Coupled thermal and pressure effects on thermal conductivity of joint-filled bentonite in engineered barrier systems

The thermal conductivity characteristics of bentonite blocks with filled joints directly influence the temperature field distribution and safety assessment within the repository. In this study, the thermal conductivity of compacted bentonite powder and joint-filled specimens was tested using the thermal needle probe method under a wide range of temperatures (20–90 °C) and pressures (0–20 MPa). A series of microstructural analyses were also conducted on representative specimens to investigate the potential influence and microscopic mechanisms of joint type and width on bentonite thermal conductivity under complex buffer conditions. Tested results show that at room temperature, the thermal conductivity of joint-filled specimens is up to approximately 27.01 % lower than that of compacted powder specimens (without joints), which is attributed to the larger total porosity and dominant pore size in joint-filled specimens leading to insufficient particle contact. The temperature effect on thermal conductivity of joint-filled specimens is significantly greater than that of specimens without joints. With increasing joint width, the thermal conductivity of joint-filled specimens decreases, while the temperature effect correspondingly increases. This is mainly because the joint-filled specimens have more pores and dominant heat transfer paths favorable for latent heat transfer of vapour. Pressure increases the thermal conductivity of all specimen types but weakens the temperature effect on thermal conductivity, with this weakening effect becoming more pronounced as joint width increases. High pressure may disrupt the dominant paths for latent heat transfer of vapour, while larger joint widths increase both the dominant inter-aggregate pore size and number, thereby enhancing the temperature effect and its attenuation under pressure. • Jointed specimens show lower thermal conductivity, decreasing with joint width. • Thermal conductivity of jointed specimens increases more with temperature rise. • Pressure enhances thermal conductivity, especially in jointed bentonite specimens. • Pressure weakens the temperature effect, more so with increasing joint width.