Effect of slag chemistry on the hydration of alkali-activated blast-furnace slag

This paper presents results of detailed investigations about the alkaline activation of ground granulated blast furnace slags whose chemical composition were modified. Sodium hydroxide and potassium silicates were used as alkaline activators. The influence of the CaO/SiO2 ratio, the Al2O3 and TiO2 content of the slag composition on the hydration reaction was analyzed utilizing isothermal heat flow calorimetry, compressive strength development of binder samples, 29Si MAS NMR spectroscopy and molybdate method. It could be shown that the TiO2 content of the slag has only minor influence on the hydraulic reactivity and compressive strength of the alkaline activated binders.The variation of the Al2O3 content of the slag leads to different results. A rise in Al2O3 content enhanced the strength if activating with NaOH but resulted in a lower strength if activating with potassium silicate. The results of the 29Si MAS NMR spectroscopy verify the decrease in the reaction degree with increase of Al2O3 content. This is associated with the rise of the chain length of the C-(A)-S–H phases by incorporation of Al–O tetrahedrons resulting in a lower Si/Al-ratio. A decrease in the C/S-ratio yielded to a lower heat evolution, whereas the reaction was delayed if activated with potassium silicate with higher silicate content. The increase of the C/S-ratio caused in less condensed slag glass and therefore an enhancement of the reaction degree with a simultaneous decrease of silicate chain length, as seen by 29Si NMR. These outcomes fit well with the results measured by the molybdate method.