A comparative study on geopolymers synthesized by different classes of fly ash after exposure to elevated temperatures

Due to more restrictions in disposal regulations, limited accessibility of landfills, and growing disposal costs, more sustainable methods are needed to recycle coal fly ash. Coal fly ash-based geopolymer is regarded as an environmental-friendly alternative cementitious material to ordinary Portland cement for its sustainable characteristics such as less CO₂ emissions, less energy consumption, and less leaching of toxic elements. Mechanical properties and fire-resistance of geopolymer concrete have been investigated in previous studies. However, there lacks a systematic study on the thermal-mechanical behaviors of geopolymer derived from different classes of fly ash after exposure to high temperature (>800 °C). This study presents a detailed comparative study concerning the effect of elevated temperatures (up to 1200 °C) on Class C and Class F fly ash-based geopolymer pastes. The thermo-physical behavior, engineering performance, and microstructures of the geopolymers before and after the exposure have been investigated. In addition, sustainability analysis of the raw materials and geopolymer was also presented in this study. The findings demonstrate that Class F fly ash-based geopolymer pastes have better mechanical properties and thermo-physical performance when the heating temperature is below 500 °C. On the other hand, Class C fly ash-based geopolymer specimens have better thermal performance and higher retention ratio of strengths after being exposed to higher temperatures (>800 °C). The results can provide more detailed guidance for engineers to sustainably use the fly-ash based geopolymer in different specific temperature ranges.