Utilization of CO₂ in direct aqueous carbonation of concrete fines generated from aggregate recycling: Influences of the solid–liquid ratio and CO₂ concentration

Development of an effective aggregate recycling process for concrete waste is an important issue. During aggregate recycling, the hydrated cement fraction in concrete that decreases the quality of recycled aggregate is removed, and this yields concrete fines as a by-product. Appropriate treatment of the concrete fines is needed to improve the feasibility of aggregate recycling. In this study, concrete fines from aggregate recycling were used for CO₂ utilization through direct aqueous carbonation with a low energy input using low-purity CO₂ at atmospheric pressure. Two samples of actual concrete fines and one of synthetic concrete fines were compared in these experiments. The differences between CO₂ uptake efficiencies were explained according to the characteristics of the different concrete fines. Solid–liquid ratio and concentration of introduced CO₂ were studied and their effects on the reaction were determined and explained according to the three main steps of direct aqueous carbonation, Ca extraction, CO₂ dissolution, and CaCO₃ precipitation. The results are supported by and compared to the literature based on the carbonation mechanism and kinetics. Additionally, the mechanisms of particle collision and solidification were observed, and their effect was determined by several characterization analyses. The CO₂ uptake efficiency was compared with those from other studies, which proves concrete fines are a promising candidate for direct aqueous carbonation. The present method under atmospheric pressure with a low CO₂ concentration was effective for CO₂ utilization with concrete fines and had a CO₂ uptake capacity of 0.13 g-CO₂/g-concrete fines.