Grouts with highly thermally conductive binder for low-temperature geothermal applications

Abstract

This paper presents the results of a laboratory study on grouts with a novel binder characterized by high thermal conductivity, designed for low-temperature geothermal applications. The highly thermally conductive binder was prepared by milling expanded graphite together with Portland clinker to obtain a homogenous mixture containing exfoliated graphite. Cement grouts with the obtained binder were prepared to determine their thermal and mechanical properties and durability in both dried and water-saturated state. Thermal properties were determined by thermal conductivity and thermal diffusivity measurements. Compressive and flexural strength tests were carried out to evaluate the mechanical properties of grouts. The durability of grouts was determined by evaluating the impact of wet-dry and heating–cooling cycles on their mechanical strength and thermal conductivity. Furthermore, the quality of the interface between grouts and HDPE pipe was determined by bond strength, hydraulic conductivity and thermal efficiency tests. The results showed that milling expanded graphite together with Portland clinker is an effective way to obtain a binder that consists uniformly dispersed graphite particles, providing high thermal conductivity. The use of the binder in grouts preparation allows to obtain good properties in terms of their use in low-temperature geothermal applications. Especially, this binder improves the thermal efficiency of the grouts. The value of thermal conductivity of grouts based on HTCB is about 24% higher than that of typical grouts with sand. Furthermore, the grouts are characterized by good mechanical properties and durability. The binder also enables to achieve a good quality of the interface between the grout and HDPE pipe.