Upcycling mixed plastic waste as a replacement for natural aggregates in concrete: a critical review

Upcycling mixed plastics as natural aggregate replacements in concrete offers a promising pathway for resource conservation and waste management. While most studies have examined single-type plastics, fewer have focused on mixed plastics. This paper provides the first comprehensive review of this approach, with comparisons between mixed and single type plastics. Recycling methods and challenges of mixed plastics are outlined, followed by an evaluation of applications in concrete. Physical and mechanical properties were assessed through quantitative data synthesis using normalisation ratios, alongside a review of durability performance. Findings indicate that density variations are mainly governed by the density of plastic aggregates, with minimal influence from production methods or additives. Mechanical performance generally decreases with plastic addition, and single-type plastics often perform better than mixed plastics, except for end-of-life vehicle plastics, which show higher strength retention and therefore appear viable as a feedstock. Shredded plastics also outperform artificially manufactured aggregates in flexural strength. Strategies to mitigate strength loss, such as surface modification of aggregates, have been identified. Environmental and economic aspects, including life cycle assessment and cost implications, are also reviewed alongside case studies of practical applications. The review highlights current limitations and research opportunities for mixed plastics in concrete. In particular, fewer mechanical properties and durability aspects have been investigated for mixed plastics than for single-type plastics in concrete. Overall, this review advances the use of non-recyclable mixed plastics in concrete from technical, environmental, and economic perspectives, supporting the valorisation of residual waste into sustainable construction materials.