Upcycling imperfectly sorted polyolefin-polyester waste and industrial fly ash into fire-safe building materials

The economic and technical burdens of perfectly separating municipal plastic waste, particularly polyolefins and polyesters, remain major bottlenecks in global recycling. This study proposes a "triple-valorization" framework by directly utilizing an imperfectly sorted blend of recycled polypropylene (rPP) and polyethylene terephthalate (rPET) (90/10 wt%) reinforced with industrial fly ash (FA) to develop passive fire-resistant building composites. The introduction of 2 wt% maleic anhydride-grafted polypropylene (PP-g-MA) as a dual compatibilizer overcame the thermodynamic immiscibility of the polymers and chemically anchored the inorganic FA. Comprehensive characterization (ATR-FTIR, SEM-EDX, TGA, and DMA) confirmed that PP-g-MA formed critical hydrogen bonds with the FA surface hydroxyls while simultaneously bridging the rPP/rPET interface. This structural synergy yielded a Young's modulus of 1.42 GPa at 20 wt% FA (36.5% enhancement) and an optimal flexural strength of 18.4 MPa at 5 wt% FA. Crucially, the aluminosilicate phases of FA induced a condensed-phase char heat shield, extending the burning time by 96.6% and reducing the combustion rate by 49.1% at a loading of 20 wt %. By transforming a co-mingled plastic waste stream and a hazardous coal combustion byproduct into a cohesive, structurally sound composite with significantly extended structural integrity under fire conditions, this study provides a scalable, eco-efficient material solution for sustainable construction.