Effect of silica fume content on the mechanical strengths, compressive stress–strain behavior and microstructures of geopolymeric recycled aggregate concrete

•Silica fume enhanced the mechanical properties of geopolymeric recycled aggregate concrete.•The concrete matrix and aggregate-matrix ITZs were improved by the addition of SF.•10% SF content is more recommended to improve the mechanical properties of GRAC. As a low-carbon sustainable concrete material, geopolymeric recycled aggregate concrete (GRAC), fabricated with industrial wastes (e.g. slag) and recycled concrete aggregate (RA), has an extensive application prospect in practice engineering. In this study, silica fume (SF) was incorporated to improve the mechanical properties of GRAC. The basic mechanical strengths, compressive stress–strain behavior and microstructures of GRAC with various SF and RA contents were investigated. The modification mechanism of SF was analyzed by combining the microstructural measuring results. The results indicate that the SF content has complicated influence on the mechanical properties of GRAC. The compressive, splitting tensile and flexural strengths as well as elastic modulus both increase first with the SF content increases to 10% but then decrease. The mechanical strengths of GRAC with 50% RA and 10% SF contents are comparable to GC without RA due to the filling and pozzolanic effect of SF. The 10% SF content decreases the total porosity of GC paste by 43.33% compared to the pure GC paste. While the small reduction in the porosity of GC paste with 15% SF content leads to a slight decrease in the mechanical performance of GRAC. From the present study, 10% SF content in GRAC is more recommended to improve the mechanical properties of GRAC, which provides strong supports for the basic researches and applications of GRAC modified with SF.