The microstructure evolution of alkali-activated modified recycled aggregates and the strengthening mechanism of recycled UHPC

To address the high porosity, low strength, and poor bonding performance of recycled coarse aggregate (RCA), this study proposes an alkali-activated synergistic modification method. In this approach, RCA were first immersed in a sodium silicate solution and then wrapped with volcanic ash paste. The synergistic modification effect was evaluated by analyzing the water absorption, crushing value, elastic modulus, and other properties of RCA, as well as the mechanical properties of the ultra-high performance concrete (UHPC) incorporating them. Furthermore, the reaction products and pore structure of RCA were qualitatively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), nanoindentation (NI), mercury intrusion porosimetry (MIP), and X-ray computed tomography (X-ray CT) to elucidate the underlying modification mechanism. The results indicate that the alkaline solution dissolves active ions from the RCA surfaces, particularly from the old mortar and the added pozzolanic materials, thereby significantly enhancing the reaction degree between the old mortar and the pozzolanic materials. Compared with the untreated RCA group, the average elastic modulus of aggregates modified with sodium silicate and fly ash increased by 22.8% to 99.6%, while that of aggregates modified with sodium silicate and silica fume increased by 55.1% to 72.5%. In terms of pore structure, the porosity and proportion of large pores in aggregates modified with the 6% sodium silicate solution and volcanic ash slurry were significantly lower than those of unmodified RCA. When UHPC was prepared with a 100% replacement of modified aggregates, the 28-day compressive strength of the UHPC containing RCA treated with higher sodium silicate concentrations (4% and 6%) improved accordingly. As the sodium silicate immersion concentration increased, the amount of hydration products in the RCA-incorporated UHPC also increased. Among these products, the gel phases improved the pore structure of the interfacial transition zones (ITZs) between the old and new phases through both filling and bonding effects. Finally, data fitting verified that enhancement of the elastic modulus of RCA had a significant positive effect on improving the interfacial transition zone and mechanical properties of UHPC, thus providing a theoretical basis for the efficient application of recycled aggregates in UHPC.