Roles of carbonated recycled fines and aggregates in hydration, microstructure and mechanical properties of concrete: A critical review

CO2 mineralisation by recycled concrete is a novel approach of carbon capture and utilisation to improve the sustainability of concrete products, which has attracted increasing attention in recent years. This paper presents a comprehensive review on carbonation mechanisms related to CO2 mineralisation and hydration, microstructure and multiscale mechanical properties of concrete containing carbonated recycled concrete fines and aggregates, with special focus on the chemistry-microstructure-property relationships. The carbonation kinetics of recycled fines in aqueous environment exhibit a typical particle size effect, while the high pozzolanic reactivity of carbonated recycled fines can facilitate the nucleation and stabilization of hydration gels and contribute to the rapid hydration of cement paste. Carbonated recycled aggregates are favourable in enhancing the micro-mechanical properties of interfacial transition zone in concrete through the synergic effects of physical interlocking and chemical bonding. Compared to concrete with plain recycled aggregates, up to 33.0%, 12.1% and 28.7% rises in compressive, splitting and flexural strengths of concrete can be achieved under 100% replacement of carbonated recycled aggregates. Due to the Stefan effect, concrete with carbonated recycled aggregates has higher dynamic peak stress but lower strain-rate sensitivity of elastic modulus than concrete with plain recycled aggregates. The global CO2 mitigation capacity of recycled concrete varies from region to region and thus reasonable transportation network and high-efficient carbonation process are essential to ensure a balance between the environmental and economic benefits. This review summarises the recent advances in the field and discusses some future research opportunities to develop low-carbon concrete with recycled concrete as a carbon sink.