Mechanically Recycled and Upcycled Mixed Plastic Waste for Construction Units: FTIR Characterization and TCLP-Based Environmental Safety Assessment

Post-consumer mixed plastic waste comprising PET, HDPE, and PP was mechanically recycled and upcycled into brick and interlocking construction units using a locally fabricated batch melter. Four specimen groups were produced, each with three replicates: recycled brick, upcycled brick, recycled interlock, and upcycled interlock. The recycled formulation contained 100 wt% mixed plastic waste, whereas the upcycled formulation contained 88 wt% mixed plastic waste, 10 wt% SiO₂, and 2 wt% carbon black masterbatch. Fourier transform infrared spectroscopy confirmed retention of the principal PET, HDPE, and PP functional groups after melt-processing. The diagnostic Si–O–Si triplet at 1097–1100, 1015–1025, and 795–810 cm⁻¹ was detected in all upcycled specimens and was absent from recycled-only specimens, confirming silica incorporation. One upcycled brick specimen showed an isolated, shifted carbonyl band at approximately 1689 cm⁻¹. Toxicity Characteristic Leaching Procedure testing showed cadmium below the detection limit in all specimens and no significant difference in chromium concentrations among groups. Lead concentrations were significantly higher in upcycled specimens than in recycled-only controls, although all measured concentrations remained below the applicable regulatory limit. The recycled interlocking group recorded the highest mean compressive strength at 4.268 ± 0.072 MPa. Overall, the results indicate that the fabricated mixed-plastic construction units were chemically distinguishable by formulation and environmentally compliant under the reported test conditions, while highlighting the need to verify additive-related contamination.