Polyethylene terephthalate (PET) waste plastic as natural aggregate replacement in geopolymer mortar production

This work investigates the recycling potential of polyethylene terephthalate (PET) bottle wastes as natural sand substitute in geopolymer (GP) mixtures to reduce plastic pollution and transition to a circular economy. Fresh and hardened properties of coal fly ash-based GP mortars with replacement of quartz sand by grinded fine PET particles of 0.315–1.25 mm in size (20%, 40%, 60%, 80% and 100%) were evaluated. It was found out that an increase in plastic aggregate content leads to a decrease in compressive strength and flexural strength of geopolymer mortars. In turn, the splitting tensile strength increased slightly when up to 40% of the sand volume was replaced by plastic aggregate. At this replacement level, the fresh geopolymer mixes had workability close to that of conventional mortar. The flake-like PET particles contributed to the reduction of cracking of the specimens and more ductile failure modes. Furthermore, GP mortars containing recycled PET bottle wastes at the full replacement level of natural aggregate showed advantages in lightweight (up to 15%), water absorption (up to 26%) and thermal insulation properties (up to 59%), enabling production of sustainable construction materials with environmental and economic benefits.