the oxidizing gas sufficient heat to melt at least the surfaces of the refractory particles so that the refractory particles and at least some of the at least one refractory oxide bond together and particles of porosity inducing material comprised of at least one of hollow and porous particles which become incorporated in the refractory mass formed on projection and which have at least one of a composition and a particle size selected so that incorporation of the porosity inducing material in the powder mixture results in the formation of porosity within the refractory mass formed on projection application_number US 74882591 A description various preferred embodiments of the invention will now be described by way of example an internal insulating wall must be repaired in a cracking furnace in the petrochemical industry which has suffered relatively extensive damage without having to stop the plant this wall consists of silico aluminous insulating bricks which have the following composition 47 sio 2 38 alumina 15 lime the bricks have a relative bulk density of 0 77 the repair consists in forming a refractory mass on the damaged parts of the wall to do this oxygen is projected onto this wall together with a mixture of refractory particles of finely divided particles of at least one element capable of forming a refractory oxide when being oxidized in an exothermic manner and of hollow particles in this embodiment of the invention the said hollow particles are borosilicate glass microbubbles with a diameter of the order of 25 u03bcm to 125 u03bcm and which have an apparent density of 0 19 g cm 3 bulk density of the microbubbles measured according.