The global plastic crisis has generated significant interest in repurposing waste plastics as asphalt modifiers, presenting both environmental and engineering advantages. This study offers a comprehensive review of the applications of waste plastics in asphalt, focusing on their types, modification mechanisms, incorporation techniques, and environmental impacts, alongside proposed mitigation strategies. Commonly utilized plastics include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET), each affecting asphalt performance differently—enhancing high-temperature stability and fatigue resistance while exhibiting varying levels of compatibility and environmental risks. The incorporation techniques, namely wet and dry processes, differ in terms of efficiency, cost, and environmental footprint: the wet process enhances durability but requires more energy, whereas the dry process is more cost-effective but may lead to uneven dispersion. Environmental concerns associated with these practices include toxic emissions (such as polycyclic aromatic hydrocarbons and volatile organic compounds) during production, microplastic generation through abrasion and weathering, and ecological contamination of soil and water. Mitigation strategies encompass optimizing plastic selection, improving pre-treatment and compatibilization methods, controlling high-temperature processing, and monitoring the spread of microplastics. This review highlights the need for balanced adoption of waste plastic-modified asphalt, emphasizing sustainable practices to maximize benefits while minimizing risks.

The disposal of construction and demolition waste (CDW) and the rehabilitation of water-rich goafs present two pressing challenges in sustainable mining and geotechnical engineering. This study introduces a bio-carbonated reactive magnesia cement (RMC) technology to solidify CDW for goaf backfilling applications. A comprehensive experimental program was conducted to evaluate the mechanical properties, water stability, and durability of bio-carbonated RMC-solidified CDW under simulated water-rich goaf environments. The unconfined compressive strength (UCS) of the 16% RMC specimens reached 3.12 MPa after 28 days, which was approximately 21% higher than that of the 12% Portland cement (OPC) control. Under dynamic water erosion conditions of 1.0 m/s for 72 h, the 16% RMC specimens showed a mass loss rate of 2.68% and a strength retention rate of 84.2%, both of which were superior to those of the 12% OPC control. The water–land strength ratio of the 16% RMC specimens reached 0.90 after 28 days of immersion, and the strength retention rate remained at 73.1% after 12 wet–dry cycles. Compared to the OPC control, these two indicators respectively represent a precise increase of 25% and approximately 60%. Microstructural analysis revealed that hydrated magnesium carbonates (HMCs), including nesquehonite and hydromagnesite, formed a dense spatial network that binds CDW particles and blocks pore channels. Additionally, a random forest model quantified the relative importance of RMC content, curing age, and carbonation degree on UCS, confirming that RMC dosage and the extent of carbonation are the dominant controlling factors. The optimal RMC content was determined to be 16%, balancing performance and cost. This study demonstrates that bio-carbonated RMC-solidified CDW is a technically viable and low-carbon backfill material with substantial CO₂ sequestration potential for water-rich goafs.