Geopolymer (GP) foam as a fire protective coating was synthesized, deposited on a steel plate, hardened and evaluated using a burn-through test at a reduced scale. It was shown that the GP foam acts as an efficient fire barrier (with 250°C reduction compared to virgin steel evaluated in the same conditions). A numerical model using Comsol Multiphysics® (finite element code) was performed to simulate the fire behavior of the GP foam. It was based on the complete characterization of the GP foam to provide accurate input data for the model. The latter captures well the temperature rise, including the endothermal effect due to water vaporization. A parametric study of the porosity and the emissivity at the surface of the GP foam brings new insights to optimize the performance of the GP foam. It is shown that a porosity of 90% and an emissivity lower than 0.75 should provide the highest performance to GP foam. The fabrication of an optimized GP foam is feasible using a technology of low emissivity thin coating and by adjusting the synthesis of the GP foam to increase its porosity.