This research investigates the fire resistance of novel geopolymer (GP) foams, based on alkali-activated metakaolin and silica fume (SF). Fresh GP foams are applied as coatings on steel plates. After one week curing, the foams are subjected to a flame burn-through test. Changes in their physico-chemical properties are characterized before and after fire test, mainly with XRD, quantitative MAS NMR, electron probe micro-analysis, quantitative X ray micro-computed tomography and heat conductivity. Results show that GP foams are excellent thermal barriers, providing up to 251 °C less than for uncoated steel plate. Their porosity ranges between 25 and 81%, for typical pore sizes d50 from 0.5 to 3.0 mm 29Si MAS NMR shows that the proportion of GP cement only decreases from 60-68% to 53–58% after fire. SF expands and creates small pores in the coating, which is favorable to decrease heat conductivity by a factor of 2 whatever the foam.