XRD analyses on silica-amended Class G cement samples exposed to clean and impure CO2 in the presence of water.

<ns3:p> Background and methods This paper presents XRD analyses on samples of silica-amended Class G Portland Cement exposed to CO <ns3:sub>2</ns3:sub> -saturated water and wet supercritical CO <ns3:sub>2</ns3:sub> , using clean CO <ns3:sub>2</ns3:sub> , CO <ns3:sub>2</ns3:sub> with 1.6 mol% H <ns3:sub>2</ns3:sub> S, and CO <ns3:sub>2</ns3:sub> in equilibrium with concentrated H <ns3:sub>2</ns3:sub> SO <ns3:sub>4</ns3:sub> , under conditions relevant for the geological storage of CO <ns3:sub>2</ns3:sub> . The results complement microstructural and chemical analyses on the same samples published previously, for an enhanced insight into the mineralogical changes induced in these samples by exposure, focusing on the impacts of exposure environment (CO <ns3:sub>2</ns3:sub> -saturated water vs. wet supercritical CO <ns3:sub>2</ns3:sub> ) and key impurities (H <ns3:sub>2</ns3:sub> S and H <ns3:sub>2</ns3:sub> SO <ns3:sub>4</ns3:sub> ) on cement alteration and carbonate formation, as well as on the fate of introduced impurities within the altered cement matrix. Results The results show that initial Ca-carbonate precipitation was dominated by metastable vaterite, which subsequently dissolved again and re-precipitated, partly, as more stable calcite and aragonite. During exposure to CO <ns3:sub>2</ns3:sub> -saturated water, vaterite dissolution also resulted in the leaching of significant Ca <ns3:sup>2+</ns3:sup> into the aqueous exposure fluid, especially in the presence of H <ns3:sub>2</ns3:sub> S, while during exposure to wet supercritical CO <ns3:sub>2</ns3:sub> (with or without H <ns3:sub>2</ns3:sub> S), overall carbonate contents continued to increase. During the exposure to CO <ns3:sub>2</ns3:sub> with H <ns3:sub>2</ns3:sub> SO <ns3:sub>4</ns3:sub> , samples were affected to the presence of H <ns3:sub>2</ns3:sub> SO <ns3:sub>4</ns3:sub> and desiccation effects, leading, in particular, to relatively high aragonite contents. Exposure in the presence of both H <ns3:sub>2</ns3:sub> S and H <ns3:sub>2</ns3:sub> SO <ns3:sub>4</ns3:sub> resulted in the formation of, mostly minor quantities of, pyrite and pyrrhotites. In addition, exposure to CO <ns3:sub>2</ns3:sub> with H <ns3:sub>2</ns3:sub> S resulted in the precipitation of significant quantities of gypsum, most likely as a result of the oxidation of sulphide to sulphate through the reduction of Fe <ns3:sup>3+</ns3:sup> from brownmillerite. Conclusions Even though, overall, the impacts of impurities were relatively minor, these results reiterate the need for a thorough, experiment-based understanding of potential impurity-mineral interactions for accurate long-term forecasting of cement integrity during CCS. </ns3:p>