###### Abstract
The gantry enables the construction of large cross-sections in a cartesian coordinate system, whereas the robotic arm operates on a polar coordinate system and is mostly used for slightly smaller prints [18]. In both cases, the geopolymer mix is pumped through a hose before reaching the nozzle. In contrast, systems are also used where the material supply is positioned just above the nozzle and a feed is achieved using extruder screws instead of direct force input. This principle is mainly used to develop and manufacture smaller components and has become established due to its ease of handling [1; 16]. The flow behavior of materials can be summarized under the term rheology. By testing the material with a rheometer, it can be divided into different fluid registers, each of which has different properties [19; 20]. Especially for systems in which the material is pumped, it is important that the paste behaves like an easily pumpable liquid under pressure while increasing its viscosity as stress decreases. This behavior can be described as shear-thinning. When viscosity decreases not only under shear strain but also as time progresses, about the material exhibits thixotropy [20]. The activator solution plays a vital role in determining the viscosity of geopolymer mixes. Sodium-based activators lead to higher viscosities and their molarity is an important regulator for flowability [21]. Another necessary property for clean processing is the good extrudability of the paste. A low plastic viscosity and optimal yield stress help to convey a material string that neither tears off nor leads to lumping. [PERSON] et al. [22] investigated the effect of Si/Na ratio of the activator on the extrudability of geopolymer and found that decreasing this ratio leads to better results in terms of paste structure [22]. The workability of a material is characterized by how little resistance it offers to processing by conventional methods. This property is greatly influenced by the particle size and shape of the raw materials. Particle size reduction, for example with fly ash, generally improves the workability of the mixture and the even particle shape also enhances machinability compared to slag-based geopolymer [8; 23]. The irregular particle shape and compact structure of slag-based geopolymers lower their workability. It can be improved by replacing ground granulated blast furnace slag (GGBFS) with metakaolin to prevent early polymerization and by adding additives such as superplasticizer to the mix [24; 25]. The open time of the material, i.e., how long it remains workable, is closely related to buildability. Because fresh properties can change over time, it is important to know the time window during which the paste has optimal characteristics for extrusion. For geopolymers, this typically ranges up to 30 or 60 min [21; 26]. However, variations in precursors, temperature, and additives have retarding or accelerating effects on the material [21; 27; 28]. Several studies have been conducted to test the effects of different additives on paste rheology, which is extremely important to ensure good flowability through the printer and the other factors mentioned above. Table 1 provides an overview of the most common ones and their function regarding the paste. The level of knowledge and experience in the field of additive manufacturing with geopolymers is still limited due to the short history of the topic. However, there has been a strong increase in interest in this area in recent years [36], largely due to a shift in thinking in the construction industry. It is important to point out that geopolymer materials are not yet economically competitive. In most cases, they are still more expensive than the ordinary cement option, mainly because the activators are produced with a high degree of purity, which is not completely necessary for their use in geopolymers [37].