Properties of a backfill material prepared by cementing coal gangue and fly ash through microbial-induced calcite precipitation

Microbes can induce the production of calcium carbonate deposits, which cement solid wastes such as coal gangue to form backfill materials. Filling such backfill materials in underground goafs can facilitate the disposal of solid wastes and control surface subsidence. In view of this, the idea of preparing a backfill material by cementing coal gangue and fly ash through microbial-induced calcite precipitation (MICP) induced by B. pasteurii was proposed and the preparation procedure and test methods of properties of the backfill material were introduced. Changes in the slump flow and bleeding rate of the backfill material under conditions of different bacterial concentration, urea concentration, and mass concentration were studied. Influences of factors including the bacterial concentration, urea concentration, mass concentration, curing time, and curing temperature on the compressive strength of the backfill material were estimated. Results show that as the bacterial concentration increases, the slump flow increases, the bleeding rate decreases, while the compressive strength of the backfill material increases. An increase in the urea concentration reduces the slump flow and bleeding rate of the backfill material while first increasing, then decreasing the compressive strength; excessive concentrations of urea suppress bacterial activities. An increase in the mass concentration decreases the slump flow and bleeding rate of the backfill material, while increasing the compressive strength. As the curing temperature is increased, the compressive strength of the backfill material first increases, then decreases and the temperature range of 30 ∼ 40℃ is more conducive to bacterial hydrolysis of the urea and chemical reactions in the coal gangue-fly ash slurry. The prolonged curing time can significantly improve the compressive strength of the backfill material. The research results provide a theoretical basis for the application of microbial-induced calcite precipitation (MICP) in backfilling mining of coal mines.