The overall objective of this research project was to determine the expansive properties for Reclaimed asphalt pavement (RAP) materials, especially the materials including recycled steel slag aggregates, with respect to those of the virgin aggregates, and evaluate their potential use as pavement base materials in Illinois. Seventeen RAP materials and virgin aggregates were tested for their expansive characteristics in the laboratory following the ASTM D4792 "Potential Expansion of Aggregates from Hydration Reactions" test method. The specimens in California Bearing Ratio (CBR) test molds were submerged into a high alkali cement water solution (pH of 12) and kept constantly soaked at 70°C to accelerate hydration reactions. The expansion percentage of the CBR specimens and the temperature and pH levels of the solution were measured continuously on a daily basis during the soaking period for a minimum of 7 days and maximum 60 days until the expansion curve flattened or the expansion rate slowed down.