Progress,  page 103
I placed clean sharp sand inside the space between the firebrick and the concrete block in layers
about four inches thick and tamped the each layer of sand.  I couldn't believe how much sand it
took ... almost a full cubic yard.
Once the sand was in place, I needed to make a concrete slab on top of it as a base to continue
the rest of the fireplace.  I used cement paver bricks to make a form around the damper.
Here the concrete platform has been poured and the damper installed over an expansion gasket
I made from heavy rock wool.