The thermodynamic and poromechanic crysallization pressure of sodium sulfate heptahydrate : an NMR study

Sodium sulfate is known as one of the most destructive salts leading to the deterioration of porous materials such as monuments, sculptures and civil engineering structures. While sodium sulfate crystals are growing in a porous material, a crystallization pressure will build up. In this study we compare two approaches for determining the crystallization pressure (i.e., a thermodynamic and a poromechanic). The thermodynamic pressure is related to the supersaturation of the crystals growing in a material, whereas the poromechanic pressure is related to the expansion of the material as a reaction to the crystal growth. In this study we have combined the non-destructive measurement of the concentration of the pore solution in a material by using Nuclear Magnetic Resonance (NMR) with an optical measurement of the dilation of the material during crystallization. This gives the possibility to simultaneously determine the crystallization pressure using both approaches. Here we have focused on the crystallization induced by cooling of a saturated fired-clay brick. In the experiments, first heptahydrate is formed which is supersaturated with respect to decahydrate and a transformation to decahydrate has to be induced. It was found that, for fired-clay brick, the thermodynamic crystallization pressure is larger than the poromechanic crystallization pressure. In a final equilibrium condition with no dynamical crystallization and a constant concentration, the thermodynamic and poromechanic crystallization pressure are equal. The highest crystallization pressure for sodium sulfate is obtained just after the transformation from heptahydrate into decahydrate.