| Summary: | The carbonation of recycled aggregate was accelerated by sparging with supercritical carbon dioxide (scCO<sub>2</sub>) to reduce the amount of time needed for carbonation, which is necessary for the pH neutralization of recycled aggregate. To accelerate the carbonation process, pressurized scCO<sub>2</sub> was sparged into two different types of recycled aggregates immersed in water for 1 h, followed by standstill for 2 h (in total, a 3 h treatment process). The reduction of the pH of the treated aggregates due to carbonation was investigated using batch extraction experiments. A continuous column extraction experiment for the scCO<sub>2</sub>-sparged recycled aggregate was also performed to identify the effect of pH reduction under the condition of non-equilibrium reaction. From XRD, SEM/EDS, and TG/DTA analyses, much of the portlandite in the recycled aggregates was consumed. In its place, calcite was created as a secondary mineral during only 3 h of treatment (1 h scCO<sub>2</sub> sparging and 2 h stationing), indicating satisfactory carbonation of the aggregate. The results of the batch extraction experiments for both of the two recycled aggregate types also showed that the average pH of scCO<sub>2</sub>-sparged aggregate decreased from 12.0 to <9.8 (the tolerance limit for recycling). The pH of the eluent from the column packed with the scCO<sub>2</sub>-sparged aggregate also remained as <9.8, suggesting that a 1 h scCO<sub>2</sub> sparging process is sufficient to carbonate waste concrete aggregate and to create an alternative construction material resource.
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