Bentonite-based grout fluids and methods of using the grout fluids are provided. A method of using a grout fluid includes placing a geothermal conduit in at least one hole in the earth, providing a grout fluid consisting essentially of water and a bentonite-based grout, wherein the bentonite-based grout consists essentially of calcium carbonate, a bentonite, one or more grout-setting modifiers, and one or more thermally conductive materials, introducing the grout fluid into a space between the geothermal conduit and sidewalls of the at least one hole so that the grout fluid is in contact with the geothermal conduit, and after introducing the grout fluid, allowing the grout fluid to set to fix the geothermal conduit to the at least one hole, wherein after setting, the grout fluid has a hydraulic conductivity of between about 1×10−7 cm/s and about 1×10−9 cm/s.