The effect of salt on the freezing temperature of saline soil

Abstract

The freezing temperature jumping phenomenon in saline soils caused the freezing temperature activity model to fail. In this study, the freezing temperatures and the rapid freezing water change Δw_ff of three typical saline soils (NaCl, Na₂SO₄, and Na₂CO₃ saline soils) were measured under different water and salt contents using temperature change curves during cooling. The effects and mechanisms of water content, salt content, and salt type on the freezing temperatures of saline soils were analyzed from three aspects: water activity of pore solutions, salt precipitation, and the rapid freezing water change Δw_ff. The results indicated that the mechanism by which water and salt content affected the freezing temperature of soil was through changes in the water activity of the pore solution. For salt solutions with solubility insensitive to temperature changes, the activity model adequately described the influence of water activity on the solution’s freezing temperature. For salt solutions with solubility highly sensitive to temperature changes, the activity model was applicable when the salt concentration was less than or equal to the jump characteristic concentration. When the salt concentration exceeded the jump characteristic concentration, the freezing temperature jump phenomenon occurred, rendering the activity model inapplicable. Solution salt precipitation caused the freezing temperature jump phenomenon. For saline soils with salt solubility significantly affected by temperature, salt precipitation should be considered when calculating freezing temperatures. The soil’s freezing temperature, as measured by the freezing temperature curve, was influenced by the rapid freezing water change Δw_ff. When the rapid freezing water change Δw_ff was greater than or equal to the critical change in frozen water content Δw_crf, the soil’s freezing temperature equaled the equilibrium temperature of the pore solution. When the rapid freezing water change Δw_ff was less than the critical change in frozen water content Δw_crf, the freezing temperature gradually decreased from the equilibrium temperature of the pore solution to the supercooling temperature as the rapid freezing water change Δw_ff decreased.