whatever you do, DONT use portland, best thing to use is fireclay, most building supply places have it,
i have crushed up firebricks and used it as grog for refractory, what i used was grog(crushed firebricks) fireclay and silica sand, i cant remember the amounts of each, but mostly grog
bentonite works but from what i have been told it wont take the temps that real fireclay will, and over time will fall apart
Ron
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