Fly ash-based geopolymers are being developed as sustainable alternate binders for producing concrete. The consistent production of a stable geopolymeric binder suitable for use in structural applications from alkaline activation of low-calcium fly ash was explored in this paper. The role of working solution and total reactive oxide ratios in consistently achieving high compressive strength in fly ash-based geopolymers were evaluated using different source fly ashes. The primary source variability was identified with the reactive silica and alumina contents in the fly ash. The maximum strength achieved from the activated fly ash was determined by the reactive alumina content in the fly ash. Due to source variability of reactive species contributed by fly ash, maintaining a constant composition of the activating solution resulted in varying compressive strength from the activated fly ash. Keeping constant reactive oxide contents in the activated system produced consistent strength from the fly ash-based geopolymers. The composition of the aluminosilicate gel depended on the reactive oxide ratios, and it varied with the fly ash composition for identical solution ratios. Global reactive oxide ratios, which are calculated based on the reactive oxide contents of the fly ash and the alkaline solution, were established. The link between strength and product formation was established, and the global reactive oxide ratios resulted in a larger reaction product content.