Modelling the phase change of salt dissolved in pore water – Equilibrium and non-equilibrium approach

Salts strongly influence the durability of porous building materials. There are two main processes, which are responsible for the damage of the materials related with salt presence in the pore system. The first one is the corrosion of steel bars triggered by chlorides. Chlorides exist in two forms: free chlorides dissolved in pore water and bound chlorides. It is believed that only free chlorides are responsible for the corrosion. Therefore it is very important to define and model properly the chloride binding isotherm and its influence on free chloride transport. The linear, Freundlich and Langmuir isotherms were tested. Another adverse phenomenon is the salt crystallisation, which is associated with the supersaturation. This phenomenon is modelled using the kinetics of salt crystallisation. The model and its numerical solution describing the salt and moisture transport including the equilibrium and non-equilibrium phase change of salt in non-isothermal condition is proposed. To model the kinetics of salt crystallisation the simple approach proposed by Espinosa et al. was utilised. The drying, cooling and warming of cement mortar sample, during which the salt phase change occurs, were simulated using the developed software. The changes of salt concentration and the amount of precipitated salt due to variation of boundary conditions are presented. The model including kinetics of salt phase change is found to be reliable.