Evaporation-driven growth of large crystallized salt structures in a porous medium

Subflorescence refers to crystallized salt structures that form inside a porous medium. We report a drying experiment revealing major development of subflorescence in the dry region of the porous medium away from the liquid zone. Using a combination of image analyses and numerical computations, we show that the growth is directly correlated to the evaporation flux distribution along the boundary of the growing salt structure. This indicates that the salt is transported into the domain occupied by the salt structure in the porous medium up to the structure periphery, where salt deposition takes place. This is confirmed when a growing salt structure encounters dry subflorescence formed earlier during the drying process. The dry subflorescence is reimbibed and resumes its growth. The analysis also suggests that the solution within the growing subflorescence is in equilibrium with the crystallized salt wall. These results shed light on the growth mechanisms of subflorescence, a phenomenon that can play a fundamental role in several important issues such as carbon dioxide sequestration or salt weathering.