Revisiting the carbonation of recycled concrete fine: A pH-cycle carbonation method

Recycled concrete fine (RCF) has been demonstrated as a promising CO2 absorbent owing to its high-calcium and high-alkali contents. For the high-efficiency utilization of bubbled CO2 for the carbonation of RCF, this paper revisits the carbonation process by proposing a pH-cycle carbonation method. The carbonation of RCF was suspended when the pH of RCF suspension dropped to 7, and the subsequent carbonation was started when the pH reached constant values. The effect of the pH-cycle carbonation method on the performance of RCF-added mortars was investigated by preparing mortars using RCF suspension with different carbonation cycles. Experimental results showed that RCF spontaneously dissolved and released Ca2+ to the suspension during the intervals of carbonation cycles, which could save some CO2 that should be spent on attacking RCF in continuous carbonation approaches. In this method, the CO2 sequestration potential of RCF is approaching depletion by the bubble of 7 min CO2. The carbonation of RCF enabled the formation of CaCO3 crystals and nano-silica, leading to accelerated hydration of cement and refined pore structure of RCF-added mortars. The 28d compressive strength of RCFadded mortars was increased by 10.27-12.57% when RCF was subjected to different carbonation cycles. The two cycles of carbonation for RCF could be the optimal one.