Unraveling the microstructural evolution of cement paste in different zones after CO2 curing and subsequent hydration

This study examines how CO 2 curing and subsequent hydration affect the microstructural evolution of cement paste in different zones. Compressive strength tests revealed that at 1 d (day), the CO 2 curing group exhibited a 63.0 % higher compressive strength than the control group. However, by 28 d, the strength increase was only 33.2 %, significantly lower than the 90.6 % gain observed in the control group. To explore the underlying reasons, specimens were divided into three zones from the surface to the interior based on pH measurements and TG analysis, with carbonation degrees of 55.3 %, 39.1 %, and 5.6 %, respectively. Subsequently, phase assemblages and microstructural characteristics of each zone were analyzed. The findings indicate that the difference in mechanical properties results from variations in porosity evolution across zones. In highly carbonated zone I, the transformation of calcium carbonate polymorphs, intense pozzolanic reactions, and restricted hydration of unhydrated clinker led to increased porosity over time. In contrast, zones II and III exhibited ongoing hydration, reducing porosity and improving densification. Compared to the CO 2 curing group, the continuous hydration in the control group reduced porosity and enhanced strength. This study offers insights to optimize CO 2 curing for balancing early strength enhancement and long-term hydration development, ensuring sustained mechanical performance in cement-based materials. • DOC variation in zones I, II, and III caused microstructural heterogeneity and strength differences. • Different DOC drove the divergence in porosity through CaCO 3 polymorphic transformations and silica gel pozzolanic reactions. • CO 2 curing enhanced early strength gain but slowed later-stage development compared to the control group.