Suppression of salt weathering of porous limestone by borax-induced promotion of sodium and magnesium sulphate crystallization

Abstract

AbstractThe effects of borax on the crystallization of sodium and magnesium sulphate, two of the most damaging salts affecting porous stones, have been studied. Borax promotes the crystallization of mirabilite and inhibits epsomite crystallization in open glass beakers. The additive is preferentially adsorbed onto {140}mirabilite and {111}epsomite faces, thus acting as an effective habit modifier. In contrast, in the presence of a calcitic support (either Iceland spar single crystals or a porous limestone – a biocalcarenite) crystallization is promoted in the presence of borax, irrespective of the salt tested. Apparently, this is due to a high stereochemical affinity between borate molecules adsorbed (and/or co-precipitated) onto calcite, and mirabilite and epsomite crystals. Salt weathering tests using a biocalcarenite show a significant damage reduction upon borax addition to the saline solutions. Borax promotes the crystallization of both mirabilite and epsomite within the pores of the stone, reducing its porosity. Crystallization promotion favours nucleation at a low supersaturation, thereby resulting in very low crystallization pressure and minimal damage. Application of borax to porous limestones affected by mirabilite and/or epsomite crystallization could be a new means of suppressing salt weathering.