Deep insight into reactive chemical structures of Class F fly ash

Reactive amorphous aluminum silicates dominate the reactivity of fly ash in alkali cement; however, the quantitative characterization of valid chemical compositions and structures information is highly challenging due to the heterogeneity and complexity of these phases. To acquire this information, in this work, Class F fly ash was treated with 1% hydrofluoric acid, followed by step-by-step analysis using a scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR), and Fourier transform infrared (FT-IR) spectroscopy. Based on the acidolysis method, along with SEM-EDS statistical analysis, four amorphous aluminum silicate phases with relatively higher reactivity were successfully separated and quantitatively showcased, along with their respective average atomic percentage compositions (i.e., Si, Al, Ca, Fe, Na, K, and Mg%). In addition, the use of MAS-NMR and FT-IR analyses allowed for further quantitative insight into the reactive chemical structures and their associations with reactivity. The proposed characterization techniques have unique advantages for acquiring detailed reactive amorphous chemistries, which are difficult to obtain using conventional analysis procedures in alkali systems.