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Low-lime calcium silicate cement develops strength mainly through carbonation curing. However, long curing times can limit precast productivity. This study examined whether recycled coarse aggregates promote carbonation in CSC concrete via porous adhered mortar, which facilitates CO2 transport. Two mixes (CSC replacement 50%, W/B 0.45) were prepared: NCA-CSC50 and RCA-CSC50 (100% NCA replacement). After steam curing, the specimens were carbonated in 20% CO2 at 20 °C and 60% RH for 1–14 days. The carbonation degree was quantified from phenolphthalein-sprayed cross-sections by image binarization, and depth-dependent phase evolution and ITZ changes were assessed by XRD and SEM–EDS. RCA-CSC50 exhibited a higher carbonation degree and coefficient and achieved higher compressive strength, exceeding those of NCA-CSC50 after 3 days. XRD analysis performed after 14 days of carbonation curing revealed that portlandite peaks remained in NCA-CSC50 at depths of 35–50 mm, whereas they were not detected at the same depths in RCA-CSC50, indicating more extensive carbonation penetration in the RCA-containing mixture. This result is consistent with the quantitatively higher carbonation degree and carbonation coefficient of RCA-CSC50 compared with NCA-CSC50. SEM–EDS observations further revealed multiple ITZs around the recycled aggregate. Although the ITZs were not directly quantified as CO2 diffusion paths, their presence is likely associated with the enhanced carbonation observed in RCA-CSC50 by providing additional connected zones for CO2 ingress. These findings suggest that RCA can be considered not only as a recycled aggregate source but also as a potential means of facilitating CO2 transport in carbonation-cured CSC concrete. Furthermore, the combined use of carbonation-reactive binders and recycled aggregates is expected to contribute to the broader application of low-carbon concrete technologies by reducing construction waste and expanding the implementation of CCUS-based approaches.
Sang-Rak Sim (Mon,) studied this question.