Salt transport in plaster/substrate layers

We have investigated how transport and accumulation of salt in a plaster depends on the underlying masonry material. To this end, moisture and sodium profiles have been measured non-destructively with a Nuclear Magnetic Resonance (NMR) technique during drying of plaster/substrate systems. The same plaster is applied on two substrates of which the pores are either an order of magnitude larger or smaller than those of the plaster. The moisture and salt transport and the salt accumulation differed significantly for these two systems. In a plaster/Bentheimer sandstone system (the pores of the plaster are smaller than those of the substrate) all salt is removed from the substrate and accumulates in the plaster. In a plaster/calcium-silicate brick system (the substrate has a considerable amount of pores that are smaller than those of the plaster) some salt crystallizes in the plaster layer, but a significant amount of salt remains within the substrate itself. The salt transport from substrate to plaster is quantified in terms of an efficiency number e, which can be estimated from the pore-size distributions measured by mercury intrusion porosimetry.