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Just to clarify (from post 12), when I said slab, I meant the base foundation of the entire entertainment area...not the stand and top oven supporting slab (called the hearth by most folks). I had my ~16' x 16' (14.7 m square) slab forms set then concrete poured, and finished professionally. Once that was cured, I dry stacked the concrete blocks on to form the oven stand and then built the top hearth form for the hearth slab. I tried mixing bags of cement myself, but the shear volume and time involved was beyond me...so I hired a cement truck to come out and deliver/pour into my top hearth form. (Well worth it to me! 
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As David noted, you want to create a moisture/water barrier between the hearth concrete and your ceramic board. Also you'd like to incorporate some form of drainage into the top hearth slab (in case of water seeping under the oven perimeter or down through the structure). You can (1) slightly mound the area under the oven site - so water drains away to the edges of the hearth, (2) set some plastic pipe into your hearth pour or drill an existing hearth to provide weep holes, (3) use mosaic or porcelain tile sheets to lift/separate the ceramic board from the concrete of the hearth slab, or (4) lay Foamglas between the hearth and ceramic board. Water barriers provided by painting on membrane compounds are also effective, but you need to provide the weep holes so the membrane doesn't trap and hold moisture from above.
)As David noted, you want to create a moisture/water barrier between the hearth concrete and your ceramic board. Also you'd like to incorporate some form of drainage into the top hearth slab (in case of water seeping under the oven perimeter or down through the structure). You can (1) slightly mound the area under the oven site - so water drains away to the edges of the hearth, (2) set some plastic pipe into your hearth pour or drill an existing hearth to provide weep holes, (3) use mosaic or porcelain tile sheets to lift/separate the ceramic board from the concrete of the hearth slab, or (4) lay Foamglas between the hearth and ceramic board. Water barriers provided by painting on membrane compounds are also effective, but you need to provide the weep holes so the membrane doesn't trap and hold moisture from above.
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