The rheological behavior of fly ash suspensions in alkali-silicate solutions used to prepare geopolymers is investigated. The transient stress response of fly ash suspensions at a constant applied strain rate is influenced by both the solids loading and the rheological behavior of the activating solution. The alkali-silicate solution itself behaves like a Newtonian fluid. The fundamental response of alkali-silicate fly ash suspension under constant applied shear strain rate exhibits a transition from a yield type to Maxwell flow. The variability in the Maxwell flow to yield type behavior depends upon the solids loading given by the solution to binder ratio and the composition of the activating solution. In both Maxwell flow and yield type responses, the maximum stress before initiation of flow is directly influenced by the viscosity of the activating solution. At specific solid loading, the transition between the Maxwell flow to yield type behavior is controlled by the composition of the activating solution. The viscous nature of the alkali-silicate solution produces a rate dependent transient response under constant applied strain rate. © 2019, RILEM.