Controlling workability in alkali-activated Class C fly ash

Alkali-activation of Class C fly ashes can produce Portland cement-free binders that take advantage of sodium aluminosilicate hydrate (N-A-S-H) and aluminum-substituted calcium silicate hydrate (C-A-S-H) phases for strength development. However, alkali-activated Class C fly ashes suffer from poor workability and rapid setting; traditional water-reducing admixtures are not effective. In this study, different methods to modify the workability of alkali-activated Class C fly ash mixtures were investigated. Retarders decreased the mixture stiffness and delayed setting, but strength development suffered. Increasing mixing intensity slightly improved workability without significantly impacting strength. Controlling silica modulus significantly improved workability, but the impact on strength was inconsistent.