Two-step carbonation of recycled aggregate concrete incorporating calcium-rich wastes for enhanced CO2 sequestration

This study develops a two-step carbonation approach to maximize CO 2 sequestration in recycled aggregate concrete incorporating calcium-rich waste materials as partial replacements for cement. In the first phase, different combinations of fine and coarse aggregate types (i.e., natural aggregates, recycled aggregates, and pre-carbonated recycled aggregates) were integrated in carbonation-cured concrete while measuring CO 2 sequestration potential and compressive strength. The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) and a sensitivity analysis were used to identify the mix comprising natural fine aggregates and pre-carbonated recycled coarse aggregates as optimal based on CO 2 storage capacity and compressive strength. Subsequently, the second phase investigated the synergistic impact of accelerated carbonation curing and partial cement replacement by calcium carbide residue (CCR) and waste paper ash (WPA) at levels of 5%, 10%, and 20% by binder mass on the properties of the optimum concrete mix from the first phase. Performance evaluation involved CO 2 sequestration potential, compressive strength, water absorption, and volume of permeable voids. Results showed that the two-step carbonation approach achieved an estimated gross CO 2 reduction by up to 70 g/kg while maintaining performance requirements for masonry unit applications. Maximum CO 2 sequestration potential and superior properties were attained in concrete with 20% and 5% WPA replacement, respectively, with 8-hour carbonation curing. The proposed approach provides routes to reduce cement demand and upcycle recycled aggregates and calcium-rich wastes while producing concrete with enhanced CO 2 sequestration potential, supporting decarbonization and circular economy efforts in the construction sector.