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

Chi Sun Poon, S. C. Kou, L. Lam, Z. S. Lin

Research output: Journal article publicationJournal articleAcademic researchpeer-review

149 Citations (Scopus)


A number of studies had been conducted on the activation of fly ash using gypsum and sodium sulfate. Anhydrite, another form of calcium sulfate, has not been used for this purpose. This paper presents an exploratory study on the effectiveness of anhydrite in activating fly ash cement systems. Anhydrite (10%) was added into cement mortars with up to 55% fly ash replacement. The prepared mortars were allowed to cure in steam at 65°C for 6 h before normal room temperature water curing. Significant strength increases (up to 70%) compared to the control mortars were observed as early as after 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) analysis show that, with accelerated curing, a large quantity of ettringite (AFt) was formed during the early stage of hydration of the anhydrite-activated fly ash cement pastes. This might be the main cause of the high early strength of the activated fly ash cement systems. A comparison was made using anhydrite and gypsum as activators. For an equivalent SO3content, anhydrite 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 gypsum.
Original languageEnglish
Pages (from-to)873-881
Number of pages9
JournalCement and Concrete Research
Issue number6
Publication statusPublished - 1 May 2001


  • Acceleration
  • Anhydrite
  • Compressive strength
  • Fly ash
  • Pore size distribution

ASJC Scopus subject areas

  • Industrial and Manufacturing Engineering
  • Ceramics and Composites


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