I'm building out of Perlite and Rapid Set Cement All.
I wet the Perlite,let it drain,then I mix the two ingredients at 1 part Rapid Set to 6 parts Perlite.
I stack square wooden forms around a round metal duct,mixing and packing as I go.
I have not fired these cores yet, but they do set up hard.
The Rapid Set Cement All is available at Home Depot at about $20.00 american for 50lbs.
I chose it because it is a Calcium Sulfoaluminate cement , with at least one msds document listing the decomposition tempature at 2460°,which is a little low for use in a rocket core,but time will tell,and you can't beat the price.
EDIT: 2190° is the expected highest tempature in the core.
The Perlite costs me $4.00 for 2 cubic feet at my locally owned feed n
seed nursery type place.
Perlite sinsters at 1598°,melts at 2300°.
I was going to substitute one or more part of the Perlite with fireclay,to assure strength at high temperatures,but around here it's hard to source and about as expensive as the Rapid Set.
Being mostly made of Perlite, the core are pretty light.
Of
course, insulative refractory bricks would work as well, but they are neither cheap or easy to come by.