Phase changes of salts in porous materials: Crystallization, hydration and deliquescence

The objective of this work was to deduce a mathematical model for the kinetics of phase changes of salts in pores to be coupled with a simulation program for heat, moisture and salt transport in capillary porous materials.It could be proven that a unique equation describes the kinetics of all five phase changes (hydration, dehydration, deliquescence, crystallization and dissolution). This equation is based on the fact that the driving force for any phase change is the difference between the chemical potentials of the start and end phases. The difference in chemical potentials is described by means of the thermodynamic supersaturation ratio of the pore solution UaP. Furthermore two kinetic parameters K and g are also involved in this equation.The time-dependent phase change of salts in porous materials was investigated experimentally by means of a self-developed measuring temperature differences (crystallization and dissolution) and weight differences (hydration, dehydration and deliquescence).In all cases a supersaturation ratio greater than 1 is necessary for the phase change to start. In case of the crystallization, a “starting supersaturation ratio” Ua,startP larger than 1 was found for each combination of material and salt.The values of K and g were obtained by fitting the experimental results to the proposed equation for kinetics. It was found that the kinetics of each phase change generally depends on both salt and porosity.