Microstructure of Portland cement paste subjected to different CO2 concentrations and further water curing

Understanding the effect of formed calcium carbonate and highly polymerization of silicates on the microstructure of cement paste exposed to early CO2 curing and further hydration process is crucial to comprehend the mechanisms of CO2 curing. In this paper, the cement pastes were further cured in water after CO2 curing under two CO2 concentrations (3% and 20%) and the carbonation depth, compressive strength, composition and morphology of the formed products were characterized. The results showed that calcite was the main product generated in CO2-cured pastes and its content was increased with the increasing CO2 concentration. The formed calcium carbonate provided additional nucleation sites and accelerated the hydration of C3S in further water curing. Furthermore, calcium carbonate was consumed by C3A to form calcium aluminate monocarbonate, which delayed the transformation of ettringite to monosulfate during further water curing. The decarbonation temperature of the formed calcium carbonate was increased with the increase of the CO2 concentration, but it was decreased in further water curing due to participating the further hydration process. The early carbonation curing improved the polymerization of silica gel, which was decreased in the subsequent hydration due to the formation of C-S-H. The CO2-cured sample showed higher early compressive strength and comparable long-term compressive strength compared to the conventional samples due to the lower porosity.