Research on the influence and mechanism of recycled UHPC fine aggregates containing fibers on the performance of mortar

The recycling and utilization of waste Ultra-High-Performance Concrete (UHPC) is often overlooked due to its smaller quantity compared to traditional waste concrete. This study investigates the influence of recycled UHPC fine aggregate (R-UHPC-FA) replacement ratios (0 %, 25 %, 50 %, 75 %, and 100 %) on the physical, mechanical, shrinkage, and frost resistance properties of mortar. The results revealed that increasing R-UHPC-FA content reduced the flowability of mortar. The mortar density increased initially and then decreased, while water absorption followed an inverse trend. At 28 days, compressive and flexural strengths of the mortar containing 100 % R-UHPC-FA improved maximally by up to 8.43 % and 25.16 %, respectively, compared to mortars without R-UHPC-FA. Furthermore, a 100 % replacement ratio reduces drying shrinkage and enhances frost resistance, which is regarded as the best replacement rate. Microscopic analysis indicated that the improvements in mechanical properties and durability of mortar were attributed to the dual effects of steel fibers and highly active substances in R-UHPC-FA. • Recycled UHPC fine aggregate (R-UHPC-FA) is a future quality construction solid waste. • Physical-mechanical properties, drying shrinkage and freeze-thaw resistance of mortar with R-UHPC-FA were investigated. • Microscopic properties of mortar with R-UHPC-FA were investigated by SEM, XRD and MIP. • The optimal replacement rate of R-UHPC-FA is expected to reach 100 %.