Alkali-activated slag cements (AAS) are considered as an alternative to portland cement (PC) in many studies. A significant number of studies have focused on their mechanical and durability properties, but very few studies have concentrated on their rheological behavior. In this study, the effects of sodium silicate and sodium hydroxide on the rheological properties and structural build-up of alkali-activated GGBFS mixtures have been investigated. The sodium silicate-activated GGBFS mixtures exhibited a lower yield stress and a higher plastic viscosity than sodium hydroxide-activated GGBFS mixtures. The small-amplitude oscillatory shear (SAOS) tests indicated a negligible colloidal interaction between GGBFS particles in sodium silicate-activated system; therefore, the early increase in structuration was associated with the formation of reaction products due to the interaction between the dissolved calcium ions and the silicates originated from the activator solution. On the other hand, the stiffness and the early increase in structural build-up of sodium hydroxide-activated GGBFS system were attributed to the formation of well-percolated network.