## 3 Results and Discussion
### Phase Analysis  
As already seen from the SEM images in Figures 5 and 6, the hydration of the CSA cement linker led to the formation of large etrringite crystals that enveloped the HGMSs and formed solid bridges among the spheres when the HGMS content was comparatively low. However, with an increasing amount of HGMSs, the amount of large crystals dropped. In order to thoroughly analyze the differences in phase formation, X-ray diffraction (XRD) was applied. First, the non-hydrated CSA cement was analyzed. Figure 9 shows the diffractogram of the cement powder, which was investigated at room temperature. The diffractogram shows diffraction peaks of the clinker phases ye'elimite and anhydrite. Ye'elimite is characteristic of CSA cement [24], while anhydrite is added as a sulfate source. Depending on the ratio of these two cement clinker phases, either etrringite (calcium-trisulfate) or calcium-monosulfate will form during hydration, as can be expressed by  
\[m=\frac{n_{\rm A}}{n_{\rm Y}}, \tag{3}\]  
where \(m\) is the weight ratio between anhydrite (\(n_{\rm A}\)) and ye'elimite (\(n_{\rm Y}\)). For complete hydration of anhydrite and ye'elimite to etrringite, the ratio \(m\) has to be greater than 0.45 [25]. For the used CSA clinker, the ratio was found to be \(m=0.53\) according to Rietveld refinement. Therefore, the formation of etrringite is most likely to happen, at least until the clinker phase ye'elimite is fully depleted. Figure 10 shows the XRD diffractogram of the hydrated CSA cement (sample 100C), which was prepared using a water/cement ratio of 0.45. Apart from the clinker phases ye'elimite (Y) and anhydrite (A), etrringite (E) was detected as expected from the calculated ratio \(m=0.53\). However, ye'elimite and anhydrite were still present, which can be explained by the relatively short time of hydration of only 14 days. Complete hydration of ye'elimite and anhydrite phases to etrringite would probably take a couple of months or even years. However, the amount of etrringite that had already formed in the first 14 days of hydration led to a high early flexural strength of sample 100C of about 4 MPa, as shown in the above bending tests (Figure 7). Figure 8: Thermal conductivity as a function of the cement content using surface probes. For each sample composition, a large prism (\(N=1\)) was measured at three different spots (\(S=3\)). Data fit to \(f(\phi)=a_{0}+a\phi^{p}\), with \(a_{0}=0.0404\), \(a=1.234\), and \(v=1.48\). Even though the addition of the amorphous HGMSs reduced the overall intensity of the diffractograms, ettrinigite formation was easily recognizable for the composite samples. Exemplarily, the diffractograms of samples 90C and 40C are shown in Figure 10. Apart from ettrinigite, ye'elimite as well as anhydrite were still detected. However, the peak intensities for ye'elimite (Y) and anhydrite (A), e.g., at 23.5\({}^{\circ}\)\(2\theta\) (Y) or 26\({}^{\circ}\)\(2\theta\) (A), decreased clearly (sample 90C) or almost vanished (see Y in sample 40C), while the ettrinigite \(\epsilon\) peaks, e.g., at 16\({}^{\circ}\)\(2\theta\), clearly stand out. We assume that the ettrinigite formation is, on the  
Figure 10: XRD diffractograms of the following samples after 14 days of hydration: 100C (solid line), 90C (dotted line), and 40C (dashed line). Figure 9: XRD diffractogram of the CSA cement clinker used in this study. The cement was measured as a powder that was stored in a dry environment until the measurement. one hand, preferred by the increasing water-cement ratio, and on the other hand by the huge amount of surface area that is provided by the HGMSs and which is presumably a preferred place for nucleation of ettrinigite crystals. Considering that nucleation outweighs the crystal growth of ettrinigite with an increasing amount of HGMSs, it could explain the decreasing proportion of those large needle-like ettrinigite crystals in the SEM micrographs in Figures 5 and 6.