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

• By incorporating a suitable amount of HPMC as a partial replacement for cement, the self-supporting capacity and mechanical properties of the paste backfill were improved. • The hydration mechanism through which HPMC influences the mechanical behavior of the backfill was elucidated. • Applicable scenarios for the use of HPMC-modified paste backfill were proposed. 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.