Carbonated calcium silicates as pozzolanic supplementary cementitious materials

This study examined the carbonation mechanisms and products of various precursors, including hydrated cement pastes and calcium silicate clinkers. Regardless of the materials investigated, all exhibited rapid carbonation reactions and achieved high degrees of carbonation. Calcium carbonate and siliceous gel were consistently formed as carbonation products. The starting materials significantly influenced the characteristics of the siliceous gel. Carbonated concrete pastes displayed a higher presence of alumina in the gel, resulting in greater surface area and nano-porosity compared to gels formed from calcium silicate clinkers. The pozzolanic reaction products also differed depending on the precursor, with carbonated concrete pastes forming C-S-H and AFm phases, while carbonated calcium silicate clinkers produced only C-S-H phase. Such variations impacted the evolution of mechanical performance in composite cements. Notably, carbonated composite cements containing recycled concrete pastes exhibited higher early compressive strength compared to carbonated calcium silicate clinkers. However, the final compressive strength is similar.