Phase transition of pore solution in saline soil during cooling process
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摘要: 降温过程中,盐渍土中孔隙溶液的相变过程直接影响着土体的物理力学性质。以大同盆地的盐渍土为研究对象,研究了不同类型盐渍土的孔隙溶液在降温过程中的相变过程。结果表明NaCl可以显著降低土体的冻结温度,NaCl盐渍土的冻结温度与溶液的冻结温度相比有一定的偏差,且随含盐量的增加,这种偏差逐渐增大。当浓度超过二次相变点浓度后,土体的冻结温度保持不变,冻结温度的偏差达到最大。NaCl盐渍土二次相变温度受冰结晶量的影响显著,冰晶含量越多,二次相变的温度越低。Na2SO4和Na2CO3对冻结温度的影响较小,当浓度低于二次相变点的浓度时,土体的冻结温度主要受盐分浓度的影响；若浓度高于二次相变浓度,盐分结晶量使得土体二次相变的温度进一步降低。受孔隙半径的影响,土体孔隙溶液中盐分的溶解度比一般溶液中的更高。通过比较相图与试验结果,发现土体的相变规律和溶液的相变规律有一定的相似性,盐渍土孔隙溶液二次相变过程为盐晶体和冰晶体共同生成的过程。在较高含盐量的情况下,Na2SO4盐渍土和Na2CO3盐渍土的冻结温度点实际上是土体的二次相变点的温度。Abstract: The phase transition of pore solution directly affects the physical and mechanical properties of soil during the cooling process. Taking the saline soil of Datong Basin as the research object, the phase transition processes of different saline soils are investigated. The results show that sodium chloride can significantly reduce the freezing temperature of soil. There is a certain deviation between the freezing temperatures of soil and those of free solution, which increase gradually with the increase of salt content. When the concentration exceeds the eutectic point, the freezing temperature of soil remains unchanged, and the deviation of freezing temperature reaches the maximum. The second transition temperature of sodium chloride saline soil is affected by the amount of ice formation. The more the ice crystal content is, the lower the secondary transition temperature is. The sodium sulfate and sodium carbonate have little effect on the freezing temperature. When the concentration is lower than that of the eutectic point, the freezing temperature of saline soil is mainly affected by the salt concentration. If the concentration is higher than that of the eutectic point, the salt crystallization makes the eutectic point temperature of soil further reduce. Under the influence of pore radius, the solubility of salt in pore solution is higher than that in general solution. By comparing the experimental results with the phase diagram, there is a certain similarity between soil and solution. It can be concluded that the secondary phase transition process in saline soil is the formation of salt crystal and ice crystal simultaneously. In the case of high salt content, the freezing temperature of sodium sulfate and sodium carbonate saline soil is actually the eutectic point temperature of the soil.
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图 3 NaCl盐渍土在降温过程中的温度变化曲线
Figure 3. Variation curves of temperature of NaCl saline soil during cooling process
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图 4 NaCl盐渍土中孔隙溶液相变温度与相图的对比
Figure 4. Comparison of phase transition temperature and phase diagram of pore solution in NaCl saline soil
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图 5 Na2SO4盐渍土在降温过程中的温度变化曲线
Figure 5. Variation curves of temperature of Na2SO4 saline soil during cooling process
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图 6 Na2SO4盐渍土中孔隙溶液相变温度与相图的对比
Figure 6. Comparison of phase transition temperature and phase diagram of pore solution in Na2SO4 saline soil
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图 7 Na2CO3盐渍土在降温过程中的温度变化曲线
Figure 7. Variation curves of temperature of Na2CO3 saline soil during cooling process
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图 8 Na2CO3盐渍土中孔隙溶液相变温度与相图的对比
Figure 8. Comparison of phase transition temperature and phase diagram of pore solution in Na2CO3 saline soil
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