Synergistic enhancement of recycled aggregate concrete using cement slurry-treated aggregates and graphite nano/micro platelets

The use of recycled coarse aggregates (RCA) in concrete is an effective strategy for reducing construction and demolition waste; however, their inferior quality often leads to reduced mechanical and durability performance. This study investigates the synergistic effects of cement slurry-treated RCA and graphite nano/micro platelets (GNMPs) on the fresh, mechanical, durability, and microstructural properties of concrete. RCA replacement levels of 0%, 30%, 60%, and 100% were combined with GNMP dosages of 0.1%, 0.3%, and 0.5% by weight of cement. Results indicate that untreated 100% RCA reduced compressive strength and increased water absorption by more than 25-30% compared to natural aggregate concrete, whereas cement slurry treatment significantly improved aggregate quality and partially restored performance. The incorporation of GNMPs led to compressive strength increases of up to ~ 20-25%, along with corresponding improvements in split tensile and flexural strengths. Durability performance was also enhanced, with reductions in water absorption (~ 15-20%), sorptivity (~ 15-20%), and chloride ion permeability (shift from "low" to "very low" category). Freeze-thaw resistance improved significantly, with mass loss reduced by up to ~ 30-40% in GNMP-modified mixes. Mixes containing 30% RCA and 0.5% GNMPs exhibited the most balanced performance, achieving strength and durability comparable to or exceeding conventional concrete, while maintaining reduced permeability and refined microstructure. Microstructural analysis confirmed improved matrix densification and interfacial transition zone (ITZ) characteristics due to the combined action of slurry-treated RCA and GNMPs. The findings demonstrate that the synergistic use of treated RCA and GNMPs provides an effective pathway for producing high-performance and sustainable concrete suitable for structural applications.