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

## TL;DR

This paper explores using coal gangue from tunneling as a highway base material, finding the best mix for strength, durability, and sustainability.

## Contribution

The study introduces a new evaluation model combining Delphi, entropy weight, and TOPSIS methods for optimizing coal gangue mixtures.

## Key findings

- Increasing coal gangue content reduces mechanical properties like UCS and fatigue life.
- Coarse aggregates reduce drying shrinkage, while fine aggregates improve long-term strength.
- Gradation T1~3 enhances microstructure and is optimal for expressway bases.

## Abstract

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.