Crystallization and Dissolution of Common Salts - Damage Potential to Porous Media

Abstract

Growing crystals of soluble salts could cause degradation of porous building materials due to generation of crystallization pressure inducing tensile stress inside porous system. Considerable damage potential has been observed in case of sodium sulfate through phase change and rapid formation of hydrated phase mirabilite from highly supersaturated solution rising from dissolution of anhydrous phase thenardite after changing of surrounding conditions. Crystallization of sodium chloride can also lead to damage but the intensity is not as evident in comparison with sodium sulfate. The extent of salt attack strongly depends particularly on the environmental conditions and salt content in the material. The morphology of crystals (NaCl, Na₂SO₄ and mixture of both in ratio 1:1) and phenomena related to dissolution were studied with optical microscope. Conclusions from microscopic observation were applied to real porous system - sandstone subjected to salinization and wetting-drying cycles. The massive damage (>50%) showed the specimen containing single sodium sulfate crystals which are during wetting subjected to phase transition accompanied by volume change. The damage caused by sodium chloride and by mixture was much lower - 1% and 3% respectively. Such low mass change could be explained by greater amount of efflorescence and also by lower damage potential of NaCl and the mixture.