Mid-infrared spectroscopic studies of alkali-activated fly ash structure

Infrared spectroscopic techniques were used in the present study to acquire a deeper understanding of mid-range order in the alkaline aluminosilicate (zeolite precursor) formed in the alkali activation of fly ash. Experiments were conducted with three type F fly ashes containing different proportions of vitreous and crystalline (primarily mullite and quartz) phases, activated with an 8 M solution of NaOH. All the mixes were alkali activated at 85 °C for the following reaction times: 2 h, 5 h, 8 h, 20 h and 7 days. The results indicate that the amount of alkaline silicoaluminate formed as the main reaction product, and its Si/Al ratio, induce a shift in the T-O stretching band appearing at 950-1100 cm−1. Moreover, the presence of zeolite-type crystalline phases generates substantial changes in the 800-500 cm−1 area of the spectra. Selective chemical attacks were used to facilitate identification of the spectra bands.