## 2 Materials and Methods  
### Experimental Tests  
To realize the geopolymer composites to be tested experimentally, as described in the following, two different steps were performed. In the first one, the matrix was prepared by selecting metakaolin as the precursor (silicon-to-aluminum molar ratio equal to 1.3:1 with a grain size distribution ranging between 1 \(\upmu\)m and 100 \(\upmu\)m). The alkaline activator was a 7 M potassium peroxide alkaline solution, obtained by mixing KOH pellets (99% purity) and potassium silicate (K\({}_{2}\)O-nSiO\({}_{2}\)) powder with deionized water. The reagents to obtain the alkaline solution were supplied by Carlo Erba Reagents S.r.l. (Cornaredo, Italy). The selected aggregate was river sand with a nominal maximum diameter of 2 mm. In the second step, the sisal fibers were suitably mixed by replacing different weight contents (0.5%, 1% and 2%) of the aggregate. The sisal fibers, obtained from plants cultivated in northern Sicily, were prepared by washing and subsequent drying at 25 \({}^{\circ}\)C for 48 h; the adopted fibers were not treated, with the role of treatment in the mechanical behavior being the object of an ongoing study. For a review about the influence of the fiber treatment on the mechanical behavior of geopolymer matrices, see [42]. Following the results reported in [15], the sisal fibers were shortened by a suitable cut to the selected final length equal to 2.5 mm. The upper limit of the replacement of the aggregate with fibers is because, as reported in [39], regardless of the fiber types, the increase in compressive strength is more expectable when the fiber content is less than 2%, while above this value, an adverse effect of the fiber is possible. The geopolymer composite samples, realized as described before, were cured at room temperature for 2 -days and identified with an \"MK-x\" acronym, where \"MK\" refers to the precursor and \"x\" to the sisal weight content (e.g., MK-0 refers to the reference material that is unreinforced geopolymer mortars).  
The mechanical behavior of the geopolymer samples with and without sisal fibers was assessed through a wide campaign of experimental tests. The selected tests were those prescribed by the actual standards (UNI EN 1015-11 [43] and ASTM D3967 [44]) for mortars, consisting of compression, bending and splitting (Brazilian indirect tensile) tests. Following [43], the specimens consisted of prismatic samples with nominal dimensions (\(b\times h\times\mathbb{J}\)) equal to 40 mm \(\times\) 40 mm \(\times\) 160 mm to be tested in a three-point bending configuration, while the specimens for the compression test (cube specimens with side dimension of about 40 mm) were derived by cutting the fragments obtained at the end of the bending test. Finally, the span length in the bending test was set equal to 100 mm. Following [44], the specimens for the splitting test were cylindrical with a nominal diameter (\(d\)) and height (\(h\)) equal to 50 mm and 30 mm, respectively (\(h/d\) ratio equal to 0.6). The mechanical tests were carried out using the universal testing machine (UTM) model ETM-C (WANCE, Shenzhen, China), equipped with a 50 kN load cell. Bending and compression tests were performed in displacement control with a pre-load equal to 10 N, the bending one with a rate of 1 mm/min, while the compression one had a rate of 0.5 mm/min. The splitting test was performed in stress control with a rate equal to 0.05 MPa/s. Three specimens were tested for each mechanical test, and in Figure 1, the corresponding test set-ups are reported.