In this study, three granulated blast furnace slags with different chemical compositions were activated with sodium carbonate and sodium sulfate solutions, and the reaction kinetics were studied by isothermal calorimetry.The reaction products forming in the hardened pastes were determined after 28 days of curing using X-ray diffraction and scanning electron microscopy.The results showed that the counter-ion in the alkali activator plays a significant role in defining the nature of the main reaction products forming.In the case of sodium carbonate activated slags, the formation of a carboaluminate type phase, along with an A1 substituted calcium silicate hydrate (C-A-S-H) type phase, was identified.In sodium sulfate activated binders the formation of ettringite was observed instead.Resolved X-ray diffraction reflections of a layered double hydroxide with a hydrotalcite type structure were observed in sodium carbonate activated binders as the content of MgO in the slag increased;conversely, hydrotalcite is not identified as a reaction product in the sodium sulfate activated slag cements.This elucidates that there is a systematic dependence of the reaction mechanism and the content of magnesium in the slag, which prevails for both sodium carbonate and sodium sulfate activate binders.This demonstrates that the effectiveness of these near neutral activators is strongly dependent on the slag chemistry,and so any generalised assumptions that all slags activated with near-neutral salts will react in similar ways is misleading.Thus, desirable setting time and strength evolution can be achieved if the activator is selected and tailored to match the slag chemistry.