Synergistic enhancement effect of recycled fine powder (RFP) cement paste and carbonation on recycled aggregates performances and its mechanism

The recyled aggregate (RA) prepared from construction and demolition (C & D) waste is used for concrete materials, which helps to save natural resources and solve the problem of construction waste disposal. However, the defects of RA with the loose surface, cracks, and pores limit its resource utilization in building materials. In this study, RA was reinforced by wrapping with recycled fine powders (RFP) cement paste. On the basis, the synergistic effect of RFP cement paste and carbonation on the performance of RA was studied, and its strengthening mechanism was also analyzed. The results showed that the apparent density and crush value of RA were improved to some extent after RFP cement paste treatment, but the water absorption was increased significantly. RA showed a more significant effects after the synergistic treatment of 20% RFP cement paste and carbonation: the apparent density of RA increased by 5.6% and the crushing value decreased by 44.87%, while the water absorption increased by only 11.2%. Microscopic analysis showed that the treated RA with RFP cement paste and carbonation to generate a large amount of calcium silicate hydrate (C-S-H) gels and CaCO3 products, which filled the pores, microcracks, and interfacial transition zone (ITZ) of RA and also reinforced the loose particles on the RA surface. The parameters showed that the porosity of RA decreased by 25.1%, and the CaCO3 content increased from 11.1% (only wrapped paste treatment) to 20.75% (synergistic treatment). Adding an appropriate amount of RFP can not only save cement and reduce cost, but also improve the strengthening effect.