SALT WEATHERING OF SANDSTONE DURING DRYING: EFFECT OF PRIMARY AND SECONDARY CRYSTALLISATION

In this study we show that the key to understanding why the same salt can cause damage in some conditions and not in others is the kinetics of recrystallisation. Salt-contaminated porous materials are known to deteriorate with environmental fluctuations. Salts can be naturally present in the materials used for construction or can be derived from external sources. With changes in climate, entrapped salt crystals can once again form a salt solution after contact with liquid water (dissolution) or with water vapour (deliquescence). The solution is mobilized in the porous network and then salts re-crystallize with drying. We present both macroscopic and microscopic experiments assessing the recrystallisation dynamics of NaCl in sandstone with wetting-drying and humidity cycling. Advanced techniques such as high resolution X-ray computed tomography and Scanning Electron Microscopy are used to study the recrystallisation process in the porous network in parallel with the drying kinetics. Our results show that recrystallisations following deliquescence and dissolution lead to very different crystallisation patterns. With humidity cycling, recrystallisation promotes the formation of localised bigger cubic crystals in the subsurface of the stone, whereas rewetting leads to efflorescence in the form of large localised ‘cauliflowers’ at the surface. The different growth dynamics have direct consequence on the drying behaviour of the sandstone and its deterioration.