Effect of Fly Ash on the Sulfate Resistance of Concrete Containing Fly Ash

The durability of concrete is determined by its ability to endure the physical and environmental surroundings without losing the functional properties and structural integrity of the original design. Concrete containing fly ash can be proportioned to meet the durability requirements of a wide range of applications, such as concrete for mass structure, pavements, structural members, and high strength applications. One area where the long term performance of concrete containing high calcium fly ash has been suspect is in sulfate environments. The sulfate attack mechanism in concrete containing fly ash has been related in recent years to the mineralogical and chemical characteristics of portland cement and fly ash and their proportions, and the sulfate resistance of concrete containing fly ash. This report summarizes the results of the first year of the study. Fourteen fly ashes and four portland cements have been studied at four cement replacement levels and two different workability levels. A wet chemistry analysis was performed to determine the bulk chemical properties of both the fly ashes and the portland cements. The mineralogy of the fly ash was determined by XRD analysis. Over 500 concrete specimens have been exposure tested in a 10 percent sodium sulfate solution and monitored for mass change and linear expansion. The study has revealed a possible correlation between the tricalcium aluminate content of the high calcium fly ash and sulfate deterioration. In addition, the results to date indicate that lignitic fly ashes results in sulfate resistant concrete. This is an interim report of an ongoing investigation to the problem of sulfate attack of concrete containing fly ash.