Modification of recycled aggregate by spraying colloidal nano silica and silica fume

In this study, modification methods of recycled coarse aggregate (RCA), namely spraying colloidal nano silica (NS) and silica fume (SF) on the surface of RCA, were adopted to enhance the performance of recycled aggregate concrete (RAC). In the RCA modification methods, colloidal NS with different particle sizes (12 nm, 54 nm and 106 nm) and colloidal SF with a particle size of 420 nm were considered. To understand the effectiveness of silica with different particle sizes, mechanical properties and durability of RAC prepared with the modified RCA were evaluated. Microhardness and scanning electron microscopy (SEM) tests were conducted to investigate the microstructural properties. Test results showed that the mechanical properties and durability of RAC were enhanced remarkably after spraying colloidal NS and SF on the surface of RCA, whilst the performance enhancement was larger at a larger particle size of NS. Moreover, it was found that spraying colloidal SF on RCA enhanced both the new mortar and the new interfacial transition zone (ITZ) in RAC, while spraying colloidal NS only enhanced the new ITZ, as evidenced by microhardness investigations. This study deepens the understanding of enhancing the performance of RAC to approach that of natural aggregate concrete.