1582 Figure 100 - Properties of high-porosity carbon-negative cemented backfill materials As illustrated in Fig. 7, the compressive strength of the high-porosity carbon-negative backfill samples exhibits a negative correlation with both the carbonated steel slag content and aggregate content. Specifically, as the carbonated steel slag content increased from 50% to 65%, the strength of the samples decreased from 1.00 MPa to 0.49 MPa, representing a reduction of over 50%. Moreover, the strength declined significantly with increasing porosity, reaching only approximately one-third of the strength of a dense backfill body. Regarding carbon sequestration, as the aggregate content rose from 52.2% to 62.8%, the carbonation rate of the samples dropped from 1.35% to 1.00%. Conversely, an increase in the carbonated steel slag content from 50% to 65% led to an improvement in carbonation performance, rising from 1.01% to 1.33%. Nevertheless, most existing studies utilize ionic foaming agents in conjunction with foam stabilizers to prepare high-porosity materials. This approach frequently encounters challenges such as poor bubble stability in the slurry, relatively low mechanical performance, and an underdeveloped understanding of the material consolidation mechanism(Zhang et al. 2021). Consequently, further in-depth research is required to elucidate the synergistic mechanisms of carbonation and consolidation, as well as to enhance the overall performance of high-porosity backfill materials (Ngo et al., 2023). 4.2 Microbial Carbon-Negative Cemented Backfill Materials and Their Properties Currently, high-porosity materials face challenges such as low mechanical strength and inefficient carbonation. These issues are primarily attributed to the slow crystallization of calcium carbonate precipitates, poor cementing properties, and a slow CO2 hydration rate. Research indicates that a specific microbial strain, known as carbonic anhydrase (CA) mineralizing bacteria, can significantly mitigate these issues. The CA enzyme produced through microbial metabolism is a metalloenzyme with a zinc active center. It can increase the CO2 hydration rate from 5×10-2 s-1 to 1.6×106 s-1, which is approximately 3.2×107 times faster than under natural conditions, thereby drastically accelerating the CO2 mineralization reaction (Han et al., 2022). Microbial carbon-negative cemented backfill materials can be prepared using MICP, the principle is illustrated in Fig. 8. Mechanical characterization Carbon sequestration characterization specimens 52.2% 56.4% 59.8% 62.8% 0.9 1.0 1.1 1.2 1.3 1.4 Aggregate content Carbonation rate 1.349% 1.243% 1.114% 1.004% 50 55 60 65 0.4 0.6 0.8 1.0 1.2 1.4 Carbonated steel slag content(%) UCS of the full-steel-slag group Average UCS UCS 52.2% 56.4% 59.8% 62.8% 0.4 0.6 0.8 1.0 1.2 1.4 1.6 UCS (MPa) Aggregate content UCS of the full-steel-slag group Average UCS UCS 50% 55% 60% 65% 0.9 1.0 1.1 1.2 1.3 1.4 Carbonated steel slag content Carbonation rate 1.009% 1.119% 1.243% 1.325%
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