The effect of polycarboxylate (PC) superplasticizers with different structure on the rheological properties and hydration process of slag-blended cement pastes with a slag content between 0 and 75% has been studied. Fluidizing properties of PCs admixtures are significantly higher in slag-blended cement with respect to non-blended Portland cement. Also, it has been observed that the rise of the fluidity induced by the PCs on the cement pastes increases with the slag content. This effect is mainly attributed to a decrease in the amount of CA available to adsorb and consume admixture to form an organo-mineral phase. Consequently, the PC admixtures are absorbed onto the silicate phases of the clinker and onto the slag particles, inducing a repulsion and the concomitant reduction in yield stress despite a reduction in the zeta potential. The rheological results allow us to conclude that the highest increase of the fluidity is caused by the admixtures with highest molecular weight due to the higher steric repulsion induced. As a consequence of the adsorption of the PCs, a delay of the hydration process of the pastes has been observed.