When building your pizza oven base hearth slab, you will have a section to place your Vermiculite (or Perlite) insulation mix. This mix consists of 5 parts Vermiculite to 1 part Portland Cement.
The purpose of this mix to provide a layer of insulation below the firebrick cooking surface. By having insulation under the firebrick, your oven will get hotter faster, stay hotter longer and the cooking surface will be much hotter than surfaces without insulation.