Phase transition of salt mixtures in porous media during drying using advanced optical microscopy, spectroscopy and X-ray tomography

Salt crystallization and dissolution induced by changes in air relative humidity stand out as a significant contributor to damage in construction materials [1, 2]. Aside from the visual impact on building structures, appearing as efflorescence, subflorescence may significantly influence the durability and structural integrity of materials as a result of stress-induced crystallization pressure [3]. Drying of building stones filled with salt-containing moisture is a common example of salt weathering [4]. Despite extensive studies on drying processes, precise forecasts of the evaporative drying rate and a comprehensive understanding of the various stages in drying remain major challenges, particularly when drying is coupled with salt deposition [5]. Effectively addressing the respective challenges requires a thorough understanding of the salt crystallization mechanisms within porous media [6]. In this study, we explored the response to humidity cycles in PDMS transparent micromodels and sedimentary porous rocks containing brine solutions of various compositions [7]. We specifically consider the case where air with different levels of humidity and at a constant temperature passes along one side of the porous media, leading to the formation of a drying front—a defined interface separating liquid-saturated and partially gas-filled domains. Digital optical microscopy, confocal Raman microscopy/spectroscopy, and 4D X-ray micro-computed tomography (μ-CT) [8] were used to visualize and characterize salt crystals in the porous media. We successfully detected the crystallization of the solution upon evaporation and the deliquescence of the crystal with increasing humidity. Systematic investigations were carried out to understand how the conditions for crystallization and deliquescence depend on pore space morphology and type of salt solutions. The results revealed the formation and growth of monocrystals as well as of opaque polycrystalline aggregates in the form of both efflorescence and subflorescence. The observed partial and complete pore blockage was restricted by throat and pore size and topology. The image data enabled us to accurately observe the interface between air, brine, crystal, and grain assisting in quantification of phase morphology. The findings highlight the potential of various visualization techniques for enhanced understanding of the transport-crystallization-dissolution coupling through in-situ experiments and hence for constructing more accurate prediction models and conservation strategies.