Effect of freeze-thaw on freezing point of a saline loess

Assessment of frozen range in seasonal frozen ground necessitates a rational determination of soil freezing point and particular concerns should be given to water, salt and freeze-thaw cycles. As revealed by the cooling curves for saline loess after preset freeze-thaw cycles, the conventional equilibrium freezing stage for determining the freezing point can be more precisely located based on the cooling rate of 0 degrees C/min. The equilibrium freezing duration varied insignificantly when the water content was lower than 17.0% or salt content lower than 1.0%, beyond which it positively increased. A narrow range of 2.0-4.0 min was noted with freeze-thaw cycles. Moreover, the freezing point grows positively with water content and approaches to that of pure water at standard atmospheric pressure. An approximately linear relationship between freezing point and salt content was noticed and the slope of the curve obviously decreases at higher water contents. The Clausius-Clapeyron equation, when used to estimate the freezing point of saline loess, is valid at salt contents lower than 0.5%. Otherwise, a maximum deviation of 4.09 degrees C will occur at a salt content of 5.0%. By comparing the indexes quantified from images for both surface crack and internal structure after freeze-thaw cycles, the freezing point exhibits stronger dependency on pore area ratio. Considering the coupled effect of water, salt and freeze-thaw, an empirical equation for estimating freezing point of saline loess was proposed with four parameters required to be determined. Comparisons of measured and calculated data prove that the deviation for most of data points ranges from - 0.03 to 0.05 degrees C for the tested saline loess, while it varies from - 0.19 to 0.15 degrees C for silty clay with calcium chloride and - 0.16 - 0.21 degrees C for Lanzhou loess with sodium sulfate.