Effect of silica fume on frost resistance and recyclability potential of recycled aggregate concrete under freeze-thaw environment

In this study, the effects of silica fume (SF) dosages (0 %, 3 %, 5 %, 8 %, 11 %, 13 %, and 16 %) on the mechanical properties (compressive strength and splitting tensile strength) and frost resistance (relative dynamic elasticity modulus and mass loss rate) of recycled aggregate concrete (RAC) prepared with 100 % recycled concrete aggregate (RCA) under freeze-thaw (F-T) environment were investigated. To assess the recyclability potential of RAC, the apparent density, water absorption, crushing index, and attached mortar content of second generation RCA after F-T cycles were also studied. The phase composition and microstructure near the interfacial transition zone (ITZ) of RAC were characterized by X-ray diffraction, thermogravimetric, scanning electron microscopy, and microhardness. The results indicate that the incorporation of SF effectively improves the mechanical properties and frost resistance of RAC, with an optimal SF dosage around 13 %. After 400 F-T cycles, the compressive strength of RAC with 13 % SF dosage can still reach 40.1 MPa, which is classified as medium strength concrete. All second-generation RCAs achieves Class III, but RCA obtained from RAC containing 13 % SF has higher potential to upgrade to Class II and reuse in 40 MPa structural concrete. During the F-T process, the ITZ width of RAC with 13 % SF dosage only increased by 3 mu m, and the average hardness of ITZ decreased by 6.7 %. The microscopic analysis showed that an appropriate dosage of SF is conducive to denser ITZ with higher hardness and narrower crack width, thereby hindering crack propagation within RAC during F-T cycles.