Advancing concrete technology by two-step CO2 mineralization for enhanced performance and sustainability

Concrete carbonation typically reduces alkalinity, leading to issues with durability and reinforcing corrosion. Conversely, early-age carbonation and cement hydration combine to produce micro-level calcium carbonate, which densifies the concrete, fills pores, and increases ettringite volumes, all of which significantly enhance the material's strength and durability. Conventional CO ₂ use techniques, such as carbonation curing and conditioning of recycled aggregates, have drawbacks that limit their efficacy to surface layers, the need for large chambers and limited diffusion rates. To overcome these restrictions, the present research introduces a novel CO ₂ mineralization method during mixing, which is carried out in two steps. To produce CO ₂ -mineralized concrete, CO ₂ was first sequestered into cementitious materials, and then CO ₂ -sequestrated cementitious material is combined with the other ingredients of the concrete. The fresh properties of the CO ₂ -added concrete act equivalently to reference concrete with a marginal decrement, which can be deemed acceptable. The optimal CO ₂ mineralization dose of 0.2% enhances the compressive strength of concrete by 60.69% compared to conventional concrete. Microstructural investigations, carried out using scanning electron microscope, Fourier transform infrared spectroscopy, and thermogravimetric analysis, confirm the formation of additional calcium carbonate and refined hydration products in CO ₂ -mineralized mixtures. The strength improvement due to CO ₂ addition can be leveraged to design concrete with the same strength using a reduced cement content, and it was found that a 7.5% reduction in cement content in the optimum CO ₂ mineralized concrete can be achieved without compromising compressive strength. This method can reduce carbon emissions by 8%, saving 7.5% of Portland cement and maintaining equivalent durability to conventional concrete. Hence, this research demonstrates that incorporating CO ₂ as an admixture in concrete is a feasible practice for attractive concrete properties and mitigating CO ₂ emissions, providing double benefits.