<p>Flyash and GGBS are very popular precursors used in both one-part and two-part alkali-activated binder systems. This study explores the feasibility of utilising ferrous-rich steel slag slurry, a by-product of the steel industry, as a supplementary material in binary and ternary blended, one-part Alkali-Activated Binder (AAB) systems. The slurry, characterised by fine particle size, is predominantly composed of ferrous oxide, along with aluminium and silica oxides. Conventional precursors were partially replaced with the slurry in varying proportions (0%–20%) to assess their influence on the fresh and hardened properties of AAB systems. Two groups of mixes were developed: Group 1 (ternary blends) with a control mix of 60% flyash, 30% ground granulated blast furnace slag (GGBS), and 10% microsilica; and Group 2 (binary blends) with a control mix of 50% flyash and 50% GGBS. Key parameters such as flowability, setting time, compressive strength, water absorption, sorptivity, and acid resistance were evaluated. Results indicate that a 5% replacement level of steel slag slurry optimally enhances the binder’s overall performance in both groups. The optimal mixes yielding approximately 10% improvement in mechanical strength and durability attributes compared to the control mixes. Ternary blends demonstrate superior mechanical and durability properties compared to binary blends; however, the binary system shows better early-age strength. Microstructural analyses using Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM–EDS) and X-ray Diffraction (XRD) provided further insights and validated the mechanical performance trends at the optimal level of replacement. The study confirms that steel industry by-products can be effectively valorised to enhance the sustainability and performance of AABs.</p>