## 3 Results and Discussion
### Carbon Fiber Reinforcement of Geopolymers  
Figure 13 shows the average compressive strength values for fly ash and sand-based geopolymers and geopolymer/cement hybrids (composition given in Table 5) reinforced with carbon fiber. The average compressive strength values for the reference samples--geopolymer and geopolymer-cement hybrid--were 48.4 MPa and 51.6 MPa, respectively. The highest average compressive strength values were obtained for geopolymer-cement hybrids with 0.5% addition of chopped carbon fiber, 5 mm in length (53.2 MPa). A decrease in compressive strength values, in comparison to the reference samples, was recorded for the geopolymer with 1% addition of chopped carbon fiber (46.4 MPa) and the geopolymer-cement hybrid with 1% addition of chopped fiber (49.4 MPa). The remaining compressive strength results oscillated at a similar level, approximately 48 MPa. In Figure 14, geopolymer specimens with chopped carbon fiber are shown after compression testing. Figure 14: Fly ash and sand-based geopolymer samples (10 M NaOH solution) after compression test, reinforced with short carbon fiber (5 mm). Figure 13: Compressive strength of geopolymers and cementitious geopolymers reinforced with carbon fiber. Figure 15 shows the average flexural strength values for fly ash and sand-based geopolymers and geopolymer-cement hybrids (composition given in Table 5), reinforced with the addition of carbon fiber. The highest flexural strength values were obtained for the reference samples, without the addition of reinforcement. For the geopolymer, the average value was 10.4 MPa, and for the geopolymer-cement hybrid, it was 11.4 MPa. The 0.5% addition of long carbon fiber to the geopolymer-cement hybrid slightly reduced the flexural strength values (11.0 MPa) compared to the reference sample. In addition, the 0.5% addition of chopped carbon fiber reinforcement to the geopolymer-cement hybrid obtained similar flexural strength values as the previously described material (10.8 MPa). The lowest average flexural strength value was obtained for the geopolymer material with a 1% addition of chopped carbon fiber (8.3 MPa). As a result of the study, it was observed that the best compressive strength results for specimens with the addition of carbon fibers were obtained for geopolymer-cement hybrids with 0.5% fiber addition with a length of 5 mm. Compared to the sample without reinforcement (reference sample), this value increased from 51.6 to 53.2 MPa. The addition of carbon fiber to the geopolymer matrix did not improve the compressive strength results. The values for all the geopolymer sample variants oscillated around 48 MPa (a similar value was obtained for the reference sample). In the flexural strength tests, the addition of carbon fiber as reinforcement to the geopolymer-cement hybrid did not significantly affect the result values compared to the reference sample. The values oscillated around 11 MPa. In the case of geopolymer samples, the addition of carbon fiber contributed to a decrease in the flexural strength values compared to the reference sample (10.4 MPa). [PERSON] et al. [56] conducted a study on the effect of adding short carbon fibers in a geopolymer matrix based on high-calcium fly ash. The carbon fiber (CF) content varied from 0 to 1%, and the liquid to ash ratio was 0.5. The workability, mechanical, and electrical properties of the geopolymer composites were investigated. The results showed that the addition of 0.5% CF and above caused the ductile failure of the samples. Increasing the CF content also resulted in reduced workability and mechanical strength. [PERSON] and his team of researchers, on the other hand, studied a geopolymer cement (Pyromeral) reinforced with carbon fiber layers. The mechanical and physicochemical properties of both neat and reinforced cement were tested using the Charpy impact test and three-point bending test. A five-fold increase in flexural strength was observed for fiber-reinforced geopolymer specimens and a three-fold increase in impact strength was observed as well [58].