Hi Craig, this looks like efflorescence, water-borne salts. This occurs when the water evaporates and the salt is left behind. Remove the salt with a stiff, dry brush when conditions are warm and dry.

What you have is called efflorescence and typically a muriatic acid solution (diluted with water) from what I recall.

It is efflorescense or the leaching of salts, mostly calcium hydroxide, from the concrete.

On something like a firepit, I wouldn't bother trying to remove. Otherwise there are prepared effloresence removers. Or you can try muriatic acid or 20% acetic acid.

Thanks guys!