# The Incorporation of Steel Slag into Belite-Sulfoaluminate Cement Clinkers
## 1 Introduction  
The co-generated ferrous slag stream is an integral part of the steel production process. Slag is the biggest by-product stream of the steel sector, accounting for 90% with regards to the mass of by-product and waste generated [1]. In total, 30% of the world's steel is produced using an Electric Arc Furnace (EAF), with scrap steel as the input and electricity as the energy source. On average, around 200 kg of co-products are generated from one ton of steel produced in this way [2]. In Europe, 15.7 million tons of steelmaking slag were produced in 2018, with 34.8% coming from EAF-based production and 12.5% from other processes, probably secondary metallurgical processes [3]. The most common use of EAF slag is as aggregate in road construction, for which EAF slag from carbon steel production is predominately used (EAF C slag). EAF C slag has excellent mechanical properties, which justify its use as an aggregate in the manufacturing of bituminous mixes [4]. Other uses for steel slags (EAF C slag, EAF stainless steel slag (EAF S slag), and ladle slag coming from secondary metallurgical processes) are as aggregate for concrete, waste water treatment, embankments and fill, and ground stabilization [5]. Their use in cement and other hydraulic binders has also been reported [6, 7, 8, 9]. Although steel slag is highly recyclable, there is still room for improvement, and efforts in the steel industry are being made to move closer to the \"zero-waste\" goal. In order to achieve the \"zero waste goal\", it is important to look for alternative applications for the use of steelmaking slags that are not yet in widespread use. In the cement industry, the possibility of producing low-carbon and low-energy cements using secondary mineral raw materials rich in aluminiumis a promising solution, which would save natural resources and reduce the environmental impact of the production processes.  
To date, only a few studies have demonstrated the use of steel slags in the production of belite-sulfoaluminate (BCSA) clinker [10; 11]. BCSA clinkers are one of the most promising low carbon and low energy potential alternatives to ordinary Portland cement (OPC) clinkers, as they use a sintering temperature that is around 200 \({}^{\circ}\)C lower and less energy is consumed for the grinding process [12; 13; 14]. They consist of three main phases: Belite (C\({}_{2}\)S); calcium sulfoaluminate (C\({}_{4}\)A\({}_{3}\)S); and ferrite (C\({}_{4}\)AF). By definition, the most abundant phase is belite, which represents more than 50 wt.% of BCSA clinker [14]. Among the minor phases, periclas (M), gehlenite (C\({}_{2}\)AS), mayenite (C\({}_{12}\)A7), perovskite (CT), arcanite (KS), acermanite, magnetite, and anhydrite (CS) are the most commonly reported [15; 16]. Although the targeted amounts of the main clinker phases can be well-predicted, the formation of minor phases strongly depends on the minor elements which are present in the raw materials [17; 18]. Moreover, foreign elements can be incorporated into the main clinker phases, influencing the dynamics of crystal formation [19; 20; 21] and consequently affecting the hydraulic properties of the cement [22; 23; 24].  
The aim of this study is to demonstrate and discuss the use of steel slag in BCSA cement clinker. BCSA clinkers of two targeted phase compositions were sintered at 1200, 1250, and 1300 \({}^{\circ}\)C, in order to describe the general microstructure, phase composition, and incorporation of minor elements into the hydraulic clinker phases. To reveal any potentially unfavorable influence of the material-derived impurities on the hydraulic properties, BCSA cements were prepared from clinkers sintered at 1250 \({}^{\circ}\)C. Early age hydraulic properties were investigated by isothermal calorimetry and compressive strength development was measured at 7 and 28 days of hydration.