Several groups studied the hydration kinetics of slags activated with Na2CO3, NaOH or SiO2.Na2O and showed that the calorimetric curves present at most three hydration peaks . On the basis of the knowledge of the hydration of a Portland cement, these peaks were attributed to the initial dissolution of the slag under the effect of OH -ions and water, to a pre-induction period, and finally to the acceleration of the dissolution driven by the precipitation of hydration products. In SiO2.Na2O-activated slag, there is usually a rapid setting which could be due to the reaction of the silicates of the activating solution with the calcium coming from the slag to form a primary amorphous C-S-H phase gel. The other hydrates (C-A-S-H, AFm, hydrotalcite, etc.) precipitate after an induction period. The use of NaOH leads to rapid dissolution, and calorimetry curves often contain only one heat peak . Finally, when slag is activated with Na2CO3, a calcium and sodium carbonate precipitates a long time before C-A-S-H appears .