Understanding the mechanism for initial deposition under the coupling effect of sodium and calcium during boiler operation: The first-principles study and thermodynamic analysis

•The mechanism for initial deposition is investigated by the first-principles study.•Calcium interacts more efficiently with Fe3O4(001) surface than sodium.•The thermodynamic properties of sodium and calcium on Fe3O4(001) are calculated.•The coupling of sodium and calcium can promote the deposition on Fe3O4(001).