344 Upon completion of the injection process, the molds were demolded to obtain standard specimens (Deng et al., 2021). 3.1.3 Mechanical Properties The mechanical properties of microbially cemented backfill materials are influenced by multiple factors, primarily including the preparation method, aggregate characteristics, and mix proportions. Tests were conducted with an aggregate mass fraction of 87.5% and six injection batches. Nutrient concentration ranged from 1.5 to 3 mol/L, and the volumetric ratio of bacterial suspension to nutrient solution ranged from 2:1 to 1:5. The results are shown in Fig. 4. As shown in Fig. 4(a), the strength of the mixing-type material increases with increasing nutrient concentration. In contrast, the strength of the injection-type material first increases and then decreases as the nutrient concentration increases, reaching a maximum unconfined compressive strength of 3.26 MPa at a nutrient concentration of 2 mol/L. With increasing nutrient concentration, CaCO3 precipitation is promoted. Specimen strength is increased. However, excessively high Ca²⁺ concentrations can inhibit urease activity and reduce the efficiency of urea hydrolysis, leading to a decreased CaCO3 production and strength of specimens (Deng et al., 2020). As shown in Fig. 4(b), with a gradual decrease in the volumetric ratio of bacterial suspension to nutrient solution, the strength of the mixing-type material first increases and then decreases, exhibiting a two-stage trend. In the first stage, as the dosage of nutrient solution increases, the amount of calcium carbonate produced by the MICP reaction correspondingly increases, resulting in enhanced specimen strength. The strength of the mixing-type specimens reaches a maximum value of 0.69 MPa when the volumetric ratio decreases to 1:2. In the second stage, with a further decrease in the bacterial suspension– to–nutrient solution ratio, the dosage of bacterial suspension in the mixing-type specimens is reduced. The consequent decrease in microbial population leads to a reduction in urease content, which slows the MICP reaction rate and reduces the amount of calcium carbonate precipitation, thereby causing a decline in specimen strength. For the injection-type specimens, the dosage of bacterial suspension remains constant. With a continuous increase in nutrient solution dosage, the amount of calcium carbonate precipitation increases, leading to a continuous enhancement in specimen strength. A strength up to 25.12 MPa was achieved for the injection-type material.
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