The rheology of waterglass-(Na 2 O·nSiO 2 ·mH 2 O) and NaOH-activated slag pastes and mortars depends on the nature of the alkaline activator used: in waterglass-activated slag pastes and mortars, the extensive structural breakdown under shear makes the Herschel-Bulkley model a better fit to the down ramp of the flow curve, whereas NaOH-activated pastes and mortars, such as portland-cement pastes and mortars, behaved like Bingham fluids. Admixtures were unable to reduce the yield stress of waterglass-activated slag pastes, but the inclusion of a naphthalene derivative admixture in NaOH-activated slag pastes reduced the yield stress by 80%. The problem of undesirably short setting times for waterglass-activated slag mortars and concretes could be overcome by an extended mixing time, giving an initial set of nearly 3 hours.