We report on the crystallization behavior and the salt weathering potential in natural rock and porous stone of single salts (NaNO3, Na2SO4) and salt mixtures in the ternary NaNO3–Na2SO4–H2O system. Geochemical modeling of the phase diagram of the ternary NaNO3–Na2SO4–H2O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Experiments were carried out in order to study the phase changes during dissolution either induced by deliquescence or by the addition of liquid water. In situ Raman spectroscopy was used to study the phase transformations during wetting of pure Na2SO4 (thenardite) and of Na3NO3SO4·H2O (darapskite). In both experiments crystallization of Na2SO4·10H2O (mirabilite) from highly supersaturated solutions is demonstrated confirming the high salt weathering potential of thenardite and darapskite wetting. In order to study the damage potential of darapskite experimentally, wetting–drying experiments with porous sandstone with the two single salts (Na2SO4, NaNO3) and two NaNO3–Na2SO4 salt mixtures were carried out. Different destructive and non-destructive techniques were tested for damage monitoring. NaNO3 was found to be the least damaging salt and Na2SO4 is the most damaging one. The classification of the two salt mixtures was less obvious.