the invention relates to a powder composition based on a calcium / magnésium compound having formula , xcaa . mgo ], in which a is a group ═ 2 or ═ co 3 and x and y are mole fractions where 0 & lt ; x = 1 and 0 = y = 1 , containing an amount of mineral solid flow agent that is less than 5 wt .-% of the composition , said agent being selected from the group containing vermiculite , perlite , diatomaceous earth and silica , in the form of particles having a size greater than 90 μm .