The greening of steel: Modifying a novel open bath furnace (OBF) slag for use in cement

Abstract

In response to the need for lower CO₂ emissions in ironmaking, steelmakers explored alternatives to traditional blast furnace (BF) technology. One such alternative was the Direct Reduced Iron Open Bath Furnace (DRI-OBF). As with BF slag, which has been widely used as a clinker substitute in cement, this study evaluated the potential of DRI-OBF slag for cementitious applications. OBF slag was produced in a 1 MW prototype furnace, chemically adjusted during smelting, and water-quenched to form a glassy slag comparable to ground granulated blast furnace slag (GGBFS). Two composite OBF slag samples were prepared and compared with BF slags from ArcelorMittal (South Africa) and Thyssenkrupp (Germany), with fineness levels standardized around 275–296 m²/kg. Mortar and paste mixes were tested for workability, strength development, drying shrinkage, chemical reactivity, and hydration phase evolution. Cementitious performance was influenced by slag fineness, chemical composition, glass content, and substitution ratio. With similar composition and glass content across samples, performance differences were primarily attributed to fineness. Increased fineness enhanced early strength gain but raised water demand and reduced workability. Slag reactivity—governed by fineness and composition—influenced heat evolution and strength development at later stages of curing. The hydration phases in slag–cement blends were similar for both BF and OBF slags, forming C-S-H, C-H, and C-A-S-H. Due to its lower sulphur content, OBF slag formed hydrotalcite or hydrogarnet rather than ettringite. Mortar specimens containing OBF slag exhibited a reduced greening effect, consistent with the lower sulphur levels. Environmentally, OBF slag demonstrated strong potential as a clinker replacement. Its production route via electric smelting offered CO₂ reductions of up to 50% with natural gas, or up to 90% when green hydrogen was used. In cement applications, OBF slag reduced emissions by up to 60% per ton and lowered energy demand associated with clinker production. It also allowed broader use of lower-grade iron ores and aligned with emerging low-carbon steel routes such as DRI–OBF and DRI/Scrap–EAF. As the availability of fly ash and GGBFS declines, OBF slag and calcined clays were identified as promising supplementary cementitious materials (SCMs). For optimal performance, the slag was recommended to meet key criteria: ≥95% glass content, ≥300 m²/kg fineness, 80% of particles < 40 μm, and <5% impurities. To improve refractory lifespan, a second-generation slag (OBF Gen2) was proposed, produced in a slag valorisation furnace (SVF). This MgO-saturated slag was less corrosive and was expected to deliver improved cementitious performance, though further testing was required. A techno-economic comparison between DRI-OBF and DRI-OBF-SVF routes was also suggested. In summary, OBF slag proved to be a sustainable SCM with potential benefits for both the steel and cement industries. Continued research was recommended to optimize its chemistry and performance as a viable replacement for conventional GGBFS.