This study focuses on the development and optimisation of polyethylene terephthalate (PET) composite boards incorporating spent garnet (SG) and sand as fillers to enhance water resistance and overall performance. The experimental design was conducted using Response Surface Methodology (RSM) under the Central Composite Design (CCD) framework to evaluate the effects of two key parameters: the binder-to-filler ratio and the SG-to-sand ratio. A total of thirteen experimental runs were performed to examine compressive strength, thermal conductivity, and water absorption. The inclusion of SG up to an optimum proportion improved matrix densification and reduced water uptake. Among the developed statistical models, the water-absorption model achieved an R² value of 0.97, demonstrating strong predictive reliability. The optimum mix composition produced a minimum water-absorption value of 0.30%, significantly lower than the >0.50% reported for conventional PET composites. The optimisation process yielded a desirability value of 1.000, confirming model adequacy. The optimised PET–SG–sand composite exhibited enhanced moisture resistance and thermal stability, demonstrating the potential of utilising industrial-waste SG as a sustainable filler for PET-based construction materials.

PET composite, spent garnet, sand, water absorption, optimisation