# Synergistic Utilization of Recycled Asphalt Pavement and Fly Ash for High-Ductility Coal Mine Backfill: Performance Optimization and Mechanism Analysis

## TL;DR

This study explores using recycled asphalt pavement and fly ash to create a high-ductility coal mine backfill material, improving both performance and environmental sustainability.

## Contribution

The novel contribution is the synergistic use of RAP and fly ash to achieve a brittle-to-ductile transition in backfill materials, enhancing mechanical properties and environmental compliance.

## Key findings

- RFCPB slurry follows the Herschel–Bulkley model, with rheology regulated by the F/B ratio.
- RAP enhances backfill ductility, enabling a brittle-to-ductile transition and improving UCS.
- RAP’s Dolomite/Albite synergizes with FA to generate gels, optimizing pore structure and UCS.

## Abstract

What are the main findings?
RFCPB slurry follows H-B model, with rheology regulated by F/B ratio.28-day UCS of RFCPB varies with F/B ratio (first rise, then fall).RAP enhances backfill ductility, realizing brittle-to-ductile transition.RAP’s Dolomite/Albite synergizes with FA to generate gels, optimizing pore structure and UCS.

RFCPB slurry follows H-B model, with rheology regulated by F/B ratio.

28-day UCS of RFCPB varies with F/B ratio (first rise, then fall).

RAP enhances backfill ductility, realizing brittle-to-ductile transition.

RAP’s Dolomite/Albite synergizes with FA to generate gels, optimizing pore structure and UCS.

What are the implications of the main findings?
RFCPB meets pipeline transportation and mine backfill technical requirements.Brittle-to-ductile transition mitigates goaf collapse risk, protecting overlying highways.Clarifies RAP-FA chemical synergy in cement-based composites, filling knowledge gaps.

RFCPB meets pipeline transportation and mine backfill technical requirements.

Brittle-to-ductile transition mitigates goaf collapse risk, protecting overlying highways.

Clarifies RAP-FA chemical synergy in cement-based composites, filling knowledge gaps.

To enhance the ductility of coal mine filling materials using recycled asphalt pavement (RAP) and address the limitations in RAP recycling and utilization, this study processed RAP into crushed materials (CMs) and ball-milled materials (BMs). Supplementary with fly ash (FA) and cement, RAP-fly ash cement paste backfill (RFCPB) was prepared. For 1000 g of RFCPB slurry, the composition was 365 g CM, 73 g cement, 270 g water, and a total of 292 g of FA and BM, with an F/B ratio ranging from 1:7 to 7:1. A systematic test program was carried out, including rheological property tests, unconfined compressive strength (UCS) tests combined with deformation monitoring, microstructure analysis, and leaching toxicity tests. Based on these tests, the influence of F/B ratio on the action mechanism, workability, mechanical properties, ductility and environmental compatibility of RFCPB was comprehensively explored. The results show that the rheological behavior of RFCPB slurry conforms to the Herschel–Bulkley (H-B) model; with the decrease in F/B ratio, the yield stress and apparent viscosity of the slurry increase significantly, while the slump and slump flow decrease correspondingly, which is closely related to the particle gradation optimization by BM. For mechanical properties and ductility, the 28-day UCS of RFCPB first increases and then decreases with the decrease in F/B ratio, all meeting the mine backfilling strength requirements; notably, the increase in BM proportion regulates the failure mode from brittle to ductile, which is the key to improving ductility. Microstructural analysis indicates that Dolomite and Albite in BM participate in hydration reactions to generate N-A-S-H and C-A-S-H gels, which fill internal pores, optimize pore structure, and thus synergistically improve UCS and ductility. Additionally, the leaching concentration of toxic ions in RFCPB complies with the environmental protection standards for solid waste. This study provides a theoretical basis for enhancing backfill ductility and advancing the coordinated disposal of RAP and fly ash solid wastes.