## 4 Conclusions and Outlook  
Our investigations aimed at characterizing a two-component lightweight composite mortar consisting of hollow glass microspheres (HGMSs) and CSA cement as the binder with varying amounts of the components. After mixing and casting, the prepared prisms rested for 14 days under a controlled temperature and moisture to achieve hydraulic setting. The prepared samples were finally characterized in terms of porosity, density, flexural strength, thermal conductivity, and phase formation. As expected, the addition of HGMSs led to a decrease in the density, flexural strength, and thermal conductivity of the solidified composites. Each of the parameters dropped by approximately one order of magnitude by increasing the amount of HGMSs from zero to 70 wt.%. By contrast, the open porosity of the solidified samples was only somewhat influenced by the addition of HGMSs, as the open porosity was increased from about 25% without addition of HGMSs to 32% with 70 wt.% of HGMSs. Generally, the properties of the solidified composites should be considered as a three-component system consisting of cement, HGMSs, and porosity (air), while the remaining water is mainly chemically bound to the cement phases. The material properties of the composite might be interpolated by the volume-related contribution of each of these components by the help of mathematical mixing rules. While the trend of the curves for bulk density, flexural strength, and thermal conductivity indicated some non-linearity, a power-law-based mixing rule is justified. Micrographs and phase analysis demonstrated that, in composites with up to 50 wt.% of HGMSs, the spheres were surrounded by a void-filling, loose network of large, up to 10-um-long, needle-like ettrinigite crystals. Ettrinigite is the preferred hydration product according to the given ratio of anhydrite and ye'elimite in the clinker. With an increasing amount of HGMSs, the network of large crystals gradually vanished. The decrease in the embedding matrix phase was accompanied with a loss of mechanical strength and heat conduction, respectively. Micrographs revealed that at high loads of HGMSs (60 or 70 wt.%), the glass spheres were covered predominantly by tiny ettrinigite seed crystals instead of columnar crystals. Phase analysis confirmed, however, that ettrinigite formation was proportionally even more pronounced at high loads of HGMSs. Therefore, we conclude that the high surface area provided by HGMSs contributed preferentially to heterogeneous nucleation of ettrinigite on the glass surface at the expense of crystal growth. The increase in nucleation rate led to an even faster consumption of the cement phases ye'elimite and anhydrite. Hence, the clinker phases, such as ye'elimite, almost vanished after 14 days of hydration. In contrast, the mixtures containing less HGMSs still contained a considerable share of ye'elimite after the same period of time. The mixture of CSA cement and HGMSs generally meets the requirements for a lightweight insulating plaster. Compositions consisting of 50 to 60 wt.% of HGMSs are lightweight with densities ranging from 0.15 to 0.2 g\(\cdot\)cm\({}^{-3}\) and heat-insulating with thermal conductivities ranging from 0.043 to 0.046 W\(\cdot\)m\({}^{-1}\)K\({}^{-1}\), while also offering a sufficient flexural strength of 0.4 to 0.5 MPa for facade application. The thermal conductivity approaches that of the most commonly used materials for facade insulation, EPS and XPS. The thermal conductivity of EPS ranges from 0.035 to 0.049 W\(\cdot\)m\({}^{-1}\)K\({}^{-1}\), while its density is about 0.03 g\(\cdot\)cm\({}^{-3}\)[7; 8]. Due to its purely mineral composition, the HGMS/CSA composites are fire resistant without addition of flame retardants. Its spraying applicability was demonstrated elsewhere [9]. As stated above, a further improvement in thermal insulation to less than 0.040 W\(\cdot\)m\({}^{-1}\)K\({}^{-1}\) would require the composites to have more open pores.