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

• The cement pastes were further cured in water after two different CO 2 concentrations (3% and 20%) curing. • The calcite was the main product generated in CO 2 -cured pastes. • The cement pastes cured at a higher CO 2 concentration exhibited a higher compressive strength at all curing ages. • The CO 2 -cured cement pastes, the porosity and pore size distribution became lower because of the formation of CaCO 3 . Understanding the effect of formed calcium carbonate and highly polymerization of silicates on the microstructure of cement paste exposed to early CO 2 curing and further hydration process is crucial to comprehend the mechanisms of CO 2 curing. In this paper, the cement pastes were further cured in water after CO 2 curing under two CO 2 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 CO 2 -cured pastes and its content was increased with the increasing CO 2 concentration. The formed calcium carbonate provided additional nucleation sites and accelerated the hydration of C 3 S in further water curing. Furthermore, calcium carbonate was consumed by C 3 A 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 CO 2 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 CO 2 -cured sample showed higher early compressive strength and comparable long-term compressive strength compared to the conventional samples due to the lower porosity.