Pore-scale freezing of a sandy saline soil visualized with micro-computed tomography

Sandy saline soils are widely distributed and commonly experience seasonal or long-term freezing, yet the freezing process in these soils is rarely studied. This research utilized in situ X-ray computed tomography (CT) to visualize pore-scale freezing processes in sandy saline soils under various initial water and salt contents. Micron-resolution observations of pore ice and unfrozen water produced new insights into the preferential orientation and grouping of pore ice crystals, intersections between different pore ice crystal groups causing anisotropic behavior, decreasing pore ice crystal size under faster freezing rates, and the formation of interconnected networks of both pore ice and unfrozen water upon freezing. From a thermodynamic perspective, the salt content in the unfrozen liquid is dependent on the local temperature as described by water-salt phase diagram. Furthermore, the local volume ratio between unfrozen water and pore ice reflects the initial salt content and salt mass transfer occurring due to both diffusion and fluid flow processes. This work improves the understanding of complex freezing phenomena in sandy saline soils through high-resolution evidence of crystallization patterns, transformation mechanisms, and coupled heat-mass transfer.