The importance of the network-modifying element content in fly ash as a simple measure to predict its strength potential for alkali-activation

Six different Class F fly ashes were examined to identify the material properties that determine the strength development in geopolymerization. All of the fly ashes displayed typical features of Class F fly ash in chemical and mineral composition, amorphous phase (glass) content, and X-ray diffraction pattern profile; however, the strength developments of the ashes were quite different from each other. The results suggest that the strength is higher under the following configuration: greater content of network modifying elements, greater glass content, lower silicon content, lower intensity of quartz-related 29Si NMR peak, and higher fraction of Al(IV) in the 27Al NMR spectrum. Among the possibilities, the network-modifying elemental content may be the simplest and most accurate measure for strength development of alkali activation of fly ash.