Estimating the freezing-thawing hysteresis of chloride saline soils based on the phase transition theory

Knowledge of the freezing-thawing feature and hysteretic phenomenon is required to investigate frost heave, thaw collapse, and thermo-hydro transfer in soil-water systems in seasonally frozen regions. The freezing and melting points are important indexes to judge whether a soil-water system is in a frozen state or not, which are not constant but vary over a range and are determined by soil texture, water, salt content and etc. In this paper, a series of experimental studies have been carried out to test the freezing and melting points of three types of soils, i.e., sand, silt and silty clay, with various NaCl salt contents. Then, the theoretical models for estimating the freezing and melting points of the saline soils are proposed based on the Pitzer ion model and thermodynamic theories. In order to evaluate the theoretical models, the comparison between the predicted results by these models and the experimental results is made, and the agreement between them is seen to be good. In addition, a hysteretic coefficient is proposed to quantitatively describe the hysteretic phenomenon, which is affected by the pore size and water activity. And then, possible mechanisms are discussed for the hysteresis of phase transition temperatures and volumetric unfrozen water contents. Various pore shapes are assumed to explore the effect of pore geometry on the hysteretic phenomenon, and it can be found that soil-water systems have the smallest phase transition point depressions with needlelike pores and the largest with discoid pores. As a preliminary study, the research results may provide a basis for future investigations of the freezing-thawing process of soil-water systems in seasonally frozen regions.