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The excessive accumulation of polyethylene terephthalate (PET) plastic waste and the urgent need for sustainable, energy-efficient construction materials have led to the exploration of alternatives for use in building composites. This study investigates the use of shaped recycled PET plastic aggregates, referred to as popcorn aggregates (PA), as eco-lightweight aggregates for producing lightweight bricks aimed at improving thermal comfort in hot climates. Ten concrete mixes were prepared with varying partial replacements of natural aggregates: 30%, 60%, and 90% using fine popcorn aggregates (FPA) and coarse popcorn aggregates (CPA), including hybrid blends. A comprehensive experimental program was designed to evaluate mechanical properties, specifically compressive and flexural strength. Physical characteristics such as bulk density, water sorptivity, and microstructure are assessed, and failure modes under thermal exposure are observed. Additionally, the thermal performance was investigated through thermal conductivity measurements and heat transfer analysis, along with a feasibility economic study. The mechanical tests indicated that higher popcorn coarse aggregate (PCA) content generally resulted in decreased strength. The hybrid mix, containing 30% FPA and 60% CPA, maintained a compressive strength of 14 MPa and a flexural strength of 2.37 MPa, which aligns with the ASTM standards for non-load-bearing masonry units. The 60% FPA–60% CPA blend achieved the lowest bulk density of 1,286 kg/m3, marking a 41.4% reduction, while sorptivity decreased by as much as 26.4%, due to the nonabsorbent nature of the plastic disrupting capillary pores. Moreover, thermal conductivity significantly decreased, reaching 0.98 W/m·K in the 90% CPA mix (a reduction of 28.98%). The 60% FPA–30% CPA blend showed a similar 26.64% reduction while maintaining mechanical integrity. Thermal exposure tests confirmed that mixes containing up to 60% PCA withstood temperatures of up to 300°C. The 60% FPA–60% CPA mix demonstrated superior passive thermal performance, reducing surface temperatures by 4.5°C and delaying heat peaks by 1.15 h. Economically, this mix provided the best balance, offering the lowest production cost ($16.74/m3), the highest energy savings ($1.94/m3/day), and the shortest payback period (8.65 days). These results underscore the potential of hybrid PCA aggregates for producing lightweight, energy-efficient, and environmentally sustainable bricks.