346 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, bacteria attach to the surfaces of aggregate particles. Subsequently, as the nutrient solution is continuously injected, large amounts of Ca²⁺ react with CO2- 3 produced from urea hydrolysis to form CaCO₃ precipitates, which progressively fill the pore spaces. This process ultimately results in high-strength consolidation of the microbially cemented backfill materials (Deng et al., 2024). 3.2.2 Consolidation Mechanism Model The schematic model of this consolidation mechanism is illustrated in Fig. 6. In the consolidation reaction system of microbially cemented backfill materials, bacteria typically possess negatively charged cell surfaces, which electrostatically attract Ca²⁺ ions in solution and provide favorable nucleation sites for subsequent calcium carbonate crystallization. When urease produced by the microorganisms catalyzes urea hydrolysis, urea is decomposed into NH+ 4 and CO2- 3 , resulting in an increase in system alkalinity. This elevated alkalinity promotes the combination of CO2- 3 with Ca²⁺ and induces the precipitation of calcium carbonate crystals on the bacterial surfaces or in their immediate vicinity. As the crystals continuously grow and deposit, calcium carbonate gradually fills the interparticle pore and cements adjacent aggregate particles, ultimately forming a backfill body with good integrity. Figure 15 - Schematic model of the consolidation mechanism of microbially cemented backfill 4. CARBON-NEGATIVE CEMENTED BACKFILL MATERIALS AND THEIR PROPERTIES 4.1 High-Porosity Carbon-Negative Cemented Backfill Materials and Their Properties A technical framework known as carbon negative high efficiency backfill mining has been proposed by the research team (Xie et al., 2024). The fundamental mechanism of this system lies in the development of novel composite backfill materials characterized by high porosity, robust mechanical strength, and superior carbon sequestration performance. Two preparation processes were developed for high-porosity carbon-negative cemented backfill materials, namely the pre-carbonation type and the post-carbonation type. Utilizing fly ash, steel slag, and cement as primary raw materials and incorporating foam NH3+H2O→NH4 ++OHCO2+H2O→HCO3 -+H+ CO(NH2)2+H2O→2NH3+CO2 Ca2 ++ OH- +HCO3 - →CaCO3 +H2O Ca2 + Ca2 + Ca2 + CO3 2Ca2 + Gangue Particles CaCO3 H2O Ca2 +
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