How to evaluate pozzolanic activity of sulfate-rich fly ash

Abstract

The Frattini test and strength activity index (SAI) test have been commonly used methods to assess the pozzolanic activity of pozzolan by measuring its reaction extent, which would be affected by the sulfate content in pozzolan and the cement used. This study investigates the effect of sulfate content in sulfate-rich fly ash on the test results regarding pozzolanic activity by means of the Frattini test and SAI test; three fly ashes with different sulfate contents are used and their sulfate contents are further increased by adding calcined gypsums. There is significant correlation between the test results of the pozzolanic activity and the sulfur trioxide content of the cement–fly ash system, no matter whether the sulfates came from cement or fly ash. However, current methods may confound the test results regarding the pozzolanic activity of sulfate-rich fly ash. It is suggested that the sulfur trioxide content of the cement–fly ash system should be kept constant when the Frattini test and SAI test are selected to evaluate the pozzolanic activity of sulfate-rich fly ash or to compare with other fly ashes and pozzolans.