关键词: 盐渍土, 相变温度, 三元溶液, 盐胀, 冻胀

Abstract:

Saline soil is widely distributed in cold and arid regions of Northwest China. Detrimental frost heave and salt expansion occur in these areas due to harsh natural conditions， and then cause great damage to the stability of the engineering structures. Salt expansion and frost heave are caused by salt crystallization and ice formation， respectively； then the phase transition temperatures become very important to judge the water freezing-thawing and salt crystallization-dissolution in saline soil. Multiple ions exist in the pore solution of natural saline soil， but the phase transition mechanism of the multi-component pore solution in saline soil is studied less. In order to explore the water and salt phase transition mechanism of multi-component pore solution in the freezing process， remolded loess on Datong basin （in Shanxi Province） was taken as research object， and two kinds of salts （sodium chloride and sodium sulfate） were chosen； by changing the mass ratio of sodium chloride and sodium sulfate under different total salt contents， the phase transition temperatures of ternary pore solution during cooling were investigated. In the cooling tests， the temperature was controlled by cold bath （TMS8035-R40， precision： ±0.01 ℃）. Considering the salt solubility， the cold bath was first set to 30 ℃ for a specific period of time to maintain the consistent temperature of each soil samples， and then the cold bath was adjusted to -30 ℃ for cooling the soil samples. The temperatures of soil samples were measured by high-precision temperature sensor （precision： ±0.05 ℃） at intervals of 10 s， and CR300 was used to collect the measured data for later analysis. If there is no phase change occurs in saline soil during the cooling process， the cooling curve is continuous. And temperature mutation appears on the cooling curve， when phase change occurs. The temperature mutation indicates the phase transition of pore solution （ice formation or salt crystallization）， and the latent heat of phase change compensates the heat lost from the system to the environment. By drawing the cooling curves of soil samples， the temperature mutation points are obtained， then the phase transition mechanism are determined based on the phase diagram of aqueous solution. The experimental results showed that the crystallization temperature varies with the mass ratio of sodium chloride and sodium sulfate under the same total salt content. With adding an appropriate amount of sodium chloride， sodium sulfate is easier to crystallize under the common-ion effect of sodium ion. However， the eutectic temperature of ice and mirabilite in soil decreases， reducing the proportion of solid phase in pore solution， thus restrain the salt expansion and frost heave of sulfate_{saline soil. When only sodium sulfate exists in saline soil， the eutectic temperature of ice and mirabilite first increases， and then decreases slowly with sodium sulfate increasing. The leading cause of this tendency is the accumulation of ice and hydrated salt before the second phase transition stage. The three-phase eutectic temperature of saline soil increases with the increase of sodium chloride content. This is because the solid phase content in pore solution decreases with the addition of sodium chloride before three-phase eutectic point， which makes a less impact of pore structure. In addition， for saline soil containing sodium chloride and sodium sulfate， water and salt may precipitate in single solid phase， double solid phase and three solid phase state. The diversity of phase transition temperature of different salt content brings great uncertainty for modeling the water and salt migration and deformation of saline soil during cooling. The result of this study will further enrich the related theory of saline soil in cold area， and it is helpful for understanding the phase transition mechanism and properties of saline soil in freezing process.}

Key words: saline soil, phase transition temperature, ternary solution, salt expansion, frost heave