Optimizing the mechanical properties of concrete incorporating metakaolin and recycled PET

Cement manufacturing in the construction sector is a major driver of worldwide carbon dioxide emissions, and also causes natural resource depletion due to the significant production of natural aggregates. Recent efforts have therefore focused on reducing cement consumption and dependence on natural aggregates. One promising approach is to partially replace cement and natural aggregates with low-carbon or waste-derived materials. This study examined the use of metakaolin and recycled PET as partial replacements for cement and coarse aggregate, respectively, and evaluated their combined effects on concrete properties. Metakaolin was used as a partial cement replacement at 0–30%, while recycled PET replaced natural coarse aggregate at 0–25% by volume. Response surface methodology was used to develop statistical models to predict mechanical properties and to conduct multi-objective optimization of the mix parameters and responses. Moderate replacement levels (around 10% metakaolin and 10% PET) gave the best overall performance, increasing compressive and split tensile strengths by about 5% and 2%, respectively, compared with the control mixture. Multi-objective optimization identified an optimal mix with 9.52% metakaolin and 10.18% PET as replacements for coarse aggregate. This study addresses the gap in mechanical behavior of PET–metakaolin concretes within a unified RSM framework, while durability and long-term performance remain topics for future research.