Moisture and salt transport in three-layer plaster/substrate systems

We have investigated whether a plaster which has two plaster layers with different pore sizes can act as a salt accumulating plaster system, in which salt crystallizes in the base layer of the plaster and not in the substrate or at the external surface. We used two substrates; fired-clay brick and Bentheim sandstone. On the substrate we applied two plaster layers, of which the first (base) plaster layer has pores that are significantly smaller than the pores of the second (external) plaster layer. These systems were either saturated with pure water or a NaCl solution. The moisture and salt transport during drying were investigated with a Nuclear Magnetic Resonance technique. In our experiments, only a system consisting of a coarse porous gypsum layer and a fine porous lime–cement layer on a Bentheim sandstone showed, under certain conditions, salt accumulation in the base layer.