Comparative Study on the Carbonation-Activated Calcium Silicates as Sustainable Binders: Reactivity, Mechanical Performance, and Microstructure

Calcium silicate minerals can react with CO2 to form calcium carbonate and have been proposed to be a sustainable binder as a potential CO2 sinker. In this study, the carbonation characteristics are comparatively assessed among calcium silicates having different calcium/silica (Ca/Si) ratios and polymorphs (CS, C3S2, γ-C2S, β-C2S, C3S). Calcium silicate compacts exposed to a 100% CO2 environment at a 0.4 MPa pressure were tested for carbonation temperature evolution, degree of carbonation (DOC), mechanical properties, and microstructural characterization. Results indicate γ-C2S is the most reactive, reaching a DOC of 50% in 24 h, followed by C3S2, CS, β-C2S, and C3S, which generally agrees with the pattern of the cumulative normalized temperature increase. Meanwhile, carbonated β-C2S compact attains the highest compressive strength of 80 MPa in 24 h, followed by γ-C2S, C3S2, and C3S, while CS only reaches 20 MPa. Calcite and aragonite are the preferable polymorphs of calcium carbonate in the carbonated C3S, γ-C2S, β-C2S, and C3S2, while only the carbonation of CS generates vaterite in addition to calcite and aragonite. The unreacted grains coated by a thin rim of calcium-modified silica gels are encapsulated by the continuous calcium carbonates, which composes the skeleton of the carbonated calcium silicates.