Study on multi-scale dynamic mechanical properties of metakaolin modified recycled aggregate concrete under coupling conditions of sulfate attack and dry-wet cycles

To explore the potential of modified recycled aggregate concrete for use in construction projects, the evolution of its properties in the combined effects of environment and loading was investigated. This paper considered various factors, including the replacement amount of metakaolin (0 %, 15 %), dry-wet cycles (0, 20, 60, and 120 cycles), and the dynamic strain rate (ε˙ = 10−5/s, 10−4/s, 10−3/s, and 10−2/s), and conducted uniaxial compression, scanning electron microscopy (SEM), and X-ray diffraction (XRD) tests. The experimental findings demonstrated that the peak stress and elastic modulus of recycled aggregate concrete (RAC) exhibited a pattern of initial increase followed by a decrease before and after 20 dry-wet cycles, a concurrent upward trend was observed in both parameters with an escalation in strain rate and the incorporation of metakaolin (MK), and no uniform trend was observed for the peak strain. The test results from SEM and XRD revealed that the persistent formation of sulfate crystals and expansive substances like gypsum and ettringite were the primary factors contributing to the deterioration of specimen damage as cycle counts escalated. A quantitative analysis was conducted to investigate the mesoscopic damage mechanism associated with the dynamic mechanical properties and durability evolution process of metakaolin-based RAC under sulfate erosion. The correlation between the macroscopic mechanical properties and the microscopic changes of RAC was explored using statistical damage theory.