343 indicators to different factors, then apply the NSGA III algorithm to perform multiobjective optimization on the analytical models of multiple performance indicators, yielding a Pareto optimal mix proportion design within the reasonable range; (4) Rank factors based on sensitivity discrimination indices, then optimize the backfill material composition for different target properties to derive the optimal formulation. 3. MICROBIALLY CEMENTED BACKFILL MATERIALS FOR COAL MINES AND THEIR CONSOLIDATION MECHANISM 3.1 Microbially Cemented Backfill Materials for Coal Mines 3.1.1 Raw Material Composition (1) Microorganisms. Various microorganisms can be employed for microbially cemented backfill materials, among which urease-producing bacteria, such as Sporosarcina pasteurii, are most commonly used. (2) Nutrient solution. A commonly used nutrient solution is a mixed solution of calcium chloride and urea. The concentrations of urea and calcium chloride typically range from 0.25 to 3 mol/L, with the molar ratio of urea to calcium chloride generally maintained at 1:1. (3) Aggregates. The aggregates used in microbially cemented backfill materials are mainly solid wastes, such as coal gangue, and aeolian sand. 3.1.2 Preparation Methods The preparation methods of microbially cemented backfill materials mainly include the mixing method and the injection method, as illustrated in Fig. 3. Figure 12 - Preparation methods of microbially cemented backfill materials (1) Mixing method. According to the designed mix proportions, the aggregates, nutrient solution, and bacterial suspension were weighed and mixed uniformly and then placed into molds. The molds filled with the fresh mixture were cured at room temperature for 7 days, after which the molds were demolded to obtain standard specimens. (2) Injection method. The aggregates were first packed into molds, with an inlet and an outlet arranged at opposite ends. The bacterial suspension and nutrient solution were then injected sequentially into the molds in multiple batches through the inlet. After each injection batch, the inlet and outlet were alternated before the subsequent injection. Inoculation Bacteria Bacterial suspension Cultivation Urea Ca²⁺ coal gangue Fly ash Nutrient solution Injection method Mixing method Bacterial preparation Aeolian sand Yellow River sand Preparation procedure Specimens aggregates
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