Mesoscopic damage behavior of recycled aggregate concrete modified with metakaolin under the combined effects of freeze-thaw cycles and sulfate attack

The study investigated the damage evolution of recycled aggregate concrete (RAC) following various freeze-thaw cycles in a mass fraction of 5% sodium sulfate solution. Additionally, modified RAC specimens were created by partially substituting cement with metakaolin (MK) at substitution rates of 0% and 15%. The specimens were characterized using uniaxial compression testing, nuclear magnetic resonance (NMR) testing, scanning electron microscopy (SEM) testing, and X-ray diffraction (XRD) testing. The findings demonstrated that in the early cycles, physical damage caused by freeze-thaw primarily dominated, the limited amount of sulfate products filled the pores and generated a positive effect. In the later stages, the generation of numerous sulfate crystals and expansive substances like gypsum and ettringite were the primary factors contributing to the deterioration of specimen damage, increasing porosity and coarsening of pore size distribution. Furthermore, the inclusion of MK compacted the internal structure and alleviated the deterioration of RAC. Finally, based on statistical damage theory, it was believed that there were two damage modes at the mesoscopic: fracture and yield, and the failure mechanism of the specimens under freeze-thaw conditions was analyzed from the perspective of effective stress. By analyzing the evolution law of characteristic parameters, the connection between macro-, micro- and meso-cross-scales was constructed.