## 3 Results and Discussion
### Structural Analysis  
To better assess the results and explain the causes of the material properties, the structure of the samples is analyzed. This is done before and after the load tests, based on the macrostructure (Figures 14-16), and with SEM to visualize microstructural features (Figures 17 and 18). Figure 13: Flexural strength. Figure 14: Samples: (**a**) 3 DP samples after cutting and measuring (#20 AB); (**b**) #20 AB CBHT after 28 days; (**c**) Efflorescence after 14 days at #20 AB CC; (**d**) Efflorescence at #20 AB 3 DPB after 14 days. Figure 15: Printed volume after 49 days underwater: (**a**) The appearance of the samples after removing the water; (**b**) The plate with hole after cut element; (**c**) The cut element with visible delamination. Figure 16: Comparison of the fracture surfaces after the flexural strength test: (**a**) #21 AB—orientation 1; (**b**) #21 AB—orientation 2; (**c**) #21 AB—orientation 3; (**d**) #20 AB—orientation 1; (**e**) #20 AB—orientation 2; (**f**) #20 AB—orientation 3. Figure 17: SEM images (magnification 200\(\times\)): (**a**) #21 AB casted; (**b**) #21 AB casted, heat-treated; (**c**) #21 AB 3D printed; (**d**) #20 AB casted; (**e**) #20 AB casted, heat-treated; (**f**) #20 AB 3D printed. Figure 14 shows different geopolymer samples at different times. Figure 14b shows how the cast samples deviate from the ideal geometry due to voids and cavities. These deviations can be observed equally in all cast samples but are more pronounced in material #20 AB and the heat-treated samples. This can be explained by the reduced processability of pastes specialized for extrusion and the higher early shrinkage from heat curing. Figure 14c,d show specimens that developed significant efflorescence after a certain time. Efflorescence results from the reaction of alkalis present in the geopolymer and atmospheric carbon dioxide. Free alkalis like Na+ or K+ travel through the pore network of the geopolymer until they reach the surface [54; 55]. Efflorescence has a negative effect on the mechanical properties of the samples [54]. This phenomenon occurred in all specimens, although it was most noticeable in the casted, not heat-treated specimens and in the ones cut from the printed volumes. Leaching also occurred in the samples produced underwater, causing the water to become cloudy and forming a white layer between the component and the water. The dimensions of the body printed underwater do not deviate excessively from those printed in the air (Figure 15). The individual layers and contours on the sides and on the top are clearly recognizable. However, there are gaps between the different layers, resulting in poor or no bonding between them. Although the paste is extruded and placed according to the g-code, the composition is not suitable for producing mechanically resistant test specimens. Despite additives, the mix is not adapted to the conditions of underwater printing. Since there is no literature on underwater 3D printing of geopolymers, it is not possible to compare the results. When looking at the fracture surfaces (Figure 16), the layer bonding within a component is differently defined. For example, a significantly poorer homogeneity can be observed in the bodies printed from material #21 AB compared to those from #20 AB. Dis  
Figure 18: Analysis of microstructural details: **(a)** #20 ABCHT, magnification 170\(\times\)—removal of a sand grain; **(b)** #20 ABCHT, magnification 200\(\times\)—poor connection sand–matrix; **(c)** #21 ABCHT, magnification 500\(\times\)—pore with cracks; **(d)** #20 AB3 DP, magnification 550\(\times\)—pores within the matrix; **(e)** #21 AB3 DP, magnification 1000\(\times\)—pore and partially dissolved fly ash; **(f)** #21 AB3 DP, magnification 1200\(\times\)—undissolved fly ash. coloration on the fracture surface, which indicates incomplete drying, is also particularly evident in #20 AB samples. Since the structure of the specimens in #20 AB is homogeneous, the values are close to each other.