Multiscale experimental and simulation analysis on the synergistic modification mechanism of nano-SiO₂/EVA at the recycled concrete interface

To enhance the reuse of construction waste in structural materials and improve the interfacial transition zone (ITZ) in recycled concrete (RC), this study employed two modification strategies and conducted a multiscale analysis to compare the effects of combined Nano-SiO2 (NS)/ethylene-vinyl acetate copolymer (EVA) modification and EVA-only modification on the mechanical properties, hydration characteristics, and microstructure of RC. The aim was to better overcome the interface defects in recycled concrete through synergistic modification. Experimental results show that the incorporation of EVA and NS significantly improves the mechanical properties of RC, with the combined modification outperforming the single modification. Specifically, when EVA content is 5% and NS is 2%, the 28-day compressive, shear, and flexural strengths of concrete increased by 15.7%, 17.6%, and 18.5%, respectively. Microscopic tests, including scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR), revealed that the cement matrix became denser, the porosity decreased, and the content of hydrated calcium silicate (C-S-H) increased. Molecular dynamics (MD) simulations indicate that EVA-only modification enhances interfacial bonding through hydrogen and ionic bonds, while the combined modification further strengthens the interface by forming additional Si-O-Si bonds, thereby improving the overall cohesion between the new and old concrete.