Modifying recycled aggregate concrete by aggregate surface treatment using sulphoaluminate cement and basalt powder

•Recycled aggregate (RA) was surface treated by sulphoaluminate cement (SAC).•Quality of RA and performance of recycled aggregate concrete (RAC) were improved.•Addition of basalt powder (BP) to the SAC slurry improved the modifying efficiency.•The ITZ1 and ITZ3 in the modified RAC presented a sandwich-like structure.•The enhanced ITZ1 and ITZ3 explains the mechanism of RAC’s improvement. Recycled aggregate (RA) was surface treated by two strengthening slurries prepared with sulphoaluminate cement (SAC), with or without the addition of an industrial waste, basalt powder (BP). Test results show that the quality of the surface treated RA was improved, reflected by the increased apparent density and the reduced crushing value and water absorption (by weight), meanwhile the mechanical properties and the resistance against chloride and sulphate attacks of the prepared RAC were enhanced. Based on the microhardness test results, the newly formed ITZ1s and ITZ3s in RAC using the treated RA demonstrated a sandwich-like structure. The significant enhancement of ITZ1s and ITZ3s and the surface strengthening of the old adhering cement mortar explain the mechanism of RAC’s improvement at the macro scale. It was also found that by adding a certain quantity of BP to the SAC slurry, RAC demonstrated better performance at both the micro and the macro scales. By also considering the environmental benefits by consuming BP as an industrial waste in some regions of China, the SAC-BP composite slurry deserves more exploration in further studies.