*2.1. Experimental Procedure*

The sand/polymer composite sheets were prepared via melt-blending, followed by compression molding. For all the experiments, American Standard Test procedures were adopted.

### *2.2. Synthesis of Sand/Polymer Composite Material via Melt-Blending*

Weighed amounts of sand at two different particle sizes (5 and 25 μm) and different fractions (0 wt%, 20 wt%, 35 wt%, and 50 wt%) were mixed with HDPE and melt-blended in a twin-screw extruder (MiniLab HAAKE Rheomex CTW5, Germany) for 15 min at 170 ◦C and at a screw speed of 100 rpm. These conditions were selected after several trials of varying the time, temperature, and rpm for the process. The total feed amount was kept constant at 4 g as required by the extruder setup [23]. The control sample was prepared by blending pure HDPE using the same conditions as in numerous studies [31–34]. For the samples prepared with the addition of the compatibilizer (C), the composition of the compatibilizer was kept constant at a filler-to-compatibilizer ratio of 2:1. Compatibilizers are used as additives to enhance the properties of the fabricated polymer composites prepared by various techniques. Studies report polyethylene grafted with maleic anhydride (PP-g-MA) as a compatibilizer to increase the dispersion of the clay in the nonpolar polymer matrices and improved properties [14], which is why it was chosen. PE-g-MA and its ratio was also chosen after several trials of varying the compatibilizer type and amount. Table 1 represents chosen weight percentages for the three components for the preparation of the sand/polymer composite sheets.


**Table 1.** Chosen weight percentages for HDPE, sand, and compatibilizer.

Synthesis of Sand/Polymer Composite Sheets via Compression Molding

The compounded composites were chopped into small pieces (~1 g) and compressed (hot-pressed) for 10 min using a Carver's press (Carver™ Lab Presses) under 5000 psi pressure at 170 ◦C to prepare the flat sand/polymer composite sheets. Sand/polymer composite sheets were prepared from two different sizes (25 μm and 5 μm) of sand as the filler. The same sets were prepared with the addition of the compatibilizer as well. Figures 2 and 3 show images for sand/polymer composite sheets prepared from 25 μm sand particles without and with compatibilizer, respectively, whereas Figures 4 and 5 represent the same for sheets prepared with 5 μm, respectively. All the sheets were roughly 1 mm thick and 100 mm in diameter. It could be visually seen that increasing filler addition made the sheets darker in appearance. However, addition of compatibilizer produced sheets that were relatively lighter in appearance and softer. Furthermore, based on a visual inspection, it can be said that the dispersion was random in all sheets, with a more homogeneous dispersion in 20 wt% and 35 wt%, and a non-homogeneous dispersion was observed in both cases of 50 wt% filler before and after compatibilization. The overall process is illustrated in Figure 6. A similar process was followed with the addition of compatibilizer.

**Figure 2.** Images of prepared composite sheets for (**a**) pure HDPE, (**b**) 20 wt%, (**c**) 35 wt%, and (**d**) 50 wt% prepared from 25 μm sand particles.

**Figure 3.** Images of prepared compatibilized composite sheets for (**a**) pure HDPE, (**b**) 20 wt%, (**c**) 35 wt%, and (**d**) 50 wt% prepared from 25 μm sand particles.

**Figure 4.** Images of prepared composite sheets: (**a**) pure HDPE, (**b**) 20 wt%, (**c**) 35 wt%, and (**d**) 50 wt% with 5 μm sand particles.

**Figure 5.** Images of prepared compatibilized composite sheets: (**a**) pure HDPE, (**b**) 20 wt% + C, (**c**) 35 wt% + C, and (**d**) 50 wt% + C, with 5 μm sand particles.

**Figure 6.** Process of preparing the composite sheets.

### *2.3. Characterization*

2.3.1. Scanning Electron Microscope (SEM)

A JEOL/EO scanning electron microscope (SEM), operated at 2 kV, spot size of 40, was used to image the sand of both 25 μm and 5 μm, and neat HDPE at their surface. To improve conductivity and quality of image, samples were coated with Au/C using a vacuum sputter coater. Likewise, SEM analysis was used to observe the surface morphology of the selected composite sheets. Selected composite sheets were placed on an aluminum pin-mount adapter using double-sided carbon tape and then were sputter-coated with gold using a sputter-coater to avoid electrostatic charging during examination. The SEM was operated at high vacuum mode with an acceleration voltage of 15 kV and the images were acquired.
