Construction using sustainable materials has become widespread in the last decades in the building sector where the CO2 emission transmitted is reduced and the contribution towards sustainability and green buildings are enabled. Geopolymer represents the binder part of the concrete mix through a series of polymeric interactions where the cement utilization was significantly reduced. The flexural behavior of one-way reinforced concrete (RC) slabs with slag and fly-ash-based geopolymer concrete were investigated in this study using the nonlinear finite element analysis (NLFEA) adopted and the structural performance was assessed. Slabs were simply supported with 2100 length, 500 mm width, and 120 mm depth tested with four-point loading. The effect of the slab’s depths was examined using three different values (120, 150, and 180) mm, along with the effect of the main flexural reinforcement (2∅10, 2∅12, and 2∅16) using 9 NLFEA models. The strength, durability, and serviceability measurements were all addressed and reported with proper relation to the presence of the geopolymer concrete. Results showed the contribution of the slab depth and the reinforcement ratio toward the slab’s flexural performance, including the cracking, yielding, and ultimate stages. The study outcomes promote the use of geopolymer concrete for structural members with dominant flexural behavior.