Optimizing paste proportions to enhance early age strength of high volume fly ash (HVFA) concrete.

Use of higher proportions of fly ash as a cement replacement in concrete has obvious environmental benefits such as reducing cement consumption and increased use of fly ash (alleviating issues with fly ash disposal). HVFA concrete benefits from enhanced workability and cohesion, lower heat of hydration and increased long term strength and durability. However, 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. It was found that CKD was much more effective in activating the fly ash than gypsum and appreciable early age compressive strengths were achieved for fly ash contents up to 60% of the binder material.