Enhancing the treatment efficiency of recycled concrete fines with aqueous carbonation

Recycled concrete fines (RCFs, 0.3 mm ≪ 2.36 mm) are the fine fractions of waste concrete generated during the recycling process. RCFs contain less natural aggregate and more residual cement paste than their coarser counterparts, thus more prone to carbonation. Although carbonation is beneficial for the microstructure and reactivity of RCFs by refining the pores and changing the phase compositions of the residual cement paste, carbonation rate is usually too slow for practical applications. In this study, two critical modifications (i.e., manipulating the initial carbonate concentrations and using elevating temperatures) were applied to aqueous carbonation of RCFs, aiming to improve the carbonation efficiency. Particularly, the rate of calcium carbonate (Cc) precipitation, the advance of Cc deposition front, the mineralogy, the development of microstructure, and the aqueous chemistry during carbonation were investigated using techniques of multiple scales. Based on the results, the rate of Cc precipitation and advancement of the Cc deposition front was significantly improved. The pore structure was drastically refined. Using elevated temperatures not only activated carbonate transport but also enhanced silicate polymerization, leading to the formation of a larger amount of silica gel as well as the increase in Q⁴/Q³ ratio, thus improving the reactivity of RCFs. A lab-scale production of carbonated RCFs was conducted, and the carbonation products were used for preparing cement mortars. Based on the compressive strength results, the carbonation treatment duration for RCFs could be reasonably reduced to hours and even minutes.