Abstract
In this study, time-resolved X-ray microtomography is employed to improve the understanding of coupled hydro-chemo-mechanical processes involved in saline weathering of natural stone. A Savonnières limestone sample, hydrophobically treated at its upper surface, was saturated with a NaCl solution and subjected to a controlled drying and deliquescence cycle while being imaged at a resolution of 9.4 µm/voxel. The volumetric deformations, induced during the cycles, were extracted from the time-resolved 3D images using Digital Volume Correlation (DVC). In combination with deep-learning-based segmentation of distinct pore families, including those below the image resolution, the associated transport and crystallization mechanisms are elucidated at the pore scale. The results reveal that salt crystallization in the hydrophobized region of the sample generates internal stresses leading to fracture formation, while deliquescence promotes partial crack closure due to matrix swelling and salt dissolution. In the untreated part of the sample, shrinkage and swelling was observed during drying and deliquescence, respectively. A chronology of the kinetics of these transport processes was further identified across the different pore families. Overall, this work demonstrates that the combination of X-ray microtomography with advanced image-processing techniques enables in-depth analysis of complex, intertwined phenomena in porous media, even when they occur partly in pores smaller than the image resolution.
WHY THIS PAPER MATTERS
Salt silently cracks stone buildings and monuments from the inside out, threatening cultural heritage worldwide. Using X-ray imaging without a contrast agent, we developed a novel analysis workflow that reveals how salt is transported and creates cracks at a microscopic scale. This workflow opens new ways for scientists to study similar complex processes in porous materials. The insights inform conservation experts and engineers as they develop strategies to protect materials, buildings, and structures from salt weathering.