A Novel Approach of Bulk Strength Enhancement through Microbially-Mediated Carbonate Cementation for Mylonitic Coal

Mylonite coal is known to be highly tectonically deformed coal and is a result of crushing original coal into fine-grained coal under multiple tectonic events. Because of its granular nature and the resultant inferior mechanical property, the borehole stability during both drilling and gas drainage is challenging due to unpredictable failures. This study explored a potential approach to enhance the bulk strength of mylonitic coal via microbiologically-mediated CaCO3 cementation which effectively increases the cohesion of the fine coal particles through the bio-cementation process. The experimental results indicated that the mechanical strength was significantly enhanced after a short period of biotreatment with 12 cycles of biotreatment yielding a maximum stiffness of 32 KN/mm and maximum uniaxial compressive strength of 9.3 MPa. The strength evolution behavior demonstrated that the macroscopic failure behavior of mylonite coal evolves from plastic failure to brittle failure as the increase of bio-treatments. The results from XRD indicated that the generated CaCO3 crystals mainly occurs in the form of calcite and vaterite. Imaging by SEM further indicated the cementation process for CaCO3. The generated CaCO3 precipitation first occurs on random spots on the particle surfaces, and then occupies the interstitial space until particle-particle bonds are generated.