Improvement of High-Volume Fly Ash Cementitious Material Using Single Alkali Activation

This research aimed to present the improvement of the cementitious material in high-volume fly ash using only one alkali activator. Fly ash was used as a partial replacement for Portland cement type I, varying from 0 to 60% by weight of the binder. Concentrations of NaOH varying from 0.00 to 1.25 molar were used as alkali activator. Paste properties and mortar compressive strength at the ages of 3, 7, 14, 28, 60, and 90 days of water curing were investigated. The results reveal that fly ash paste with an alkali activator provides shorter initial setting time when compared to control paste without alkali activator. The use of 0.50 molar NaOH concentration in mortar containing fly ash not exceeding 50% by weight of binder provides the highest compressive strength at any age of curing. At that concentration, there is a significant increase in the 28-day compressive strength of up to 45% over that of the control mortar. In addition, higher NaOH concentration (not exceeding 1.00 molar) has a significant positive effect on the compressive strength of mortar with higher fly ash content, especially over longer curing periods.