Effects of Alkalies in Class C Fly Ash on Alkali-Aggregate Reaction

Abstract

The effects of alkalies in Class C fly ash on alkali-aggregate reaction were studied in this paper by using two cements, a type I high-alkali cement and a type II low-alkali cement, and three Class C fly ashes. Mortar bar expansion was measured according to ASTM C 441 and the reaction products of alkali-aggregate reaction were examined. This paper describes how (1) the significance of the standard mortar bar test in determining the degree to which high and low-alkali cement could be replaced by Class C fly ashes, and (2) effect of fly ash alkali contents on alkali reactivity. Expansion of mortar bars prepared using high-alkali cement increased at low replacement levels but decreased at high replacement levels for curing periods up to 12 weeks at 38°C; whereas expansion of mortars prepared using low-alkali cement increased at all levels of fly ash replacements up to 40% by volume. A critical equivalent Na²0/Si0² mole ratio was identified and found to characterize alkali reactivity.