The change mechanism and a prediction model of unfrozen water content in sodium chloride soil

The amount of unfrozen water reflects the soil water potential and is of importance to explore the water/salt migration in saline soil. Using low-field NMR technology, we measured the amount of unfrozen water content in sodium chloride soil under several initial salt contents. The results reveal that there are two phase transition processes in sodium chloride soil between 20 to -30 degrees C, the unfrozen water content decreases as ice crystals form in the first phase transition stage, and further decreases as ice and hydrated salt precipitate in the second phase transition stages. Supercooling phenomenon, caused by ice nucleation, delays the ice formation in the freezing process, thus leads to variation of unfrozen water content in soil. Through distribution of the unfrozen pore water in soils, we find that water in large pores is prone to freeze into ice. Considering the real phase transition process of soil, we propose a theoretical model to evaluate the unfrozen water content of sodium chloride soil at a wide temperature range. Not only unfrozen water content is well predicted in the two phase transition stages, but also the change of eutectic temperature can be explained under different initial salt concentrations. Based on this model, we discuss the influence of our parameter alpha and the supersaturation degree on our calculated results. This work provides a theoretical reference for investigating the deformation mechanism of saline frozen soil.