<p>This article analyzed the rise in global plastic waste production and its environmental impact, while exploring opportunities to repurpose such waste in the construction materials sector. The study highlighted the promising use of plastic waste specifically polyethylene terephthalate (PET) and natural pozzolan (PZN), a porous volcanic rock, to produce lightweight, energy-efficient composite mortars. This innovative approach aimed to volumetrically replace natural sand with 25%, 50%, 75% and 100% thermally synthesized composite sand (WPPz) in the formulation of eco-composite mortars, in order to assess its influence on yield stress, thermo-physical and mechanical properties, and durability. The production of these composite mortars was also evaluated through a Life Cycle Assessment (LCA) using the Ecoinvent 3 database, while the mechanical properties were partially examined through analytical modeling. Results showed that increasing WPPz content significantly influenced the fresh-state properties, particularly yield stress. The WPPz100 formulation achieved a 27% reduction in density, a 62% decrease in dynamic elastic modulus, and a 53% reduction in thermal conductivity compared to the reference mortar, with only a slight decrease in mechanical strength. The optimal dosage, WPPz25, exhibited the highest mechanical performance, suggesting a balance between strength and lightness. This research contributed to mitigating environmental impact by promoting the sustainable management of plastic waste and the use of volcanic pozzolan, while supporting a circular economy of natural resources. The main objective was to develop industrially applicable eco-composite materials for construction (such as lightweight facades, insulating screeds, or energy-efficient envelopes), offering a method to reduce emissions, protect natural resources, minimize the carbon footprint of sustainable buildings, and lower overall construction costs.</p>