Impact of recrystallization cycles on drying of salt contaminated porous media

In this study we show that the key to understand why the same salt can cause damage in some conditions and not in others is the kinetics of re-crystallization. Salt contaminated porous materials are known to deteriorate with environmental fluctuations causing the weathering of rocks and buildings. Salts can be naturally present in the materials used for construction (such as mortars and bricks) or can derive from external sources (capillary rising ground water, eolian origin, sea water along rocky coasts, and atmospheric pollution). With the variation of outdoor climate such as relative humidity, temperature or rain, the entrapped salt crystals can form again a salt solution after contact with liquid water (dissolution) or with water vapor (deliquescence). The so formed salt solution is subsequently mobilized in the porous network and re-crystallizes with drying. We present both macroscopic and microscopic experiments assessing the recrystallization 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 recrystallization process in the porous network in parallel with the drying kinetics. Our results show that recrystallization following deliquescence and dissolution 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 have direct consequence on the drying behaviour of the sandstone and its deterioration. This work is part of the European Project KISADAMA, a JPI- Cultural Heritage : Joint Heritage European Programme.