Factors Affecting the Kinetics of Salt Crystallization and Related Damage in Sandstone

Salt contaminated stones or masonry materials are often observed to deteriorate under environmental conditions due to crystallization of salts, the importance of which is expected to increase in the future due to global climate change. When environmental conditions such as humidity or exposure to rain or rising damp vary, salts in contact with water (liquid or vapor) can dissolve and cause damage to the material by re-crystallization upon drying. The detailed mechanisms of the damaging processes have been investigated for several decades now. However there are still open questions such as why a given salt can lead to damage in some environmental conditions and not in others.

We present both macroscopic and microscopic experiments assessing the importance of the kinetics of salt crystallization for NaCl on damage in sandstone with environmental fluctuations, i.e. wetting/drying and humidity cycling. NaCl contaminated sandstones are put in contact with liquid water and water vapor until complete saturation, followed by drying at different relative humidities. Advanced techniques such as high resolution X-ray computed tomography and Scanning Electron Microscopy are used to study the kinetics of salt growth in the porous network.

Our results show that the kinetics of recrystallization after deliquescence (contact with water vapor) and rewetting of salt contaminated sandstones leads to very different crystallization patterns. With humidity cycling, recrystallization promotes the formation of localised bigger cubic crystals at the subsurface of the stone whereas rewetting leads to efflorescence in the form of large localised cauliflowers at the surface. The different growth dynamics also affects the drying behaviour of the sandstones. These results reveal for the first time the major role of the crystallization dynamics in the way a given salt causes damage in some environmental conditions and not in other.