The thermal conductivity and viscosity of bentonite grouts have been evaluated and compared each other to determine the suitability of these materials for backfilling vertical boreholes of ground heat exchangers.Seven bentonite grouts from different product sources were considered in this paper.Two additives,silica sand and graphite were added in bentonite grouts to enhance thermal performance.The bentonite grouts indicate that both the thermal conductivity and the viscosity increase with the content of silica sand and graphite.Therefore,it is recommended to select cautiously the amount of silica sand and graphite considering not only thermal conductivity but also viscosity for the optimum condition of backfilling.Finally,the effect of salinity in the pore water on the change of swelling potential of the bentonite-based grouts has been quantitatively evaluated to show the feasibility of bentonite grouts in the coastal area. The thermal conductivity and viscosity of bentonite grouts have been evaluated and compared each other to determine the suitability of these materials for backfilling vertical boreholes of ground heat exchangers. Seven bentonite grouts from different product sources were considered in this paper. Two additives, silica sand and graphite were added in bentonite grouts to enhance thermal performance. The bentonite grouts indicate that both the thermal conductivity and the viscosity increase with the content of silica sand and graphite. Herefore, it is recommended to select cautiously the amount of silica sand and graphite considered not only thermal conductivity but also viscosity for the optimum condition of backfilling. Finally, the effect of salinity in the pore water on the change of swelling potential of the bentonite-based grouts has been quantitatively evaluated to show the feasibility of bentonite grouts in the coastal area .