Castiel said it's an explosive powder.

[ She glances over at him for confirmation. He's the one who could actually tell what it was, and she's just kept it safe, and dry as possible. ]

I think you just need to ignite it.

But if you're holding back those vines to keep the webbing open, then destroying it should stall the web reforming longer... theoretically. So you can make a run for it.