Mechanism of carbonating recycled concrete fines in aqueous environment: The particle size effect

Processing waste concrete in recycling facilities inevitably generates by-products such as very fine particles (<2.36 mm) and powders (<0.15 mm). The resourcing of these low-value recycled concrete fines (RCFs) has attracted increasing interest from the construction industry. This study attempts to elucidate the underlying mechanisms of particle size effects on carbonating RCFs via aqueous route. The results suggested that the relatively coarse particles (0.6–1.18 mm and 1.18–2.36 mm) experienced two positive effects i.e., i) improvement of surface properties by the formation of a reactive shell, and ii) significant densification of the microstructure. This was due to an initial carbonation mediated in the bulk solution, and then the internal carbonation due to the inward diffusion of carbonate species. Meanwhile, the finer particles (<0.15 mm) showed significantly different effects i.e., being totally disintegrated and converted to a calcium carbonate and silica gel composite. Such a difference was attributed to the long alkalinity maintaining ability and extensive dissolution and leaching associated with the high fineness. Whereas 0.15–0.6 mm was regarded as a transition particle size where balanced decomposition and densification were observed. Finally, after carbonating for only 6 h, the carbonated RCFs was found to enhance the compressive strength of pastes (as cement substitute) and mortars (as fine aggregate substitute) by an average of 13.2% and 9.0% in comparison with those prepared with raw RCFs.