Effect of Fly Ash as a Partial Replacement for Sand on Sulfate Resistance of Mortar

This study investigates the influence of fly ash as a partial replacing sand material on the sulfate resistance of mortar. Fly ash replacement are 0%, 10%, and 20% by volume of sand. Compressive strength and expansion of mortar due to sulfate attack were examined. In order to clarify the effect of fly ash as a fine aggregate on the sulfate resistance of mortar, the secondary electron images of cracked surface of mortar were analysed by SEM. Based on the test results, the compressive strength of mortar is enhanced by using fly ash when submerged in both water and sodium sulfate solution. The expansion due to sulfate attack of mortar is reduced when the amount of fly ash as a fine aggregate increased. Massive ettringite particles form and fill up the pore in the mortar matrix without fly ash, as seen in SEM images. Ettringite formation in small pores of mortar without fly ash leads to a higher expansion due to sulfate attack when compared to mortars with fly ash.