Effects of high activator content on fly ash-based geopolymers exposed to elevated temperatures

Abstract

This article reports the influence of high alkaline activator content on the compressive strength and microstructure characteristics of a fly ash (FA) geopolymer system after exposure to elevated temperatures of 400 °C, 600 °C, and 800 °C. The sequential changes in the geopolymer gel structure after exposure to elevated temperatures and their effects on the residual strength were investigated through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that the high strength of the FA geopolymers reduced after exposure to 400 °C and 600 °C and failed in return any strength after exposure to 800 °C. The SEM results showed that the high activator content generated large quantities of unreacted crystals composed mainly from silicate underwent viscous sintering process at range of temperatures of 600 °C to 800 °C, and swelling resulting in system failure. XRD results showed that the geopolymers exposed to 800 °C exhibited significant decomposition in the aluminosilicate phase and amorphous hump compared to unexposed pattern.