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.