Effects of CO2 curing methods on frost resistance and mechanical properties of recycled aggregate concrete

Recycled aggregate concrete, as a construction material, can be applied to structural components (beams, slabs, columns, and foundations), auxiliary components, and temporary facilities with strength grades ranging from C30 to C50. Today, the world is facing an increasing pressure of greenhouse gas emissions. Utilizing carbon dioxide for the curing of recycled aggregates contributes to achieving carbon reduction goals. Freeze damage can cause deterioration of mechanical properties and surface spalling of recycled aggregate concrete (RAC). This study investigated the influence of three different CO2 curing methods, standard CO2 curing (CC group), CO2 curing with pre-saturated Ca(OH)2 immersion (HH group), and whole specimen CO2 curing (CRR group), on the properties of RAC subjected to freeze-thaw cycles. The results revealed that all the three CO2 curing methods improved the peak stress and elastic modulus of RAC by 12.3%-13.7% and 19.2%-31.5%, respectively. The HH group suffered the least mass loss rate and experienced the most improvement in the relative dynamic modulus. The improvement in the RAC impermeability was in the descending order of HH group > CC group > CRR group. It should be pointed out that RAC with a lower initial water-cement (w/c) ratio exhibited better permeability resistance. Compared with the reference group, the CC, HH, and CRR groups showed a respective 49.0%-58.7%, 55.8%-72.6%, and 45.5%-46.8% reduction in the capillary water absorption coefficients after being subjected to different CO2 curing methods. Among the three groups, the HH group with a w/c ratio of 0.35 and 0.55 showed better resistance to chloride ion penetration (which was further reduced by 9.4%-31.6% and 19.2%-41.1%, respectively).