The carbon sinking-corrosion dilemma in concrete: insights from early-age CSA-PC mortar

Early-age enforced carbonation of cementitious materials presents a promising pathway for CO2 sequestration and potential microstructural enhancement. However, its impact on the durability of reinforced concrete, particularly the risk of steel corrosion, remains a critical and unresolved dilemma. This study investigates the corrosion behavior of steel bars in a hybrid calcium sulfoaluminate-Portland cement (CSA-PC) mortar subjected to deep enforced carbonation at early ages (4, 24, and 72 h). Through a combined approach of electrochemical monitoring (OCP, PR, CCD) during 43 weeks of chloride exposure, and multi-scale characterization (XCT, BSE/EDS, XPS, TGA, NAD), we unravel the conflicting effects of carbonation. Results demonstrate that carbonation significantly accelerates steel corrosion, with the mean corrosion cluster volume doubling from 0.5 mm3 to 1 mm3 after just 4 h of carbonation at early age. While carbonation refined the pore structure of the mortar matrix, it also neutralized the alkaline environment, leading to the depassivation of steel and a one-order-of-magnitude increase in corrosion current density. Corrosion products extensively migrated into the mortar, with invasion distances up to 2 mm. These findings highlight a critical trade-off: although early-age carbonation densifies the microstructure, it concurrently creates a corrosive environment that severely compromises the passivation and long-term durability of embedded steel. Therefore, cautions should be alerted when early-age enforced carbonation is used to treat reinforced concrete.