The rheology of fly ash-slag geopolymer binders is evaluated for different forms and dosages of dissolved silica in the alkaline activating solution. The addition of silica decreases the elastic resistance and the yield stress of the binder. The binder rheology is influenced by the polymeric form of silica in the activating solution determined by the silica modulus (MS = SiO2/Na2O molar ratio). There is a larger decrease in the elastic component and a larger increase in the viscosity at a lower MS. Silica addition at low MS transforms the yield behavior of the binder to viscous Maxwell flow response. The changes in the fresh state influenced by thixotropy and setting are related to the early kinetics and reactivity of the slag in the binder. The silica content and the MS in the activating solution influence reaction kinetics in the binder. While the addition of silica delays the hydration of slag, there is a higher level of early reactivity associated with gel formation from the dissolved silica at a lower MS. The early chemical reactivity produced by the silica in the activating solution influences the buildup of storage modulus (G/) and produces set. However, the influence of early reactivity in the systems on the measured increase in G/ and penetration resistance varies with silica content. While high early reactivity produces a faster setting, G/ does not scale with the kinetics. Higher silica content produces a more uniform distribution of reaction products, not producing a proportional increase in stiffness. © 2022, RILEM.