## 1 Introduction  
CO\({}_{2}\) emissions reduction is the most pressing topic to mitigate global warming. The European Green Deal sets the ambitious goal of a 55% reduction in greenhouse gas emissions by 2030 [1]. This applies in particular to the building sector. In the European Union, around 33% of greenhouse gases are currently emitted by buildings [2]. These are generated especially by the use of fossil fuels for heating. Therefore, CO\({}_{2}\) emissions can be lowered by improving building insulation in order to reduce heat losses through the building shell [3]. Since new buildings generally have good insulation, the impact of the energy-efficient renovation of old buildings is even greater [4]. Apart from the general need for better insulation of old buildings, renovation has to be sped up in order to meet the requirements of the European Union by 2030. The renovation rate is nowadays around 1% of the building stock per year, but it is estimated that this rate has to be at least doubled in the near future [5].  
Building insulation materials range from purely mineral insulation materials, such as rock wool over polymer foams, such as EPS or XPS, to organic materials such as wooden or straw-based materials. EPS boards have a market share of about 80% in Europe [6] because of their low thermal conductivity (0.035-0.049 W\(\cdot\)m\({}^{-1}\)K\({}^{-1}\)) [7] and low density (30 kg\(\cdot\)m\({}^{-3}\)) [8], which are combined with low costs. EPS boards are applied directly onto the wall, resulting in two challenges. First, planar boards need to be adjusted and fixed manually on the facade, which is labor- and time-intensive. Secondly, EPS boards must cover the entire facade to avoid thermal bridges, which is problematic for some older buildings as their facades are often uneven, decorated, or even landmarked. Since EPS boards cover and conceal any individual facade design, there is a growing interest in form-following materials and application techniques for facade renovation.  
An attractive solution to these problems is a mineral-based composite insulation plaster that can be applied, for example, by wet spraying [9]. Earlier investigations on lightweight concrete reported an admixture of hollow glass microspheres embedded in a Portland cement and geopolymer cement matrix [10; 11]. By embedding hollow glass microspheres (HGMSs) in concrete, the thermal conductivity and the density of the composite concrete were reduced. However, cement cluster production emits an average of 0.842 t of CO\({}_{2}\) for every ton of cement clinker [12]. Many of these emissions result from the calcination of limestone to produce Portland cement clinker [13]. Over the past few decades, several alternative cementitious binders have been developed [14]. One of them is calcium sulfoaluminate (CSA) cement. Its production emits only 50% of the CO\({}_{2}\) compared with Portland cement [15]. This is due to its limited lime content and the reduced firing temperature of the clinker of only 1250 \({}^{\circ}\)C [16], which is 200 K lower than for Portland cement. In addition, CSA cement also settles very quickly [17]. This is mainly due to the formation of ettrinigite, which provides early strength. The shrinkage of CSA cement can be controlled by adjusting the sulfate content in the clinker [18]. After mixing with water, a transportable and processable mass is created. This mass can be conveyed through pipes using cement pumps and applied by plastering machines. These properties make CSA cement a superior binding agent for a mineral insulating plaster [9].  
Based on the above considerations, this study investigated a composite plaster based on CSA cement and HGMSs only. Varying compositions of HGMSs and CSA cement were prepared by setting the amount of HGMSs from 0 to 70 wt.% in relation to the dry mass. After hydraulic setting, the mixtures were analyzed in order to report the thermal and mechanical properties as well as the phase composition.