Sodium was removed from alumina-extracted coal fly ash (CFA) by using diluted NaOH solution. The effects of
reaction temperature, reaction time, and NaOH concentration on sodium removal were studied. Result shows
that the sodium(Na2O) content of the alumina-extracted CFA was decreased from 20 wt.% to 0.6 wt.% under optimal
conditions: temperature of 185 °C, NaOH concentration of 50 g/L, residence time of 2 h, and liquid-to-solid
ratio of 40 for water washing. The possible mechanism of sodium removal was also investigated using Fourier
transform infrared spectroscopy, X-ray diffraction analysis, and nuclear magnetic resonance spectroscopy. The
results indicated that NaCaHSiO4 in the residue was decomposed by NaOH to form NaSiO3 and Ca(OH)2, and
then converted into tobermorite [Ca5Si6O16(OH)2·nH2O] and NaOH. The sodium obtained through the reaction
in the residue was mainly combined with the silanol group and then washed with water