Carbonation Reaction Mechanisms of Portlandite Predicted from Enhanced Ab Initio Molecular Dynamics Simulations

Geological carbon capture and sequestration (CCS) is a promising technology for curbing the global warming crisis by reduction of the overall carbon footprint. Degradation of cement wellbore casings due to carbonation reactions in the underground CO₂ storage environment is one of the central issues in assessing the long-term success of the CCS operations. However, the complexity of hydrated cement coupled with extreme subsurface environmental conditions makes it difficult to understand the carbonation reaction mechanisms leading to the loss of well integrity. In this work, we use biased ab initio molecular dynamics (AIMD) simulations to explore the reactivity of supercritical CO₂ with the basal and edge surfaces of a model hydrated cement phase—portlandite—in dry scCO₂ and water-rich conditions. Our simulations show that in dry scCO₂ conditions, the undercoordinated edge surfaces of portlandite experience a fast barrierless reaction with CO₂, while the fully hydroxylated basal surfaces suppress the formation of carbonate ions, resulting in a higher reactivity barrier. We deduce that the rate-limiting step in scCO₂ conditions is the formation of the surface carbonate barrier which controls the diffusion of CO₂ through the layer. The presence of water hinders direct interaction of CO₂ with portlandite as H₂O molecules form well-structured surface layers. In the water-rich environment, CO₂ undergoes a concerted reaction with H₂O and surface hydroxyl groups to form bicarbonate complexes. We relate the variation of the free-energy barriers in the formation of the bicarbonate complexes to the structure of the water layer at the interface which is, in turn, dictated by the surface chemistry and the degree of nanoconfinement.