## 2 Materials and Methods
### Experimental Program  
In this section, the tests conducted to carry out the physical and mechanical characterization of the composites developed in this research are described. Figure 3 outlines the developed experimental program in a schematic manner. Figure 3: Outline of the developed experimental program. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Series**} & \multicolumn{3}{c}{**Volume Ratios [\%]**} & \multicolumn{3}{c}{**Mass Quantities [g]**} \\  & **Gypsum** & **Water** & **Powder** & **Fiber** & **Gypsum** & **Water** & **Powder** & **Fiber** \\ \hline G0.65 & 60.6 & 39.4 & — & — & 1000 & 650 & — & — \\ G0.65–15\%P & 51.5 & 33.5 & 15.0 & — & 850 & 553 & 30 & — \\ G0.65–30\%P & 42.4 & 27.6 & 30.0 & — & 700 & 455 & 60 & — \\ G0.65–15\%F & 51.5 & 33.5 & — & 15.0 & 850 & 553 & — & 25 \\ G0.65–30\%F & 42.4 & 27.6 & — & 30.0 & 700 & 455 & — & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Formulations used for the preparation of gypsum composites. Figure 2: Matrices of the various gypsum composites that have been prepared and cured. As depicted in Figure 3, in the initial phase of the research, a mechanical characterization of the developed gypsum composites was conducted. Three samples of each type of gypsum, measuring 4 \(\times\) 4 \(\times\) 16 cm\({}^{3}\), were used for the following tests and methods:  
* **Surface hardness:** Measured using a Shore C durometer. It reflects the resistance of gypsum composites to surface scratching. The test is conducted according to the UNE 102042:2023 standard [46], using five measurements on two plane-parallel faces separated by at least 2 cm from each other and from the ends. * **Dynamic elastic modulus:** Employing a C368 ultrasonic emission-reception device by Matset (Treviolo, Italy), with 55 kHz contact transceiver probes measuring longitudinally along the 16 cm dimension of the specimen. * **Flexural strength:** Utilizing a hydraulic press by Ibertest, model AUTOTEST 200-10 SW (Madrid, Spain). It consists in a simple three-point bending test conducted in accordance with the EN 13279-2:2014 standard [44]. A loading speed of 10 N/s was applied, testing the samples until fracture. * **Compressive strength:** In accordance with the EN 13279-2:2014 standard [44] and using the aforementioned equipment. The test is performed on the half specimens obtained after the flexural test, with a loading speed of 20 N/s until the final fracture of the gypsum composites. * **SEM analysis:** Conducted on a sample extracted from the matrix of the cured gypsum composite and previously coated with a thin gold film using a Cressington 108 auto metallizer (Watford, UK). The analysis is performed using a TESCAN Vega 4 scanning electron microscope (Watford, UK), with color cathodoluminescence and two EDX Bruker detectors (30 and 60 mm\({}^{2}\)) operating at 20 kV. In the second phase, the behavior of the composites developed in this work against water exposure was assessed. For this purpose, the following tests were conducted on sets of three specimens measuring 4 \(\times\) 4 \(\times\) 16 cm\({}^{3}\):  
* **Capillary water absorption:** Determined according to the EN 1925:1999 standard. It relates the mass of water absorbed by the gypsum composites per unit area and as a function of time [47]. * **Maximum height reached by water in the capillarity test:** Determined as the average of three values measured on each specimen, reflecting the final height reached by the water after 40 min of capillarity test. Infrared images were also taken to understand the water evolution inside the composites. * **Total water absorption:** Performed adapting the indications in the EN 14617-1:2013 standard [48]. In order to do this, the samples were completely submerged in water for a period of 8 h, after which the percentage change in mass experienced by the gypsum composites due to water absorption was obtained. * **Open porosity:** Defined as the ratio between the volume of accessible pores and the apparent volume of the material.