Production and evaluation of alkali-activated binders of low-calcium fly ash with slag replacement

Formulating alkali-activated binders requires a clear understanding of the role of activator concentration and source material composition on product formation and strength gain. Alkali activation of a low-calcium fly ash (FA) binder with slag replacements at 30% and 50% by mass was evaluated. The reactivity and compressive strength gain of the activated binders were evaluated using activating solutions with different sodium hydroxide molarities (SHMs). An increase in SHM produced a higher compressive strength of the activated binder. The primary reaction product formed in the activated binder was calcium silicate hydrate with aluminium substitution (C-(A)-S-H). An increase in SHM led to an increase in C-(A)-S-H content and a reduction in porosity. The slag contributed to early reactivity of the binder and the initial strength development, while the FA contributed to the later age strength by silica enrichment of the C-(A)-S-H. While increasing the SHM produced an increase in early reactivity, the sodium did not directly contribute to the formation of the reaction product. The sodium from the alkaline activator was not chemically bound to the C-(A)-S-H and could be removed by leaching in water. An increase in basicity with SHM enhanced the contribution of the low-calcium FA, leading to higher silica enrichment of the C-(A)-S-H.