Recycled HDPE–Sand Composite Blocks: Strength and Moisture Resistance

Abstract

Portland cement production is associated with high carbon emissions, while plastic waste particularly high-density polyethylene (HDPE) continues to accumulate in landfills and natural environments. This work-in-progress study investigates the feasibility of using recycled HDPE as a primary binding phase in plastic–sand composite materials for non-structural construction applications. HDPE–sand composites were produced at plastic-to-sand mass ratios of 2:8, 3:7, and 4:6 and compared with conventional cement–sand control specimens. Mechanical performance was evaluated using compressive strength and a comparative impact resistance (drop-energy) test, while durability-related behavior was assessed using water absorption, apparent water porosity, exploratory thermal exposure, and short-term pH immersion monitoring. The results indicate that HDPE–sand composites can achieve compressive strengths within ranges commonly reported for non-structural masonry units and paving blocks, with the 3:7 ratio providing the most favorable balance between strength and ductility. All HDPE-based composites exhibited significantly lower water absorption and higher impact resistance than cement–sand controls due to the hydrophobic and ductile nature of the polymer binder. Exploratory thermal exposure highlighted a clear temperature sensitivity that defines the current service envelope of unmodified HDPE composites. Overall, recycled HDPE–sand composites show strong potential for paving blocks, pedestrian surfaces, and partition elements, offering a practical pathway for plastic waste valorization and reduced reliance on cement-based materials.