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• Addition of MgAlCO 3 -LDHs and CaAlCO 3 -LDHs improved thermal insulation of IFR coating. • Metal ions from LDHs were chelated with carbon materials to form 3D reticular char. • LDHs can delay oxidation and improve degradation performance of char layers. MgAlCO 3 -layered double hydroxides (LDHs) and CaAlCO 3 -LDHs were respectively added as a synergistic agent to intumescent fire retardant (IFR) coatings. The 2.4 wt% of MgAlCO 3 -LDHs and the 6.7 wt% of CaAlCO 3 -LDHs were found to possess the best thermal insulation according to the fire performance tests. Further characterization by a cone calorimeter shows that the MgAlCO 3 -LDHs are superior to decreasing the specific extinction area of IFR coating, and the CaAlCO 3 -LDHs are advantageous in some other properties such as heat release rate, effective heat combustion and total heat release. Moreover, the X-ray diffraction (XRD) results infer that the flame retardant of char layer is primarily dependent on the amorphous structure formed after exposure to fire, the X-ray photoelectron spectroscopy (XPS) measurements of O 1s , C 1s , P 2p and N 1s reveal that the LDHs fillers suppress the oxidation of the IFR coating, and the Fourier transform infrared spectroscopy (FTIR) tests demonstrate that the LDHs protect the bridge frame structures. It is concluded that the LDHs can prevent the degradation and improve the thermal insulation of IFR coatings. [ABSTRACT FROM AUTHOR]