Influence of recycled coarse aggregate quality on carbonation behavior of compression-cast fully recycled aggregate concrete

Compression-cast concrete (CCC) is an innovative precast concrete solution with great potential to enhance the overall durability of recycled aggregate concrete (RAC), including its carbonation resistance. However, there is currently no research on the carbonation performance of compression-cast RAC. This study explores the carbonation behavior of compression-cast RAC, considering the effects of recycled concrete aggregate (RCA) type, casting method, and w/c ratio. A series of characterization techniques was adopted to elucidate the impacts of these parameters on pore structure, hydration products, carbonation compounds, and microstructural features of concrete. The results revealed that compared to the normal-cast concrete (NCC) counterparts with the same mix parameters, CCC specimens possess a denser microstructure and reduced porosity, which consequently endow them with notably improved carbonation resistance. High calcium hydroxide content in the mortar matrix does not guarantee enhanced carbonation resistance, with porosity affecting the carbonation process far more than alkaline substance content. Whether it is NCC or CCC, the inferior quality of RCA will significantly increase the carbonation depth of concrete. Based on the existing carbonation depth prediction model, a modified models for compression-cast RAC were proposed by incorporating a compression casting pressure parameter that influences both the aggregate mixture’s equivalent water absorption and the w/c ratio. For the CCC dataset, the modified model demonstrated strong predictive capability, with a correlation coefficient of 0.93 between calculated and experimental results.