Low-content graphite powder for improving the compressive–tensile strength and fracture behavior of recycled aggregate concrete

Abstract

To mitigate the inherent drawbacks of Recycled Aggregate Concrete (RAC) produced from Construction and Demolition Waste (C&DW)—such as relatively low strength and pronounced microstructural defects—this study proposes the incorporation of a low content of Flake Graphite Powder (GP) to improve its mechanical properties. Through uniaxial compressive, splitting tensile, and three-point bending tests coupled with Acoustic Emission (AE) monitoring, the effects of varying GP contents on RAC’s strength, fracture toughness, and damage modes were investigated. Results reveal a non-monotonic effect of GP on concrete mechanical properties—enhancing them at low dosages but degrading them at high dosages. Notably, GP yields a significantly greater improvement in RAC compared to Natural Aggregate Concrete (NAC). With the addition of 1.0 wt% GP, the compressive strength and splitting tensile strength of RAC increase by 14.71% and 27.18%, respectively. Microstructural analysis reveals that GP improves interfacial bonding by filling surface pores and forming reinforcing elements within the Interfacial Transition Zone (ITZ). Furthermore, the bridging effect and crack deflection mechanism of the graphite platelets significantly enhance the fracture energy. AE parameters indicate that an optimal dosage of GP induces a transition in the failure mode of RAC from sudden brittle failure to a more progressive, damage-tolerant failure process, substantially broadening the pre-peak damage evolution interval. This study not only demonstrates the simultaneous enhancement of the strength and toughness of recycled concrete, but also provides a theoretical basis and technical pathway for the high-value utilization of construction and demolition waste.