Existing Concrete Hearth Slab


I already have an existing 60" x 72" reinforced concrete hearth slab. Is there any issue with simply making an all perlite/cement 47" x 60" monolithic slab on top of that? My oven would then be built on top of that layer. I think making a single all perlite/cement slab will be easier to make than three panels, but would like to learn about potential issues if any.

Thanks very much.