Utilization of weathered basic oxygen furnace slag in the production of hydraulic road binders

This paper deals with the use of a weathered basic oxygen furnace (BOF) slag as a main constituent in hydraulic road binder. The original idea of this study was to associate the BOF slag with ground granulated blast furnace slag (GGBFS). The free lime contained in BOF slag activated the GGBFS. Small amounts of catalyst were added to the mixtures in order to accelerate the setting of this ternary blended binder. Before studying the characteristics and the influence of the proportion of each constituent, the chemical, mineralogical and physical characteristics of BOF slag were analyzed. X-ray diffraction showed that the main minerals present were calcium silicates (βC2S and C3S), dicalcium ferrite (C2F), lime (CaO), a solid solution of magnesium iron oxide (MgOxFeOy), an alumino ferrite calcium phase (C4AF) and, due to the weathering, calcite (CaCO3) and portlandite (Ca(OH)2). The activity of BOF slag in cement-based mortars was evaluated and the results show that BOF slag has little hydraulic activity and apparently no pozzolanic properties. These results were confirmed by XRD on hydrated BOF slag. Only CaO, C3S and C4AF react with water and, as the C3S and C4AF contents were relatively low, the hydraulicity of the BOF slag under study was low. However, BOF slag contains 7% of Ca(OH)2 and, in spite of the weathering, still 5% of CaO. The production of a ternary blended binder containing a mixture of BOF slag, GGBFS and catalyst was a success. The optimization of the binder composition showed that it was possible to produce an HRB10 (the compressive strength of standardized mortars higher than 10 MPa at 56 days) by mixing 52.5% of GGBFS, 42.5% of BOF slag and 5% of catalyst. Moreover, the stability study showed that there was no problem of expansion with such a binder.