## 3 Results and Discussion  
### Experimental Results  
In Figure 3, the obtained results for compression, three-point bending and splitting tests are respectively reported, while in Figure 4, images of one tested specimen are reported for each test. The results sketched in Figure 3 for each test are the mean of those obtained for each sample (three specimens for each test) and allow us to draw the following remarks: (a) in the compression test, the main contribution of the fibers is related to the great increment in the softening phase, which, on the contrary, is limited in the case of specimens without fibers; (b) the ultimate strength \(f_{cu}\) increases when comparing the results of no-fiber specimens with those with a 2% of fiber content; (c) for intermediate fiber contents, the results show a decrement of \(f_{cu}\) and a limited increment in the yield strength \(f_{cy}\) with the fiber content. These results are different from those reported in [45] where, however, the length of the fiber is about 35 mm, 14 times greater than that adopted in the present study, also suggesting a dependency of the compressive strength on the fiber length. It must be noted that in [45], the replacement was performed as a volume fraction, while in the present paper, it was performed in terms of a weight fraction. As a result, the compression Young' modulus is very close in all the cases except for the case of MK3-05, in which it is lower. The compression results confirm that for this type of loading, the contribution of the fiber content is not meaningful and the behavior is similar. The exception to this remark is the case of 0.5% of which the peculiar behavior should be related to some \"defects\" in the tested specimens. The reason of these \"defects\" must be related to the role of the porosity above the fiber's critical concentration (usually between 0.2% and 2%) that, in the case under examination, is around 0.5% [15]. This aspect will be further investigated in the development of the research. Analogous remarks can be made in the case of the three-point bending test. The fiber content causes a significant increment in the softening phase with respect to the no-fiber content, an important increment in the ultimate strength \(f_{fu}\) and a limited increment in the yield strength \(f_{fy}\). Further, it causes the onset of the hardening phase and a huge increment in the softening phase with an important consequence on the energy fracture as it will be described in the following. The analysis of SEM images (which are not reported in the paper only for brevity sake but are available as Supplemental Material) confirms the good bonding between fibers and the matrix for all examined cases. The Young' modulus has been determined as the slope of the straight line interpolating the experimental results in the range 0.1-0.3 of the strength for both compression and bending tests. Figure 4: Images of the tested specimen: (**a**) compression test; (**b**) splitting test; (**c**) three-point bending test. Figure 3: Experimental results: (**a**) compression test; (**b**) three-point bending test; (**c**) splitting test. To characterize the dependency of the mechanical behavior of the geopolymer composite on the fiber content yield and ultimate strengths, the Young' modulus was identified for each batch and each test and plotted vs. the fiber content. In Figure 5, the limit elastic strength and the ultimate ones are respectively reported in terms of mean values and deviations. These results suggest that in the case of the splitting test, the dependency of both the yield and ultimate strength is lowest and that of the three-point bending test is the highest, while in the compression test, the dependency is intermediate. These results are coherent with those reported in [15; 27; 46]. This peculiar behavior can be ascribed to the presence of fibers, due to which the initiation and extension of cracks of mode I fracture, and potential shear stresses are reduced.