Perl-crete volume question

[danjmath]

I have 4 cubic feet of expanded perlite left over from gardening, and am wondering how much volume of p-crete that will make, assuming 4-1 or 5-1 perlite/portland mixture(even a rough estimate would be nice). I had planned on just using FB board(or other CaSi), and will probably still use at least one layer.
Is it worth making a batch of p-crete with what I have, or should I just save it for gardening in years to come?
Thanks.

[Toomulla]

I would expect a smaller amount possibly 3.5 in general a mixed concrete occupies less volume then the dry ingredients

[david s]

Yes that's right. Once you mix cement and water with it you lose around 20% of the volume of the dry vermiculite.