## 3 Results and Discussion
### Comparison of the Properties of the Samples Made by Different Methods of Manufacturing  
The various specimens are tested for compressive strength and flexural strength after 28 days (#20 AB) and 35 days (#21 AB). To better classify the results, the densities are calculated. The results are presented in Figures 12 and 13 for the samples from material #21 AB and #20 AB, for compressive strength (Figure 12) and flexural strength (Figure 13). CC (CB) stands for casted cubes (casted blocks), CCHT for casted cubes heat treated (analogous to CBHT), and 3 DPC1-3 for the printed samples in the corresponding orientation.  
The average values for compressive strength are between 1.58 and 9.16 MPa. The casted samples made from #21 AB that are heat treated, have the highest compressive strength, and the samples printed with #20 AB that are tested in orientation 3 have the lowest. Material #21 AB has more than three times higher compressive strength values, compared to #20 AB. Both the density and the test values achieved are clustered and can be easily separated visually. The only exception is sample #21 AB CCHT, which has the second-lowest density and the highest fracture strength. At 1.55 g/cm\({}^{3}\), the density is far below the average value of 1.86 g/cm\({}^{3}\) for the other samples of the same material. The lower density of the test series #20 AB compared to #21 AB of 12% on average, can be attributed to the increased water content in the activator. The lower values of samples marked as CCHT in both groups can be caused by loss of water during the heat treatment process.  
Figure 12: Compressive strength.  
Additional water in the mixture is released during hydrolysis, geopolymerization, and condensation. The decrease in density as the curing temperature increases is also consistent with the findings of other studies [51].  
The flexural strengths are between 0.533 and 4.154 MPa, with the value of the sample printed from #21 AB in the second orientation being seven times higher than the rest. Apart from this exception, material #20 AB has a higher flexural strength than #21 AB. The same densities as in the compressive strength test are used, but the test series #21 AB CBHT is not carried out.  
Even after 49 days, the cuboid printed under water has not yet hardened to the point where it can be removed from the tub. The volume does not offer enough resistance to the penetration of a small stick, so the performance of mechanical stress tests is perceived as impossible.  
The different densities of the individual sample specimens can be attributed to the difference in water content between the test series and the type of curing. [PERSON] et al. [52] confirmed a correlation between increasing water content and decreasing mechanical properties. The formation of pores and a decrease in the degree of polymerization due to water in the network leads to a strong reduction in strength [52]. The values achieved in the work for compressive and flexural strength are significantly lower than those of comparable documented studies. [PERSON] et al. found compressive strength values around 35 MPa for printed and cast cubes of fly ash-based geopolymers, with flexural strength from 4 to 10 MPa [49]. Other works also show the possibility of obtaining compressive strength up to 55 MPa using 3D printing technology [47; 53]. The reduced properties of the samples tested here can be attributed to the use of other additives. These require a high addition of water (total water content 21% in #20 AB and 12% in #21 AB), which inhibits strength-forming processes and promotes destructive pore formation.