Viscosity and salinity effect on thermal performance of bentonite-based grouts for ground heat exchanger

Due to its high swelling potential and low hydraulic conductivity, the bentonite-based grout has been popularly used to backfill a borehole constructed for a closed-loop vertical ground heat exchanger in a geothermal heat pump system. In this study, three types of bentonites were compared in terms of viscosity and thermal conductivity. With the consideration of the groundwater inflow, the viscosity and thermal conductivity of the bentonite grouts having bentonite contents of 5%, 10%, 15%, 20% and 25% by weight were also examined. To evaluate the effect of salinity in the coastal area on the swelling potential of the bentonite, volume reduction tests were performed. And in order to reproduce the particle segregation of bentonite–sand mixture when the bentonite-based grout has relatively low viscosity, the segregation was observed using a ring-type separable test apparatus.