Effect of Landfilled Fly Ash as a Partial Replacement of Cement on the Compressive Strength of Mortars

Abstract – The aims of this study are to find the optimum percentage of landfilled fly ash that could be used as partial replacement cement and the effect of curing methods on the compressive strength of mortars. The percentages of landfilled fly ash used were 0%, 10%, 20%, 30%, 40%, 50%, 60%, and 70% of cement weight. The cube specimens used were 50 mm x 50 mm x 50 mm. The specimen after curing was tested at the ages of 1, 3, 7, 28, and 56 days. The curing methods applied to the mortars were air drying and water curing, which involved immersing them in a water tank. In the air drying method, the results showed that the compressive strength of the mortar incorporating landfilled fly ash was lower than that of the control mortar in all ages of mortars. At 28 days, only mortar containing 10% landfilled fly ash showed a higher compressive strength than that of similar mortar, but at 56 days, mortar containing up to 30% landfilled fly ash showed a compressive strength close to 45 MPa, which achieves the target of a compressive strength of 40 MPa. Furthermore, 70% of landfilled fly ash mortar at ages of 28 and 56 days achieved compressive strengths of 19 and 20 MPa, respectively. On the other side, using the water curing method, mortars containing up to 30% landfilled fly ash at ages of 28 and 56 days outperformed control mortars. However, mortars containing up to 40% landfilled fly ash had a compressive strength higher than that of the control mortar at 56 days. A 70% landfilled fly ash mortar at ages of 28 and 56 days showed the lowest compressive strengths of 23 and 28 MPa, respectively. Improvements in compressive strength of mortar containing 10%, 20%, and 30% landfilled fly ash in water curing compared to that of control mortar are 13, 17, and 17%, respectively. The 40% partial replacement of cement with landfilled fly ash by either water curing or air curing produced compressive strength mortars above 20 MPa, which is still in the range of normal strength mortar. The effect of the water curing method on the mortars decreased their porosities, and then the mortar became denser, increasing their compressive strength. The different porosities of mortar which partially replacement of cement with up to 40% landfilled fly ash and the control were less than 4%, but the compressive strength of the mortar could increase up to 12% due to water curing