Molecular simulation of CO2 and H2O adsorption/deformation on model cement materials

Carbon capture and storage (CCS) is not only a highly effective method for reducing atmospheric CO2 levels but also a necessary solution for achieving carbon neutrality. This technology has the potential to capture and trap several billion tons of CO2 annually. However, ensuring the secure and cost-effective geological storage of carbon requires a deep understanding of potential well cement degradation, the carbonation process, and wellbore integrity, especially under CO2 injection conditions. In this study, we use Monte Carlo simulations to explore CO2 and H2O interactions at the microscopic level within cement, using simplified models of portlandite and tobermorite under specific aging conditions (66 °C, 100 bar). This approach helps us better understand the thermodynamic mechanisms at play before any chemical degradation occurs. Our models are validated by comparing the isosteric heat of adsorption of water molecules (Qst) with experimental data from existing literature, showing good agreement for a cement paste model where a surface water clustering analysis allows a comprehensive understanding of the variation of this property with respect to the relative humidity. Our findings emphasize that surface water content is critical in influencing CO2 adsorption behavior, and modifications in this water content can significantly affect the material's interaction with CO2. Local density profile analyses revealed a higher concentration of CO2 molecules at the interface with the water film, indicating a liquid-vapor interfacial enrichment for both materials. These results underscore the significant impact of surface water content on the intergranular equilibrium distance in portlandite and tobermorite. The insights from these simulations provide essential information for developing more accurate phenomenological models to predict the impact of CO2 on cement integrity, particularly in evaluating the effect of local humidity on the material's ability to adsorb CO2 molecules before any chemical degradation reactions occur.