<p>Geopolymer concrete, produced from the alkali activation of aluminosilicate-rich by-products such as fly ash, presents a sustainable, low-carbon substitute for standard Portland cement. Despite its acknowledged mechanical and durability advantages, researchers have not thoroughly examined the flexural performance of reinforced GC slabs particularly hollow-core designs. This paper offers a comprehensive experimental and theoretical assessment of reinforced GC solid and hollow-core slabs to fill this gap. Seven slabs were tested under four-point loading tests: two solid specimens (OPC and GC) and five hollow-core GC slabs featuring varying void sizes and shear-span-to-depth ratios (a/d). The investigation concentrated on reaction to cracking, ultimate flexural capacity, stiffness characteristics, and deflection behavior, facilitating a systematic evaluation of both material and geometric effects. The results indicate that the GC solid slab exhibited marginally superior flexural performance compared to the OPC slab, demonstrating advantageous bond properties and material uniformity. In hollow-core slabs, elevated void ratios resulted in significant decreases in cracking load, ultimate capacity, and effective stiffness, whereas alterations in a/d caused pronounced differences in strength and deflection characteristics. Reduced shear spans improved load capacity and stiffness, while extended spans led to a more pliable load–deflection response. Analytical predictions derived from traditional flexural theory closely aligned with experimental outcomes, validating the appropriateness of classical models for GC and voided slab systems. The results show that geopolymer concrete is a structurally sound and environmentally friendly alternative to regular Portland cement (OPC). Its efficacy in both solid and hollow-core configurations facilitate its wider implementation in contemporary low-carbon structural applications.</p>