Functional upcycling of nanomodified cementitious waste into recycled aggregate with multiscale reinforcement and environmental performance assessment.

The growing accumulation of cementitious waste and continued extraction of natural sand have increased the need for high-value recycling of cement-based materials. However, recycled cementitious particles are often limited by old hydration products, residual pores, and crushing-induced microcracks. Nano-modified cementitious waste may offer an alternative pathway because residual nanomaterials, if retained after crushing, could still influence pore structure and interfacial bonding. In this study, graphene oxide-modified recycled concrete particles (GO-RCP), produced from crushed GO-modified mortar, were used to partially replace natural sand in fresh mortar. At 40% sand replacement, metal-accessible cumulative porosity decreased from 18.87% to 14.38%, mean interfacial transition zone (ITZ) porosity within 0-40 μm decreased to 18.75%, and the normalized pore enrichment index declined from 9.27% to 4.37%. Compared with the control, compressive and flexural strengths increased by 27.39% and 21.95%, respectively. Deep Taylor decomposition revealed distinct classification-relevant regions, while the extracted-feature fractal dimension showed an exploratory correlation with compressive strength (R2 = 0.94). Molecular dynamics simulations further indicated that residual GO nanosheets can improve stress transfer, crack bridging, crack deflection and energy dissipation at the C-S-H/RCP interface. Under the baseline cut-off scenario, at 40% replacement, direct production cost and PED decreased by 15.97% and 1.23%, respectively, while absolute GHG emissions remained nearly unchanged (+0.08%). After strength normalization, PC, GHG, and PED decreased by 34.05%, 21.43%, and 22.46%, respectively. These findings demonstrate a potential route for converting GO-modified cementitious waste into a recycled fine aggregate with improved 28-day mechanical and microstructural performance, together with resource-saving and strength-normalized performance-efficiency benefits.