Calorimetric Studies of Powder Additions to Mitigate Excessive Retardation in High Volume Fly Ash Mixtures
Dale P. Bentz
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 hydrate, calcium hydroxide, cement kiln dust, condensed silica fume, limestone, and a rapid set cement. The additions of either 5 % calcium hydroxide or 10 % of the rapid set cement by mass of total solids are observed to provide significant reductions in the retardation measured in mixtures based on either class of fly ash. These two materials may provide viable solutions, extending the utilization of HVFA mixtures in practice.