Steel slag is a byproduct of iron and steel production by the metallurgical industries. Annually, 21 million tons of steel slags are produced in the United States, and most of this slag is landfilled. Landfilling represents significant economic loss and uses valuable land space. Although steel slag has great potential for use in highway applications, especially as a granular highway base or subbase material, it has not been used extensively because of its high swelling potential and alkalinity. Swelling potential deteriorates the structural stability of highways, and high alkalinity poses an environmental challenge. This study seeks a methodology that promotes the use of steel slags in highway base and subbase layers by minimizing these two main disadvantages. Two treatment methods were used. In the first method, steel slag material was coated with bituminous material. In the second method, the slag was mixed with water treatment residuals at various percentages by weight. The mixtures prepared in this study were subjected to accelerated swelling tests and batch water leach tests. Results of the swelling tests indicated that the addition of both water treatment residuals and bituminous material into steel slag decreased the swelling rate significantly. Furthermore, bituminous-coated mixtures did not exhibit any swelling. These two methods also decreased the effluent pH of steel slag from 12.3 to 11.65 (bitumen-coated slag) and 9.8 (slag mixed with water treatment residuals). The batch test results did not satisfy the pH 8.5 limit regulated by the Maryland Department of Environment for placement of industrial byproducts in highways.

The use of industrial by-products in highway embankments is becoming popular due to their large-volume use in such systems. Steel slag is one of these by-products. Previous research shows that steel slag has mechanical properties comparable to those of traditional embankment fill material; however, the environmental impacts of using steel slag need to be carefully evaluated. In this study, a series of sequential water leach and column leach tests are performed in order to assess the trace metal leaching characteristics from pure or treated steel slag highway embankments into surface waters. In addition, a numerical model is used to determine the metal concentrations within the surface waters with respect to distance from the embankment. The results show that placing a clayey embankment soil around pure or treated steel slag significantly decreases the metal concentrations of the leachate. In addition, the numerical model results indicate that, after the slag leachate meets the surface waters, the concentrations decrease to levels significantly lower than the regulatory limits.

An experimental research program was carried out to evaluate the environmental impacts of embankments constructed with steel slag on groundwater. A series of sequential water leach tests and sequential column leach tests were performed to study the leaching of aluminum, copper, and zinc into groundwater due to the flow of percolating rainwater through treated steel slag. The results indicated that treating steel slag prior to use by mixing with an alum-based water treatment residual resulted in lower effluent pH and metal concentrations. Metal leaching was greatly affected by the effluent pH and total metal content, as well as the natural pH and buffering capacity of the subgrade. The results of the employed numerical model showed that the field metal concentrations were expected to be considerably lower than those measured in laboratory column leach tests due to dispersion within the vadose zone.