This paper reports the results of a research study aimed at identifying the compositionalfeatures of fly ash that influence the sulfate resistance of fly ash concrete. The sulfate resistance ofcementitious systems consisting of a Type I portland cement blended with Class F and Class C fly ashesof varying composition was evaluated by monitoring the length change of mortar bars stored in 5%sodium sulfate solution (ASTM C 1012). The glass phases of the fly ashes were characterized usingscanning electron microscopy and electron dispersive X-ray analysis. The position occupied by the glasswhen plotted on a CaO-SiO2-Al2O3 ternary was identified as belonging to one of the fields occupied by themineral phases mullite, anorthoite or gehlenite. The glass showed a transition from alumino-silicate inClass F fly ash to a calcium alumino-silicate or mixed calcium-aluminate/alumino-silicate in Class C flyashes with higher calcium contents. Fly ashes containing high amounts of calcium-aluminate glassshowed a distinct reduction in sulfate resistance when tested in mortars. Blends of Class C and Class Ffly ashes showed improved sulfate resistance over the mixes made with only Class C fly ash. Arelationship between the calcium oxide content of the blended fly ash and sulfate resistance of mortarwas established.
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