I am about to start my pizza oven and I’ve noticed that there are some ovens on your site that people have used one concrete form set up on top of the oven as opposed to using three separate forms on the ground that allow for a small space in the middle for the “insulated” mixture of concrete.
Some ovens online have done what I’m planning on doing - and just put the hearth surface (cooking surface) firebricks directly on the concrete slab. What advantage is there to using the special mixture of concrete and vermiculite or/and whatever else there is in the mixture as opposed to putting the firebricks directly on the concrete?
The 3-slab process (our design) gives oven builders with little to zero construction experience the ability to make a perfect oven base slab w/out the hassle (or dangers) of making a pour-in-place single monolithic slab. Making a monolithic slab frame can be difficult for some - plus, you are going to put 800+ lbs of wet concrete in that frame. Better hope you got it braced correctly or your are going to have a big, big mess if it falls, breaks or begins to lean… And if it does break, you have a big mess that is going to turn in to a rock-hard big mess in no time.
Our simple-to-build 3-step base slab frames (that are all identical in size) allow you to build your base slab on a nice flat surface with little-to-zero possibility of failure. In addition, building the void for the insulating vermiculite / perlite layer is super-easy by gluing Styrofoam into each frame that you will remove once the 3 slabs have cured. Inserting this void into a single monolithic slab is kinda difficult…
To answer the 2nd part of your question - you MUST have that insulation layer in the slab! If you place your firebrick directly on the concrete slab, the slab will absorb the heat from those firebrick like a sponge! In addition, you don’t want the concrete slab to get too hot as concrete is unable to handle high-heat for extended bake times. The vermiculite is an inexpensive way to insulate the slab from that high-heat AND keep your hearth firebrick super-hot as you bake the perfect pizza!