## 4 Conclusions  
This article deals with the investigation of raw materials and the examination of paste properties and the additive manufacturing of geopolymers in different environments. Additionally, printed samples are compared with conventionally manufactured ones in terms of mechanical strength and structural properties. The utilized production methods include manual extrusion with a cartridge press, casting in molds, and paste extrusion modeling in air and underwater. Based on this research, the following findings have been formulated:  
* Paste extrusion modeling can be used to generate solid bodies from fly ash-based geopolymer. * By adding additives such as xanthan gum and superplasticizer, the rheological properties of the paste are modified to retain shape while being well extrudable. * 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 and 35 days, respectively, have been produced. Compared to the average of the cast samples, the compressive strength of the printed samples was 5% (#21 AB) and 9% (#20 AB) lower. The flexural strength was, on average, 38% (#20 AB) and 64% (#21 AB) lower for printed samples. * The sample series #21 AB showed strong anisotropy between the tested orientations of the flexural strength samples. Values achieved for the \"favorable\" orientation (4.15 MPa) were almost 7.5 times as high as those for the other orientations (0.56 MPa). For a reliable prediction of strength, anisotropies that occur during the printing process must be avoided. * The addition of more water (20% total in #20 AB versus 11% in #21 AB) resulted in better layer bonding and less anisotropy in sample series #20 AB. Excessively adding water leads to pore formation and insufficient development of the gel phase and should, therefore, be avoided. * The 3D-printed samples showed strong anisotropy, including the samples 3D printed underwater. * The extrusion of layers under water is possible with the tested paste, but the desired hardening did not occur after 7 weeks of immersion in water. * One of the problems was the printer's discontinuous process path. In the future, directly generated and optimized process paths should be used instead of sliced solids. Such g-codes can be created using the program Rhinoceros and the Grasshopper plug-in. Based on this work, key topics for subsequent experiments are derived. To test the effect of the extrusion device on the used printer, it is advisable to replace the progressive cavity pump with an auger. The findings on the compositions of the printable pastes can also be used as a starting point for further optimizations regarding the strength and water resistance of printed components. Finally, the influence of nozzle geometries and an intensified search for additives for underwater 3D printing should be considered for further optimization. Conceptualization, A.F.B., H.Z. and K.K. ; methodology, A.F.B. and K.K. ; software, A.F.B. and S.G.; validation, H.Z. and K.K. ; formal analysis, A.F.B. ; investigation, A.F.B. and S.G.; resources, K.K. ; data curation, A.F.B. ; writing--original draft preparation, A.F.B. and K.K; writing--review and editing, H.Z. and S.G.; visualization, A.F.B. ; supervision, H.Z. and K.K. ; project administration, K.K. ; funding acquisition, K.K. All authors have read and agreed to the published version of the manuscript. This research was funded by the project titled \"Development of geopolymer composites as a material for protection of hazardous wrecks and other critical underwater structures against corrosion\" under the M-ERA.NET 3 program by the Polish National Centre for Research and Development, grant number M-ERA.NET3/2021/71/MAR-WRECK/2022, and co-financed with tax funds based on the budget passed by the Saxon State Parliament (SAB). Not applicable. The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author. The authors declare no conflicts of interest.