Study of Heat–Mass Transfer and Salt–Frost Expansion Mechanism of Sulfate Saline Soil during the Unidirectional Freezing Process

Abstract The water and salt movement and crystallization deformation of saline soil in cold and arid regions is a complex hydrothermal–salt–mechanical coupling problem. Based on the law of mass conservation, the law of energy conservation, and the theory of permafrost mechanics, the hydrothermal–salt–mechanical coupling mathematical model of unsaturated sodium sulfate saline soil is established. The model takes into account the latent heat of phase transition, crystallization impedance, consumption of crystallization, ice crystal self-cleaning behavior, and temperature as mechanical parameters. Numerical simulations using COMSOL Multiphysics (version 5.5) software were carried out, and the outcomes were analyzed and compared with unidirectional freezing test data. The findings show that the coupled model accurately simulates heat–mass transfer, crystallization, and salt freeze–thaw deformation in unsaturated saline soil. In the unidirectional freezing process, the temperature, salt freezing deformation, and freezing depth within the saline soil showed a three-stage rule of change, and the migration of water and salt to the freezing front made the water and salt content in the freezing zone increase significantly and form a laminar distribution, and the peak of the water and salt content appeared at the freezing front. The migration of water and salt aids in forming ice and salt crystals that rapidly grow within the soil pores of the freezing zone, leading to salt freeze–thaw deformation. Furthermore, the models and results of this study offer crucial insights into the mechanisms of soil salinization, desertification, and salt freeze–thaw deformation in cold and arid regions.