Performance of sustainable ultra-high-performance concrete with microplastics: comparative strategies of nano-coated microplastics and direct nanomaterial incorporation

Microplastics (MPs) are pervasive environmental contaminants with increasing urgency for sustainable management strategies. Their incorporation into cementitious materials offers a promising pathway for waste valorization; however, the weak interfacial transition zone (ITZ) between MPs and the cement matrix severely limits mechanical performance and hinders structural applications. This study presents a comparative nano-engineering framework to overcome this limitation by systematically evaluating two fundamentally distinct strategies: (1) direct surface nano-coating of MPs using a nano-silica and cellulose nanocrystal system, and (2) matrix nano-densification through direct incorporation of the same nanomaterials into ultra-high-performance concrete (UHPC). Because these two strategies deliver nanomaterials in fundamentally different ways, a fair comparison required equating their nanomaterial content. This was achieved by using thermogravimetric analysis to quantify the nanomaterial mass retained on the MP surface and applying the same content to the nano-densification mixtures. A comprehensive experimental program was conducted across multiple MP replacement levels (10–50%) to elucidate the effects of these strategies on mechanical performance and interfacial behavior. Results show that MP incorporation significantly reduces strength due to weak interfacial bonding and increased porosity; however, both nano-engineering approaches effectively recover performance by enhancing ITZ integrity. Notably, despite a 50% MP replacement, the optimized mixtures achieved a compressive strength of 91 MPa, demonstrating structural-level performance using high volumes of plastic waste. While both strategies yielded comparable mechanical improvements, matrix nano-densification provided slightly higher strength gains, whereas nano-coated MPs exhibited superior workability and required substantially less high-range water reducer to achieve equivalent workability, indicating superior and practical scalability. The study advances the field by providing the first direct comparison between ITZ-targeted nano-coating and matrix-level nano-densification for MP-modified UHPC, revealing that although both strategies achieve comparable mechanical performance, they differ substantially in admixture demand. These findings establish an application-oriented pathway for integrating microplastic waste into high-performance structural materials, contributing to circular material design and sustainable infrastructure development.