Damage potential and supersaturation of nitrates relevant to salt damage of porous building materials

Salt weathering is one of the most threatening issues for the preservation of our built cultural heritage comprised of porous materials. After salt accumulation in the pores, cyclic crystallization and dissolution processes can lead to material damage when the crystallization pressure, which is directly influenced by the supersaturation of the pore solution, acts on the material. Following the presentation of the damage potential and achievable supersaturation of potassium nitrate in a previous paper, this study reports results for the nitrates of sodium, magnesium and calcium for the first time. Two different types of damage tests with impregnated sandstone specimens were performed to assess their actual damage potential, also including the well investigated sodium chloride for comparison. Significant deterioration was revealed for all investigated salts as a result of a high supersaturation of the pore solution at the time when crystals grow against the pore wall. Surface roughness measurements were used to compare the extent of damage and to rank the damage potentials of the salts. Investigations on the supersaturation, which is directly linked to the crystallization pressure and is suggested to be used as a quantitative and comparable measure for the damage potential, revealed high values at the onset of crystallization for all salts in cooling (also in porous host materials) and evaporation experiments. The results of this comprehensive study clearly reveal a high damage potential of nitrate salts, formerly often neglected in discussions on damage-relevant salts.