Formulating alkali-activated binders requires a clear understanding of the role of activator concentration and source material composition on product formation and strength gain. Alkaline-activation of a low-calcium fly ash binder with slag replacement at 30 and 50% by mass is evaluated. The reactivity of the binder and the compressive strength gain are evaluated using activating solutions of different NaOH molarities. Increasing the NaOH molarity produces a higher compressive strength in the activated binder. The primary reaction product formed in the activated binder is a calcium silicate hydrate with aluminium substitution (C-(A)-S-H). There is an increase in the C-(A)-S-H content, and a reduction in the porosity, with an increase in NaOH molarity. Slag contributes to early reactivity in the binder and the initial strength development while fly ash contributes to later age strength by silica enrichment of C-(A)-S-H. While increasing the NaOH molarity produces an increase in the early reactivity, the Na does not directly contribute to the reaction product formation. The Na from alkaline activator is not chemically bound to the C-(A)-S-H and can be removed by leaching in water. The increasing basicity with NaOH molarity enhances the contribution of low-calcium fly ash leading to higher silica enrichment of C-(A)-S-H. © 2023 ICE Publishing: All rights reserved.