Effect of calcium-rich compounds on setting time and strength development of alkali-activated fly ash cured at ambient temperature

This paper investigates the effect of calcium-rich compounds on setting time and compressive strength development of alkali-activated fly ash (FA) pastes. Three calcium-rich compounds viz., Portland cement (PC), calcium hydroxide (CH) and calcium oxide (CaO) were used as additives to replace FA at the dosages of 5, 10, and 15% by weight basis. Sodium hydroxide and sodium silicate solutions were used as alkali activator liquids in all mixtures. Alkali activator liquid/solid binder (FA and calcium-rich compound) ratio of 0.60, sodium silicate-to-sodium hydroxide ratio of 2.0 and curing at ambient temperature (25 °C) were used for all mixes. Test results showed that the setting time and compressive strength of alkali-activated FA depended on the type and content of calcium-rich compound. The incorporation of all three calcium additives resulted in the acceleration of setting time. Alkali-activated FA with CH and PC could improve the strength development; however, the use of CaO resulted in the compressive strength reduction. The 28-day compressive strengths of alkali-activated FA with 15% PC and 5%, 10% and 15% CH met the strength requirement for repair material as specified by ASTM standard. The alkali-activated FA with additives can thus be used as an alternative repair binding material.