Ground granulated blast furnace slag (ggbf slag) and metakaolin were blended and the combination was activated by sodium hydroxide solution. Two mix series were investigated, one with low NaOH concentration (9-16 wt%) and the other with a high NaOH concentration of 25 wt%. The reaction progress of the alkali-activated pastes was indirectly measured by isothermal calorimetry as well as by ultrasonic measurements. Both methods show an acceleration of the condensation reaction of the alkali-activated blends compared to both single phases. The acceleration effect is more considerable at the higher activator concentration related to a higher reaction degree of the metakaolin.