In order to address the issue of reduced compressive strength in plastic mortar caused by the hydrophobic nature of plastic, this study utilizes the thermoplasticity of discarded polypropylene (PP) plastic to prepare silica fume@plastic particle composite particles (SF@PP) by welding silica fume on the surface of PP through the thermal welding process and tests the hydrophilicity and volcanic ash activity of the composite particles. The results indicate that the modified plastic through silica fume heat welding improves the hydrophilicity of PP and enhances its volcanic ash activity, promoting the deposition of hydration products at the interface and increasing the compactness of the interface. Consequently, the porosity of the plastic mortar is significantly reduced, and the compressive strength is significantly increased by 26.07% to 31.04%. The results of the study are conducive to the efficient utilization of waste plastics and provide a basis for the preparation of high-performance plastic mortar.