Improvement of crushed returned concrete aggregates by wet carbonation

A considerable quantity of pre-mixed concrete is returned from construction sites which is usually dumped directly into pits. While this concrete may possess lower quality attributes due to inadequate compaction and curing, the crushed hardened concrete can be potentially used to produce recycled aggregates. Carbonation is regarded as a significant way of improvement of recycled concrete aggregates. This study investigated the effectiveness of wet carbonation method for the improvement of crushed returned concrete aggregates (CRCA) to reuse in concrete productions. Recycled concrete aggregate (RCA) obtained from crushing of old conventionally cast concrete was also subjected to the same treatment, and its properties were compared with those of CRCA to understand the improvement in each type of recycled aggregate. The effectiveness of wet carbonation was evaluated by the aqueous chemistry during carbonation, development of microstructure, depth of calcium carbonate deposition, the mineralogy, water absorption, and compressive strength for both treated and untreated CRCA and RCA for different carbonation durations. The results showed that, although the water absorptions of both uncarbonated and carbonated CRCA were higher than those of RCA, the rate of reduction of water absorption of CRCA after carbonation was higher than that in RCA. Due to the long exposure to open environment, CRCA showed less initial alkalinity and conductivity than RCA but at the end of carbonation, both pH and conductivity were almost same as those of RCA. Wet carbonation successfully produced well shaped calcite in both CRCA and RCA where the deposition depth of calcite was higher in CRCA than in RCA. Mineralogical analysis showed higher calcium carbonate formation in CRCA indicating its potential of sequestrating more CO 2 than by RCA. The improvement of concrete compressive strength was more pronounced for CRCA, with a 28 % enhancement observed after 2 hours of carbonation, compared to an 18 % enhancement for RCA. These findings demonstrated that wet carbonation can be regarded as an effective strategy for enhancing CRCA. • For the same carbonation duration, more CaCO3 was produced in CRCA than in RCA. • The produced CaCO3 crystals were larger in RCA than in CRCA. • The rate of reduction of water absorption for carbonated CRCA was significantly greater compared to that of RCA.