Reclaimed asphalt pavement (RAP) is reprocessed hot-mix asphalt pavement material that contains asphalt and aggregates. A viable solution for disposing of large quantities of RAP is to incorporate it into base and subbase applications for highway construction. This paper compares the expansive properties of RAP materials, especially the ones including recycled steel slag aggregates, with those of the virgin aggregates to evaluate their potential use as pavement base materials. Seventeen RAP materials and virgin aggregates collected in Illinois were tested for their expansive characteristics in the laboratory, following the ASTM D4792 test method. The specimens in California bearing ratio test molds were submerged into a high-alkali cement–water solution and kept soaked at 70°C to accelerate hydration reactions. Some steel slag aggregates showed considerably high expansion potential, up to 6.2% swell, when compared with other virgin aggregates, such as siliceous gravel and crushed dolomite, which had minor or almost no expansion. The RAP materials, which often had lower densities, exhibited more of an initial settlement or contraction before any expansion with time. Two RAP materials - surface RAP, with 92% steel slag aggregates, and steel slag RAP - gave the maximum expansion amounts of 1.69% and 1.46%, respectively. Although the RAP materials had much lower tendencies to expand than did the virgin steel slag aggregates, the use of RAP materials containing high percentages of steel slag aggregates may have to be avoided in the pavement substructure layers, depending on the level of expansion.