An experimental investigation of fly ash activated with sodium-based alkali-silicate activating solutions is presented. As the first step a detailed characterization of the low-calcium fly ash is performed. The reactive SiO2 and Al2 O3 in fly ash are shown to be smaller proportions of the total SiO2 and Al2 O3 indicated by the oxide analysis. In the experimental program, the roles of reactive oxide ratios, and the molarity of sodium hydroxide in influencing the compressive strength of alkali-activated fly ash are evaluated. The reactive silica content in the alkali-activated fly ash is taken as the sum of the reactive silica contributed by the fly ash and the soluble silica from the activating solution. The reactive alumina in the fly ash and the total sodium in the activating solution are used in determining the reactive oxide ratios. The ratio of reactive Al2 O3 /Na2 O close to 2.5 produces the highest compressive strength. The maximum ultimate compressive strength is attained for a reactive SiO2 /Al2 O3 mass ratio equal to 2.0 in the activated system. The minimum required molarity of NaOH in the alkali-activated mixture is 3M. The results presented in this paper indicate that the composition of the activating solution for achieving the highest compressive strength would depend on the reactive oxide composition of the fly ash. Producing high strength geopolymers requires optimizing the activating solution for the specific reactive oxide ratios based on the reactive oxide content in the fly ash. © 2020, Associated Cement Companies Ltd.. All rights reserved.