Microstructural characterization of alkali-activation of six Korean Class F fly ashes with different geopolymeric reactivity and their zeolitic precursors with various mixture designs

This study presents the mechanical strength testing results for alkali-activation of six different Class F fly ashes under varying activator/binder ratios, activator concentrations and curing temperature histories. It also reports on the synthesis of two functional zeolitic precursors formed in the fly ash geopolymerization: (1) nitrate sodalite, which may possess a relatively good Cation Exchange Capacity (CEC) for heavy metals; and (2) (basic) nitrite sodalite, which can be used for CO2 absorption. The mechanical strengths of the alkali-activated fly ashes are found to be more affected by the activator/binder ratio than the other variables. The nitrate sodalite and basic nitrite sodalite are identified in both X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) images by showing 'balls of yarn' shapes. The presence of these crystalline products not only influences strength development in the alkali-activated fly ash samples but may also provide value-added properties for the geopolymer binder.