Use of gypsum and CKD to enhance early age strength of High Volume Fly Ash (HVFA) pastes

•Fly ash content up to 60% of the binder can be used in a HVFA structural concrete.•Gypsum as an activator for fly ash is not effective at EAS but can improve 28 days.•CKD as an activator for fly ash reduces setting times and increases EAS.•60% Fly ash + 30% CEM 1 + 10% CKD is optimum binder proportion in HVFA concretes.

Use of higher proportions of fly ash as a cement replacement in concrete has obvious environmental and performance benefits but high volumes of fly ash are not commonly used due to perceived lower early age strengths. In this investigation, addition of cement kiln dust (CKD) and gypsum to activate the fly ash was studied and the proportions used in the paste mixes were designed to optimize the mixture ingredients to achieve the highest early age compressive strength. Change of mineral phase composition and micro structure of the composites was analyzed. It was found that CKD was much more effective in activating the fly ash than gypsum. Appreciable early age compressive strengths were achieved for fly ash contents up to 60% of the binder and these observations were supported by analysis of the mineral phases.