Development and properties investigation of recycled plastic solid interlocking blocks

Plastic waste accumulation threatens the environment, yet high-density polyethylene (HDPE), a major constituent of this waste can be recycled into building materials. This study fabricates solid interlocking blocks (SIBs) entirely from recycled-HDPE waste using extrusion molding. Molds were developed for manufacturing specimens for characterization of material. Standardized mechanical tests (compression, shear, flexure, tensile) were conducted on recycled-HDPE as per ASTM standards. One-inch-thick molds were pre-pared for manufacturing SIBs and then one-third and two-third fractions were fabricated. Uniaxial compression tests were performed stacked configuration and 9 different SIBs. Bottom block SIBs exhibited the highest compression strength of 51.2 MPa, compared to full blocks 43.1 MPa and top blocks 12.1 MPa. In plane and out-of-plane shear tests were performed at three different confinement stresses as per EN 1052–3. At every confinement level, in-plane shear resistance was ∼20–30 % higher than out-of-plane. Crack pattern was analysed for both compression and shear tests. The elevated calcium in shear areas acted as stress concentrators, explaining the lower shear strength relative to compression. X-ray diffraction (XRD) showed sharp HDPE crystalline peaks at ∼21.5° and 23.9°, in the extruded blocks with intensities roughly an order of magnitude greater than those of the as-extruded pellets. Scanning Electron Microscopy (SEM) revealed the effect of extrusion on the microstructure of the HDPE. Thermal analysis of SIBs exhibited one main decom-position stage, and the degradation onset was ∼385 °C about 15 °C lower than virgin HDPE. An empirical equation was developed for the prediction of shear strength at different confinement with mean absolute percentage error (MAPE) value less than 2 %. These blocks show ductile energy absorption in shear, indicating robustness under mechanical loading. By avoiding virgin polymer and mortar i.e. no cement, they offer clear sustainability advantages for low-carbon mortar free masonry.