Crystallization behavior of Na2SO4-MgSO4 salt mixtures in sandstone and comparison to single salt behavior

We report on the crystallization behavior and the salt weathering potential of Na2SO4, MgSO4 and an equimolar mixture of these salts in natural rock and porous stone. Geochemical modeling of the phase diagram of the ternary Na2SO4-MgSO4-H2O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Model calculations include stable and metastable solubilities of the various hydrated states of the single salts and the double salts Na2Mg(SO4)(2)center dot 4H(2)O (bloedite), Na2Mg(SO4)(2)center dot 5H(2)O (konyaite), Na12Mg7(SO4)(13)center dot 15H(2)O (loeweite) and Na6Mg(SO4)(4) (vanthoffite). In situ Raman spectroscopy was used to study the phase transformations during wetting of pure MgSO4 center dot H2O (kieserite) and of the incongruently soluble salts bloedite and konyaite. Dissolution of kieserite leads to high supersaturation resulting in crystallization of higher hydrated phases, i.e. MgSO4 center dot 7H(2)O (epsomite) and MgSO4 center dot 6H(2)O (hexahydrite). This confirms the high damage potential of magnesium sulfate in salt damage of building materials. The dissolution of the incongruently soluble double salts leads to supersaturation with respect to Na2SO4 center dot 10H(2)O (mirabilite). However, the supersaturation was insufficient for mirabilite nucleation. The damage potential of the two single salts and an equimolar salt mixture was tested in wetting-drying experiments with porous sandstone. While the high damage potential of the single salts is confirmed, it appears that the supersaturation achieved during wetting of the double salts at room temperature is not sufficient to generate high crystallization pressures. In contrast, very high damage potentials of the double salts were found in experiments at low temperature under high salt load.1 (C) 2016 Elsevier Ltd. All rights reserved.