I think I read on that Ask the Builder guy site once that concrete mix
should be used for a chimney cap. Then I think I saw on this NG where folks
have successfully used mortar mix.

If so, how long has it been withstanding the test of time? I ask because I
have a half a bag of type-s mix.

I assume when you put it on you slope it slightly away from the flue vs
flat.

Thanks,
Al...