## 2 Materials and Methods
### Dosages and Sample Preparation  
To conduct this study, original plaster material was progressively replaced by HDPE granular waste. This partial substitution was conducted in percentages of 2-4-6-8-10% by volume, as shown in Table 3. The nomenclature used for naming the samples is as follows: P0.65-(%), where P refers to the binder (Plaster); 0.65 refers to the water/plaster ratio by weight; (%) refers to the percentage of substitution of the original compound by recycled HDPE material.  
The production process starts with the manual dry mixing of the plaster powder with the polyethylene residue. After that, the kneading process begins by pouring the material  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Standard**} & **Thermal Conductivity** & **Water Vapour** & **Pureness Index (\%)** & **Flexural Strength** \\  & **(mW/m-K)** & **Diffusivity (\(\mu\))** & & **(MPa)** \\ \hline UNE-EN 13279-1 & 300 & 6 & \textgreater{}90 & \textgreater{}3 \\ \hline \multirow{3}{*}{**Colour**} & **Fire Reaction** & \multirow{3}{*}{**Granulometry (mm)**} & **Water/Plaster Rate** & \multirow{3}{*}{**pH**} \\  & **According to CTE** & & & **by Weight** \\ \cline{1-1}  & **DB-SI**[37] & & & \\ \hline White & A1 & 0–0.2 mm & 0.7–0.8 & \textgreater{}6 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Technical characteristics of the binder (Plaster type E-35).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Appearance**} & **Particle Size (mm)** &
\begin{tabular}{c} **Hardness (Units** \\ **Shore D)** \\ \end{tabular} & **Tensile Strength (MPa)** & **Tensile Modulus (MPa)** \\ \hline Granulated & 1.4 & 70 & 31.7 & 1.38 \\ \hline
**Elongation at Break (\%)** & **Bending Strength (MPa)** & **Bending Modulus (GPa)** & **Compressive Strength (MPa)** & **Compressive Modulus (GPa)** \\ \hline
400 & 31.7 & 1.2 & 31.7 & 0.689 \\ \hline \hline \end{tabular}
\end{table}
Table 2: HDPE properties.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{3}{c}{**Weight (g)**} & \multicolumn{3}{c}{**Volume Percentage (\%)**} \\ \cline{2-7}  & **E-35** & **Water** & **HDPE** & **E-35** & **Water** & **HDPE** \\ \hline P0.65 & 1000.0 & 650.0 & — & 60.60 & 39.40 & — \\ P0.65-2\% & 980.2 & 636.8 & 14.5 & 59.40 & 38.60 & 2.00 \\ P0.65-4\% & 960.4 & 623.6 & 29.0 & 58.20 & 37.80 & 4.00 \\ P0.65-6\% & 940.6 & 610.4 & 43.5 & 57.00 & 37.00 & 6.00 \\ P0.65-8\% & 920.8 & 597.2 & 58.0 & 55.80 & 36.20 & 8.00 \\ P0.65-10\% & 901.0 & 584.0 & 72.5 & 54.60 & 35.40 & 10.00 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Dosages used: quantities by weight and by volume.  
into water, following the guidelines set out in the UNE-EN-13279-2:2014 standard [31]. Once samples are hardened, they are kept for a week in laboratory conditions (temperature 23 \(\pm\) 2 \({}^{\circ}\)C and relative humidity 50 \(\pm\) 5%). Finally, the process concludes with placing the samples in a drying oven for 24 h at a temperature of 45 \({}^{\circ}\)C \(\pm\) 1 \({}^{\circ}\)C.  
The materials developed have been subjected to physical and mechanical characterisation tests, aiming to determine the viability of their use in prefabricated materials for building. Two series of three samples of each proposed dosage have been prepared. Table 4 shows the dimensions and tests to which the different composites have been subjected.