Carbonation rate and microstructural alterations of class G cement under geological storage conditions

Abstract

Cementitious grouts are a vital component for the economically-viable implementation of the geological storage of CO2 in providing an engineered long-term seal. In this study a class G cement was carbonated at 80 bar, at either 60 °C or 120 °C, whilst immersed in a synthetic brine for durations of up to 5 months. X-ray computed tomography was used to evaluate the advancement of carbonation depth, whilst SEM/EDXA and XRD were used to characterise microstructural alteration of the cement phases. The microstructure of the ‘main carbonation front’ was found to be representative of the governing reactive transport mechanism. An ill-defined ‘main carbonation front’ during carbonation at 80 bar/60 °C showed a carbonation mechanism controlled by the rate or precipitation/dissolution reactions; diffusion in that case was not the controlling factor. The faster local supersaturation conditions in the pores at 60 °C (with respect to Ca2+ and HCO3−) created a dynamic system of aragonite precipitation from the carbonated to the inner regions of the cement. At 80 bar/120 °C a clearly defined ‘main carbonation front’ with higher compositional density than at 60 °C, was correlated with the fast reactions and diffusion limited evolution of the ‘main carbonation front’. Calcite, as the main result of those fast reactions at 120 °C, filled ubiquitously previously unmineralized voids, creating a system less prone to compositional alterations by chemical changes due to the CO2 plume. This study showed, that the formation of calcium carbonate polymorphs depends on the kinetics of carbonation reactions for a class G cement that is determined by temperature and time. The findings of the current paper can be further used for the understanding of reaction processes within the cements of the CO2 injection wells and assess their long-term chemical stability.