Mechanical properties of recycled plastic composites reinforced with construction and demolition waste using various particle sizes

Large volumes of plastic waste and construction and demolition waste (CDW) are generated globally. While the mechanical recycling of plastics often results in deteriorated material properties and the formation of polymer blends, the fine fraction of CDW lacks viable applications. This study addresses both challenges by converting these waste streams into valuable composite materials. Two CDW-derived fine fractions were examined: a mineral fraction composed of concrete and brick materials sourced from a demolition site, and a mixed CDW side-stream from a waste treatment plant containing concrete, ceramics, bricks, asphalt, gypsum, glass, plastics, and wood, with organics removed before filler processing. Both materials were subsequently ground into three particle sizes (∼74–85 μm, 18–19 μm, and 9–13 μm), yielding six distinct filler materials. These fillers were then incorporated into a heterogeneous plastic waste matrix (LDPE/HDPE/PP/PET; 35/35/20/20 wt%) at various loadings (1, 2.5, and 5 wt%), to assess the influence of particle size, filler content, and material characteristics on the overall reinforcing performance. The resulting composites, regardless of filler type and loading, exhibited an approximately 50% reduction in the standard deviation of tensile properties. Mechanical testing further revealed that reducing the particle size enhanced the mechanical performance of the composites, yielding up to an 11% increase in tensile strength and a 7% increase in elastic modulus. CDW-based composites also outperformed commercial nanoclay, which suffered from severe agglomeration and weak interfacial bonding with the recycled matrix. These outcomes demonstrate that CDW-origin fillers hold potential as reinforcing fillers for various polymeric applications.