Effect of porosity on anhydrite swelling

Swelling in anhydritic claystones remains a major tunnelling risk. Although this phenomenon has been widely studied, knowledge gaps persist regarding its swelling behaviour. One of them being the effect of porosity on the volumetric strains that develop during the anhydrite to gypsum transformation. To address this gap, a series of laboratory tests was carried out on artificial specimens made from highly compacted mixtures of anhydrite and kaolin powders. The initial porosity was varied between 0.22 and 0.35, and volumetric strain development was monitored during gypsum formation. The experiments show that transformation-induced strains decrease with increasing initial porosity. The observations further suggest two distinct mechanisms: in more porous specimens, gypsum precipitation occurs largely within the existing pore space, reducing porosity and limiting bulk expansion; in more highly compacted specimens, gypsum growth forces matrix expansion, leading to larger macroscopic swelling. These results are applicable to porous media where crystallisation may occur within pores. Overall, the experimental campaign provides observations and a dataset that can support the development and calibration of coupled chemo-hydro-mechanical models for anhydrite swelling, enabling more realistic predictions of strain development due to gypsum growth in tunnelling applications.