## 4 Conclusions  
In this work, a feasible method for the reincorporation of HDPE waste aggregates in the design of new sustainable building materials has been presented. To this end, the design of different plaster composites has been proposed, with a progressive substitution of the original raw material by this recycled plastic material. The results show that it is possible to apply these composite materials to prefabricated building material development. The most relevant conclusions can be listed as follows:  
* Progressive HDPE waste incorporation allows a reduction in both thermal conductivity and final bulk density. In this sense, reductions of up to 26.7% and 7.35% have been achieved for each of these properties, respectively.
* A decrease in the water absorption capacity of the gypsum composites is observed as the amount of added plastic residue increases. In this sense, the P0.65-10% sample managed to reduce the height reached by the water after the capillary test by up to 23.1%.
* Regarding mechanical strength, a linear decrease in flexural and compressive strength, as well as surface hardness, was observed as more HDPE waste was added. Nevertheless, all the tested composites exceeded the minimum strength values required by UNE-EN 13279-2. However, after SEM analysis, irregular adhesion between the HDPE aggregate and the plaster matrix was revealed, as the recycled materials used did not undergo any prior surface treatment.
* Finally, in the plates study, all samples exceeded the flexural strength limit of 0.18 kN, except for the composite with 10% HDPE residue added. These results highlight the importance of conducting full-scale tests to better understand the performance of the materials developed.  
The main limitations of this research, which in turn serve to propose future lines of work, are twofold. Firstly, it would be interesting to conduct an economic study together with a carbon footprint analysis of the developed products, which would allow us to delve into the possibilities of the commercial exploitation of these gypsum materials produced under circular economy criteria. On the other hand, this work has not considered fire resistance  
Figure 12: Qualitative classification of the different compounds evaluated according to the results obtained in the tests conducted.  
and sound absorption tests, which would be interesting to set in accordance with European regulations, which would allow us to define these materials' application possibilities.  
Conceptualization, D.F.; methodology, B.C.-P. and D.F.; software, D.F. and M.A.; validation, D.F., M.A. and A.Z.-B.; formal analysis, D.F. and M.A.; investigation, A.Z.-B. and B.C.-P.; resources, D.F.; data curation, D.F.; writing--original draft preparation, D.F. and M.A.; writing--review and editing, A.Z.-B.; visualization, A.Z.-B.; supervision, D.F.; project administration, D.F.; funding acquisition, D.F. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The authors would like to thank [PERSON] and [PERSON] from Escuela Tecnica Superior de Ingenieria y Diseno Industrial de Madrid for supplying the recycled HDPE aggregates uses in this research.  
The authors declare no conflicts of interest.