Field Scale Geoenvironmental Performance of Steelmaking Slag and Soil Mixtures in Unpaved Road Layers

Abstract The reuse of steelmaking slags in road engineering requires an integrated evaluation of their geotechnical and geoenvironmental performance under real exposure conditions, particularly because of the high alkalinity commonly associated with these materials. This study investigates a slag–soil mixture composed, by dry mass, of 35% Linz–Donawitz (LD) slag, 35% Kambara Reactor (KR) slag and 30% local lateritic soil, used as the primary surfacing layer on an unpaved section of a federal highway in tropical Brazil. Laboratory characterisation included particle-size distribution, Atterberg limits, intermediate Proctor compaction and California Bearing Ratio (CBR) testing, together with chemical classification based on Brazilian leaching and solubilisation standards. Slag incorporation improved grading continuity, eliminated plasticity and substantially increased bearing capacity relative to the natural soil, resulting in an HRB A-1-b classification and compliance with relevant Brazilian road material specifications. Although the laboratory leaching extract exhibited elevated alkalinity, no exceedances of the applicable regulatory limits for the analysed inorganic and organic constituents were identified in the leaching and solubilised extracts. An 18-month field monitoring programme encompassing soil, groundwater and surface-water compartments was conducted to evaluate the field-scale geoenvironmental performance of the compacted layer under seasonal tropical conditions. The monitoring results did not indicate measurable propagation of alkalinity from the road layer into the monitored environmental compartments. For Fe, Al and Mn, concentration variations were more consistent with the natural geochemical background of local lateritic materials and seasonal hydrogeochemical variability than with systematic enrichment attributable to the slag–soil road layer. Overall, under the monitored field conditions and within the analytical reporting limits, the LD/KR slag–soil mixture showed favourable geotechnical performance without measurable adverse geoenvironmental responses during the monitoring period. These findings provide field-scale evidence that can contribute to the controlled and site-specific reuse of properly characterised steelmaking slag–soil mixtures in low-volume unpaved road infrastructure, while highlighting the importance of compaction control and environmental monitoring.