Use fire bricks... those used for lining chimney fire boxes. If your
"slab" needs to be thinner than a regular size brick, saw the fire
bricks in half (or whatever size). Fire bricks are mated to one
another with a type of mud, for fire boxes... very thin coat of mud,
1/8" or less, so the fire bricks are, essentially, butted against one
These days, there may be a special concrete for that application, but
I don't know what specific kind.