This study investigates the influence of varying amounts of the alkaline activator Geosil® on the mechanical performance and microstructural properties of alkali-activated mortars (AAMs) incorporating oil refinery waste i.e., spent equilibrium catalyst (ECAT) and limestone. Experiments included assessments of flowability, crushing and flexural potency at 7 and 28 days, as well as microstructural analysis using XRD and SEM techniques at 28 days. The results demonstrate that an optimal Geosil® dosage, enhances both workability and mechanical strength, achieving a compressive strength of 14.7 MPa and flexural strength of 1.7 MPa at 28 days. This dosage promotes effective activation and matrix densification, whereas excessive dosages, lead to increased porosity and reduced strengths. Microstructural analyses confirm these findings, highlighting the role of ECAT's reactive aluminosilicate phase. The study accentuates the potential for using Geosil® as a sustainable substitute activator in AAM systems, thus supporting circular economy principles and reducing reliance on traditional alkali-activators. These insights are pivotal for designing durable and eco-friendly construction materials.