Long-term Sulfate Resistance Performance of Fly Ash Blended Cement

Abstract

 Evaluation of long-term sulfate resistance was carried out using various fly ash blended cements. Specimens after extended immersion in sodium sulfate solution were examined to determine expansion as well as hydration products by XRD and sulfur distribution by EPMA. It was found that higher fly ash replacement ratios affected strength development in early age but also retarded penetration of sulfate ions. Addition of limestone powder or increase in SO₃ content resulted in a reduced quantity of internal monosulfate, thereby limiting the formation of expansive ettringite with external sulfate ions. Excessive addition of SO₃ was found to cause internal expansion due to the reaction with Al₂O₃ in the glass phase of fly ash. However, addition of limestone powder and appropriate increase in the amount of SO₃ provided fly ash blended cement with excellent sulfate resistance after 6 years of exposure, without causing internal expansion or significant strength loss.