# The Incorporation of Steel Slag into Belite-Sulfoaluminate Cement Clinkers
## 3 Results
### Cements  
#### 3.2.1 Isothermal Conduction Calorimetry  
The hydration heat of the cements analysed is shown in Figure 5. The results showed that the kinetics of the two cements differ significantly.  
Due to the external mixing method used, the first peak assigned to the heat of wetting and the early reaction during hydration [27, 42, 43] was not included. As can be seen from the results, the KMO1 cement reacted faster than the KMO2. Namely, the induction period for the KMO1 was very short following the first peak, lasting up to about 2 h of hydration in comparison to KMO2, where the induction period lasted until about 6 h of hydration  
Figure 4: The incorporation of minor elements into the main clinker phases of the KMO1 and KMO2 clinkers sintered at 1200, 1250, and 1300 °C, as determined by EDS analysis (wt.%): (a) Belite (C\({}_{2}\)S) in KMO1; (b) C\({}_{2}\)S in KMO2; (c) calcium sulfoaluminate (C\({}_{4}\)A\({}_{3}\)S) in KMO1; (d) C\({}_{4}\)A\({}_{3}\)S in KMO2; (e) ferrite (C\({}_{4}\)AF) in KMO1; and (f) C\({}_{4}\)AF in KMO2. The number of grains measured is given below the temperature in each individual diagram.  
before the main peak occurred. This period is attributed to the dissolution of clinker and the early formation of ettringite [27; 43; 44]. As follows, the main peak then occurred, which is attributed to the setting and hardening of the paste and the dissolution of calcium sulfoaluminate, together with gypsum, and the formation of ettringite and aluminium hydroxide [42; 45; 46]. This peak occurred after 4 h in KMO 1 and after 7 h in KMO2. Despite the fact that KMO2 contained a higher amount of highly reactive calcium sulfoaluminate phase, and that it had slightly finer particles compared to the KMO1, according to PSD measurement of the cement, which enhances its reactivity [47], the KMO1 reacted faster. However, this could be due to the presence of alkali sulfates (arcanite, aphthitalite, and Ca-langebeinite) in the KMO1. As already reported elsewhere, alkali sulfates have a significant effect on the hydration rate and setting time of cement. Namely, according to data in the literature [48; 49; 50], the reactivity of cement is accelerated in the presence of alkali sulfates. In addition, the KMO1 sample in comparison to KMO2 contains more ferrite, which is an active hydraulic phase, while it contains less perovskite, which is an inert phase [28; 29; 30]. Furthermore, the KMO2 sample displayed a second heat maximum after 11.5 h of hydration, which was attributed to the depletion of gypsum and the subsequent precipitation of monosulfate [42; 43; 51]. In KMO1, on the other hand, where a higher calcium sulfoaluminate amount was present, only a shoulder was observed at 6.5 h of hydration. This was probably due to the lower gypsum content in this mixture, which favored the formation of ettringite over monosulfate, and the peak was shifted to an earlier time [42; 52].  
The cumulative heat results are shown in Figure 6 and Table 6. After 7 days of hydration, the amount of cumulative heat released was smaller in the KMO1 cement (176.6 J/g) than in KMO2 (223.8 J/g), indicating that more heat was released during the hydration of KMO2. This was attributed to a higher amount of calcium sulfoaluminate and a higher degree of hydration in the KMO2 cement.