During alkali activation, the alkalinity of the activator leads to the release of Ca, Si and Al from the slag. The ions that are released from the slag glass particles must then diffuse through the exterior layer of hydrate reaction products which rapidly forms around the unreacted slag particles [37]. The alkaline conditions accelerate the reaction process of activation [38], as the presence of OH-enhances slag dissolution, and also increases the solubility of silica and alumina. This suggests that activators with lower Ms (and therefore higher OHconcentration)contribute more strongly to the dissolution of the slag, leading to reduction of setting times. These results are coherent with those observed by Bakharev et al. [39]. The setting times observed here are in general lower than those reported by Fernández-Jiménez and Puertas [38] for similar activation conditions, which is most likely attributable to differences in the chemical compositions and particle size distributions of the slag sources. The slag studied here has a lower Mg content and a lower Si/Al ratio than the slag studied by those authors, which may be significant.