# The Incorporation of Steel Slag into Belite-Sulfoaluminate Cement Clinkers
## 3 Results
### Clinkers
#### 3.1.2 SEM/EDS  
Clinkers showed a clear decrease in porosity as the sintering temperature increased (Figure 2). In both mixtures, a temperature of 1200 \({}^{\circ}\)C was sufficient for developing the main reactive phases. However, X-ray Powder Diffraction (XRD) analysis revealed that the targeted amounts were not achieved at 1200 \({}^{\circ}\)C. Subhedral to euhedral C\({}_{2}\)S and C\({}_{4}\)A\({}_{3}\)S grains were already clearly developed at 1200 \({}^{\circ}\)C in both KMO1 and KMO2 clinkers. However, at 1200 \({}^{\circ}\)C, the clinkers showed an inhomogeneous structure, as C\({}_{4}\)AF was not present in the interstitial areas, but formed in individual nests (Figure 2). In both mixtures, sintering at 1250 \({}^{\circ}\)C resulted in a homogeneous structure, formed by subhedral to euhedral C\({}_{2}\)S and C\({}_{4}\)A\({}_{3}\)S grains and C\({}_{4}\)AF in the interstitial area (Figure 2). Raising the sintering temperature to 1300 \({}^{\circ}\)C led to structure densification, and increased the number of euhedral C\({}_{2}\)S and C\({}_{4}\)A\({}_{3}\)S grains (Figure 2). Figure 3 shows elemental EDS maps for the KMO1 and KMO2 clinkers sintered at 1200, 1250, and 1300 \({}^{\circ}\)C.  
Furthermore, the amounts of minor individual elements measured (EDS point measurements and SEM statistics), incorporated in the main clinker phases, are presented as sequences, in order of the maximum to minimum quantity detected. Belite enables various element substitutions [12,36,37,38,36]. It exhibited Al, Mg, Fe, S, K, Cr, Ti, Na, Ni, and Mn in the KMO1 clinker and Al, S, Fe, Mg, Cr, Ti, K, Na, Ni, and Mn in the KMO2 clinker. The higher amount of S detected in KMO2 corresponds to a higher gypsum content (Table 3). On the other hand, the higher Mg content of the KMO1 belites, as detected by EDS analysis, cannot be explained by a higher input from the raw materials (Tables 1 and 3). However, the highest amount of Mg in KMO1 belites was detected at 1200 \({}^{\circ}\)C, where the sintering was not complete, as evident from the EDS maps (Figure 4a). In both clinker mixtures, the highest amount of Al was incorporated into belite at 1250 \({}^{\circ}\)C (Figure 4a,b). On the other hand, the amount of Mg incorporated varied between the clinker mixtures. In KMO1, the amount of Mg in the belite phase clearly decreased with an increase in the sintering temperature (Figure 4a), while in KMO2 belites, the maximum amount of Mg was detected at 1250 \({}^{\circ}\)C (Figure 4b). In both clinkers, belites included the maximum amount of S at 1250 \({}^{\circ}\)C. The amount of Fe in the KMO1 belites decreased with a rise in the sintering temperature (Figure 4a), while in KMO2, the maximum amount of Fe incorporated in belite occurred at 1250 \({}^{\circ}\)C (Figure 4b). The maximum amount of K incorporated into belites occurred at 1200 \({}^{\circ}\)C in KMO1, while in KMO2, the K content of belites did not change as a result of the sintering temperature. In both clinkers, calcium sulfoaluminate included minor elements in the same sequence, with Si, Fe, and Mg, followed by K, Cr, Ti, Na, Mn, and Ni. The quantity of Si decreased with rising temperatures in both KMO1 and KMO2, while the maximum amount of Fe and Mg occurred at 1250 \({}^{\circ}\)C (Figure 4c,d). The high Si content can be attributed to the fact that calcium sulfoaluminate is an end member of sodalite solid solution, in which Si is replaced by Al [39,40]. Consequently, increasing the sintering temperature resulted in decreased amounts of Si (Figure 4c,d). Figure 2: The phase compositions of clinkers KMO1 (**a**) and KMO2 (**b**), sintered at 1200, 1250, and 1300 \({}^{\circ}\)C and shown at different magnifications (Scanning Electron Microscope (SEM)/ Backscattered-Electron (BSE) microphotographs). Clinker phase designations: C\({}_{2}\)S = belite; C\({}_{4}\)A\({}_{3}\)S = calcium sulfoaluminate; and C\({}_{4}\)AF = ferrite.