Circular concrete through acid leaching and carbonation of cementitious materials

Cementitious waste, especially the fine fraction (< 4.75 mm) of waste concrete, is either downcycled or landfilled. In this research, waste concrete fines and cement paste are treated by acetic acid leaching and carbonation for upcycling. The optimal leaching conditions are determined, which maximise the Ca2+ extraction and minimise acid use and dissolution of SiO2. The recycled sand extracted under the optimal leaching conditions are comparable to virgin sand. Total replacement of virgin sand with recycled sand produces cement mortars with improved compressive strength. The silica-rich residue (SR) from acid leaching has similar or higher pozzolanic reactivity than coal fly ash. After heat treatment under 900oC, the specific surface area of SR from cement paste is reduced by 99 %. The mortars with 20% cement replacement of this material have good workability and comparable compressive strength to mortars containing traditional supplementary cementitious materials. The Ca2+rich solution is carbonated, and 99.1% pure vaterite CaCO3 is produced. Vaterite can be transformed to aragonite in a solution containing Mg2+ and Sr2+ under 60 ℃, forming a strong fibrous interlocked structure. The properties of aragonite binder can be controlled by the liquid/solid (L/S) ratio. Lower L/S ratios generate higher compressive strength. Higher L/S ratios lead to lower density, higher porosity and lower thermal conductivity. Aragonite binder can form novel lightweight load-bearing thermal insulating materials. Currently, this process is costly and carbon positive. However, this can be improved by acid/alkali recovery, using ammonium salts and developing products with higher value. Based on laboratory results, this process can achieve a net CO2 sequestration of 93.8 kg/tonne of waste concrete with the use of sustainable acetic acid and ammonia and zero-carbon energy. This research demonstrates the potential for the scaled-up application of indirect carbonation and the development of circular concrete.