A range of anionic and cationic polycarboxylate ether (PCE) plasticizers with different molecular architectures (molecular weights, side chain lengths, and ratios of side chain density to backbone charge) are synthesized and tested to determine their effects on the rheological properties of fresh alkali-activated slag (AAS) pastes. A higher density of long side chains in the lower molecular weight polymers can provide a noticeable yield stress reduction, indicating a mild increase in workability compared to that of an unmodified AAS paste. It is hypothesized that side chains may have two important roles, i.e., providing steric hindrance to disperse particles after PCE adsorption on a particle surface, and also providing partial protection of the backbone charges against attachment of one PCE molecule to two or more slag particles, which is called bridging. This enhances the likelihood of adsorption on single particles, and thus increases the plasticizing action. A very similar plasticizing mechanism is observed for PCEs with similar structures but differing charge signs (cationic/anionic), which indicates that both anionic and cationic adsorption sites are available on AAS particle surfaces. The measured flow curves of all pastes are well described by the Herschel-Bulkley model with shear thinning behavior.