The impact of fly ash and slag on the microscopic interface of recycled concrete and its destruction evolution

The interfacial transition zones in recycled concrete are the main factors affecting their durability and strength. The interfacial transition zones (ITZS) in recycled concrete are the main factors affecting durability and strength, yet their response to combined use of fly ash and slag remains insufficiently understood. This study investigates the influence of different replacement levels of recycled aggregate, fly ash, and slag on ITZS properties through both microscopic and macroscopic testing, combined with numerical simulation of the failure process. The results show that the minimal ITZS effect occurred when the recycled aggregate content was 40% and slag content was 10%, under which the compressive strength reached 32.8 MPa and the splitting tensile strength reached 3.21 MPa, representing reductions of only 4.5% and 5.2% compared with ordinary concrete, respectively. Microscopic observations revealed that cracks initiate predominantly in porous ITZS regions and propagate inward, a process well captured by the cohesive zone model simulations. These findings indicate that a balanced combination of recycled aggregate, fly ash, and slag can achieve mechanical performance comparable to conventional concrete, offering a viable approach to promote sustainable utilization of industrial by-products in structural applications. Specifically, under these conditions, the compressive strength reached 32.8 MPa and the splitting tensile strength reached 3.21 MPa, representing reductions of only 4.5% and 5.2% compared with ordinary concrete, respectively. These results indicate that partial substitution of recycled aggregate combined with moderate slag content can produce concrete with mechanical performance comparable to that of conventional concrete, while promoting sustainable use of industrial by-products in structural applications. In addition, the failure of recycled concrete was demonstrated based on microscopic results, from which the failure of recycled concrete was found to be a gradual failure from the outside to the inside. The failure zone first appeared in the interfacial transition zones with large porosities and large area, where internal cracks were generated and gradually spread. Structural failure occurred after the cracks had penetrated.