Engineering Properties of Cement Mortar Containing Polyethylene Terephthalate Powder as a Partial Sand Replacement

The growing use of PET strapping in packaging has increased plastic waste, prompting interest in its reuse in cement-based materials. This study evaluates waste PET strapping powder as a partial replacement for natural sand in cement mortar. Mortars were prepared with three water–cement ratios (0.4, 0.5, and 0.6) and four replacement levels (0%, 5%, 10%, and 15%). Slump, flow, compressive strength, flexural strength, ultrasonic pulse velocity (UPV), four-point resistivity, and sulphate resistance were examined at different curing ages. Increasing PET content reduced slump and flow, indicating lower workability. It also decreased compressive and flexural strengths, with flexural strength showing a greater reduction. UPV declined as PET replacement increased, which was attributed to the combined effects of acoustic-property contrast between PET and the cementitious matrix, interfacial wave scattering, and local microstructural defects. However, PET increased electrical resistivity and reduced sulphate-related weight loss, indicating lower continuity of ion-conducting pathways and potentially improved resistance to the transport of aggressive ions under the adopted test conditions. SEM observations showed limited interfacial defects at 5% PET, whereas higher contents produced a more porous interfacial transition zone. Overall, the mix with 5% PET and W/C = 0.5 showed the best balance of properties, supporting the reuse of PET strapping powder as a sustainable sand replacement.