Mechanical activation of fly ash: Effect on reaction, structure and properties of resulting geopolymer

Geopolymerisation of mechanically activated fly ash was studied at ambient (27 °C) and elevated (60 °C) temperatures by isothermal conduction calorimeter. Under both the conditions, mechanical activation enhanced the rate and decreased time of reaction. It was interesting to observe that in the samples milled for 45 min (median size ∼5 μm), a broad peak corresponding to geopolymerisation initiated at 27 °C after 32 h. The rate maxima at 60 °C, a measure of fly ash reactivity, showed a non-linear dependence on particle size and increased rapidly when the median size was reduced to less than 5-7 μm. Improvement in strength properties is correlated with median particle size, and reactivity of fly ash. The characterisation of the geopolymer samples by SEM-EDS, XRD and FTIR revealed that mechanical activation leads to microstructure and structural variations which can be invoked to explain the variation in the properties.