Effect of Freeze-Thaw and Wet-Dry Cycles on the Mechanical and Hydraulic Characteristics of Geothermal Mortars

The appropriate performance of a vertical geothermal exchanger depends not only on the thermal properties of the elements that composed it, but also on the close contact between them to ensure the heat transfer. This contact can be affected by temperature, moisture and seismic tremor variations within the ground, generating cracks and leaks in the grout and thus compromising the integrity of the system. Grouts usually have a high water/cement ratio to provide an appropriate fluidity during the borehole filling, and bentonite is used to keep the cement of the mixture on suspension. The main purpose of this research is to determine according to the bentonite amount, the effects generated by the seasonal freeze-thaw and wet-dry cycles on the mechanical properties of a geothermal borehole grout. Adherence tests from the grout to the exchanger, as well as compressive and flexural strength tests to a mortar with bentonite in percentages of the cement weight (0%, 1%, 2%, 3%, 4% and 5%) has been carried out during the curing period and under the effects of 7 freeze-thaw and wet-dry cycles, which simulates the annual freezing during the heating period and the movements of ground waters, respectively. According to the results, it is concluded that mechanical properties of the mortar decrease as the bentonite amount increases and the pipe-mortar adherence is the most affected by the freeze-thaw and wet-dry cycles.