361 Figure 8 – Results of uniaxial compressive strength and CO₂ fixation 4. CONCLUSION Injection trials using CO₂ MNB water and BFS slurry were successfully conducted in a goaf of an underground coal mine in Mikasa City, Hokkaido, aiming at CO₂ storage and mineralization. The main findings are as follows: • Borehole drilling confirmed the presence of a goaf around depths of 400 m. • The goaf exhibited permeability ranging from 8.86 × 10⁻⁵ to 1.04 × 10⁻⁴ cm/s, indicating good injectivity. • The injection trials confirmed that CO₂ MNB water and BFS slurry could be effectively injected into the goaf. • The permeability of the goaf decreased over time after injection, likely due to fracture closure from solidification of BFS slurry, suggesting that reactions occurred between CO₂ MNB water and BFS slurry in situ. • The strength and CO₂ fixation of the solidified slurry are influenced by subsurface pressure conditions, indicating that high-pressure environments may enhance the hydration and carbonation reactions of the slag, promoting long-term CO₂ immobilization. Overall, these results demonstrate the potential of using CO₂ MNB water combined with BFS slurry for effective CO₂ storage and mineralization in underground mined-out areas. As a next step, further investigation is required to evaluate the CO₂ storage capacity of such environments and to assess the feasibility of large-scale, economically viable injection operations. ACKNOWLEDGEMENTS This study was supported by JSPS KAKENHI Grants JP 25K01715; Module Research Program of the Kyushu University Institute for Energy Research and Education
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