Chimney top needs refinished

I have a chimney with what looks like sandy mortar top. (the very top, on top of the last layer of bricks over the roof) This doesn't look like it's going to stop water very well, and I'd like to paint it with something to prevent water from seeping through.
What can I use that isn't flammable... like a non-flammable roofing tar or something?
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