Development and characterization of volume-stabilized grouts used for borehole heat exchangers

Abstract

This study aims to develop volume-stabilized grouts made of ordinary Portland cement (OPC)-calcium sulfoaluminate (CSA) cement-calcium sulfate (CS) ternary systems for borehole heat exchanger (BHE) applications. A series of experimental tests were carried out to characterize their properties and evaluate the applicability of grouts used in BHE. The grouts made of ternary systems exhibited satisfactory flowability within four hours and excellent volumetric stability compared with the grouts made of OPC only. Moreover, the thermal conductivities of grouts composed of ternary systems reached a range of 0.582–0.628 W/mK under oven-dry conditions and 1.940–1.949 W/mK under saturated conditions. In addition, grouts exhibited high unconfined compressive strength within a range of 14.19-16.31 MPa, indicating that the grouts have sufficient capacity to maintain the stability of boreholes. Overall, the grouts made of ternary systems exhibited good flowability, remarkable volumetric stability, and high thermal conductivity under saturated conditions. Among them, the grout composed of 70% OPC, 15% CSA cement and 15% gypsum showed superior volumetric stability and satisfactory performance in other aspects, indicating that it has the potential for BHE applications.