Weathering Effects and Reactivity Restoration in BOF Slag for Use as a Sustainable Binder

The valorization of basic oxygen furnace (BOF) slag as a binder offers a promising route to reduce the material and energy footprint of the steel and building sector. However, the reactivity of BOF slag is still poorly understood and highly variable. Even small amounts of moisture and carbonation can severely inhibit its hydraulic activity. This study examines three slags (K2, WK2, K3) with varying lime contents and storage histories. The influence of oven drying (40 and 105 °C), controlled rewetting (0.5–1 wt.% water), and milling with triisopropanolamine (TIPA, 0.05 wt.%) on the slag reactivity is investigated in addition to citrate activation. The results are analyzed by isothermal calorimetry, XRD, TGA, and SEM after 3 days. Results show that drying restores reactivity in weathered slags, originally containing 0.64 and 0.91% moisture; a process that can be reversed by rewetting the slags, although there seems to be an optimum moisture content. TIPA promotes hydration in low-lime slag but is insufficient for high-lime slag. These findings stress the critical role of storage and pre-treatment in enabling BOF slag to serve as a consistent sustainable binder.