Microbial-modified coal-based solid waste backfill material: Mechanical improvement and its effect on the water environment

This study explored the application of microbially induced calcite precipitation (MICP) technique for enhancing backfill microbial-modified material strength and reducing cement use. Laboratory tests assessed the strength of microbial-modified materials and their environmental impact by characterizing harmful elements speciation in the material and examining pH and concentrations of harmful elements in different water environments after soaking. The results revealed that microbial-modified materials achieve higher strength than traditional ones composed of coal gangue, fly ash (FA), and cement without microbial modification, with optimal performance at 30% FA compared to 25% FA in traditional materials. Cement addition does not alter the interaction between Bacillus pasteurii and coal-based solid wastes, but increasing cement content from 3% to 5% further boosts strength through combined effects of cement hydration and microbial modification. Microbial-modified materials without cement achieve a strength of 471.1 KPa, similar to traditional materials with about 3.5% cement, and require 36% and 42.67% less cement for target strengths of 1000 KPa and 2000 KPa, respectively. Additionally, Microbial-modified materials improve water pH, ensuring all tested water types and harmful elements meet quality standards within the Class I-III range. This approach not only reduces cement use and enhances material strength but also improves environmental safety, making it a promising option for backfill applications.