Direct aqueous mineral carbonation of cementitious fines derived from hardened ready-mix concrete waste: Process performance and carbon accounting.

The growing need to mitigate anthropogenic CO2 emissions is driving the development of low-energy carbon capture and utilization technologies. Hardened ready-mix concrete waste represents an underexplored alkaline feedstock for direct aqueous mineral carbonation, offering opportunities for simultaneous CO2 sequestration and waste valorisation. This study investigates direct aqueous mineral carbonation of concrete fines derived from hardened ready-mix concrete waste under ambient conditions using a CO2 gas blend representative of cement plant flue gas. The influence of solid-to-liquid (S/L) ratio on dissolved Ca evolution, CO2 uptake, CaCO3 production, and degree of carbonation was evaluated, while a gate-to-gate carbon accounting framework assessed the environmental performance of the process. Geochemical equilibrium modelling was used to elucidate the mechanisms governing dissolved Ca evolution during carbonation. Feedstock and carbonation products were characterised. Surface morphology, thermogravimetric, and phase analyses confirmed successful carbonation and the formation of crystalline calcite. An S/L ratio of 1:10 achieved the highest total CO2 uptake of 100.9 g-CO2/kg-concrete fines (20.2 g CO2 per 2 L batch), with 84.5 g-CO2/kg-concrete fines permanently fixed as crystalline calcite. The modelling attributed the observed Ca rebound to continued decalcification of Ca-bearing phases coupled with evolving carbonate speciation. Carbon accounting indicated a net-positive footprint under laboratory conditions; however, substituting technical gases with industrial waste flue gas shifted the process toward a near net-zero carbon footprint. These findings establish hardened ready-mix concrete waste as a promising feedstock for direct aqueous mineral carbonation and highlight process integration with industrial flue gas and product valorisation as key pathways towards industrial implementation.