Relation between activator ratio, hydration products and mechanical properties of alkali-activated slag

•Hydrates assemblage and kinetics of hydration are determined by initial activator content.•In situ FTIR acquisitions during the first three days allowed to measure hydration kinetics.•Portlandite and Hydrotalcite-like hydrate precipitation are favored for activator contents above 10% of slag mass.•The activator diffusion into slag grains is low even for highest activator content. The use of cement substitute, such as alkali-activated slag, is of great interest in the field of earth-based construction materials to preserve their environmental benefit. Nonetheless, the cationic activity of clayey particles of earth-based materials induces new issues by affecting the alkaline conditions in which slag is activated. This work aims to present how alkaline conditions modify the nature and the amount of hydrates formed, the kinetics for reaction and the resulting mechanical properties. A minimum of 5 wt% of activator with respect to the slag was needed to trigger consolidation of the samples within the first 24 h, while an activator content of 15% led to an excess of sodium, especially remarkable at early age. Besides, hydration kinetics were followed in situ during the first 3 days by infrared spectroscopy. The formation of portlandite and hydrotalcite-like hydrates were only observable from 10% of activator and the presence of hydrotalcite is correlated with an increase in mechanical properties. Finally, these results are of interest for the formulation of earth-based materials by predicting hydrated phases formed during slag activation depending on alkaline conditions.