MICROSCOPIC STUDY OF ALKALI-ACTIVATED FLY-ASH

The activation mechanism of fly ash in a basic environment was studied as a means to improve the reactivity of fly ash in blended cements. T he experimental program included activation of fly ash by a strong bas e (NaOH) at different concentrations, temperatures, and water-to-fly a sh ratios. It was found that the degree of reactivity, as shown by the compressive strength, increases with increasing concentration of the base (up to 4 mol of NaOH) and curing temperature (up to 90 degrees C) . Lowering the sodium hydroxide to fly ash ratio by lowering the water /fly ash ratio, while maintaining the solution concentration constant yielded a lower compressive strength in spite of the lower porosity, a nd the high concentration of the solution. These results indicate that activation of fly ash in blended cements depends not only on the pH o f the activating ambiance but also on the ratio between the latter and the fly ash. (C) 1998 Elsevier Science Ltd.