The effect of different types of class F fly ashes on the mechanical properties of geopolymers cured at ambient environment

This research investigates the effect of the physicochemical characteristics of five cognate fly ashes on the mechanical properties of Class F fly ash-based geopolymers synthesized with the same mix proportion and cured at ambient conditions. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and low-field nuclear magnetic resonance were conducted to create the quantitative connection for geopolymers between micro-characteristics and macro-mechanical properties. With detailed experimental study, the research reveals that the difference in the inherent characteristics of fly ash such as specific gravity, median particle size, and amorphous phase content causes the formation of geopolymers with varied micro-characteristics and hence the discrepancy in the mechanical properties. The compressive strength increases with increasing geopolymers' amorphous phase content and decreasing porosity and Si/Al molar ratio of geopolymer binder, expressed with the reliable numerical relationships. Besides, a single and comprehensive reactivity index is proposed to assess the suitability of fly ashes for geopolymers. The results show that reactivity index of more than 20 (corresponding amorphous phase content, specific gravity, and median particle size of fly ash is >84%, >2.35, and