Synergistic application of weathered granite and recycled coarse aggregate in concrete: Mechanical properties and microstructural mechanisms

The simultaneous use of weathered granite (WG) and waste concrete in concrete can significantly increase the recycling rate of construction and demolition waste. However, the mechanical properties and microstructure of waste concrete crushed into recycled coarse aggregate (RCA) synergistically with WG in concrete remain unknown. This paper examines the mechanical properties and microstructure of using WG as coarse and fine aggregate, replacing RCA and natural fine aggregate, respectively. The results indicated that when the replacement rate of weathered granite coarse aggregate (WGCA) reached 60 %, the mechanical properties of recycled concrete (RC) were optimized, with 28-day compressive strength, splitting tensile strength, and flexural strength of 36.0 MPa, 2.76 MPa, and 5.0 MPa, respectively. In particular, the splitting tensile strength surpassed that of natural concrete by 0.31–0.36 MPa. This is mainly attributed to the synergistic effect produced by the differences in the properties of the two coarse aggregates. Specifically, part of the WGCA is crushed during the concrete mixing process, filling in the gaps between larger aggregates with smaller particles. This increases the concrete's compactness and results in a denser microstructure. Additionally, the presence of WG facilitates the generation of calcium silicate hydrate (C-S-H), enhancing the interfacial transition zone (ITZ) and improving aggregate strength. Conversely, the RCA inhibits the generation of surrounding calcium hydration products due to the presence of aged mortar, thereby weakening the strength of the ITZ. The 28-day mechanical properties of RC were optimized at 60 % replacement of weathered granite fine aggregate (WGFA), with compressive, splitting tensile, and flexural strengths of 37.2 MPa, 2.60 MPa, and 4.7 MPa, respectively. This peak was attributed to the synergistic effect of both WGFA and natural fine aggregate, which reduced pore production while promoting the production of C-S-H and improving the ITZ of RCA. It is recommended that WGCA and WGFA be applied separately in RC in practical projects to achieve the dual benefits of high performance and high utilization.