Roles of subsequent curing on the pH evolution and further hydration for CO2 cured cement pastes

While CO2 curing has been found to improve the strength and resistance to external ion penetration of concrete, the corrosion susceptibility of rebar in precast reinforced concrete can be affected by the change in pH of the concrete. This study aims to improve the knowledge on the pH evolution and further hydration of carbonated cement pastes at different depths upon subsequent standard curing (hydration). For this purpose, the influence of water-cement (w/c) ratio (0.3 and 0.4) and standard curing time on the pH changes at different layers of cement paste (<5 mm, 5-10 mm and 10-20 mm from the exposed surface) was systematically investigated. The results showed that after 7 d CO2 curing, the pH value of cement pastes with 0.3 w/c ratio remained above 12.0, while the pH value of the outer layer in the carbonated cement pastes with 0.4 w/c ratio decreased to 11.6. This reflects that the higher w/c ratio tend to improve the carbonation reaction of cement pastes. However, with 1 d of subsequent standard curing, the pH of both specimens increased significantly. Based on the TGA, XRD and SEM results, it is inferred that the further hydration of unreacted cement particles in the specimens with 0.4 w/c ratio was inhibited, attributed to the reduced contact and reaction between unreacted cement particles and water caused by the large amount of encapsulating carbonation products and the dense surface by CO2 curing. In general, it can be said that the pH reduction by CO2 curing has little or no adverse effect on the durability of reinforced concrete due to the pH recovery of the cement matrix upon subsequent hydration.