The degradation mechanism induced by the reaction of a cement and CO2 is called carbonation. Given the CO2 concentration in the air is at its highest in human history, it poses a tremendous threat to the longevity of infrastructure. This study is the world’s first time-resolved carbonation XRD-CT evaluation of novel low carbon cements, and provides new fundamental knowledge about factors controlling changes in mineralogy, reaction products, and microstructure upon CO2 exposure. Specifically this study will identify changes in porosity and microcracks formation occurring in specimens upon CO2 exposure; elucidate carbonation extent of low carbon cements, tracing changes in pore structure and formation of crystalline reaction products (e.g. carbonate polymorphs) when exposed to CO2; and will aid validating/ refuting existing carbonation mechanisms proposed for low carbon cements. These unique outcomes will enable optimisation of cement formulations for increasing carbonation resistance.