The alkaline nature of cementitious materials has high potential of carbon sequestration. Research has shown that carbonation curing to cementitious materials at early age can improve their strength and densify their microstructure. Consequently, it is expected that carbonation curing can enhance the durability of concrete structure. However, carbonation can naturalize the alkaline environment of concrete, raising risks of steel corrosion. The debate between the carbonation-induced microstructure improvement and the neutralization-raised corrosion risks has not been resolved yet. Therefore, it provides great incentives to understand if carbonation of cementitious materials at early age accelerates corrosion of steel bar or not.

A hybrid calcium-sulphoaluminate and Portland cement (CSA-PC) mortar was prepared to coat steel bars of 0.7 mm. Three carbonation durations of 4, 24 h and 72 h were designed to treat the CSA-PC mortar. The alkalinity of the composite mortar after carbonation was characterized by phenolphthalein chromatography, and the water sorptivity of non-carbonated and carbonated mortar specimens was measured by using a contrast-enhancing X-ray computed tomography (XCT). The corrosion process of the steel bars was measured by open-circuit potential and corrosion current density up to 40 chlorine-salt drying-wetting (CSDW) cycles. The microstructure of the mortar and the corrosion rust distribution of the steel bars after the CSDW action were characterized by BSE-EDS and image analysis.

Early-age carbonation significantly reduced the alkalinity of the mortar matrix, which recovered after standard curing up to 28 d but remained below the alkalinity threshold for the formation of passivation of steel (pH=11.5). Carbonation densified the pore structure of the hybrid cement mortar and reduced the capillary water absorption. The water sorptivity of the carbonated mortar specimens was significantly lower than that of the uncarbonated specimens as observed by XCT, indicating an improvement in the impermeability of the mortar matrix. After 10 CSDW cycles, the open circuit potential of the carbonated specimens decreased significantly and the corrosion current density increased by nearly one order of magnitude. The early-age carbonation significantly increased the probability and rate of corrosion of the steel bars in the mortar under cyclic CSDW actions. BSE-EDS analysis further showed that after carbonation, the rebars were severely corroded with corrosion pits up to 200 µm in depth and 20% in area; corrosion products migrated and filled the mortar matrix around the rebars, generating cracks.

The alkalinity of the mortar matrix decreased after early age carbonation, following recovery to a certain value after standard curing, which remained below the alkalinity threshold for steel passivation. Carbonation was able to improve the compactness of the mortar matrix and enhance the material's impermeability. The open circuit potential of the carbonated material was lower and the corrosion current density was one order of magnitude higher after 10 CSDW cycles. Serious corrosion of the rebar occurred in the mortar after carbonation, the corrosion products migrated to the surrounding mortar matrix, and the volumetric expansion led to cracking of the matrix. The findings suggest that although early age carbonation may help to improve the performance of cementitious material matrix, it may be detrimental to the durability of reinforced concrete.