Research on the Activation and Enhancement Mechanisms of Recycled Concrete Powder in Alkali-Activated Cementitious Materials and Their Carbon Emission Characteristics

Recycled concrete powder (RCP) utilization as an auxiliary cementitious material absorbs construction waste and promotes low-carbon transition in construction by replacing high-carbon materials. This study optimized RCP’s particle size and amorphous SiO2 content through physical activation, systematically investigating its effects on alkali-activated cementitious materials (AACMs). The results demonstrated that 20% activated RCP enhanced compressive strength by 9% (34.2 MPa), only 12.7% lower than that of the reference samples. Hydration analysis revealed activated RCP delayed exothermic peaks but increased total heat via active particles. Life-cycle assessment showed substituting 20% ground granulated blast-furnace slag (GGBS)/fly ash (FA) with RCP reduced carbon emissions from 169.3 to 165.9 kg CO2-e/ton (−2.1%). Although activation slightly raised emissions to 166.6 kg CO2-e/ton, RCP’s carbon contribution remained at 9% versus GGBS’s 83% dominance. Crucially, the activation’s 0.7 kg CO2-e/ton increase was offset by 4.7 kg CO2-e/ton reductions from material substitution and waste recycling benefits, confirming its net carbon-neutral potential.