CO2 exposure age: a key factor to enhance cementitious composites’ flexural strength through induced CO2 mineralization treatment

The use of CO 2 exposure in hardened cementitious composites can be employed as a treatment to enhance mechanical performance and capture CO 2 in precast elements. Understanding the role of exposure age on the effectiveness of treatment is crucial to leveraging this approach. This study investigates the influence of CO 2 exposure age on the flexural and microstructural development of cement paste. The samples were divided into two groups. One group was exposed to 24-h accelerated CO 2 exposure on 6-day-old specimens, while the other was exposed to 24-h accelerated CO 2 exposure on 27-day-old specimens. Samples of both groups were tested at 28 days for flexural strength, thermogravimetric analysis (TGA), and quantitative X-ray diffraction (XRD). Besides, additional samples were tested at 7 days. The 28-day flexural strength slightly increased (+ 7%) when the CO 2 exposure was performed at 6 days, while it increased 60% with CO 2 exposure at 27 days, with respect to, in both cases, the 28-day flexural strength of unexposed samples. 7-day XRD and TGA results indicate that the samples exposed at 6 days exhibit lower belite content and lower total hydration products than unexposed samples, suggesting a carbonation of belite and a subsequent reduction in hydration due to the early CO 2 exposure. The 28-day results for TGA and XRD of the samples exposed on the 27th day showed similar CaCO 3 content to the samples exposed on the 6th day, but with more hydration products, purer calcite, and greater crystallinity. Late CO 2 exposure yields greater benefits than early exposure on hydration and carbonate formation, providing better flexural performance.