Influence of GGBS–Fly Ash Binder Composition on the Strength of Geopolymer Concrete with Natural and Recycled Concrete Aggregate

High carbon-emitting Ordinary Portland Cement (OPC) requires sustainable options in construction practices. This study explores the manufacture of high-performance, sustainable geopolymer concrete (GPC) by substituting cement completely with Ground Granulated Blast-furnace Slag (GGBS) and Fly Ash (FA) in proportion with 35% Recycled Concrete Aggregate (RCA). In order to identify the best binder proportion of GGBS:FA for obtaining the desired 25 MPa compressive strength for practical on-site applications, four GPC mixes with different GGBS:FA ratios (100:0, 90:10, 80:20, 70:30) were produced using 10M alkaline activator solutions ( NaOH and Na₂SiO₃ ) as well as cured at atmosphere (ambient) and heat (75°C). Mechanical strengths were studied at 7, 14, 28 days. All the GPC mixes showed better strength compared to the OPC control mix. GPC-2 (90% GGBS: 10% FA) was the optimal mix that attained the best 28-day compressive and flexural strengths of 33.7 MPa as well as 3.60 MPa, respectively, after applying heat curing. Strength development in GPC mixes under heat curing condition consistently outer performed from the GPC mixes under ambient curing condition. The study concludes that the GGBS–FA system, particularly at a 90:10 ratio and when heat-cured, serves as a viable, high-strength alternative to OPC-based concrete and contributes significantly to sustainable construction practices through binder optimization and RCA utilization.