Experiment study and prediction of freezing temperature of NaHCO3 saline soil

The freezing temperature of soil is the key temperature at which the soil properties change significantly. Due to the addition of salt, the freezing temperature of saline soil becomes complicated, and its prediction becomes difficult. Taking NaHCO3 saline soil as the research object, the freezing temperature of saline soil with different NaHCO3 content was measured. Considering the influence of the solute in pore solution on water activity and the curvature effect of water-ice phase transition interface, a theoretical model for predicting the freezing temper-ature of NaHCO3 saline soil was established. The Pitzer ion model was used to determine the water activity of the pore solution. The pore data of the sample obtained from the mercury intrusion test were used to quantify the curvature effect. Then the established model was applied to calculate the freezing temperatures of the samples. The results show that when the salt concentration is less than the eutectic concentration of NaHCO3, the freezing temperature of the sample decreases slowly with the increase of salt concentration; when the salt concentration exceeds the eutectic concentration, the freezing temperature of the sample first decreases sharply, then gradually becomes stable, and there is a great difference between the measured and predicted value. It is considered that when salt concentration exceeds the eutectic concentration, with the decrease of temperature, small salt crystals precipitate. Because the interfacial curvature effect can change the equilibrium concentration of the solution, the saturated concentration of solution in the soil pores increases, enhancing the influence of solute in the pore solution on freezing temperature. The prediction model was modified from the perspective of solute effect, and it shows a good effect in predicting the freezing temperature of NaHCO3 saline soil when the salt concentration of the soil is greater than the eutectic concentration.