## 3 Results and Discussion
### Mechanical Characterization Results  
The results obtained from the mechanical characterization tests are shown below. Figure 5 shows the tests performed.  
Figure 6 shows the results obtained for the flexural strength test and the dynamic Young's modulus.  
Figure 4: Results of the hygroscopic properties analysed: total water absorption test and capillary water absorption test.  
Figure 5: Mechanical characterisation tests: (**a**) determination of dynamic modulus of elasticity, (**b**) Shore C surface hardness, (**c**) flexural strength, and (**d**) compressive strength.  
As can be seen in Figure 6, there is a progressive decrease in flexural strength as the amount of recycled material added increases. Thus, a substitution of up to 10% of the original plaster material by HDPE waste has produced in the composites a decrease in flexural strength of approximately 34.0% compared to the P0.65 sample. This decrease in strength has followed a linear behaviour in the tested composites, although, for all the substitution percentages analysed, the minimum strength set by the standard of 1 MPa has been exceeded. This decrease in flexural strength is also reflected in the decrease in dynamic modulus of elasticity, as has been observed in previous studies using recycled raw materials [49]. In any case, the mechanical flexural strength values obtained show the possibility of applying these gypsum composites to produce new prefabricated products in construction. Other authors, who have used plastic waste from CDs and DVDs [15], or shredded extruded polystyrene waste [50], observed a similar decrease in flexural strength as the amount of added plastic material increased. Figure 7 shows the results obtained for compressive strength and surface hardness.  
Figure 6: Flexural strength and dynamic modulus of elasticity by ultrasonics.  
Figure 7: Compressive strength and Shore C surface hardness test results.  
Surface hardness represents resistance to surface scratching, which is quite important when defining their future applications. Figure 7 shows how surface hardness decreases as the percentage of HDPE incorporated increases. The most unfavourable decrease occurred for the P0.65-10% sample, being 14.3% lower than the P0.65 reference sample. Authors such as [PERSON] et al. have found a progressive decrease in surface hardness as recycled material content in gypsum composites increases, which agrees with the results obtained [51]. Compressive strength has also experienced a linear decrease as a higher percentage of HDPE aggregates is introduced into gypsum matrix. [PERSON] et al. observed similar behaviour when incorporating polycarbonate waste into the gypsum composite matrix [52]. This reduction in compressive strength is attributed to a higher porosity [53]. In this sense, P0.65-10% has experienced a decrease in strength of 24.2% compared to the reference sample. However, as occurred in the flexural strength test, all composites exceeded the minimum compressive strength of 2 MPa.  
To complete the mechanical characterisation, a scanning electron microscopy study was conducted. This SEM analysis allows one to determine the mixture and connection characteristics between added HDPE plastic waste and the plaster matrix. The images obtained for the P0.65-6% sample are shown in Figure 8.  
In Figure 8, the characteristic dihydrate crystals formed in the matrix can be seen. These acicular crystals are generated during the setting process [54]. On the other hand, some pores generated in the matrix because of the water evaporation from the mixing process can be observed in Figure 8a. Regarding the matrix-residue interface, an irregular adhesion between the HDPE and the plaster material is observed. It is possible to see the cracks that may cause plastic aggregate disintegration [25]. This irregular cohesion is related to the lower compressive strength in those samples containing this recycled raw material, as observed by other authors [55]. Finally, Figure 8b shows some dihydrate crystals formed on the HDPE residue surface.