Degradation mechanism of hybrid fly ash/slag based geopolymers exposed to elevated temperatures

As a promising alternative to alkali-activated fly ash (AAF) for high temperature application, the degradation mechanism of alkali-activated fly ash/GGBS (AAFS) under high temperature is not clear. This work investigates physicochemical properties of AAFS up to 800 °C and presents their synergetic influence on the thermal behavior. A quantitative assessment of the crack is developed to learn the cracking behavior. Results reveal that the crack density exhibits a linear relationship with ultrasonic pulse velocity. The crack density and compressive strength exhibit a positive correlation before 100 °C, but a negative relationship beyond 100 °C. The addition of slag into geopolymers lessons the geopolymeric behaviors such as further geopolymerization and viscous sintering, but further aggravates the thermal damage owing to the compact structure and unstable hybrid gel. The conceptual models of AAF and AAFS are proposed to explain the degradation mechanism of low slag contained geopolymers under elevated temperatures.