Detrimental frost heave and salt expansion may occur in saline soils of cold regions. The severity of these processes depends on the amount of salt precipitation and ice crystal growth in the pore solution at different temperatures. By employing nuclear magnetic resonance, the liquid water fractions in different concentrations of sodium chloride and sodium sulfate saline soils were measured at different temperatures. Based on the theory of solution phase diagram and phase change in porous media, the differences and similarities of phase change between pore solution and free solution were analyzed. The phase change sequence and effects of salt on frozen soils are also discussed. The theoretical and experimental results confirm that the phase diagram can reflect the phase change of pore solutions roughly, and pore size distribution is the cause of differences between the pore solution and free solution. Sodium chloride can mitigate the frost heave of soil, and frost heave and salt expansion occur together when the sodium sulfate soil is frozen.
The liquid water fraction is one of the most important parameters in the study of saline soils of cold regions, and has great influence on water migration. The freezing temperature and saturated concentration of pore solutions have been investigated according to thermodynamic theory, indicating that pore radius, temperature, and salt concentration are the three main influencing factors. Here, a method for calculating the liquid water fraction of saline soil is proposed, in which the liquid water fraction of saline soil can be calculated for different water contents and initial salt concentrations according to the pore size distribution. Comparison of the experimental data with the results of the model shows that the model provides reasonable calculations for the unfrozen water content of saline frozen soils, and both the first and the second phase transition can be modeled well. Considering the solubility of salt, the amount of salt crystallization and ice formation during cooling can also be obtained. The proposed method can be used to distinguish the proportion of salt expansion and frost heave, and also provide a novel approach for calculating the matric suction of saline soil during the freezing process.
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