Effect of Relative Humidity on the Carbonation Reaction Mechanism of Cement Pastes Containing CO₂ Fixing Admixture

Abstract

In this study, the effect of relative humidity on the carbonation reaction of cement (OPC) samples mixed with a CO₂ fixing admixture (CFA) consisting mainly of γ-C₂S (γ-2CaO・SiO₂) was investigated. The samples used for enforced carbonation consisted of raw CFA (hereinafter referred to as “CFA_powder”), cement paste made solely of OPC (“OPC_paste”), and a mixture of OPC and CFA (“OPC+CFA_paste”). There was no significant difference in the carbonation ratio between OPC_paste and OPC+CFA_paste, but both the carbonation ratio of CFA_powder was lower than that of OPC+CFA_paste. In addition, the amount of water adsorbed by OPC+CFA_paste was greater than that by CFA_powder, confirming that the carbonation reaction proceeds when moisture in the air is adsorbed onto the particles. Furthermore, CFA_powder contains both fine and coarse particles, including cracks due to γ-C₂S dusting, and the carbonation reaction proceeds not only on the surface of the particles but also within the cracks. Both OPC+CFA_paste and CFA_powder showed higher carbonation rates with increasing relative humidity. The reaction ratio of γ-C₂S after 7 days of carbonation is higher in the OPC+CFA_paste than in the CFA_powder. This shows that CFA reacts more with CO₂ in a mixture with OPC. XRD results show that CFA_powder has lower calcite content and higher aragonite content than OPC+CFA_paste. The FT-IR results showed a peak of calcium carbonate at 1400-1600 cm^-1. The calcite peak at 1420 cm^-1 is particularly large in OPC+CFA_paste, while CFA_powder has a broad peak at 1400-1600 cm^-1. Therefore, the CaCO₃ produced by OPC+CFA_paste has a high calcite content, which is consistent with the XRD results.