Evaluation of the Crystallization Pressure of Sulfate Saline Soil Solution by Direct Observation of Crystallization Behavior

We observed the growth of salt crystals in sulfate saline soil solution during evaporation at constant relative humidity and temperature and studied the formation mechanism of soil deformation induced by salt crystallization. The growth of salt crystals is recorded by images using a CCD camera under an optical microscope, and the solution supersaturation and crystallization pressure are calculated taking advantage of digital image processing. The growth of sodium sulfate multilayer crystals is observed conforming to the Kossel model. Moreover, it is estimated that the maximum growth rate in the longitudinal direction is almost ten times that in the lateral direction in large pore contribution to the nucleation barrier during crystal formation. The crystals act on the liquid film pushing away soil particles, achieving the "self-cleaning" effect finally. The liquid film shows elastic deformation property in a short time during crystal growth, demonstrating that crystallization pressure is exerted by the liquid film. During mirabilite crystal growth, the crystallization pressure values fluctuate within 0-12.57 MPa because the supersaturation of the film is consumed, destroying pores in sulfate saline soil and eventually expressed by salt expansion.