## 3 Results
### Mechanical Characterization  
Firstly, Table 5 presents the results obtained for the surface hardness and longitudinal dynamic elastic modulus of the different composite materials developed determined by ultrasound (MOEus). As shown in Table 5, the use of hemp additions as a partial replacement for the original plaster material results in a decrease in the surface hardness of the composites. Thus, for the samples with a higher hemp content, G0.65-30%P and G0.65-30%F, there is a reduction of around 15.1% and 12.8%, respectively, with respect to the reference material (G0.65). As confirmed in other studies, a decrease in surface hardness is generally associated with a loss of compressive strength [8]. However, the values obtained for this property in this study are in line with those obtained by [PERSON] in her research on plaster with rice husk additions, who also observed a decrease in hardness when incorporating these plant-based additions [51]. Additionally, in all cases, the obtained values exceed the recommended minimum value for this type of gypsum-based material in this surface hardness test, which is set at 45 Shore C units [52]. On the other hand, Table 5 also shows a progressive decrease in the dynamic longitudinal elastic modulus as the added hemp content increases. Thus, composites with a 30% substitution of plaster with hemp powder showed an MOEus value 30.6% lower with respect to the G0.65 sample. A similar effect can be observed in composites with hemp fiber. This reduction in the MOEus value is a consequence of the lower density and speed of ultrasound propagation through the matrix of these hemp-containing composites. A similar effect has been observed in other previous research [53], where replacing the original plaster material with recycled raw materials, such as rubber from used tires, manages to lighten the hardened composites and results in a reduction in this property. Figure 4 presents the results obtained for the flexural and compressive strengths of the different tested gypsum materials, including the minimum recommended value according to current regulations. Firstly, Figure 4a presents the results from the standardized bending test. It can be observed that the gypsum composites prepared with hemp fiber exhibited higher flexural strengths compared to their counterparts with hemp powder addition (particle size less than 2 mm). Thus, in the G0.65-15%F composite, despite having replaced 15% of the gypsum material with a light addition of hemp fiber, the flexural strength was on average 2.34% higher than the traditional material (G0.65). Similarly, the type of fracture in the composites with hemp powder addition was brittle, with the two gypsum pieces completely \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Series** & **G0.65** & **G0.65–15\%P** & **G0.65–30\%P** & **G0.65–15\%F** & **G0.65-30\%F** \\ \hline Surface Hardness [Shore C units] & 86 \(\pm\) 1 & 78 \(\pm\) 2 & 73 \(\pm\) 1 & 77 \(\pm\) 1 & 75 \(\pm\) 1 \\ MOEus [MPa] & 5196 \(\pm\) 89 & 4212 \(\pm\) 102 & 3608 \(\pm\) 34 & 4286 \(\pm\) 28 & 3788 \(\pm\) 91 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Dynamic elastic modulus measured by ultrasound (MOEus) and surface hardness of the composites. separating after the fracture. In any case, both additions used in partial substitution of the base gypsum composite surpassed the minimum value established by the current regulations of 1 MPa, making them suitable for use in building projects. Similarly, the results from the compressive strength test are presented in Figure 4b. It can be observed that, in line with the surface hardness results obtained in Table 5, all the hemp-lightened composites exhibited a compressive strength lower than the reference material. However, even in the least favorable cases, G0.65-30%P and G0.65-30%F, where the decrease in compressive strength was close to 40%, all the analyzed composites exceeded the limit of 2 MPa set by the current regulations.