Chemically engineered carbonation of recycled concrete powder for high-reactivity precursors in fly ash-based low-carbon concrete: from ionic evolution to structural performance

This study proposes a Na₂CO₃-assisted wet carbonation method to enhance recycled concrete powder (RCP) reactivity for high-volume fly ash replacement in low-carbon concrete. RCP was carbonated in deionized water or 0.5 mol/L Na₂CO₃ before replacing fly ash at 20% and 30%. Na₂CO₃-assisted carbonation increased specific surface area from 954 to 1132 m²/kg, reduced portlandite below 1%, and more than doubled Si and Al leaching, producing a highly reactive precursor. At 30% replacement, 28-day compressive strength increased from 40.47 to 53.65 MPa (32.6%), while elastic modulus rose from 21.8 to 24.3 GPa. Water penetration, chloride permeability, and carbonation depth decreased markedly, indicating improved durability. Carbonated RCP achieved CO₂ uptake of 0.058 kg CO₂/kg, reducing concrete global warming potential to 124.35 kg CO₂/m³, about 30% below conventional fly ash concrete. These findings demonstrate an effective route for valorizing RCP while improving mechanical performance, durability, and carbon reduction through sustainable utilization.