<p>This study investigates and compares the performance and environmental impact of aggregates stabilized using geopolymer and ordinary Portland cement (OPC) for pavement base applications. Incinerated biomedical waste ash (IBMWA) served as the aluminosilicate precursor in the geopolymer mix. The evaluation included mechanical properties such as unconfined compressive strength (UCS), flexural strength (FS), indirect tensile strength (ITS), and indirect tensile resilient modulus (IT Mr). Durability was assessed through wetting–drying cycles, and the potential for heavy metal leaching was also examined. Results showed that mechanical properties (UCS, ITS, IT Mr, and FS) improved with increasing IBMWA content up to 20%, beyond which a decline was observed. This trend was consistent with a fixed alkali activator ratio (Na₂SiO₃/NaOH = 1). The strength improvements in the geopolymer mix were attributed to the formation of sodium aluminosilicate hydrate (N-A-S-H) gel, as confirmed by scanning electron microscopy (SEM) analysis. In contrast, the strength enhancement in OPC-stabilized aggregates was linked to cement hydration. Geopolymer-stabilized aggregates demonstrated mechanical performance comparable to or exceeding that of OPC-stabilized counterparts, with superior durability under wetting–drying conditions and effective immobilization of heavy metals. Additionally, the geopolymer approach achieved a 13–15% reduction in CO₂ emissions for equivalent UCS values and enabled thinner base layers compared to OPC and conventional stabilization methods. These findings support the viability of IBMWA-based geopolymer as a sustainable and technically effective alternative to traditional cement stabilization in road construction.</p>