Steel slag, a byproduct of steel industry and approximately 15% of original steel input, has favorable engineering properties. However, the tendency of the material to expand due to free lime and magnesia within the elemental composition impedes its use in highways, where large volumes of the slag can be utilized, hence a pretreatment is required. For this purpose, water treatment residual (WTR), an aluminum rich by-product of drinking water plants, and a sandy borrow material provided by Maryland State Highway Administration are used as dry mixing additives by different percentages. In order to evaluate the swelling behavior of three steel slag materials with different aging properties upon the two treatment techniques, accelerated swell tests (ASTM D-4792) are performed. The results indicate that both treatment methods decrease the ultimate swelling ratio significantly. Using treated steel slag in highways results in reduction in waste disposal costs and in consumption of natural resources.