Combined effects of elevated temperature, sulfates and chlorides on performance of fly ash and metakaolin-based recycled aggregate geopolymer concrete

Previous studies have primarily focused on the effects of various supplementary cementitious materials (SCMs) on the mechanical and durability properties of geopolymers under isolated conditions such as elevated temperatures or sulfate exposure. However, the combined effects of fly ash (FA), metakaolin (MK), and recycled aggregates on geopolymers subjected to the simultaneous exposure of high temperatures, sodium sulfate, and sodium chloride have yet to be thoroughly investigated. Unlike earlier research, which tends to examine mechanical or chemical stressors in isolation, our study covers this gap by exploring the synergistic effects of these multiple stressors on key performance characteristics of FA- and MK-based geopolymer composites. This approach provides a more comprehensive understanding of the material's behavior in multi-stressor environments, closely replicating the complex conditions that real-world structures encounter. The present work investigates the mechanical, durability, and microstructural properties of recycled aggregate geopolymer concrete (RAGC) made with MK and FA, particularly under the influence of sulfate and salt following high-temperature exposure. To achieve this, five MK-based RAGC mixtures with varying FA contents were prepared. The samples were then subjected to elevated temperature in the range 200–800 °C. Afterward, the samples were immersed in 5 % sodium sulfate (SS) and 5 % sodium chloride (SC) solutions. The study assessed the effects of elevated temperatures combined with SS and SC on RAGC using mechanical strength, ultrasonic pulse velocity, mass loss measurements, capillary water absorption, scanning electron microscopy (SEM), X -ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). A statistical test was also performed to analyze the improvement in the results. Results showed that compressive strength increased with MK inclusion, achieving minimal losses even at high temperatures, with FMK-20, FMK-40, and FMK-60 showing only 11.65 %, 13.65 %, and 16.58 % losses at 200 °C. Capillary water absorption decreased with MK, with the FMK-20 blend showing a minimum absorption coefficient at 200 °C + SS. However, at 800 °C, absorption increased due to matrix degradation. After exposure to high temperatures with SS, crystalline formations like nepheline, quartz, calcite, and mullite were observed. Increased crystalline phases improved strength, but reduced quartz intensity and new cristobalite and anorthoclase phases appeared after high temperatures and SC. Conclusively, MK inclusion significantly improved RAGC's performance, particularly in elevated temperature and sulfate environments, confirming its potential for durable, high-strength concrete applications.