Class C fly ash may be activated with strong alkali to yield a novel, dense, quick-setting cement having unexpected properties. The setting time of the cement decreased as the pH of the activating solution increased. The microstructure and phase assemblage of the novel cement depended on the pH. Ettringite was absent beyond pH 14.30 (2.0N); a hexagonal plate-like crystalline form (stratlingite (gehlenite hydrate, C. sub.2 ASH.sub.8) and other compounds) became more abundant at higher pH. At higher pH the microstructure was characterized by high amounts of the plate-like crystalline phase and a dense matrix, due to higher reactivity of the glassy phase in fly ash. The novel cement will have uses in a number of areas, including fixation of hazardous wastes such as radioactive wastes, applications where rapid setting is desired, and formation of concrete in hot environments where ordinary Portland cements may tend to crack due to their heat of hydration.
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