Salt crystallization on the surface of porous media: Morphological evolution of salt crystals and growth kinetics

This paper investigates salt crystallization at the porous media surface using stacks of glass beads of varying sizes (1–50 and 400–600 µm) arranged in a Teflon cylinder and a near-saturated NaCl solution. The study focuses on characterizing the transition between crusty and patchy efflorescence structures, examining the influence of porous substrate properties on evaporation flux and efflorescence morphology. Through structured-light surface scanning and precise mass measurements under controlled humidity and temperature conditions, the results showed that substrates composed of coarse beads promoted patchy efflorescences with a clear impact on evaporation flux, while substrates made of finer beads rapidly formed crystalline crusts, significantly reducing evaporation flux. The patchy structures are porous salt structures allowing ions to move and deposit on their surface, where evaporation is more intense. Ion screening and redirection help maintain the porous patchy efflorescence morphology. On the other hand, the crust-like morphologies are also porous, allowing some vapor transport, but the specificities of their formation remain less clear. To evaluate the impact of adding a coarse bead surface layer to a fine bead substrate, a second experiment was carried out. It has been shown that adding a coarse-grained layer (400–600 µm) to a fine-grained layer (1–50 µm) accelerates the drying process compared to a medium consisting only of fine grains, while inducing the appearance of patchy efflorescence. This efflorescence is clearly observable with a 1 mm layer thickness, becoming more pronounced at 5 mm, showing the efflorescence crusty-patchy transition. Therefore, this measure could represent an effective strategy to enhance the drying process and extract more salt from a porous medium. This study enhances our understanding of the drying and desalination processes in porous media and explores methods to control these physical processes through substrate manipulation.