I built this same forge but I used a different mix for the adobe. I don't remember it exactly but you can find it on backyard metal casting. Sand, clay, portland cement and perlite. I even made bricks out of this mix and sit them on top of the forge when I need to melt small quantities of al for casting. You will be surprised how light the bricks are and the fire does not seem to touch them.
I use homemade charcoal in mine but have used lump and briquettes. Lump seems to work best.