1580 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. (a) Effect of nutrient concentration (b) Effect of bacterial suspension and nutrient dosage Figure 97 - Mechanical behavior of microbially cemented backfill materials under multi-factor ekects 3.2 Consolidation Mechanism of Microbially Cemented Backfill Materials 3.2.1 Microstructural Analysis The microstructures of the mixing-type and injection-type microbially cemented backfill materials are shown in Fig. 5. (a) Mixing-type specimen (b) Injection-type specimen Figure 98 - Microstructure of microbially cemented backfill materials In the mixing type material, CaCO3 crystals are mainly distributed on aggregate surfaces. The crystals are relatively small. Because cementation agents are blended directly into the mixture, calcium ion availability for reaction is limited. Moreover, the interparticle pores in the mixing-type specimens are filled and blocked by CaCO₃ precipitation, which restricts further transport of reactants and prevents CaCO₃ formation, leading to relatively low strength. The CaCO₃ precipitated through the MICP reaction in the mixing-type material mainly occurs in the mineral forms of calcite and vaterite. In the injection-type material, CaCO₃ predominantly precipitates as calcite within the interparticle pore spaces, forming dense crystalline layers between aggregate particles. The crystals are relatively large, with edge lengths reaching up to 26 μm, and both the crystal size and compactness are significantly greater than those of calcite observed in the mixing-type specimens. With the injection of the bacterial suspension, 1.5 2.0 2.5 3.0 0.1 1 10 0.3 3 0.1 1 10 0.3 3 UCS (MPa) Nutrient concentration (mol/L) Injection-induced UCS Average UCS (Injection) Mixing-induced UCS Average UCS (Mixing) 2:1 1:1 1:2 1:3 1:4 1:5 0.1 1 10 0.3 3 0.1 1 10 0.3 3 UCS (MPa) Bacterial suspension : nutrient solution Injection-induced UCS Average UCS (Injection) Mixing-induced UCS Average UCS (Mixing)
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