Damage of porous building stone by sodium carbonate crystallization and the effect of crystallization modifiers

Salt crystallization is an aggressive weathering mechanism affecting porous building materials. The extensive use of Portland cement, a source of alkalis, in modern buildings and restoration interventions makes sodium carbonate salts important weathering agents. Herein, we study salt damage to a porous stone commonly used in the Andalusian built heritage (Santa Pudia limestone) due to stress generation associated with the precipitation of natron (Na2CO3·10 H2O). We performed cyclic crystallization tests combined with thermodynamic and poromechanical calculations to determine salt crystallization pressure and effective tensile stress suffered by the material. The outcome reveals that in-pore natron crystallization during cooling/evaporation generates stresses exceeding the tensile strength of the wet substrate, leading to extensive damage by fracturing and material loss. Damage is reduced using aminotris(methylenephosphonic) acid (ATMP), a common phosphonate-based crystallization modifier that induces non-damaging efflorescence growth as opposed to damaging subflorescence growth, which takes place in its absence.