Thermal conductivity and pore structure analysis of alkali-activated foam cement with supercritical CO2 modified slag: Feasibility evaluation for geothermal applications

•Foam cement with AAMs modified by CO2 can be used as energy pile filling materials.•CO2 modified AAMs decrease dry density and increase water adsorption of foam cement.•CO2 modified AAMs increase the thermal conductivity of foam cement. Geothermal energy is a clean and renewable energy that can be extracted by energy piles. Low density filling materials with relatively high thermal conductivity are ideal for energy pile construction, due to the low cost and high heat exchange efficiency of filling materials. In this study, three types of low density filling materials (Class G foam cement, alkali-activated foam cement, and supercritical CO2 (ScCO2) modified alkali-activated foam cement) were tested to evaluate their performances to serve as candidate filling materials for energy piles. The thermal conductivities of the three materials were measured and showed that the minimum thermal conductivity was 0.22 W·mK−1 (foam cement without slag), and the maximum thermal conductivity was 0.32 W·mK−1 (foam cement with 5 % CO2 modified slag). By analyzing the cement hydration products and distribution of pores, adding slag improved the hydration of cement, which caused a longer foaming time and increased the porosity of samples. ScCO2 modification of slag increased the porosity and the thermal conductivities of the samples. In summary, slag with ScCO2 modification can promote the thermal conductivity of foam cement, which is beneficial to the application of shallow energy piles.