Effects of Salt Precipitation on Evaporation Rate in Porous Media

Salts in porous rocks are destructive agents that may derive from various sources such as capillary rise of groundwater, rock weathering, or atmospheric deposition; which later precipitate within or on the surface of the media. This results in clogged pore structures, and hence affects the vapor flux between the evaporation front, where subsurface evaporation takes place, and the rock surface. It is known that salt precipitation results in lower evaporation rate; however, there is still need to investigate various aspects of these processes. In the present study, sodium chloride and magnesium sulfate salts were used for evaporation experiments using loose porous media of different structure: similar grain size of natural sand and well-rounded glass beads. The combined effect of grain angularity and concentration of salt solutions were examined in cylindrical glass containers. For each experiment, the mass loss is calculated with periodic weighting, and visual changes are measured and documented. The laboratory experiments were performed in three stages: a) different type of salts under the same conditions, b) same type of salt in different concentrations, c) same type of salt solution in structurally different porous medium. We found that magnesium sulfate caused decrease in evaporation rate by a factor of 5 compare to the same concentration of sodium chloride. Comparing the sodium chloride solution in different concentrations, the solution with higher concentration showed a slower trend growth on evaporation rate. Regarding the difference of pore structure, sodium chloride created a salt crust that was covering circa 90% of the surface with superficial fractures in the case of natural sand whereas in the case of glass beads, it covered only less than 40% of the surface. Nevertheless, the evaporation rate in the described experiment from natural sand showed a faster trend growth than in the case of the glass beads, which agrees with the observation of the evaporation front, which dropped down relatively faster in natural sand than in the glass beads. This indicates that materials with more rounded grains tend to have lower evaporation rate with less visible salt crust on the surface, whereas materials with rougher grain surface tend to have higher evaporation rate with considerably thicker and wider salt crust on the surface. Therefore, the pore structure might be one of the important determinatives of salt weathering patterns in porous materials.