Pozzolanic Reactions in Synthetic Fly Ash

摘要

Synthetic fly ashes of known particle size, glass content, and chemical composition were prepared from coal-associated minerals (kaolinite, pyrite, and calcite which form over 95 wt % of the mineral matter in coal) under conditions closely approximating those in suspension-fired furnaces. Thermal history of the particles was found to play a pre-eminent role in determining pozzolanic reactivity. Although particles cooled at a slow rate contain less glass than those quenched at a rapid rate, the difference in glass content is not sufficient to explain the large reduction in pozzolanic reactivity, which may instead relate to reduction in residual strain by annealing. X-ray, light microscopic, and chemical analyses showed that pyrite added to kaolinite for production of synthetic fly ash gives rise to hematite and magnetite, whereas calcite gives rise to calcium hydroxide and calcium carbonate. Pyrite and calcite also influence the glass composition to some extent. Of secondary importance, in regard to pozzolanic strength and activity, is fineness of the fly ash particles. Provided the glass is highly strained or vitrified, the finer particles are more reactive. The chemical composition of the ash was found to be of less significance. (Author)