Influence of recycled polyethylene terephthalate on the mechanical resistance of mortars exposed to high temperatures

Abstract This study analyzes the mechanical behavior of cement mortars modified with recycled polyethylene terephthalate (rPET) fibers subjected to different curing conditions and high temperatures. Prismatic test specimens were prepared in accordance with the UNE-EN 196-1 standard, incorporating two levels of sand replacement with rPET (10% and 20%). The samples were cured for 90 days both in the air and by immersion in water, and they were then exposed to three temperature levels : 20 °C, 150 °C, and 350 °C. After heat treatment, flexural and compressive strength tests were performed. The Results show that curing in water provides significantly higher mechanical strength, reaching up to twice the strength obtained with air curing. At the microstructural level, it was observed that the rPET fibers retained their shape up to approximately 150 °C, but at 350 °C they melted completely, generating microvoids that weakened the mortar matrix. This thermal degradation caused strength losses of 80–90% compared to the initial conditions. The 10% rPET content presented the best balance between mechanical strength and thermal stability, while 20% was only advantageous at room temperature. Overall, mortars with rPET are suitable for non-structural applications in environments with temperatures below 150 °C, offering a viable alternative for recovering plastic waste and promoting the circular economy.