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.
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