Quantitative characterization of recycled cementitious materials before and after carbonation curing: Carbonation kinetics, phase assemblage, and microstructure

Carbonation curing for cement-based materials is one of the effective means to achieve carbon neutrality in the construction industry. The pure cement clinker was hydrated for 90 d to simulate recycled cementitious materials (RCMs) to further investigate the carbonation reaction kinetics, phase assemblage and microstructure. The results revealed that the carbonation rates of cement clinker's main components, i.e. C3S, C2S, C3A, and C4AF were 100%, 63.24%, 28.92%, and 14.93%, respectively. As the carbonation reaction proceeded, the calcium hydroxide (CH) would be fully consumed, while C-S-H began to become the main reactant, generating low-Ca C-S-H and even fully polymerized Ca-modified silica gel. At the same time, the unhydrated C3S and C2S were carbonized, forming carbonation products, i.e. CaCO3 and xCaO·SiO2·nH2O. Interestingly, the amount of poorly-crystalline CaCO3 was initially decreased and then increased, possibly attributed to the partial transformation into well-crystalline CaCO3 at an early stage, whereas the remaining amount was produced through the reaction of C-S-H at a later stage. Finally, the carbonation reaction was further explained through two models, the early stage was described by a chemical reaction model while the later stage was represented by a product-layer diffusion model.