Comparison of Different Methods for Testing the Pozzolanic Activity of Fly Ashes

Abstract

This paper describes how adequate pozzolanic activity of fly ash as a concrete additive in the Federal Republic of Germany has so far been insured by requiring cement mortars containing fly ash reach a certain compressive-strength ratio as compared to a reference mortar. Test results are known to be sensitive to other influencing factors, especially composition and properties of cement. A series of tests was conducted to establish the usefulness of various testing methods. Under investigation were the respective influences of cement, fly ash content, and preparation on the basis of constant water content or constant workability. Mortars were stored according to the German test guideline, to BS 3892, and to ASTM C 311. Certain physical and chemical properties of the fly ashes were also determined. This paper discusses the effects of cement, i.e., alkali content, fineness, and slag content, on the pozzolanic activity index at different ages. Also demonstrated is the suitability of accelerating strength testing methods. Finally, the strong influence of fly ash fineness on workability and strength is shown.