Powder Additions to Mitigate Retardation in High Volume Fly Ash Mixtures

While high volume fly ash (HVFA) concrete mixtures are attractive from a sustainability viewpoint, they are sometimes plagued by long delays in finishing, producing a performance that is unacceptable to contractors. In this paper, isothermal calorimetry studies are conducted to examine excessive retardation in HVFA mixtures based on both Class C and Class F fly ashes. In addition to quantifying the retardation, the calorimetric curves are also used to evaluate the performance of mitigation strategies based on various powder additions. Powder additions examined in the present study include an aluminum trihydroxide, calcium hydroxide, cement kiln dust, condensed silica fume, limestone, and a rapid set cement. The addition of either 5 % calcium hydroxide or 10 % of the rapid set cement by mass of total solids (powders) is observed to provide a significant reduction in the retardation measured in mixtures based on either class of fly ash, for the material combinations examined in this study. Thus, these two powder additions may provide viable solutions to mitigating excessive retardation, extending the utilization of HVFA mixtures in practice.