Performance evaluation of fibre-reinforced geopolymer concrete incorporating recycled coarse aggregate under elevated temperatures

The use of industrial by-products and recycled materials in geopolymer concrete has gained momentum due to its growing demand for sustainable and fire-resistant construction materials. However, the use of Recycled Coarse Aggregate (RCA) is often associated with lower mechanical and durability performance as a result of high porosity and weaker interfacial bonding. Therefore, this study aims to develop a high-temperature residual performance fly ash-based geopolymer concrete using fly ash, Metakaolin (MK), nano-alumina, RCA, and steel fibres and thereby explore its various mechanical, durability and thermal properties. In the case of RCA, 50% of the natural coarse aggregate was replaced while fly ash was replaced with MK. The results showed that the strength continued to increase until 90 days indicating the continued geo polymerization process. When compared to the control mix after 28 days, MK30 exhibited the highest increases in compressive strength 23.5%, split tensile strength 36.3% and flexural strength 35.6% out of all the mixes. Also, water absorption values, sorptivity values and RCPT values were found to decrease by 36.9%, 33.4% and 48% with the utilization of 30% MK. Moreover, better resistance to sulphuric acid and high residual strength up to 1200°C proved high thermal stability. The findings concluded that 30% MK replacement was the best-performing level for achieving balanced mechanical, durability, and thermal performance in sustainable geopolymer concrete containing RCA, nano-alumina, and steel fibres.