The construction industry contributes substantially to environmental problems through the extraction of non-renewable resources and the generation of construction and demolition waste. In concrete production, replacing natural aggregate with recycled concrete aggregate (RCA) has the potential to improve environmental sustainability. Although RCA is widely recognised as environmentally beneficial, its current use in high-performance concrete (HPC) remains limited because recycled aggregates generally exhibit poorer properties than natural aggregates, particularly in terms of porosity, water absorption, and compressive strength. This study investigates the influence of varying RCA replacement levels (15%, 30%, 50%, 75%, and 100%) and thermal loading of up to 900 °C on the key physico-mechanical properties and microstructural integrity of HPC. The methodology involved the preparation of HPC mixtures incorporating different RCA contents, followed by testing of flexural strength, compressive strength, and bulk density after exposure to thermal loading. The results showed a decline in both strength characteristics with increasing RCA content and temperature. However, certain mixtures, particularly those with 15% and 50% RCA, demonstrated comparable or improved performance under specific conditions. The 15% RCA mixture reached a compressive strength similar to that of the reference mixture after 28 d, while the 50% RCA mixture exhibited better flexural strength retention at 105 °C. Bulk density consistently decreased with increasing RCA content and temperature, with the greatest loss observed in the 100% RCA mixture at 900 °C. These findings clarify the potential and limitations of using RCA in HPC, especially where high-temperature resistance is required. Scanning electron microscopy analysis confirmed that, while internal curing at 105 °C contributed to densification of the interfacial transition zone (ITZ), the ITZ between the adhered mortar and the new cement matrix became the critical failure region at 900 °C. Thermal loading caused cracks to form in this ITZ, with their width increasing by up to four times at 75% RCA replacement. The results identify this ITZ as the weakest link in the structure and show that its degradation directly causes the loss of mechanical integrity of HPC containing RCA at high temperatures.
Gandel et al. (Tue,) studied this question.