Abstract: Improved sprayable gypsum-based fireproofing composition comprising a lightweight aggregate and a set retarding agent in an amount effective to delay the setting of said composition for at least 24 hours and preferably for at least 72-96 hours without causing the composition to swell or shrink or to cause a salt layer to form on the composition when it is spray-applied. The fireproofing composition can retain fluidity in conventional spray hoses overnight and yet is capable of being injected with a set accelerator, such as aluminum sulfate, on the next day or on a successive day when spray-applied onto a substrate.