The limited application of fly ash-based geopolymers in large-scale construction is primarily due to the slow evolution of compressive strength and low early-stage compressive strength. Our research counters this by introducing a method that harnesses the innate reactivity of finer fly ash (FF), significantly enhancing the compressive strength. Experimental and characterization results suggest that the superior performance of FF-based geopolymer is attributable to its higher content of reactive silica-alumina components, an optimal ratio of soluble silica to alumina, and a greater specific surface area, which collectively facilitate the geopolymerization process. Adding just 15 wt% NaOH, the FF-based geopolymer attains a compressive strength of 58.81 MPa after 14 d and an additional 10 wt% NaAlO2 can increase compressive strength to 70.5 MPa, showcasing a method that circumvents the need for energy-intensive milling or activation treatments. For the coarser fractions, thermal activation at 550 °C for an hour markedly improves strength to 39.40 MPa, aligning it with traditionally processed fly ash. This breakthrough paves the way for cleaner, more energy-efficient, and more substantial geopolymer materials in modern construction. Display omitted •Post-screened fine fly ash with amorphous aluminosilicates enhances the potency of low-alkali geopolymers.•NaAlO₂ enrichment optimizes silica-alumina ratio, boosting geopolymer strength.•Room temperature curing boosts sustainability and efficiency of fly ash-based geopolymers.•Screening tech enables clean, efficient fly ash utilization in geopolymer synthesis.