Sodium hydroxide substitution in slag activating mixes: A potential pathway to more sustainable slag-based binders

•Slag hydration is triggered by Ca and/or Mg-based activators that partially replace NaOH.•Slag hydration is mainly driven by the sodium concentration in the activating mixes.•Complementary activators impact hydration in relation to its dissolving behavior in alkaline solution.•5 wt% of CaCO3 or CaSO4 increased up to 18% the strength of samples with the same Na2O content.•NaOH amount can be decreased to reduce the environmental footprint of slag binders with equivalent mechanical properties. Alkali-activated slag is commonly used as a cement substitute to reduce the environmental footprint of construction materials. Nonetheless, the use of significant amount of alkaline activators, such as sodium silicate or sodium hydroxide, can be detrimental in terms of global warming potential impact. The aim of this work is to use complementary activators as partial replacement of sodium hydroxide, and therefore reduce the environmental footprint of the activated slag binder. Calcium hydroxide, calcium carbonate, calcium sulfate and magnesium oxide are used as complementary activators in binary or ternary activating mixes. The compressive strength at 28 days are higher (up to 18%) for the calcium carbonate and calcium sulfate-activated samples as compared to the reference activated with sodium hydroxide only. The characterization of the microstructure shows that the proportion of activated slag and the nature of hydrates formed depend on the nature of complementary activators. Calcium aluminum silicate hydrates and hydrotalcite are favored by the presence of calcium carbonate and magnesium oxide, and a sodium calcium aluminum sulfate hydrate is formed in the presence of calcium sulfate. The life cycle impact assessment (LCIA) performed on the different formulations shows that a partial substitution of sodium hydroxide is a potential option to develop more sustainable slag-based binders.