Valorisation of PET bottle waste in mortar: experimental investigation and prediction of properties using ultrasonic analysis

Plastic bottles are used worldwide, leading to a substantial quantity of waste being released into the environment. Including plastic bottle waste in mortars offers a sustainable solution to reduce this waste pollution. This research assesses the effect of using polyethylene terephthalate (PET) bottle waste on mortar properties. PET bottles were shredded, transformed into fine particles, and then integrated into mortar to replace natural sand at rates ranging from 5% to 50%. Six mortar formulations were designed and assessed for their workability, density, thermal conductivity, porosity, mechanical performance, ultrasonic pulse velocity (UPV). Moreover, the microstructure of PET-based mortar was analysed using scanning electron microscopy (SEM). The findings revealed enhanced workability up to 30% PET replacement, a significant decrease in density, and improved thermal insulation, with up to a 53% reduction in thermal conductivity. Besides, recycled PET particles slightly increased mortar mechanical strength at a 5% PET level but led to a gradual decline at higher levels, reaching a 48% reduction. SEM imaging showed a porous interfacial transition zone (ITZ) between PET and the cementitious matrix, indicating weak bonding, which explains the reduced mechanical performance. In addition, the study focused on predicting PET-based mortar properties through non-destructive testing by establishing accurate correlations between mortar characteristics and UPV measurements, validated using both the present findings and data from the literature. The proposed relationships, characterised by low root mean square error values and coefficient of determination (R²) ranging from 0.90 to 0.96, enable efficient assessment of this mortar’s characteristics, facilitating its use in construction practices.