Researchers are trying a different approach to composite building materials by using sand as filler and polypropylene waste as a binder.

With the cost of building materials on the rise, recycled plastics offer an opportunity for innovation and cost savings. Researchers and manufacturers have taken various approaches to creating sustainable construction materials, some commercially available. Previous studies have shown that plastic-sand composites are viable for bricks, road paving blocks, and roof tiles. Usually, these studies use plastic waste as an aggregate, fiber, or additive in concrete, rather than a binder. To expand knowledge on the subject, researchers have explored polypropylene’s binding capabilities with various types of sand.

You can also read: Building Blocks Made of Plastic Waste Spur Affordable Construction.

Creating the Composite

Because waste polypropylene takes a long time to biodegrade, it makes a promising candidate for construction applications. It is also non-hygroscopic, so moisture does not affect its physical properties during molding. Before combining it with sand, researchers ground the polypropylene into pieces ranging from five mm² to 10 mm². Using four types of sand, researchers could compare the effects of different sand grain sizes on the composite. Researchers labeled the sands A, B, C, and D, and the resultant materials A, B, C, and D. They created the plastic-bonded sand composites by making a homogenous paste using a single-screw extruder. Using a heat press, they formed the paste into brick-like specimens. The ratios of polypropylene to silica sand were 25%: 75% for all four sand types.

Researchers evaluated the materials’ water absorption, strength, and elasticity, and performed an optical analysis. The optical analysis allowed researchers to evaluate mixture quality and identify potential defects. Using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy, researchers could better understand the composition of each sand type. The insights provided by ATR-FTIR also confirmed the presence of Si–O and Si–O–Si bonds in the composites.

A Small Grain Size Makes a Big Difference

The grain size of each sand type varied, with various distributions of fine, medium, and coarse sand. Previous research indicates that fine sands result in fewer pores in HDPE/sand composites. Thus, these mixtures are more densely-packed than those using coarse sands.

Consequently, Materials B and D, with the most well-distributed particle sizes, showed lower water adsorption. These two composites also exhibited the highest load capacity, reaching approximately 3,000 N and 4,200 N, respectively. Additionally, Materials B and D performed the best during three-point flexural strength, flexural modulus, and compressive strength testing. Optical microscopic analysis revealed defects, such as pores and interconnectivity issues, in Materials A and C.

Throughout testing, the materials showed consistent performance, with Material D exhibiting the best mechanical properties. Material B followed it, demonstrating slightly lower performance. Waste polypropylene was an effective binder throughout the study. The tested sand types showed that particle size and porosity can strongly affect the material’s suitability. This research presented another use for recycled polypropylene: a binder to make construction materials in combination with sand. By “up-cycling” plastic waste for construction applications, less plastic ends up in the landfill.