Study on mechanical properties and microstructure of recycled fine aggregate concrete modified by Nano-SiO2

In this study, the effects of different contents of Nano-SiO2 (NS) on a slump, compressive strength, splitting tensile strength, flexural strength, hydration products, and microstructure of recycled fine aggregate concrete (RFAC) were investigated. The results show that the addition of recycled fine aggregate (RFA) has a significant negative impact on the mechanical properties of concrete, but with the addition of NS, the mechanical properties of RFAC are improved. When the content of NS is 3%, the compressive strength, splitting tensile strength and flexural strength at 28 d of RFAC increased by 30.7%, 35.1%, and 18.7% respectively, and the durability was also improved significantly. Meanwhile, Thermogravimetric differential scanning calorimeter (TG-DSC), and X-Ray diffraction analysis (XRD) demonstrated that the addition of NS reduced the content of calcium hydroxide (CH) crystal in RFAC samples. Scanning electron microscopy (SEM) results show that NS doping can refine CH crystal, adsorb hydration products and improve the compactness of RFAC microstructure. The maximum efficiency is obtained when the content of NS is 3%. This study aims to research the possibility of using 100% recycled fine aggregate (RFA) in concrete and helps to maintain the better mechanical properties and durability of RFAC for a longer time.