Evaporation-induced salt crystallization and feedback on hydrological functions in porous media with different grain morphologies

Abstract

Evaporation from porous media in the presence of dissolved salts is an important topic in relation to several environmental issues. In this study, we investigated the interaction of evaporation, salt crystallization and the internal structure of the porous material. Porous columns with different internal structures were generated through random packing of angular quartz sand and round glass beads. Evaporation-induced salt efflorescence on the surface of porous media was recorded by a camera with micro lens. The internal pore structure before and after salt crystallization was visualized using the x-ray computed tomography and quantified by the extracted pore networks. Hydraulic properties in the absence/presence of salt crystals were simulated numerically using the pore-network approach and partially measured experimentally. The evaporative water loss ratio in the angular quartz sand was greater than that in the glass beads with the same sieving size. The evaporation of saline water in angular quartz sand and round glass beads can be separated into three stages similar to water evaporation. Salt efflorescence was generated a crust dome architecture on the surface of the angular sand, but was in tight patchy contact with the round glass beads. The presence of salt crust interrupted the exchange of vapor between the porous media and atmosphere, hindering evaporation. Salt subflorescence clogged some of the pores, and the pore size distribution shifted to smaller sizes part. The porosity decreased and the geometric tortuosity of the porous materials increased owing to salt subflorescence. The presence of subflorescence salt crystals decreased the intrinsic permeability and relative diffusion coefficient of different porous media. The relationship between the vapor diffusion coefficients and porosity is linear in the presence/absence of salt crystallization, considering the geometric tortuosity. The findings indicate that the feedback of salt precipitation on hydraulic properties and vapor diffusion must be considered to accurately quantify hydrological processes, particularly in arid and semi-arid areas where intensive evaporation is accompanied by soil salinization and water shortage.