Total recycling of concrete waste using accelerated carbonation: A review

Our world is facing critical environmental challenges due to the overexploitation of natural resources and the extensive release of CO2. Meanwhile, huge amounts of concrete waste are generated due to accelerated urbanization and redevelopment. Carbonated concrete waste can thus both be used as alternative materials to aggregates and cement, and a carbon sink to reduce CO2 emission. In this paper, different carbonation approaches and their modified techniques to treat recycled coarse aggregates (RCAs), recycled concrete fines (RCFs), and recycled concrete powders (RCPs) are critically reviewed and discussed. The results suggest that carbonation effectively strengthens RCAs by densifying microstructures and improving physical properties through the precipitation of calcium carbonate (Cc) that fills the pores. Meanwhile, the carbonation reaction also develops surface reactivities on the recycled aggregates due to silica gel and Cc formation, which is particularly beneficial for RCFs because of the high surface area. Re-structuring and re-activation of RCPs through carbonation transforms the waste hydrated cement powders into novel supplementary cementitious materials. In addition to wet carbonation, which has a higher efficiency than dry carbonation, other modified carbonation techniques with the addition of limewater, magnesium ions, ammonium ions, and the use of elevated temperatures are found to produce RCAs/RCFs/RCPs with better qualities, and facilitate the formation of other high-value products such as nano-silica gel, micro-fiber, and vaterite, etc. Apart from their practical uses in conventional concrete, potential applications of the carbonation products in ultra-high-performance concrete, automated construction, and selfcleaning concrete were also briefly introduced. More importantly, the key issues in upscaling the carbonation technologies and limitations in current research are pointed out. Overall, carbonation could not only facilitate the re-circulating of concrete waste but also help reduce CO2 emissions and achieve carbon neutrality by permanently sequestrating CO2.