Effect of Si/Al Ratio on Performance of Fly Ash Geopolymers at Elevated Temperature

This paper presents the results of an experimental investigation on performance of fly ash-based geopolymer pastes at elevated temperature exposure. Geopolymer paste specimens having Si/Al in the range 1.7–2.2, manufactured by activating low calcium fly ash with a mixture of sodium hydroxide and sodium silicate solution were subjected to temperatures up to 900 °C. The effect of Si/Al ratio was studied on the basis of physical appearance, weight losses, residual strength, volumetric shrinkage and water sorptivity at different temperatures. Scanning electron microscopy along with energy dispersive X-ray and X-ray diffraction analysis were also conducted to examine changes in microstructure and mineralogy during the thermal exposure. Specimens gradually changed in colour from grey to light red accompanied by the appearance of small cracks as the temperature was increased to 900 °C. Loss of weight and volumetric strain due to elevated temperature exposure were higher in specimens manufactured with lesser Si/Al ratios. Geopolymer paste specimen containing maximum Si/Al of 2.2 performed best in terms of residual compressive strength after exposure to elevated temperatures.