Carbonation hardening of Portland cement with recycled supplementary cementitious materials

This study investigated composite cements with recycled concrete pastes (RCP) and the carbonated analogue, comparing them to Portland and limestone cements. The carbonation curing resulted in a carbonation degree of around 30%. The presence of supplementary cementitious materials had little impact on the carbonation degree and phase assemblage. Cement pastes consisted of ettringite, calcium carbonate, C-S-H phase and silica gel. This phase assemblage transformed upon further hydration. The alumina-silica gel from cRCP did not contribute significantly to the reactions but modified porosity. The hydrates from RCP carbonated, however did not contributed to the strength evolution. Still, replacing limestone with RCP positively contributes to environmental sustainability by increasing CO₂ sequestration. Composite cements had lower strength, but those with carbonated RCP showed higher compressive strength and faster strength evolution. This effect was related to the appreciable porosity distribution compensating for the clinker dilution impact and a fast clinker hydration during the post carbonation curing.