Study on the regulation of hydration products and mechanical properties of fly ash-mountain sand composite backfill materials by hydroxypropyl methylcellulose

Conventional paste backfilling in underground coal mining often suffers from poor self-standing ability, and low compressive strength. To address these limitations, this study introduces hydroxypropyl methyl cellulose (HPMC) into a fly ash (FA)-mountain sand-based backfill material to develop a paste with enhanced self-supporting properties. The influence of HPMC dosage on paste fluidity, self-standing capacity, mechanical performance, and microstructure was systematically investigated. Results demonstrate that as the HPMC content increases from 0 ‰ to 2.0 ‰, the self-standing performance of the paste is significantly improved. The maximum reductions in slump and consistency reach 20% and 30.9%, respectively, with the rate of decrease slowing beyond 1.0 ‰ HPMC. An appropriate amount of HPMC (0.5 ‰ - 1.5 ‰) enhances the compressive strength of the backfill, with the most notable improvement observed at 0.5 ‰. However, excessive HPMC addition (2.0 ‰) leads to a decline in strength. At 1.0 ‰ HPMC, an optimal balance is achieved among fluidity, self-standing ability, and compressive strength, the slump of the paste is 22.6 cm, and the 3-day compressive strength is 2.31 MPa. Microstructural analysis indicates that a suitable HPMC content promotes the formation of C-S-H gel and ettringite (AFt), suppresses the precipitation of portlandite (CH), optimizes pore structure, and increases compactness. The developed paste is suitable for template-free strip backfilling in goaf areas and for sealing upper and lower corners of longwall faces. It is recommended that the HPMC dosage be controlled at 1.0 ‰ of the solid mass in practical engineering applications to achieve a synergistic improvement in both self-standing capacity and strength. This study provides a valuable reference for advancing green mining backfill technologies.