Fine perlite verse loose fill for perl-crete mix?

[lplough]

Hi,
I am looking to make the insulation under the bed of my oven out of perl-crete (refractory cement and perlite). I happen to have some of the industrial concrete perlite mix (ryolex) but its the fairly fine grade....(like fake plastic snow--like panko bread crums or slightly smaller)--

Can I use this or should I use a more granulated size(similar to horticultural grade perlite/vermiculate)?

Ive read that ratio of 5:1 perlite:cement is workable. ANy thoughts?

Thanks,
Louis