Reactive magnesium oxide cement (RMC) has the potential to become a sustainable alternative to ordinary Portland cement (OPC). In this study, an approach utilizing supercritical CO 2 (scCO 2) was investigated to accelerate carbonation of an RMC-based composite and to overcome its long carbonation process under the natural environment. It was found that scCO 2 led to an extremely rapid strength gain of the composite, with a mature strength level achievable within a period of hours. CO 2 sequestration factors were also increased by three folds as compared to samples cured under a 20% CO 2 concentration environment for 28 days. It was also revealed that the carbonation phases under scCO 2 were dominated by nesquehonite followed by hydromagnesite and some other intermediate hydrated magnesium carbonates (HMCs). More uniform carbonation within the matrix was also attained under the scCO 2 condition. Despite the promising outcomes, technical and cost challenges would need to be resolved before a possible scale-up.