Sorptivity of Mortar with Alkali-Activated Slag and Fly Ash Binders

This paper presents the finding of a study aimed at evaluating sorptivity of sustainable mortars that use alkali-activated GGBS-fly blends as binders, without Ordinary Portland Cement. Sorptivity, which is a measure of matrix pore-system, is an important indicator of concrete durability. Three binder combinations were successfully activated under room temperature, without heating, namely, GGBS as sole binder, GGBS:fly ash ratio of 3:1, and equal amounts of GGBS and fly ash. The optimum combination of GGBS and fly ash is the one with GGBS:fly ash ratio of 3:1, which exhibited the lowest sorptivity of the three binder combinations when the molarity of the alkaline activator NaOH is 12M. Mortars with GGBS:fly ash ratio of 3:1 retained the lowest sorptivity over mortars that use GGBS as sole binder and the mix with equal amounts of GGBS and fly ash, after 7-, 28-, and 90-day of curing. Mixes prepared using 100% GGBS binder demonstrated decrease in sorptivity as NaOH concentration was increased from 10M to 16M, in increments of 2M. The optimum sodium silicate/sodium hydroxide ratio for mortars using 50% GGBS+50% fly ash as binder is 2.0, which resulted in lower sorptivity compared 1.5 and 2.5.