Modelling of coupled ion and moisture transport in porous building materials

In this paper, a physically based model describing the coupled ion and moisture transport is developed by combining existing theories of liquid water and water vapour transport with aqueous electrolyte theory. We derive the set of governing differential equations describing simultaneous movement of water in the vapour and liquid phases and consequent transport of ions in unsaturated porous media. The equations are developed in one-dimension, assuming isothermal conditions. It is also assumed that the movement of water in both the vapour and the liquid phases are first-order phenomena described by Fickian diffusion and Darcy’s law, respectively. The influence of ions on liquid–vapour equilibrium is modelled here by considering the water activity term. The effect of salt crystallization on the transport properties is considered. The diffusion of the ions present in the system is modelled by solving the Nernst–Planck/local electroneutrality set of equations. A computer program has been developed to solve this highly non-linear problem. The validation of the model has been performed on the basis of a comparison between predicted kinetics of moisture or ion concentration profiles and measurement obtained by NMR on plaster/Bentheimer sandstone system during drying.