NaCl crystallization in restoration plasters

Salt crystallization is one of the most recurrent causes of damage in building materials. In this paper a study of NaCl crystallization in restoration plasters is presented. The morphology of the salt crystals, the location of salt crystallization and the decay patterns are investigated and related to the properties of the plaster and of the substrate/plaster combination. The research has been part of a European research program COMPASS.Based on ESEM observations from laboratory and field samples it is shown that moisture transport is one of the most important factors determining the location of salt crystallization. In particular, the presence of a water repellent strongly influences the location of salt accumulation. Depending on the effectiveness of the water repellent, salt will accumulate either at the substrate/plaster interface (salt blocking plasters) or in the inner layer of the plaster (salt accumulating plasters). The transport can be influenced by the pore system and by chemical admixtures like water repellents. Furthermore, it is discussed that salt is usually not found in cracks. It is suggested that a mechanism of differential dilation of zones with high and low salt content may be responsible for the damage.