## 2 Materials and Methods  
Natural gypsum powder from the rocks of the Safi region (Morocco) was used to assemble the samples. Its features were deeply discussed in [17; 34]. The powder (p) was mixed with water (w) (mass ratio w/p equal to 0.8) in about 90 s, in order to obtain a fluid and homogenous porous plaster without any air bubbles. The paste was added with different components and mixed by means of a conventional stirrer, the Ika Rw 20 (helical moves for about 3 min at a speed equal to 100 rpm). The selected additives are of the following three types (Figure 1):  
- Synthetic: Polyester Fiber (PF), Paraffin (Pa), and Glass Wool (GW), selected among the most used commercial ones;  
- Natural: Stipa Tenacissima (ST) or Alfa fiber, Palm Petiol (PPa), Sisal Fiber (SF), Straw (Sw), Corn Bristles (CB), Cork (Ck), and Chip-Sawdust (ChSd), very diffused in the local Moroccan area;  
- Mineral: Cement (Ce), Clay (Cl), Sand (Sa), and Refractory Cement (R.Ce). Natural fibers were collected and shredded by hand, obtaining a homogeneous size of about 1 cm. Several tests were carried out to identify the optimal additive value among the different types to obtain stable samples; it was found to be equal to 20 wt.% of the total volume. Also, a reference panel with pure hardened gypsum plaster and water (called PS 100) was assembled. The mixture was placed in molds purposely made and left under controlled laboratory conditions (relative humidity equal to 70% and temperature in the 23-25 \({}^{\circ}\)C range) for about 3 h. Once removed, the samples were dried in a climatic chamber at 45 \({}^{\circ}\)C for 2 days to eliminate all the interstitial water. For each composite, three square-shaped panels of 300 \(\times\) 300 mm\({}^{2}\) dimensions and cylindrical specimens of 100 and 29 mm diameter were fabricated in order to investigate the thermal and acoustic properties, respectively (Table 1). Figure 1: Safi gypsum powder and synthetic, natural, and mineral additives used in the samples. Despite the use of molds, the thickness of the samples was not the same due to their different shrinkage during the drying step; the thickness of the square panels was in the 0.023-0.030 m range, slightly smaller than that of the cylindrical ones (about 0.040-0.060 m). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  
For the acoustic tests, only specimens with similar thicknesses and densities \(\rho\) were considered in the analysis for a reliable comparison. In general, the insertion of the synthetic and natural additives in the plaster matrix did not result in significant changes in density values with respect to the reference panel (Table 1); the mineral materials involved an increase in density up to about 32% (with 20% of refractory cement). According to the heat flow meter method, the thermal performance of the samples was evaluated by means of the Small Hot Box experimental apparatus available at the Department of Engineering (University of Perugia) [36]. The tests (lasting at least 2 h each) were carried out under steady-state conditions. The temperature inside the hot chamber was set at 45 \({}^{\circ}\)C to ensure a difference between the air temperature in the box (the hot side) and in the laboratory room (the cold side) higher than 20 \({}^{\circ}\)C, according to [37]. The thermal resistance of each panel was calculated by measuring the heat flux through the sample (with a heat flow meter, \(\pm\)5% precision) and its hot and cold side surface temperatures (with thermo-resistances, \(\pm\)10% precision). The relative type B uncertainties of each test were calculated in compliance with JCGM 100:2008 [38]. The sound absorption coefficient (\(\alpha\)) and transmission loss (TL) at normal incidence of each panel were measured in a conventional Kundt's tube (Bruel & Kjaer, model 4187), by means of the two- and four-microphone configurations (1/4-inch microphones, Briel & Kjaer, model 4187), according to ISO 10534-2 [39].