Composite cements with aqueous and semi-dry carbonated recycled concrete pastes

Abstract

Carbonation of recycled concrete paste (RCP) may reduce cement industry’s CO2 emissions by sequestering CO2 and by using the carbonated material in new composite cements enabling a decrease in the clinker content. This study examines effects of wet and semi-dry carbonation of RCP on the hydration reactions and performance for composite cements incorporating the carbonated material. The two carbonation methods lead to differences in composition and morphology of the carbonated RCP’s. Semi-dry conditions produce CaCO3 as aragonite instead of calcite and results in a lower carbonation degree and denser particles. Despite these differences, the effects on the hydration and performance of the two types of carbonated RCP in composite cements are similar. For both methods, the fast reaction of the alumina-silica gel results initially in a C-(A)-S-H phase with a low Ca/Si ratio, resulting in early-age microstructure densification and higher early strengths. The Ca/Si ratio of the C-(A)-S-H phase increases again when all alumina-silica gel has reacted after a few days of hydration. It is found that the differences in cement performance are associated with different carbonation degrees of the two RCP’s rather than morphological changes between the carbonated pastes.