Property enhancement of recycled concrete aggregate treated by vacuum absorption and direct soaking of silica fume slurry

This study investigates the effects of vacuum adsorption (VA) and direct soaking (DS) treatments with silica fume (SF) slurry at different dosages and durations on RAC aggregate (RCA). Properties of RCA , including apparent density, water adsorption, porosity, crushing value, abrasion value, and SF adsorption capacity, were evaluated. In addition, microhardness, XRD, FTIR, TG, SEM, and EDS analyses were conducted to examine microstructural and chemical changes. Results indicate that SF treatment effectively improves apparent density of RCA, reduces water adsorption and porosity, and lowers both crushing and abrasion values. Specifically, abrasion values decreased by 11% and 20.9%, with DS and VA treatments, respectively, while porosity was reduced by up to 15% and 24%, and water adsorption by 10% and 20%. The use of SF-treated RCA provides better strength enhancement for RAC compared to mixes with equivalent SF added directly. The strength enhancement is attributed to the filling effect of SF that fills the pores and 'repairs' microcracks in RCA. Notably, VA treatment allows more SF to accumulate in the pores and cracks, resulting in a denser surface structure. Both treatments improve the strength of the interfacial transition zone (ITZ), and reduce its width. Microstructural analyses reveal that SF introduced via both VA and DS treatments participates in secondary hydration, generating more calcium silicate hydrate (C-S-H) or calcium aluminate-silicate hydrate (C-A-S-H), while reducing CH and AFt contents. The hydration products of SF-treated RCA exhibit lower Ca content and Ca/Si ratio, and higher Si content and (Al+Fe)/Ca ratio.