Effect of nano-SiO2 modified recycled coarse aggregate on the mechanical properties of recycled concrete

Abstract

Improving the performance of the interface between old mortar and recycled coarse aggregate (RCA) is a hot topic and important to enhance the properties of recycled aggregate concrete (RAC). In this paper, the RCA was soaked by nano-SiO2 (NS-RCA) then the water absorption and crushing index of NS-RCA were investigated. The mechanical properties, such as compressive strength, flexural strength, and elastic modulus of RAC prepared by NS-RCA (NS-RAC) were studied. The Thermogravimetric and Differential Thermal Analysis (TG-DTA) and scanning electron microscopy (SEM) were applied to analyze the properties of the interface transition zone (ITZ) around unmodified RAC and NS-RAC. A modified stress–strain model of NS-RAC was proposed considering the concentration of nano-SiO2 and soaking time. The testing results showed that the concentration of nano-SiO2 and soaking time had significant effects on the strength and durability of RCA and the mechanical properties of RAC. The 2% concentration of nano-SiO2 and 48 h soaking time had the best modification effect on RCA. The compressive strength, flexural strength, and elastic modulus of NS-RAC with best modification were 31.8%, 33.2%, and 89.6% higher than those of unmodified RAC respectively. The TG-DTA and SEM analyses indicated that NS-RAC had the denser microstructure of ITZ. This study is conducive to enrich the knowledge about using NS-RAC and supplying guidance for its mix design.