This study aimed to assess the impact of Ground Granulated Blast-Furnace Slag (GGBS) on the mechanical properties and moisture resistance of cold recycled base course materials, comparing its performance to conventional additives such as cement and lime. Foam bitumen stabilization of cold recycled base course materials is an eco-friendly method for pavement rehabilitation. The process involves combining aggregates from both bound and unbound pavement layers, which are stabilized using foam bitumen to form a stabilized base course. Typically, 1% cement or lime is used as filler in cold recycled mixes to improve foam bitumen dispersion. This research explores the effectiveness of GGBS as alternative filler. GGBS is a by-product of iron production in blast furnaces. Three different additives—cement, lime, and GGBS—were tested at a 1% dosage by weight of the mix. The study evaluated the influence of these additives on the indirect tensile strength (ITS) and resilient modulus of the recycled mixtures. Foam bitumen was used as the bituminous binder in the cold recycled mixtures, and the effect of moisture on the durability of these mixtures was assessed by comparing ITS values in both dry and wet conditions. The results indicated that the addition of foam bitumen and various additives improved the mechanical performance and moisture resistance of the base course material, with the combination of 2.25% foam bitumen and 1% GGBS outperforming cement and lime in terms of both performance and moisture resistance.