Enforced aqueous carbonation of heterogeneous cementitious waste fines: CO 2 uptake and performance as supplementary cementitious materials in Portland cement

Enforced carbonation of cementitious wastes is a strategy to mitigate CO2 emissions and generate reactive supplementary cementitious materials (SCMs), yet few studies have analyzed real sources. In this study, three heterogeneous wastes were investigated: hardened cement paste (HCP), concrete truck washout sludge (RCS), and young CDW fines (YCW). The hydrated cement content of the cementitious wastes was 87%, 62%, and 28%, respectively. Enforced aqueous carbonation (100% CO2, 0.02 L·min−1·g−1, L/S 10:1, 6 h) was applied. Carbonation degrees reached 99%, 76%, and 88%, while CO2 uptake was 372, 112, and 69 kg CO2/t. Specific surface area increased up to 156%, and mesoporosity doubled. Carbonated powders tested as SCMs at 30% clinker replacement delivered 27–34% strength gains at 7 days and 40–45% at 28 days above dilution, with RCS matching HCP performance. Carbonation degree alone was insufficient to assess carbonation performance; quantity of CaO available governs both sequestration efficiency and SCM quality.