The relationship between the composition of the pore solution in alkali-activated slag cement (AAS) pastes activated with different alkaline activator, and the composition and structure of the main reaction products, was studied. Pore solution was extracted from hardened AAS pastes. The liquids were analysed by different techniques: Na, Mg and Al by atomic absorption (AA), Ca ions by ion chromatography (IC) and Si by colorimetry; the pH was also determined. The solid phases were analysed by XRD, FTIR, solid-state 29Si and 27Al NMR and BSE/EDX. The most significant changes in the ionic composition of the pore solution of the AAS pastes activated with waterglass occur between 3 and 24 h of reaction. These changes are due to the decrease of the Na content and mainly to the Si content. 29Si MAS NMR and FTIR confirm that the activation process occurs with more intensity after 3 h (although at this age, Q2 units already exist). The pore solution of the AAS pastes activated with NaOH shows a different evolution to that of pastes activated with waterglass. The decrease of Na and Si contents progresses with time. The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q3 units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass. 35 refs.