The role of powder physicochemical properties on the extinction performance of an extinguishing powder for sodium fires

•The composition of sodium fire extinguishing powders changes with storage conditions.•The efficiency of powders of different compositions are tested in dedicated facility.•Higher amounts of crystallization water in powders permit efficient fire extinction.•The ageing is found to be beneficial to the extinction of the sodium fire.•A mechanism for the sodium fire extinction with this powder is proposed.