Mixed Electrolyte or Single Salt Solution–Is There an Impact of Precipitating Salts on the Damage Potential? Investigations on the Na⁺–K⁺–Cl^––NO₃^––H₂O System

Salt damage to porous building materials, which becomes relevant in case of high pore fillings and cyclic crystallization events, affects numerous objects of our cultural heritage. For single salts, the critical threshold values for crystallization cycles are given by the deliquescence humidity and the crystallization humidity, the latter depending on the degree of supersaturation prior to crystallization. Contamination of built heritage with only one single salt is unlikely, so ion mixtures must be considered for a more realistic assessment of the damage potential. For salt mixtures, the boundary conditions, i.e., crystallization humidities and the relative humidity range relevant for crystallization and dissolution processes, are different from the conditions for single salts. This paper investigates whether the damage potential of a salt is also different when precipitating from a mixture rather than from its pure solution. For that purpose, the supersaturation by cooling and by evaporation is used as a measure for the damage potential and is determined for different systematically selected mixture compositions of the Na⁺–K⁺–Cl^––NO₃^––H₂O system and respective subsystems. The results indicate that the supersaturation of some salts is indeed affected by accompanying ions, while for others, the impact is low.