Thermal conductivity and impermeability of spheroidal graphite-bentonite mixture after compaction

Incorporating flake graphite into bentonite enhances its thermal conductivity but compromises its impermeability. To address this trade-off，spheroidal graphite was selected as a substitute for flake graphite. Thermal conductivity and saturated permeability tests were conducted on spheroidal graphite-bentonite mixtures to investigate the effects of graphite content and shape on both thermal conductivity and permeability properties of bentonite. The results show that a spheroidal graphite content of approximately 20% significantly enhances the thermal conductivity of bentonite(1.7 W/(m·K))，while ensuring the permeability coefficient(9.44×10^－13 m/s) remains below the buffer layer's threshold of ≤1×10^－12 m/s. Concurrently，the maximum swelling pressure attained is 5.76 MPa，which falls within the recommended range of the buffer layer(1–10 MPa). Furthermore，the concept of critical graphite content is introduced，with the critical contents for spheroidal and flake graphite determined to be 27.3% and 16.4%，respectively. At a spheroidal graphite content of 20%，the graphite has not yet formed a skeleton within the mixture，indicating that it remains suspended in the bentonite. Conversely，at a flake graphite content of 20%，a skeleton forms with bentonite filling the spaces within the graphite structure. By analyzing the microscopic morphology and pore distribution characteristics of graphite-bentonite，this study elucidates why flake graphite enhances thermal conductivity while reducing impermeability. Additionally，it explains the physical mechanism by which spheroidal graphite outperforms flake graphite，providing scientific guidance for improving the operational safety of the repository.