PhD defense Meysam Golmohammadi

Mitigating climate change requires scalable carbon capture and storage (CCS) solutions, among which carbon mineralization offers highly secure pathways by converting CO₂ into stable carbonate minerals. Mine tailings and industrial residues are promising candidates for surficial CO₂ storage due to their abundance and reactivity. These materials typically form unsaturated porous media, where gas, water, and minerals interact under partially saturated conditions that strongly control CO₂ transport, dissolution, and mineralization processes. However, key mechanisms governing these interactions—particularly capillarity, evaporation-driven water redistribution, and pore structure evolution—remain insufficiently understood at the column scale. This thesis aims to develop a mechanistic understanding of CO₂ storage in the unsaturated zone (UZ) by investigating the coupled effects of capillarity, water distribution, and mineral reactions. The objectives are to (i) examine the influence of capillarity on gas solubility, (ii) evaluate the capability of time domain reflectometry (TDR) to monitor cavitation and geochemical processes, (iii) quantify the impact of water content and redistribution on reactive surface area, and (iv) determine how secondary carbonate morphology controls pore structure evolution and CO₂ storage progression. To address these objectives, brucite (Mg(OH)₂)-bearing column experiments were conducted under controlled gas injection, combining in situ measurements (e.g., CO₂ breakthrough and gravimetry) with post-mortem analyses (e.g., scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP)). Prolonged evaporation induces very low water contents, where water is retained as capillary pockets. Under these conditions, TDR not only tracks water loss but also captures distinct signals associated with geochemical reactions and cavitation. Experiments performed across a range of water contents and spatial distributions demonstrate a strong control of these parameters on the extent of carbonation reactions. Post-mortem characterization using SEM-FIB (focused ion beam) and X-ray diffraction (XRD) reveals the formation of dypingite as the dominant carbonation product. Above a threshold water content, a dypingite–water assemblage develops, generating secondary micro- to nano-porosity. This newly formed pore structure significantly alters system behavior by enhancing water retention (up to ~40% compared to systems without dypingite formation) and increasing CO₂ solubility and capillary trapping through modifications of pore-water thermodynamics at small scales. Overall, this thesis provides new insights into the coupled physicochemical processes governing CO₂ storage in the UZ and offers a framework for improving the design and performance of pilot- and field-scale storage strategies in anthropogenic unsaturated materials.