# The Incorporation of Steel Slag into Belite-Sulfoaluminate Cement Clinkers
## 2 Materials and Methods  
### Materials  
The main raw materials used in this study were limestone and flysch (deep water siliciclastic rocks), steel slag, calcined bauxite, and white titanogypsum. For the correction, smaller quantities of mill scale, quartz, and calcite were also used. Steel slag, or more precisely slag mineral residue, was obtained from the processing of the mixture of EAF S slag and ladle slag. Slag processing involved weathering in the slag cooling yard, grinding, and metal recovery. The final product of the slag processing was steel granulate, which was returned to the EAF as steel scrap and slag mineral residue, the latter of which was used in this study. The treatments and processes which gave the mixture of EAF S slag and ladle slag its final properties are normal industrial practices. These processes and treatments are reported as Best Available Techniques by BREF documents. With the exception of white titanogypsum, which was dried at 40 \({}^{\circ}\)C to prevent its transformation to anhydrite, all raw materials were dried to a constant mass at 105 \({}^{\circ}\)C. In order to prepare large batches of homogeneous raw materials, 5 kg of each material was milled in a ball mill, until it met the criteria that 95 wt.% of the material could pass through a 200 \(\upmu\)m sieve. The exception was gypsum, which was not milled. The main oxide composition and loss on ignition at 950 \({}^{\circ}\)C in the raw materials for the clinker are given in Table 1.  
The proportions of raw materials in the raw clinker mixtures were calculated using the Bogue method for sulfoaluminate cements [25]. The formulation of targeted phase compositions was based on the minimum and maximum amount of C\({}_{4}\)A\({}_{3}\)S in belite-rich sulfoaluminate clinkers [26]. A medium amount of C\({}_{4}\)AF was pointed (constant for both mixtures) and the C\({}_{2}\)S content was adjusted to reach the sum 95 wt.% (minor phases were estimated to be 5 wt.%). The intended phase compositions are given in Table 2, and the corresponding proportions of raw materials in the raw mixtures KMO1 and KMO2 are presented in Table 3.  
Raw mixtures were homogenized in a laboratory ball mill (CAPCO Test Equipment Ball Mill Model 9 VS). In total, 200 g of each mixture was homogenized in 200 mL of isopropanol, together with steel balls, for 3 h. Isopropanol was removed by vacuum drying (1 medium permeable filter paper was used), followed by oven drying at 40 \({}^{\circ}\)C overnight. The raw material mixtures were then pressed into pellets 30 mm in diameter, using a force of 10.6 kN, with a weight of 15 g. The clinker mixtures were sintered at 1200, 1250, and 1300 \({}^{\circ}\)C in a Protherm furnace (PLF 160/9), at a heating rate of 10 \({}^{\circ}\)C/min. Samples were held at the final temperature for 60 min and allowed to cool naturally in the closed furnace. Cements were prepared from clinkers sintered at 1250 \({}^{\circ}\)C, as this was the lowest temperature at which the targeted phase composition was reached. Cement clinkers and white titanogypsum were ground using a laboratory disc mill to pass through a 125 \(\upmu\)m sieve. The proportion of gypsum added to the clinkers depended on the clinker phase composition intended. Gypsum was blended with the ground clinkers at a gypsum/calcium sulfoaluminate molar ratio of 1 [27]; 5.60 wt.% in the KMO1 sample and 9.90 wt.% in KMO2. The prepared cements were homogenized in a Turbula for 1.5 h and further milled to a similar Blaine specific surface area (SSA) of approximately 4900 \(\pm\) 150 cm\({}^{2}\)/g in order to eliminate the influence of the particle size on the reactivity of cement (Table 4). However, despite the uniform Blaine SSA, the measurement of the particle size distribution (PSD) indicated that the fineness of the cement was greater in the KM02 sample compared to KMO1 (Table 4, Figure 1).