussed, the relationship between freeze/thaw cycles and actual environmental temperature changes has not yet been established, and phase changes in porous materials are closely related to the speed of temperature variation. The manner in which freeze/thaw cycles correspond to an actual environmental cycle and the improvement and phase transformation of mortar under ultralow temperatures (below −20°C) are worth studying in the future. Furthermore, the volume of a specimen has a significant influence on the transition temperature, in which supersaturation and supercooling decrease as the volume of soil increases (Wan et al., 2021). Furthermore, the effects of volume on the law of phase transition in mortar remain to be addressed in future studies.

The variation in unfrozen water content with temperature substantially affects coupled heat and water transport in frozen soil, causing frost heave and thaw settlement owing to the ice and water phase change and influencing soil stability in cold regions. Thus, analyzing the mechanism of water freezing and building a predictive model for the unfrozen water content of soils is paramount. In this study, an analytical model based on equivalent contact angle was developed to predict the unfrozen water content. The relationship between the equivalent contact angle and temperature was obtained based on the assumption that the heterogeneous nucleation rate nonlinearly decreased with temperature. The proposed analytical model was validated using existing unfrozen water content data at various temperatures for a silty clay soil material from the Qinghai–Tibet Plateau, and compared to several existing numerical models which predict unfrozen water content in soil materials. The results revealed a close relationship between the unfrozen water content and equivalent contact angle, and the equivalent contact angle increased as the temperature decreased. Meanwhile, the pore water in the soil first froze when the contact angle was smaller. Moreover, the values predicted by the analytical model for the unfrozen water content agreed well with the experimental results, especially under low‐temperature conditions and during the early stage of water freezing.