Effects of grain angularity on NaCl precipitation in porous media during evaporation

Three dimensional pore-scale analysis was carried out using X-ray microtomography to investigate the effects of grain angularity on NaCl precipitation dynamics and patterns during evaporation from saline porous media. To do so, quartz sand and glass beads with almost similar average particle size and porosity were used enabling us to constrain the effects of grain angularity on NaCl precipitation since the glass beads were spherical and smooth whereas the sand consisted of irregularly shaped grains. Presence of angularity resulted in different pore sizes and shapes influencing the dynamics of evaporation and NaCl precipitation. Our results demonstrate that the preferential evaporation exclusively in fine pores at the surface of porous media results in discrete efflorescence. We observed a higher cumulative NaCl precipitation in the case of glass beads at the early stages of precipitation due to the presence of a fewer number of evaporation sites at the surface. This phenomenon resulted in formation of a thicker and more discrete NaCl crust at the surface of glass beads compared to sand grains. Also, computed water saturation profiles reveals formation of a wider unsaturated zone above the receding drying front in the case of sand compared to glass beads due to the presence of finer pores affecting the capillary flow though the partially wet zone. Our results provide new insights regarding the effects of grain angularity on NaCl precipitation dynamics and patterns during evaporation from saline porous media.