Revealing the freezing-thawing hysteretic mechanisms of soil–water system based on soil microstructure

The soil freezing-thawing characteristic curve (FTCC) can reflect the physical and mechanical properties of soil–water system during freezing-thawing (FT) process, which is of guiding significance to the study of soil moisture, heat and matter transport in cold regions. In this study, firstly, according to the evolution law of freezing-thawing hysteresis with freezing-thawing process, revealing the hysteresis mechanisms at different stages based on ice-water transformation theory. The freezing-thawing hysteresis can be divided into four stages as temperature decreasing. The hysteresis of the first three stages are due to nucleation and electrolyte effects, capillarity and pore clogging effects, structural damage effect, respectively; and the last stage is extremely weak and can be ignored. Secondly, evaluating freezing-thawing curves of soil–water system with three pore structures (cylindrical, spherical, and sphere-cylinder binary pore) based on the thermodynamic theory, quantitatively. The upper and lower boundaries of the freezing/thawing characteristic curve with natural pores are those with idealized cylindrical and spherical pores, respectively. Finally, the evaluation index (i.e., hysteresis degree) was introduced to quantitatively describe the variation of unfrozen water hysteresis degree with freezing-thawing process. The relationship between the unfrozen water hysteresis degree and temperature can be divided into four stages. The maximum hysteresis degree was found in the second stage, indicating that hysteresis was most significant in the second stage, followed by the first, third, and fourth stages. Our results provide theoretical support for studying hydrothermal characteristics and water, heat, and solute transport of geotechnical materials in seasonally frozen regions.