Experimental study on pore water phase transition in saline soils

Abstract

Sharp variations of thermophysical properties during the freezing process are caused by phase change of water and salt, which necessitates examining the water and salt transfer as well as deformation of saline soils. Meanwhile, the study on phase change in saline soils can provide the insight conditions for numerical heat-moisture coupling simulation. In order to investigate the principle of phase transformation and crystallization in saline soil with Na2SO4 and NaCl under freezing and freezing-thawing cycles, heat curves of saline soils with different salt concentrations were measured by differential scanning calorimetry (DSC). The temperature of phase transition and supercooling were obtained from the curves. According to the conservation of heat and mass, the proportions of salt crystal, ice crystal and unfrozen water produced during the phase transition were calculated, respectively. The precipitation sequence of salt crystal and ice crystal in saline soils was investigated, and the effect of NaCl on the phase transition of Na2SO4 was analyzed based on the phase change diagram of the salt solution. Finally, the effect of salt type and their concentration on soil structure was analyzed under freezing-thawing cycles, combining with soils' scanning electron microscope (SEM). The results indicated that the rules of supercooling for the soil with NaCl or mixture of Na2SO4 and NaCl were the same, whereby the supercooling diminished as the concentration or soil volume increased. Further, the addition of NaCl can significantly reduce the temperature of phase transition and ice content in sulfate saline soil, while Na2SO4 had little effect on the temperature of soil phase transition. Ice firstly generated in a small portion in soil and salt crystallized with the phase transition occurred for the most of water, and the curve of unfrozen water content in soils shifted toward low-temperature direction as the salt content increased. Phase transition sequence of water and salt will be changed for high salty saline soils under freezing-thawing cycles which has little effect on the precipitation amount of salt crystal in saline soils.