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Slag, a by-product of the iron and steelmaking industry, has been identified as an alternative construction material; however, there are currently limited engineering reuse applications for some slags. Steel slag coarse aggregates (>6 mm) has been successfully used in the asphalt mix for the surface course in pavement constructions; nonetheless, the construction applications of finer slag particles (<6 mm) are limited due to their expansive properties when hydrated. Therefore, this research aims to investigate the potential of using granulated blast furnace slag (GBFS) in controlling the swelling of fine particles of fresh basic oxygen furnace slag (BOFS). A series of laboratory tests were conducted to evaluate the engineering properties, swelling, and shear behavior of BOFS-GBFS mixtures to identify controlling factors and governing mechanisms of swelling, followed by acceptable mix proportions for potential use in pavement applications. Both accelerated swell tests and long-term swell consolidation tests demonstrated an exponential reduction in the hydration-induced expansion of BOFS with the addition of GBFS. For example, up to 77% of the swelling reduction was observed by replacing 30% of BOFS particles with GBFS. In contrast, the peak deviatoric stress of BOFS was reduced linearly with an increase in GBFS content, causing a 15% reduction of friction angle with a 30% addition of GBFS content. Optical microscopy and micro-computed tomography (CT) analysis revealed the bond formed between BOFS-GBFS particles due to the chemical reaction between the free lime in BOFS with silica and alumina in GBFS in the presence of water. The overall laboratory results were summarized by developing a design chart in which the optimal mix of GBFS to BOFS lies between 25% and 30%, volumetric swelling is less than 1%, and the peak friction angle is about 43°. [ABSTRACT FROM AUTHOR]