<p>The partial substitution of metallurgical coal with waste plastic in cokemaking presents a promising pathway for resource recycling and improved sustainability of steel production. This study examines the impact of high-density polyethylene (HDPE) addition on microstructural transitions from coal–plastic blends to the final coke, utilizing interrupted coking tests and synchrotron-based micro-CT imaging. Two Australian coking coals, differing in maceral composition and thermoplastic properties, were blended with 5% HDPE and tested in a laboratory-scale coke oven to produce plastic layer samples with characteristic layered structures formed under practical coking conditions. These samples were then subjected to 3D microstructural analysis to evaluate microstructure evolution throughout carbonisation. HDPE addition resulted in the formation of coal–HDPE mixtures in the plastic layer, disrupting pore rearrangement during resolidification and limiting the development of fine pore structures, with the extent of impact dependent on coal properties. The overall impact of HDPE addition was reduced porosity in the plastic layer and higher porosity in the resolidified layer during coal-to-coke transformation. The addition of HDPE to high fluidity coal A increased bubble growth, mean pore size and internal gas pressure in the plastic layer. During resolidification, the closure of these pores was hindered, resulting in the persistence of large pores in the semi-coke structure. In contrast, coal B exhibited lower internal gas pressure and more localized retention of the coal–HDPE mixture, attributed to its lower dilatation and higher inertinite content, resulting in limited microstructural modification. These differences were confirmed through 3D microstructure analysis of micro-CT images.</p> Graphical Abstract <p></p>