Recycled LDPE–Sand Composites as Cement-Free Construction Materials: Effects of Processing Parameters on Mechanical and Physical Properties

This study investigates recycled low-density polyethylene (LDPE)–sand composites as cement-free materials for selected construction applications. The effects of plastic content (30–50 wt.%), processing temperature (220–260 °C), and particle size (319–1015 µm) on mechanical and physical properties were evaluated using a Taguchi L9 experimental design. Mechanical properties, including compressive, flexural, and tensile strength, and physical properties, including density and water absorption, were assessed using laboratory-scale specimens prepared from moulded composite panels. Processing temperature was the dominant factor controlling strength development and water absorption reduction. The best-performing experimental condition within the investigated range was 30 wt.% LDPE, 260 °C, and 1015 µm particle size, yielding an apparent compressive strength of 65.5 MPa, flexural strength of 20.7 MPa, tensile strength of 4.4 MPa, density of 1595.2 kg/m3, and water absorption of 0.7%. Cross-validation showed good predictive capability for density, tensile strength, flexural strength, and water absorption, but only moderate predictive capability for compressive strength and compressive modulus. Therefore, the regression models are presented as screening tools within the investigated parameter range rather than as general design models. The results indicate that recycled LDPE–sand composites have potential for selected non-structural and limited semi-structural applications, subject to further product-standard testing, durability assessment, fire performance evaluation, and environmental impact analysis.