Water and salt transport in plaster/substrate systems

The transport of salt and water during drying has been studied in systems consisting of a substrate covered with either one or two plaster layers. The drying behaviour of these systems was modelled with invasion percolation (IP) algorithms. The model outcomes were compared with experimental results obtained with Nuclear Magnetic Resonance (NMR). It was found that drying behaviour of the plaster layers was strongly influenced by the properties of the substrate. When the substrate has the widest pores, the plaster layers stay wet while the substrate dries out. As a consequence most salt, present in the substrate, moves to the plaster layers and accumulates at the external surface. In the case that the substrate has the smallest pores, the plaster layers dry out first. In this case salts also crystallize in the substrate. Further we have tried to make an accumulating plaster system consisting of two layers on top of a substrate, which would function purely on the basis of differences in pore sizes between the layers. The drying behaviour in the presence of pure water was as predicted by the model. However, in the case of a salt solution the salt modified the drying behaviour such that the accumulation properties of the system were reduced. Therefore, we conclude that for transporting systems tuning the pore-sizes of the layers suffices, but for accumulating systems it seems that additives, for example water repellents, have to be used.