Thermal conductivity of steelmaking slag-based controlled low-strength materials over entire range of degree of saturation: A study for ground source heat pump systems

Abstract

Controlled low-strength materials (CLSMs) with high workability and relatively high thermal conductivity can function as a thermal grout in the heat exchanger of ground source heat pump systems. The degree of saturation of CLSM strongly influences its thermal conductivity and thus affects the heat transfer capacity of the heat exchanger system. This study investigates the degree of saturation–thermal conductivity relationship of steelmaking slag-based CLSM using two sample preparation methods: a modified high-pressure membrane extractor coupled with a thermal needle probe (HP) and air-dried (AD) methods. The effect of grinding steelmaking slag on the mechanical properties and thermal conductivity of CLSM is also evaluated. The engineering properties test results indicate that ground steelmaking slag presents a positive effect on the compressive strength and thermal conductivity of CLSMs owing to the reduction of the inherent porosity and increased specific surface area of the raw steelmaking slag by the grinding. The degree of saturation and thermal conductivity have a linear relationship for both the AD and HP test methods. However, it was found that the AD method overestimates the thermal conductivity owing to the uneven distribution of the water content in the specimen. Finally, predictive equations are proposed to estimate the thermal conductivity of CLSM for the entire range of degree of saturation. Compared to the recently proposed equation, these equations are more appropriate for thermal conductivity estimation of grouting materials based CLSM, especially at low water content.