Optimum Mix of Tunneling Coal Gangue as a Highway Base Material Through Delphi–Entropy Weight–TOPSIS and Microstructure Analysis

Using coal gangue in highway base construction provides a sustainable and high-value solid waste recycling approach. This research focused on the mechanical and durability properties of coal gangue from tunneling operations. Six experimental tests, such as unconfined compressive strength (UCS), flexural-tensile strength (FTS), etc., were carried out. The impact of aggregate gradation on coal gangue mixtures' performance was systematically evaluated. XRD and SEM were used to explore the microstructural mechanisms in cement-stabilized coal gangue-gravel mixtures (CGM). An improved evaluation model, the Delphi-entropy weight-TOPSIS (DET) method, integrating Delphi and entropy weighting, was proposed. Together with an advanced radar chart, it evaluates eight performance criteria, including mechanical, durability, economic, and environmental aspects. The results show that increasing the coal gangue content in mixtures decreases UCS, dynamic compressive rebound modulus (DCRM), FTS, fatigue life, and drying shrinkage performance. Coarse aggregates relieve drying shrinkage, while fine ones improve long-term mechanical properties. Gradation T1~3 promotes the formation of C-S-H gel and AFt crystals, enhancing compactness. Based on the DET model's quantitative evaluation, T1~3 was determined as the optimal mix for expressway bases, achieving a balance between mechanical performance, durability, and sustainability.