This study investigates the effect of replacing fly ash with ground granulated blast furnace slag (GGBFS) in lightweight geopolymer concrete. Given that geopolymer concrete eliminates the use of Portland cement, thereby contributing to reduced cement consumption and mitigating its negative environmental impacts, this topic holds particular significance. In this research, lightweight expanded clay aggregate was employed as coarse aggregate, and the activator-to-binder ratio (Al/Bi) was set at 0.5. The specimens included cubes for compressive strength, cylinders for tensile strength and elastic modulus, and notched beams for fracture testing. Curing was performed at 80 °C for 24 hours. The results revealed that a 20% slag replacement led to a 16% increase in compressive strength compared to the control specimen. Furthermore, fracture energy and fracture toughness reached their maximum at 30% slag replacement, with an improvement of approximately 63% relative to the slag-free specimen. However, further replacement levels of 40% and 50% resulted in a decline in these properties. The findings indicate that a slag replacement range of 20–30% provides the optimum balance for enhancing both the mechanical performance and fracture behavior of geopolymer concrete. These results underscore the substantial importance of such mixtures in advancing sustainable construction materials and reducing Portland cement consumption.