Modifications of basic-oxygen-furnace slag microstructure and their effect on the rheology and the strength of alkali-activated binders

Microstructure tailoring of metallurgical slags allows the production of alternative construction binders with customized properties. In this study, the variations of rheology and strength of alkali-activated basic-oxygen-furnace (BOF) slags are quantified. Two modifications of BOF slag were created adding amounts of alumina and silica at high temperature (> 1250 degrees C). The additions, defined by thermodynamic modeling, lowered the liquidus temperature facilitating the generation of amorphous when the slag was fast cooled. The first modification (SAT1) incorporated 5 wt% silica and 11 wt% alumina, while the second (SAT2) included around 13 wt % alumina. Both modifications generated a hybrid microstructure composed of cementitious and non-cementitious crystalline phases and an amorphous fraction. During alkali activation using NaOH solutions of 0.25 M, rheological measurements on fresh paste using SAT2 registered plastic viscosity values 2.3 times higher than those of SAT1. The compressive strength after 28 days for the binder developed from SAT2 slag was 10-30% stronger than the one from SAT1. These binders showed similar crystalline reaction products but compositional differences in the amorphous gel correlated to the initial slag modification. The detected differences in the binder properties are significant enough to justify BOF-slag engineering as a way to deliver customized precursors for alkali activation.