## 2 Materials and Methods  
### Materials  
The plaster mixtures were prepared by admixing the two components (CSA cement and hollow glass microspheres (HGMSs)) with water. CSA cement was used as the binding agent (Next Base Cement grey, Buzzi Unicem, Casale Monferrato, Italy). Its X-ray fluorescence analysis (XRF, ZSX Primus II, Rigaku, Japan) is provided in Table 1.  
Hollow glass microspheres (HGMSs) of type K15 from 3M Europe were used as the lightweight insulating component. According to the datasheet, the HGMSs are made of alkali-resistant borosilicate glass with a density of 0.15 g\(\cdot\)cm\({}^{-3}\), a median diameter of 60 \(\upmu\)m, and a wall thickness of 1-2 \(\upmu\)m [19].  
CSA cement and HGMSs were mixed in the ratios presented in Table 2. A ratio of HGMSs higher than 70 wt.% was not feasible due to the low mechanical strength of the  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Element** & **CaO** & **Al2O3** & **SO3** & **SiO2** & **Fe2O3** & **TiO2** & **MgO** & **K2O** \\ \hline Composition (wt.\%) & 45.7 & 23.5 & 19.5 & 6.3 & 2.4 & 1.3 & 1.0 & 0.3 \\ \hline \end{tabular}
\end{table}
Table 1: Oxide composition of the CSA cement clinker used in this study determined by X-ray fluorescence analysis.  
hydrated material. Dry mixing of CSA cement and HGMSs was performed with a tumble mixer (Turbula T2F, WAB, Muttenz, Switzerland) at 47 rpm for 15 min to ensure a good mixture and to prevent segregation.  
Since the content of the binding component varied with each mixture, the water/cement (_w/c_) ratio was increased stepwise in order to achieve good workability. The best _w/c_ ratio for each mixture was determined individually by carefully adding tap water during continuous mixing. The maximum addition of water was reached as soon as the mixture stopped absorbing water, and excess water showed up at the surface, which is known as \"bleeding\". The determined and used w/c ratios are summarized in Table 2. The prepared plaster mixtures were finally stirred for 1 min in a mixer (N50-619, Hobart, Offenburg, USA) according to the standard DIN EN 196-1 [20].  
After mixing with tap water, the plaster was immediately cast into suitable molds. For each set of samples, six prisms with a 20 \(\times\) 20 \(\times\) 76 mm\({}^{3}\) edge length and one prism with a 134 \(\times\) 160 \(\times\) 40 mm\({}^{3}\) edge length were cast. Before casting, the molds were treated with rapeseed oil as a mold-release agent to prevent sticking after hydration. The small prisms (Figure 1a) were used to measure bending strengths, density, and porosity, while the larger samples were used to analyze thermal conductivity (Figure 1b).  
After casting, the molds were placed immediately on a vibrating table (Analysette 3 Spartan, Fritsch, Idar-Oberstein, Germany) for 1 min in order to achieve form fitting and to remove large bubbles. The amplitude was set to 1 mm. Afterwards, the molds were covered and stored inside a sealed cabinet at 99% relative humidity and 20 \({}^{\circ}\)C. After 24 h, the specimens were demolded and stored inside the same cabinet for another 13 days.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Specimen** & **100C** & **90C** & **80C** & **70C** & **60C** & **50C** & **40C** & **30C** \\ \hline CSA cement & 100\% & 90\% & 80\% & 70\% & 60\% & 50\% & 40\% & 30\% \\ in wt.\% of dry matter & 0\% & 10\% & 20\% & 30\% & 40\% & 50\% & 60\% & 70\% \\ \hline water/cement ratio & 0.45 & 0.7 & 1.3 & 1.8 & 1.9 & 2.4 & 2.75 & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Composition of the dry mixtures and the used w/c ratios.  
Figure 1: Specim with 50 wt.% CSA cement (50C) after 14 days of hydration: (**a**) small prism; (**b**) big prism.