Efflorescences of confined aqueous sodiumsalt solutions

Abstract

Owing to its destructive power to porous structures such as buildings and rocks, salt weathering has attracted considerable attention in the community of civil engineers and geomorphologists, who devote their efforts to conservations of architecture and engineering structures afflicted by salt attack, and to the investigation of natural landscape caused by the same group of processes, respectively. Precipitation of dissolved salts is a direct cause of salt weathering effect. Crystallization phenomena in salt weathering can be crudely categorized under efflorescence and subflorescence with respect to the distinct precipitation sites, and the latter is believed to be able to cause more destructions to porous structure. In contrast to subflorescence for which even models of statistical dynamics have been well-established, efflorescence has drawn less attention, partly because of the complexity of constructing a sound theoretical model to describe the mass transport process there involved. As a serie of sodium salts is the main culprit of salt weathering, the current work deals with experimental study of efflorescences of the aqueous NaCl, NaNO3 and Na2SO4 solutions on the surface of porous silica gel particles. We investigate the influences of salt concentration and pore size on the crystal morphology arising in efflorescence by using scanning electron microscopy. It is found that though Na2SO4 effloresces on the specimen surface, its inclination towards subflorescence makes the whiskers appear on specimen with smaller pore radii at low concentrations, which differs obviously from the cases of NaCl and NaNO3. Moreover, unlike the upright growths of NaCl and NaNO3 crystals, the whiskers of Na2SO4 are always oblique to the specimen surface, and the large lateral stress to the specimen thus induced may become another factor of its destructive power apart from the subflorescing trend. The crystallization behaviors of Na2SO4, i.e., both the oblique whiskers and regular crystallites, indicate that mirabilite (Na2SO410H2O) is the main precipitation, which is consistent with the high relative humidity employed in this article. Remarkably, the thinnest whiskers of NaNO3 exhibit the branching and ball-chain structures, indicating that plateau-Rayleigh instablility occurs in the growth process. Our results are expected to inspire more deliberate studies for the full understanding of detailed processes and mechanism involved in efflorescence of aqueous salt solutions.