Study on the modification effect of recycled fine aggregates treated by synergistic alkali activation using volcanic ash and cement powder wrapping

The synergistic treatment of recycled concrete fine aggregate (RFA), fly ash (FA), and recycled concrete powder (RP)—bulk solid wastes—achieves efficient resource utilization through waste-to-waste treatment, significantly enhancing both environmental and economic benefits. This study employs a semi-dry wrapping method for RFA, using wrapping materials that include cement, FA-cement, and RP-cement, along with a NaOH solution (4 mol/L) for synergistic enhancement. The reinforcement effects were assessed through the crushing index of the aggregate, as well as the mechanical and durability properties of the mortar, supplemented by tests such as X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG), and nanoindentation (NI) to investigate the enhancement mechanisms. The results revealed that the RP-cement coated treatment significantly enhanced the intrinsic strength of the aggregate. Specifically, the 28d strength of the mortar prepared with the FA-cement coated treatment increased by 25.4 %, demonstrating optimal environmental and ecological benefits at this stage. Both alkali-activated synergistic reinforcement methods improved the strength of the aggregate and mortar at early ages. Notably, the crushing index of RFA was reduced by 34 % with the RP-cement coated treatment, while the 3d compressive strength of the mortar increased by 20.1 % with the FA-cement coated treatment. The hydration products formed on the aggregate surface by the coating material provided substantial strength support. Furthermore, the synergistic alkali activation effect of FA and RP enhanced the hydration degree of the coating layer, thereby improving the overall strengthening effect. This study offers a novel approach to waste-to-waste treatment, contributing to sustainable resource utilization.