portland will degrade too, it fluxes when it gets up that hot and will crumble, i have never used portland in any of my foundry furnaces or forges, but i have seen a few that have and they never lasted
the portland has nothing to do with spalling, concrete on the other hand will, but it cant take the temps that an oil burner will get up to without crumbling
there is a lot of info on this very subject at the BYMC forum, a lot about perlite as well
Ron
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