Geoenvironmental evaluation of steel slag utilization in highways using experimental analyses

Abstract

Slag, in its most general definition, is a solid by-product resulting from the metal, iron and steel production industries and is mostly composed of light oxides such as calcium oxide (CaO) and magnesium oxide (MgO), silicates and borates in terms of chemical content. Additives and alloys used to remove impurities in molten steel production make slag formation inevitable. While these wastes are formed in furnaces, they accumulate on the surface because they have a lower density than molten iron or steel. Molten steel is produced by refining peak iron obtained from raw iron ore, together with alloys, scrap metals and additives, in basic oxygen and electric arc furnaces. The steel slag wastes generated in these furnaces are called basic oxygen (BOF) and electric arc furnace (EAF) and these two wastes are chemically similar. Global steel production achieved an 1.892 billion tons as of 2023 while China produced 1.019 billion tons to lead the industry. Turkey ranked as eighth with its 33.7 million tons steel production contribution. The steel production process creates steel slag which constitutes 15%  20% per ton with substantial storage and disposal difficulties. The combination of high alkalinity and substantial toxic trace metal content in slags leads to major environmental and biological risks. Accordingly, sustainability aims to reduce resource usage while encouraging waste reuse to minimize environmental damage as much as possible. Geotechnical engineering applications play a crucial role in the complete and efficient evaluation of steel slags in this framework. As a results of the study, XRD and XRF analyses demonstrate that EAF and LS steel slags are mainly composed of light oxides while LS experiences diminished iron oxide content due to secondary processing. EAF has a specific gravity of 3.62 while LS waste has a specific gravity of 2.73 and both materials belong to GP and A-1-a classifications. Direct shear tests determined that EAF and LS have internal fiction angles of 37° and 30° respectively. Higher levels of iron oxide in wastes create hard, rough, and sharp surfaces that generate substantial friction. California Bearing Ratio (CBR) test results showed EAF = 31%, LS = 28% in unsoaked conditions; EAF 36%, LS 30% in soaked conditions were obtained. The accelerated swelling test reveals that EAF swelled by 4.6% and LS expanded by 4.4% due to the presence free CaO and MgO components. Tests on water leachate tests (WLTs) proved that EAF and LS steel slags produce leachate with elevated alkalinity levels (pH: EAF 12.4, LS 12.5). The values surpass the thresholds established by both the Ministry of Environment of Türkiye (6.5-8.5) and the US EPA (6-9). To lower the pH, EAF and LS were combined with natural soils like bentonite, sand, kaolin, and NC clay at proportions of 20%, 40%, and 60% by weight. The inclusion of 60% NC clay resulted in a reduction of 1.2 and 0.7 units in pH values of EAF and LS, respectively. The impact of sand was minimal (0.1 unit), whereas clays demonstrated useful in lowering pH, attributed to their negative surface characteristics, buffering capacity and cation exchange capacity (CEC). The environmental effects of EAF and LS steel slags combined with kaolin (K), bentonite (B), natural clay (NC), fine sand (FS), and coarse sand (CS) were assessed through sequential water leachate tests (SWLTs). NC clay achieved the most significant decrease in EAF and LS leachate pH, with reductions of 2.0 and 0.9 units, respectively, attributed to its buffering properties. Moreover, NC proved to be the most effective soil in minimizing the environmental impact, achieving a reduction in electrical conductivity (EC) of 77% for EAF and 81% for LS. Column leachate tests were carried out as water infiltration tests were insufficient to fully represent road fills, and the environmental impacts of EAF and LS steel slag were assessed using a two-stage column setup. In the experiments, leachate from EAF was channeled into the second column filled with kaolin or NC clay. In the initial column with EAF, the pH consistently averaged 11.5 over a period of 84 days, while the EC and TDS recorded values of 2.86 and 1.43, respectively. When the polluted water flowed through the second column filled with kaolin, the pH value dropped to 8, which is below EPA limits, while the EC and TDS reduced to 2.04 and 1.02, respectively. The findings indicate kaolin's potential for leachate remediation. The mitigation effect of NC clay was also investigated in the sequential column setup. Accordingly, the pH values of the polluted leachates coming out of EAF and LS decreased to 6.6 and 6.7 after passing through NC clay, respectively. The EC and TDS values of the water coming out of EAF column decreased to 2.97 and 1.49 after leaving NC, and in LS they were measured as 2.83 and 1.42, respectively. Consequently, steel slags exhibit adequate mechanical performance; however, the elements present in their chemical composition contribute to their high alkaline properties, which in turn may lead to the release of heavy metals. As a result, the mechanical performance of steel slags does not meet the necessary criteria for evaluation. It is recommended to utilize them in engineering applications while taking into account the potential for pollution and the long-term swelling behavior.