ENHANCING THE REACTIVITY OF THE INDUSTRIAL FLY ASH IN THE PROCESS OF ALKALI ACTIVATION

Abstract

Over the past few decades a new group of binding materials, geopolymers, have emerged as an alternative to traditional binding materials such as Portland cement. Geopolymers are obtained by the process of alkali activation of various alumino- silicate materials, both natural and synthetic. Of particular importance is the possibility of alkali activation of the industrial waste material such as fly ash. Fly ash (FA) is generated in the process of coal combustion in thermal power plants. In Serbia a small part of fly ash is recycled while the rest is landfilled, causing a serious environmental pollution. Alkali activation represents a process by which fly ash can safely be converted into a useful biding material, suitable for the construction purposes. Geopolymers (or alkali-activated materials) based on fly ash are known for their good compressive strength and good durability in aggressive environments, when propeerly designed. However, the limiting factor for wider use of fly ash in the process of alkali activation and geopolymers synthesis is its low reactivity and consequent low compressive strength of binding elements. Our research has shown that the reactivity of fly ash in the process of alkali activation can be enhanced by the appropriate choice of the reaction conditions – by mechanical activation of fly ash and by blending with more reactive material such as blast furnace slag (BFS). Both options were explored in this paper and comparison was performed. Mechanical activation of fly ash was conducted in a planetary ball mill, while blends of fly ash and blast furnace slag were prepared with different ratios (FA/(FA+BFS) = 1; 0,75; 0,50; 0,25; 0). Alkali activation was carried out at 95ºC by use of sodium silicate solution as an activator. In both cases significant increase of geopolymer compressive strength was observed in respect to the geopolymer based on the initial fly ash. Optimal geopolymer strength was correlated with the chemical composition of the binding gel. Empirical values of optimal gel composition could serve as a basis for tailoring properties of alkali-activated binders based on different precursors. Both alkali-activated systems represent promissing routes for geopolymer technology development.