Thermal conduction mechanism based on microstructural transformations of molten slag: The role of calcium oxide

•A rNEMD simulation and its experimental validation for thermal conductivity of were conducted for molten slag.•The existence of NBOs enhances the anharmonicity and shortens the interatomic distance to reduce phonon mean free path.•An equivalent molar concentration of the Q4 tetrahedron (XQ4) is defined.•The approximate positive linear relationships between the thermal conductivities and XQ4 are obtained.