## 3 Results and Discussion
Figure 6 shows the fracture surfaces of the broken samples, highlighting the morphology of the fibers. All of the fibers appear to be quite embedded in the matrix without the presence of large porosities at the interface or spherical-shaped porosities deriving from entrapped air during the mixing stage. Bamboo and kenaf fibers show some matrix fragments still adhering while the surface of hemp fibers is rather clean. Bamboo fibers also show evidence of disaggregation caused by the tensile breaking. The plot of the applied force vs. the midspan deflection for the reinforced 3 wt% samples is shown in Figure 7. From the completely brittle behavior of the unmodified sample, after fiber addition, the mechanical behavior becomes pseudoplastic. The effect is more intense in the kenaf and hemp samples. Indeed, the average absorbed energy for the specimens is reported in Figure 8 as a function of fiber content. As to the compression tests, the addition of fibers does not significantly alter this parameter (Figure 9). A slight increase in compressive strength takes place at the lowest wt%, but as wt% increases, the positive effect levels off and He3 specimens show a 20% decrease in compressive strength. This is, however, lower than the decrease in strength reported elsewhere [35], although with a higher fibers' content. A similar trend has been found in other papers concerning the modification of natural fibers [17; 36; 37]. Figure 5: Flexural strength of the investigated materials. Figure 6: Morphology of the different fracture surfaces of the composites: **(a)** bamboo, **(b)** hemp and **(c)** kenaf. Figure 7: Force vs. displacement for 3 wt% samples and the unmodified matrix. The dimensional stability of the unmodified material and of all the samples containing a 3 wt% of fibers is reported in Figure 10. As can be seen, an increase in stability is obtained, and this effect is independent of fiber type. Since measurements began 24 h after mortar casting, dimensional stability on the fresh state was not included in the experiment. Thus, the positive effect may be even higher than the reported one. Finally, the results concerning water permeability are shown in Figure 2:0 The plot concerns all of the 3 wt% samples, and the square root of time is reported on the x-axis. After an initial linear trend, common to all samples, an asymptotic value is reached. At the longest investigated times of the experiment (20 days, not reported in the figure), the values were still constant, confirming Fickian behavior of all materials, i.e., no swelling phenomena taking place, leading to a continuous increase in the absorbed water. Contrary to what could be expected, the modified samples have a lower absorption rate than that of the bare matrix. Figure 2:1 Absorbed energy for the different composites during the flexural test. Figure 2:2 Compressive strength of the investigated materials. This result confirms again that the fibers and the interface between the fibers and the matrix do not create a preferential path for water diffusion. Indeed, the subtraction of a small volume of porous matrix by the fiber substitution creates a slightly impervious path to water diffusion. The final water uptake of modified samples is lower than that of the MTK sample, and this agrees with what was previously found in the water absorption test (Figure 4). As a general consideration, the addition of all of the investigated fibers has, apart from workability, a positive effect on the investigated properties, or they at least do not compromise the geopolymer performance. However, the influence is different when the type of fiber is considered. Bamboo fiber-modified composites have stronger mechanical properties than hemp ones, a feature reflected by the flexural and, albeit partially, compressive strength. Contrarily, hemp fibers provide the highest toughness but do not increase compressive and flexural strengths. Kenaf samples have intermediate behavior. The adhesion between fiber and matrix plays a fundamental role.