Production of Sulfate-Resistant Concrete Containing High-Calcium Fly Ash and Sodium Sulfate Admixture


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Title: Production of Sulfate-Resistant Concrete Containing High-Calcium Fly Ash and Sodium Sulfate Admixture

Author(s): R. B. Freeman and R. L Carrasquillo

Publication: Special Publication

Volume: 153


Appears on pages(s): 153-176

Keywords: admixtures; alkalies; blended cements; durability; fly ash; sulfate resistance; Materials Research

Date: 6/1/1995

Partial replacement of a moderately sulfate-resistant cement with a high-calcium fly ash may result in either increased or decreased sulfate resistance for concrete. These effects of fly ash have been related to, among other factors, changes in the permeability of concrete and changes in the stability of hydrated calcium aluminates in the presence of sulfate-bearing solutions. The objective of this study was to investigate the effects of using anhydrous sodium sulfate as a chemical admixture in concrete made with Class C fly ash. The sodium sulfate admixture was expected to influence the sulfate resistance of concrete by increasing the availability of sulfate ions during the hydration of calcium aluminates. The admixture was also expected to increase the rate of pozzolanic reactions by increasing the concentration of alkali ions in solution. In addition to studying the effects of the sodium sulfate admixture on sulfate resistance, its effects on mixing water requirements, compressive strength, and permeability were also examined. The fly ash was introduced into the concrete by two methods: partialre placement of portland cement with fly ash at the time of mixing concrete and intergrinding of fly ash with portland cement clinker and gypsum, as in the production of blended cements. A commercially available ASTM C 150 Type II cement and five ASTM C 618 Class C fly ashes were used. The fly ashes replaced the portland cement (or the cement clinker plus gypsum) at a level of 35 percent by volume. For each source of fly ash, 14 concrete mixtures were produced; seven mixtures included Type II cement and fly ash with various amounts of the sodium sulfate admixture and seven mixtures included blended fly ash cement with various amounts of the sodium sulfate admixture. The use of sodium sulfate as a concrete admixture, in amounts ranging from two to five percent by mass of cement, resulted in improved sulfate resistance for concrete containing Class C fly ash. In many cases, sulfate resistance exceeded that of Type II cement concrete without fly ash. Additional effects of the sodium sulfate admixture included increased compressive strengths at early ages and lower permeabilities at early ages.