Chemical activation of high volume fly ash binders by selected sodium salts

Fly ash (FA), a by-product of coal combustion in thermal power plants, has been widely used in construction industry as partial replacement for Portland cement (PC). Due to environmental protection requirements, it became an imperative to increase the replacement rate of FA. Use of high volume fly ash (HVFA) binders, in which more than 50% of PC is replaced by FA, brings many ecological benefits (reduced pollution and energy consumption related to PC production, reduced waste generation, and preservation of natural resources). However, an increase of FA content in a binder can cause worsening of some of the properties of the binder, and reduction of early compressive strength and prolongation of setting time are of the greatest concern. These properties of HVFA binders can be improved by chemical activation of the binder. In this paper, three selected inorganic activators (Na2SO4, Na2CO3, and Na2SiO3) were used for chemical activation of HVFA binder (70 mas.% of FA and 30 mas.% PC). FA was mechanically activated prior to binder synthesis. Effects of the activators on the hydration of the HVFA binders were examined by isothermal calorimetry, X-ray diffraction, and thermogravimetric analyses. Influence of the chemical activation on the properties of the HVFA binders was analyzed by determination of setting time and compressive strength. The results showed that the chemical activators used in this work accelerated reactions of both PC and FA, which lead to shortened setting times and increased early compressive strength of the binders. The HVFA binder synthesized with sodium silicate as chemical activator had the best properties