STUDY OF KINETICS OF SALT CRYSTALLIZATION DURING REWETTING DRYING AND HUMIDITY CYCLING

In this paper, we reveal the major role the recrystallization dynamics plays in the way different salts cause damage to stone under different environmental conditions. There are two ways in which a salt, once crystallized, can take up water again: by bringing it in contact with liquid water (dissolution) or with water vapour (deliquescence). We compare the kinetics of salt recrystallization for different salts (NaCl. KCl, Na2SO4), after either deliquescence or dissolution followed by drying, for several cycles. We find that deliquescence/recrystallization with humidity cycling leads to the growth of salt crystals by expulsing impurities; after only a few cycles we obtain highquality single crystals for salts such as NaCl and KCl that have only a one crystalline form. In this process, the solution reaches high supersaturations before the crystal growth which in turn leads to a higher crystallization pressure and hence damages the stone. These results show why subsequent humidity cycling with NaCl leads to an irreversible dilation and the gradual expansion of the material. For Na2SO4 which has different crystalline forms, the fact that high salt concentrations are reached during deliquescence followed by drying, favors the direct precipitation of anhydrous Na2SO4. The crystallization of the anhydrous salt generates only very small stresses that hardly damage the stone. However, for sodium sulfate rewetting/drying leads to severe damage because the thenardite microcrystals dissolve very rapidly, and remaining crystals act as seeds to form large amount of hydrated crystals creating grape-like structures that expand rapidly. These clusters generate stresses larger than tensile strength of the stone. These results shed some light on how the kinetic pathway of crystallization plays an important role on the way the same salt can cause in some conditions and not in others