THE RETARDING EFFECTS OF FLY ASH UPON THE HYDRATION OF CEMENT PASTES: THE FIRST 24 HOURS

Abstract Disagreement exists in the literature as to whether or not fly ash accelerates or retards the early hydration of fly-ash blended cements. As an outgrowth of this controversy the effects of two fly ashes, a Class C and a Class F, their leachates and leached fly ash residues upon the first 24 hours of cement hydration were studied. It was found that both fly ashes (“as received and to some degree, leached”) retarded a Type I cement hydration; however the fly-ash leachates did not. The retardation phenomenon is apparently related to the presence and condition of the fly ash surfaces.