Since refractory castable cement is so expensive I was wondering if there is any additives I could use along with the refractory for my fire box lining. I read about people using fire clay or other cements mixed into it but I don't know how that would hold up. Basically I'm trying to get 10 pounds of stuff out of a 5 pound bag. Just trying to strech 50 pounds of refractory as far as possible with cheaper add-ins. Could anyone help me out here?