Effect of sodium sulfate on the alkali activation of fly ash

This paper discusses the effect of the presence of a small amount of SO42- (a 16 group oxyanion) on the nature and chemical composition of the resulting N–A–S–H gel and zeolites (main reaction products from the alkaline activation of fly ashes). To this end, fly ash was activated with different alkaline solutions containing 2.5% by weight of sodium sulfate. The paste was cured in an oven at 85 °C for different periods of time. The presence of sulfate was observed to lengthen the time needed for certain percentages of N–A–S–H gel to precipitate, while shortening the time needed for a given percentage of zeolites to precipitate. This, in turn, had an adverse effect on the mechanical strength of the material.