Unregulated sand mining in ecologically sensitive river systems like the Wainganga Basin has raised urgent sustainability concerns in construction practices. This study investigates the partial replacement of natural river sand with fine Electric Arc Furnace (EAF) steel slag in Roller-Compacted Concrete Pavement (RCCP) mixes incorporating 25% Class F fly ash as cement replacement. Five mixes were prepared by replacing fine aggregates with 0, 10, 20, 30, and 40% steel slag by weight. Compaction trials determined the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD), while mechanical properties were evaluated through compressive and flexural strength tests at 7 and 28 days. The results revealed a consistent increase in OMC and a slight reduction in MDD with higher slag content, attributed to the slag’s angularity and rough texture. Compressive and flexural strengths declined with increased slag levels. At 28 days, only the FS10 mix achieved a compressive strength of 29.88 MPa, exceeding the 27.6 MPa requirement for surface layers, while FS20 and beyond failed to meet the threshold. Accordingly, FS10 is recommended for surface layer use, while higher replacement levels may be suitable for base layers. This study demonstrates that fine EAF steel slag, when judiciously used, can reduce river sand dependency and enhance the circular utilization of industrial by-products in sustainable pavement construction.