###### Abstract  
Additive manufacturing brings many benefits to the building industry, one of them being automatization and the possibility to work in harsh environments, including underwater applications. In addition, this technology enables faster infrastructure repairs and adjustments to the scope of work to specific damage caused by, for example, biocorrosion processes. The main aim of this article is to investigate the development of geopolymers as a printable material for civil engineering, including underwater applications. For that purpose, the process of the material extrusion will be modified, and material properties will be improved. In the first step, the raw materials were investigated (SEM, EDS, XRF, particle size analysis) and the proper additives were selected based on literature analysis. Next, geopolymer paste was synthesized and fresh paste properties were investigated, including time for curing samples and workflow. The mixture composition was modified to obtain the required printable parameters through the application of different additives and the modification of the proportion of components, especially alkali solution. Finally, small-scale additive manufacturing trials were conducted in the air and with submerged containers. Additionally, samples were prepared using the casting method to compare the mechanical properties and microstructure. The obtained results show that additives such as xanthan gum and superplasticizer improve the rheological properties of the paste efficiently. With the help of additive manufacturing, geopolymer samples with compressive strengths of up to 7.5 MPa and flexural strengths of up to 4.15 MPa after 28 respectively were achieved. Compared to the average of the cast samples, the compressive strength of the printed samples was at least 5% lower, while the flexural strength was at least 38% lower for printed samples. The 3D-printed samples showed strong anisotropy between the tested orientations of the flexural strength samples. geopolymer; underwater; 3D printing; additive manufacturing 2020 ac [EMAIL_ADDRESS]/cachemic/  
conventional manufacturing methods [2; 3]. Additive manufacturing brings many benefits to building industries, including lowering the cost of investment in the building industry and lowering the environmental burden for construction [2; 4]. Additive manufacturing reduces the amount of used materials in comparison to traditional casting technology [2]. Moreover, it does not need a framework for construction, which additionally reduces the amount of materials. Another benefit is connected with automation and the possibility to work in harsh environments, including in space or underwater [5; 6]. However, this technology is relatively new and, in addition to its benefits, also brings challenges. The additive manufacturing of a pasty material differs in many respects from working with conventional methods such as molding [2; 7]. The properties of the paste must support uniform extrusion, good bonding to existing layers, minimal deformation after deposition, rapid curing, and other parameters [1; 8]. The process and material behavior are also completely different than traditional 3D printing for polymers [9; 10; 11; 12; 13]. With additive methods, geopolymers are mainly processed using extrusion-based techniques and less with the aid of binder jetting [14]. However, binder jetting gives more aesthetic products and better accuracy on a small scale [15; 16]. The extrusion technologies are more widely applied on a large scale and for prototype constructions, such as houses or bridges [1; 2]. Extrusion-based AM methods require the presence of geopolymer in a mixed, paste-like form and convey it through a nozzle using force. A continuous material flow at the nozzle can be achieved by either using a pump, a driven piston, or an extruder screw. The use of pumps or piston units for material feed is particularly suitable for larger printer systems. On the largest scale, the nozzle is guided by a structural framework (gantry) or a robotic arm [2; 17].