The rheological behavior of pastes made with fly ash-slag blends in alkali silicate activating solutions is evaluated. The link between the activating solution composition and the fundamental rheological behavior of alkali-activated pastes are studied for different fly ash-slag blends. The transient response of alkali-activated pastes prior to initiation of flow under an applied strain rate varies between yield-type or Maxwell-flow behaviors depending on the silica content in the activating solution. Adding dissolved silica in the alkaline activating solution initially produces a decrease in the yield stress of the paste, but with further increase the transient response transitions to a Maxwell-flow type behavior. Maxwell flow behavior is a fluid-dominated response with an apparent yield stress in the transient response under constant applied strain rate. The apparent yield stress in the Maxwell flow response increases sensitively with increasing silica content in the activating solution. On increasing the silica content in the activating solution, there is an increase in the plastic viscosity of the pastes. The thixotropy of the paste is influenced by the silica content in the activating solution and the slag content in the blend. Blends with yield-type behavior exhibit a rapid yield stress recovery after shearing. The yield stress and the structural rebuilding energy (SRE) increase very rapidly with age in activated mixtures containing dissolved silica. The pastes which exhibit Maxwell flow response have a significantly slower structural buildup indicated by a lower rate of increase in the SRE with age compared to material with yield type of behavior. The slag content in the blend contributes to a rapid increase in the SRE. The requirements of rheology control of the paste for different processing requirements including pumping and 3D concrete printing are evaluated.