Effects of salt precipitation during evaporation on porosity and permeability of porous media

Abstract

Salt precipitation from evaporation is a key factor for soil degradation due to salinization in arid and semi-arid regions. Evaporation-induced water movement transports dissolved salt ions to the surface of the porous medium where they accumulate. When the solubility limit is reached, salt starts to precipitate and forms crusts on top (efflorescence) or inside (subflorescence) of the porous medium depending on the type of salt ions in solution. For the development of strategies to treat salt-affected porous media and to prevent precipitation, it is important to understand the formation and the impact of the salt crusts on water flow during evaporation. In this context, it is not known how the hydraulic properties of the porous medium such as porosity, permeability, and water retention affect evaporation and crust formation in detail. Thus, the overall aim of this thesis was to improve understanding of the impact of salt crusts on evaporation with a focus on the porosity, the intrinsic permeability, and the spatial and temporal development of salt crusts. To reach this aim, a novel gas permeameter set-up was developed that allows the permeability determination of the salt crusts from different types of salt. Further, the porosity and the spatial and temporal development of the magnesium sulfate (MgSO4) crusts in sand and sintered glass was analyzed using X-ray computed tomography (XRCT) and nuclear magnetic resonance (NMR). The results showed that efflorescent sodium chloride crusts reduce evaporation by forming a barrier, of which the permeability is mostly independent of the properties of the porous medium. In contrast, it was found that the permeability and the formation of subflorescent MgSO4 crusts strongly depend on the (surface) properties of the porous medium. Further, non-invasive imaging showed that unconsolidated porous media deform due to subflorescent salt precipitation, which needs to be considered to understand the impact of subflorescent salt crusts on evaporation.