Carbonation of supplementary cementitious materials and slag aggregates: Enhancing strength, durability, and CO2 sequestration in mortar

The global cement industry faces dual challenges of reducing clinker content and mitigating carbon dioxide emissions. This study explores an integrated strategy involving the accelerated carbonation of supplementary cementitious materials, including slag powder and fly ash, and steel slag aggregates, aiming to enhance mechanical performance, freeze-thaw durability, and CO 2 sequestration in mortar systems. Cement was replaced at 50% by mass with SCMs in carbonated and uncarbonated forms, while natural sand was fully substituted with uncarbonated or carbonated slag aggregates. Pre-carbonation of materials was performed under controlled CO 2 conditions, achieving uptakes of 14 wt% for slag aggregates, 25 wt% for carbonated fly ash, and 27 wt% for carbonated slag powder. Mechanical testing shows significant enhancements in strength with carbonation. At 28 days, carbonated slag and carbonated slag aggregate mortars achieved the highest compressive strength of 6,984 psi, representing a ∼35% increase over the Ordinary Portland Cement performance despite 50% cement reduction. Carbonated systems also displayed superior ultrasonic pulse velocity values, reflecting improved matrix density and interfacial transition zone integrity. Freeze-thaw testing confirmed enhanced durability, with carbonated mixes exhibiting slower UPV degradation and reduced microcracking compared to uncarbonated systems. Scanning electron microscopy analysis supported these findings, showing refined pore structure, dense C-S-H gel networks, and calcite precipitation in carbonated binders and aggregates. This dual carbonation approach offers both structural and environmental benefits: substantial early and late age strength gains, improved freeze-thaw resistance, and large-scale CO 2 mitigation potential, estimated at 33-38 Mt of CO 2 annually if applied to 25% of global slag and 10% of fly ash production. The results demonstrate the feasibility of integrating carbonation treatments into conventional concrete production, enabling the manufacture of sustainable, high-performance mortars with significantly reduced embodied carbon.