Abstract: Understanding thermal conductivity of buffer/backfill materials is significant for the safe operation of deep geological repository for high-level radioactive waste. In this study, combined blocks of Gaomiaozi (GMZ) bentonite with 2 mm wide technological voids are prepared in constant volume condition and then exposed to hydration by distilled water or NaCl solutions (0.1, 0.5 and 1.0 mol/L) under different temperatures (20, 60, and 80 ℃). The evolutions of thermal conductivity, dry density, moisture content and microstructure at different locations of the combined blocks sample are measured. Results show that the thermal conductivity of the initial technological void area is lower than that of the block area, regardless of thermal and saline conditions. The thermal conductivity increases with increasing dry density and environmental temperature due to thermal enhanced latent heat transfer of water vapor, while decreases with increasing concentration of NaCl solution due to less effectively healed technological voids resulting from the saline attenuated diffuse double layers. Under coupled thermal-saline conditions, the thermal enhancement effect could be suppressed by the saline attenuation effect, leading to reduced thermal conductivity. After 150-day hydration, the sample is still heterogeneous with distinct dry density and water content gradients perpendicular to the block/block interface. This implies a long-term homogenization of the microstructure and thus further evolution of thermal conductivity of the sample.