Carbon dioxide uptake of recycled concrete aggregates during their carbonation treatment

Recycled concrete aggregates (RCAs) have gained significant attention as a sustainable alternatives to natural aggregates in the construction industry. Carbonation treatment of RCAs is one of the methods currently used to enhance their mechanical properties. This paper presents a spherical diffusion–carbonation model to simulate carbon dioxide transport and its reaction with cementitious materials involved in RCAs by taking into account their typical spherical geometry – a feature frequently neglected in traditional slab-based approaches. The model integrates essential parameters such as the carbon dioxide diffusion coefficient in the gaseous phase, carbonation reaction kinetics and the chemical composition of the parent concrete. The model was validated using carbon dioxide uptake data from existing experiments. The results showed that carbonation behaviour predicted by the spherical model differs noticeably from that of slab geometries. In contrast to the slab model, which yields a carbonation depth proportional to the square root of time, the spherical model exhibits a more intricate progression owing to the decreasing reactive surface area as carbonation proceeds inwards, which highlights the importance of considering the geometry and shape of particles during the carbonation treatment of RCAs.