Activation of fly ash/cement systems using calcium sulfate anhydrite (CaSO4)

A number of studies had been conducted on the activation of fly ash using g ypsum and sodium sulfate. Anhydrite, another form of calcium sulfate, has n ot been used for this purpose. This paper presents an exploratory study on tile effectiveness of anhydrite in activating fly ash cement systems. Anhyd rite (10%) was added into cement mortars with up to 55% fly ash replacement . The prepared mortars were allowed to cure in steam at 65 degreesC for 6 h before normal room temperature water curing. Significant strength increase s (up to 70%) compared to the control mortars were observed as early as aft er 3 days curing. Improvements in the pore size distribution of the mortars were also observed due to the activation. The results of scanning electron microscopy (SEM) examination and quantitative X-ray diffraction (XRD) anal ysis show that, with accelerated curing, a Large quantity of ettringite (AF t) was formed during the early stage of hydration of the anhydrile-activate d fly ash cement pastes. This might be the main cause of the high early str ength of the activated fly ash cement systems. A comparison was made using anhydrite and gypsum as activators. For an equivalent SO3 content, anhydrit e is more effective in increasing the early-age strength of the cement/fly ash mortars, but less effective in increasing the later-age strength than g ypsum. (C) 2001 Elsevier Science Ltd. All rights reserved.