Sorry for asking a question maybe all ready answered.I am building a dome oven ,i am going to use a ceramic fibre blankit around the outside of the dome, than will lay 3 inches of perlite on top.Qustion is can i mix the perlite with portland cement for this stage. Thansks in advance.
Hello Malcolm, and welcome to the BrickWood forum.
I understand your plan as far as it you have it laid out. You want to add a layer of Perlite to the outer shell of your dome, that would act similarly to the layer for which you have cast your refractory slab.
If you choose to insulate your oven this way, here are my thoughts:
You can add an additional layer of Perlite/Portland cement mix over the dome and fiber blanket, provided you follow the instructions to apply a “chicken” wire matrix over the mineral fiber blanket. If you do not do this, your shell will break up and moisture will infiltrate into the fiber blanket.
My gut says you also still need a stucco layer over this if you choose to do this. You will need to treat the Perlite/Portland cement surface with a suitable adhesive. You will also need to waterproof the stucco layer with outdoor paint, or apply a veneer to the stucco surface to finish your oven.
But my third thought is whether adding the Perlite/Portland cement layer is needed at all. The mineral fiber blanket is highly efficient at insulating the dome all by itself, combined with the refractory material underneath. You have already used Perlite in your slab, and that works fine because the slab is providing a thermal break to stop heat from conducting down into cold concrete. But the oven above is dealing with three forms of heat transfer: convection, refraction, and conduction. The refractory materials take care of the first two, and the mineral fiber effectively prevents heat energy from escaping the refractory shell. In my opinion you don’t need anything on the other side of that mineral fiber, and you risk compressing it which will make it less efficient.
I hope all of this helps, and I wish you the best of luck in building your oven! Please share more photos when you are done.
Sorry if i posted the images in the wrong place i am not a technology guru.
Your photos and construction plan look great! Thank you for these.
Looking at your build, I would just say again that you should not add a Perlite/Portland cement layer on top of the mineral fiber blanket. Feel free to look at our advice for adding the blanket, covering it with a “chicken wire” matrix (critical step!), and then sealing it with at least two layers of mortar. This will provide an insulated shell that will give your dome all the thermal insulation and protection it needs.
Good luck with your build, Malcolm!