Performance of Cementitious Materials Subjected to Low CO2 Concentration Accelerated Carbonation Curing and Further Hydration

Excessive carbonation curing can impair later-age properties; therefore, determining an appropriate carbonation duration is critical for practical low-CO2 utilization. In this study, cement paste and mortar specimens were subjected to accelerated carbonation curing under a diluted CO2 atmosphere (3%) using a concentration-controlled scheme (2.5–3.0%), followed by standard hydration curing for up to 28 d. Carbonation durations of 500, 1000, 2000, and 6000 min were examined. The results show that an early carbonation duration of 1000 to 2000 min achieves an optimal balance between performance enhancement and subsequent hydration development. Compared with the reference specimens, compressive and flexural strengths, as well as durability-related indicators (including electrical flux and EIS parameters), are improved. In addition, the surface microstructure is refined, with a higher proportion of highly crystalline CaCO3 (>70%). In contrast, prolonged carbonation (>2000 min) induces unfavorable microstructural evolution during subsequent hydration, leading to reductions in mechanical performance and durability. These findings provide a practical duration-control strategy for accelerated carbonation curing using low-concentration CO2 streams (3–10%), which are typical of light-industry flue gases.