This study aims to develop a thermally enhanced controlled low-strength material (CLSM) that functions as a heat transfer medium in the ground source heat pump (GSHP) system based on by-products. Steel-making slags (raw and ground slags) and fly ash were used as main materials of CLSM mixture. The general properties of the CLSM mixture, such as flowability, bleeding properties, initial setting time, unconfined compressive strength (UCS), permeability, thermal conductivity, and environmental impact, were investigated with different steel-making slag contents. Finally, the total construction costs of GSHP with different backfill materials were calculated to assess the benefits of the proposed CLSM mixtures. As the results, the steel-making slag based CLSM exhibited good flowability and bleeding and a significantly enhanced UCS and hydraulic resistance with respect to those of CLSM. However, the thermal conductivity, UCS, and hydraulic resistance tended to decrease when the raw steel-making slag exceeded a certain amount. Furthermore, all proposed CLSM mixtures satisfied the environmental regulations for their safe use in the underground. The cost analysis revealed that proposed CLSM mixtures slag can reduce the total construction costs of GSHP system by up to 41.36%, 0.73%, and 20.44% when compared to those of conventional materials. (C) 2020 Elsevier Ltd. All rights reserved.