Salt crystallization as damage mechanism in porous building materials—a nuclear magnetic resonance study

Salts can damage building materials by chemical reactions or crystallization, which is a serious threat to cultural heritage. In order to develop better conservation techniques, more knowledge of the crystallization processes is needed. orous material, the size of a salt crystal is limited by the sizes of the pores. It has been predicted that as a consequence, the solubility of a salt increases with decreasing pore size. This increase seems to be related to an increase of the stress generated by a crystal on the pore wall. It has been suggested that the resulting stress could become high enough to induce failure. e studied the crystallization of salts in porous materials with well-defined pore sizes. Samples were saturated at 40 °C with saturated Na2SO4 and Na2CO3 solutions. Next we have cooled the samples to 0 °C and waited for nucleation. After nucleation occurred, the solubility in the porous material was measured with nuclear magnetic resonance (NMR) as a function of the temperature. asurements on Na2CO3 indeed show an increase in solubility with a decrease in pore size. For Na2SO4, we did not observe a pore size-dependent solubility. However, we have to remark that these results show a metastable crystal phase. The results can be used to calculate the actual pressure exerted by the crystals onto the pore wall.