## 3 Results and Discussion
### Precast Samples Results  
The final section of the experimental campaign includes a study conducted on prefabricated plates for ceilings. The aim is to evaluate the newly developed composites' behaviour under the stresses they will be under when applied. The results for plate flexural strength and impact hardness are shown in Figure 9.  
Figure 8: Images obtained by SEM technique on sample P0.65-6%: (a) 500\(\times\) magnifications; (b) 1000\(\times\) magnifications.  
Figure 9 shows how the flexural strength in the plates decreases as the HDPE waste content increases, in accordance with the tests performed on standardised RILEM samples shown in Figure 6. UNE-EN 14246 standard requires a minimum strength of 0.18 kN to guarantee the optimum performance of these products [43]. It can be observed in the tests conducted on 4 \(\times\) 4 \(\times\) 16 cm\({}^{3}\) samples that all composites exceeded the standard requirements. However, when used in 40 \(\times\) 30 \(\times\) 1.5 cm\({}^{3}\) format, the P0.65-10% composites did not exceed the minimum value required by the standards (0.16 kN < 0.18 kN). This is a limit for the incorporation of these wastes in plaster materials, since, as in other mechanical properties, a progressive decrease in flexural strength is experienced as the HDPE aggregate content increases. This decrease in strength has been observed in previous research [57] and it has been necessary to incorporate reinforcing fibres to increase the ductility of the composites produced.  
Figure 9 also shows that the diameter of the footprint after the dynamic hardness test is also reduced as the HDPE waste content increases. This is a positive effect, as it implies that the incorporation of this recycled plastic material allows the reduction of the impact caused by the 50 mm steel ball. The P0.65-8% sample, which contains the most HDPE waste amount suitable for precast elaboration, reduced the footprint by up to 12.3% compared to P0.65. This effect has been observed in previous research that has used waste plastics to elaborate prefabricated panels [58].  
The plaster material's main advantage is its versatility and application possibilities [57]. It is a workable material with an aesthetic finish, which makes it an optimal solution for creating different configurations in building [59]. The results obtained in the designed materials in this research makes it possible to create specific configurations that provide other benefits, such as aesthetic vision and the possibility of combining them with other elements [60].  
Thus, the design and development of a new ceiling panel with the materials studied as the main element is proposed. Figure 10 shows an axonometric view of the proposed system, with a deeper development of the anchoring system and possible configurations in Figure 11a,b, respectively.  
Figure 9: Results for plate bending strength and impact toughness [56].  
The system shown in Figures 10 and 11 allows the user an adjustable adaptation, with this system being easy to execute, showing a typical application for the composites developed in this work. However, the possibilities for adaptation are multiple and can be explored to develop new sustainable construction systems, elaborated under circular economy criteria. In this sense, this proposal is in line with the recommendations of the European Green Deal [61], trying to expose novel solutions that are aligned with the European objectives for the sustainable development of the construction industry.