This study investigates the effect of varying ratios of fly ash as a partial replacement for fine aggregate on the performance of alkali-activated municipal slag composites. The strength and other properties of alkali-activated cementitious material (AACM) composites can be optimized by selecting the appropriate mix proportion. In this study, we used fly ash as a substitute for fine aggregate (FA/S) at varying ratios of 0.0, 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0%, mixed with 50% water (W/SL), and 20% alkali activator (AL/SL) content instead of municipal slag (SL) as a core binder, cured in steam conditions. The effects of these substitutions on the initial mixing temperature, slump flow, compressive and splitting tensile strengths, and microstructure analysis of composites cured in steam conditions were investigated at 1, 7, 28, and 91 days. The evaluation of the experimental results revealed that increasing the ratio of fly ash substitution to fine aggregate by up to 20.0% led to a higher strength attributable to the composites, whereas when the extra substitution ratio of FA/S ranged from 25.0-30.0%, significant decreases in strength were observed. The composites' strengths were estimated using the ACI 209 and ACI 318 design equations and compared to the measured strengths.