Influence of reactivity of fly ash on geopolymerisation

The reactivity of fly ash has been altered through an increase in glass content by air classification and mechanical activation using vibratory and attrition mills. The effect of the reactivity on fly ash geopolymerisation has been investigated with specific reference to edging of fly ash with alkali at 27°C and geopolymerisation schemes involving edging and thermal curing or direct thermal curing at 60°C. The effect of improved reactivity of fly ash on the resulting geopolymers was studied through determination of compressive strength, phase formation by X-ray diffraction and microstructural evaluation by scanning electron microscopy. The improvement in compressive strength is found to be related to the improved reactivity and resulting formation of compact microstructure. Selection of geopolymerisation scheme is found to be a key factor to realise beneficial effect of improved reactivity. Isothermal conduction calorimetry studies along with differential thermal analysis (TG/DTA) were carried out to elucidate the influence of improved reactivity during geopolymerisation. Finer particle size resulted in greater dissolution of fly ash during edging. However, the overall process of geopolymerisation and strength development was found to depend not only on dissolution but also on subsequent stages of geopolymerisation. Mechanically induced reactivity is found to have far greater influence on geopolymerisation and strength development vis-à-vis reactivity induced by finer particle size and higher glass content obtained through air classification.