摘要
盐渍土相变温度是判断土体中水分冻结与融化、盐分结晶与溶解的重要参数。不同盐分含量相变温度的差异,给盐渍土在降温过程中的水盐迁移过程及变形规律的模拟带来极大的不确定性。通过降温试验,研究了降温过程中氯盐和硫酸盐综合作用盐渍土中水盐相变温度的变化情况。结果表明:全盐量相同时,盐结晶温度随NaCl和Na_(2)SO_(4)比例的不同而不同。随NaCl的加入,在Na^(+)同离子效应的影响下,Na_(2)SO_(4)更容易结晶,但土体的冰和芒硝共晶点温度下降,使得冰含量显著减少,从而降低了孔隙溶液中固相的产生比例,起到抑制Na_(2)SO_(4)盐渍土盐冻胀变形的作用。当土中只含Na_(2)SO_(4)盐时,随Na_(2)SO_(4)浓度的增加,冰和芒硝共晶点的温度先上升而后缓慢下降,二次相变前冰盐的累积量是导致冰和芒硝共晶点产生这种变化的主要原因。盐渍土三相共晶点温度随NaCl含量的增加呈现上升趋势,这是因为随着NaCl的加入,在发生三相共晶前,孔隙溶液发生相变的固相含量减少,从而使孔隙结构对三相共晶点的影响减小。此外,含有NaCl与Na_(2)SO_(4)双组分的盐渍土,水分和盐分可能以单固相、双固相以及三固相状态析出。研究结果可为深入认识盐渍土的相变规律及物理性质提供理论支撑。
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 stabili⁃ty of the engineering structures.Salt expansion and frost heave are caused by salt crystallization and ice forma⁃tion,respectively;then the phase transition temperatures become very important to judge the water freezingthawing 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 or⁃der to explore the water and salt phase transition mechanism of multi-component pore solution in the freezing pro⁃cess,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 cool⁃ing were investigated.In the cooling tests,the temperature was controlled by cold bath(TMS8035-R40,preci⁃sion:±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 indi⁃cates 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 sam⁃ples,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 var⁃ies 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 so⁃dium 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 so⁃dium 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 hydrat⁃ed 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 sin⁃gle 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.
作者
肖泽岸
朱霖泽
侯振荣
董晓强
XIAO Ze’an;ZHU Linze;HOU Zhenrong;DONG Xiaoqiang(College of Civil Engineering,Taiyuan University of Technology,Taiyuan 030024,China)
出处
《冰川冻土》
CSCD
北大核心
2021年第4期1121-1129,共9页
Journal of Glaciology and Geocryology
基金
国家自然科学基金项目(41801044,51978438)
山西省应用基础研究计划项目(201901D211003)
山西省高等学校科技创新项目(2019L0304)资助。
关键词
盐渍土
相变温度
三元溶液
盐胀
冻胀
saline soil
phase transition temperature
ternary solution
salt expansion
frost heave
