Effect of salt types on salt precipitation and water transport in saline sandy soil

Abstract Salt precipitation on the surface of porous media significantly affects water transport processes. Most studies on salt precipitation mainly focused on single salts, but in nature, salt precipitation usually occurs as mixtures. Consequently, information on the crystallization of salt mixtures and its effect on water transport remains scarce. This study investigated the precipitation of mixtures (the mass ratios of NaCl:Na2SO4 were 3:7, 5:5, and 7:3, respectively) of NaCl (typical efflorescence) and Na2SO4 (typical subflorescence) in the initially saturated sandy soil columns and its effect on evaporation and compared it with the cases of the two salts individually. The results showed that salt mixtures exhibited a mixed pattern of crystals including both efflorescence and subflorescence, and the efflorescence showed granular aggregation, unlike the mono‐salts. The crystallization coverage of the salt mixtures was smaller than that of NaCl mono‐salt; high (7:3) and low (5:5 and 3:7) proportions of NaCl led to larger and smaller crystallization coverage than that of Na2SO4 mono‐salt, respectively. While the salt mixtures had less crystallization coverage than the mono‐salts, they showed lower evaporation because the salt mixtures formed a denser crystallization structure of efflorescence‐subflorescence‐soil layer, this crystallization structure exhibited greater inhibition of water vapour diffusion, thus reducing evaporation. In addition, the crystallization of the salt mixtures with higher NaCl proportion afforded greater resistance of evaporation. The mixed crystallization pattern formed by the salt mixtures significantly enhances the crystallization resistance to evaporation.