Evaluation of Coal Ash–Based CLSM Made with Cementless Binder as a Thermal Grout for Borehole Heat Exchangers

Abstract

The primary goal of this study is to assess the characteristics of coal ash–based controlled low-strength material (CLSM) mixtures made with a cementless binder for the feasible usage as a thermal grout in borehole heat exchangers. In the experimental program, a group of CLSM mixtures made with cement or cementless binder through various ratios of water to binder (W∶B) and water to solid (W∶S). The general properties (e.g., fresh density, flowability, initial setting time, bleeding, unconfined compressive strength, and microstructural analysis), and environmental impacts (e.g., corrosivity and heavy metals) were carried out on CLSM mixtures. Afterward, thermal conductivity was determined following a current specification for verification of the proposed CLSM, which is feasibly used as a thermal grout in geothermal systems (borehole heat exchangers). According to the results, all of the engineering properties and environmental impacts satisfied the specifications of CLSM. In addition, compared with the thermal conductivity values of conventional grouts, those of the proposed CLSM mixtures were higher. In conclusion, owing to good flowability and higher thermal conductivity than conventional grouts, the CLSMs made with cementless binder can be feasibly employed as a thermal grout for borehole heat exchangers.