[gjbingham]

Nope, I'm not that educated. dmun and Canuk Jim can give you lessons in mortar that will make your head hurt. Someone will chime in that knows the physkcal qualities and weaknesses of refractory mortar.

I wouldn't, for the time being expose Heat Stop 50 or the homemade lime/portland cement/water/fire clay mix to prolonged rain. Refmix - no idea.