Refractory Tiles instead of firebricks for oven floor

[MarioT]

I am starting a new build in a small town. We are on a budget so we will be using standard red bricks for the dome. Yet for the cooking floor we bought refractory tiles which are essentially firebricks measuring only 20mm thickness instead of the standard 63mm thickness of firebricks. Of course 20mm thickness will not be enough thermal mass so we should add some thermal mass. I am thinking in two possible options: making a layer of ordinary red bricks and topping it with the refractory tiles or sitting the tiles on top of a thick layer of sand (around 40mm thick). I have read that silica sand would be a better choice compared to ordinary construction sand but, again, silica sand seems to be somewaht costly and it is very easy to source.
Any comments will be greatly appreciated.

[UtahBeehiver]

What are you wanting to do with your oven? Cooking pizzas for a party or doing multi-day cooking or baking? Floor and dome insulation play a key role if oven performance so what are you plans on this?

[MarioT]

Thank you for your answer. The intended use is mostly single day pizzas, not multi day baking. I am planning to isolate the hearth by using a 3-4 inch of vermiculite concrete. Dome will be insulated by using a 1 inch layer of ceramic blanket and on top of it a 2-3 inch layer of vermiculite concrete (5:1 with Portland Cement). Finally some render.

[UtahBeehiver]

For the dome insulation, I would go with a leaner mix for better insulation value, 8 to 10 to 1. You do not need the structural strength on the dome like you do under the floor. Back to you original question, IMHO, you would be better off with bricks under tiles rather than sand under tiles. This would provide a better platform for the tiles to sit and also give you better thermal mass. You need 5 to 1 vcrete under the floor bricks/tiles.

[MarioT]

Thank you very much, Russell. Vermiculite is not cheap down here so I am trying to make an accurate calculation of quantities. I plan to place the vermiculite concrete layer only under the dome circle, entry and landing, so I basically calculated the volume of a 51" diameter and 4 inch high cilindric slab (plus entry and landing). Not sure about shrinkage of the vermiculite concrete after curing. Of course will only calculate the volume contribution of Vermiculite and Portland cement since I assume the water will go. As for the dome, since I will be using a first layer of 1" blanket, do you think an additional 2" layer of 8:1 vermiculite will do?
Thank you.

[UtahBeehiver]

Have you looked for perlite instead of vermiculite, a quick google search showed a manufacturer in BA. 2" v or p crete over 1" ceramic blanket would be bare minimum. Pcrete works similar to Vcrete.

[david s]

When calculating the volume required only use the volume of the vermiculite or perlite. Do not add the volume of the cement as that is taken up by the spaces in between the lightweight aggregate. In fact you have to add around 20% more because there is a reduction in volume when you add the water and cement and start mixing. Better to mix in a barrow than a mixer because there will be even further volume reduction from the abrasion of the mixing drum and blades. Many folk find they need to rush off and buy more vermiculite because of the failure to account for this volume reduction.

[MarioT]

Rusell, David, thank you for your valuable help. I directed my search towards vermiculite over perlite since I read it is more complicated to handle and "stick" to the walls, and most of the builders seem to favour vermiculite over perlite. Around here both seem to be available as substrates for plants and not very easy to find in large quantities at a decent price. Will have a look at it. Any tips on using perlite concrete?

[david s]

Perlite is a slightly better insulator although once you start adding cement to it that advantage disappears quickly. The very fine dust that accompanies it is an irritant to breathe, vermiculite does not have the same characteristic. I find a mix of 50/50 vermiculite, perlite makes a better mix than either of them alone, but it’s only when making a really lean mix, weaker than 8:1, that workability is a problem. The correct water addition is probably the most important factor so mix dry ingredients first then slowly add by volume, 3 parts water for every 10 parts vermiculite or perlite. A handful of powdered clay to every litre of cement powder assists greatly to making a lean mix more workable.

[MarioT]

Thank you both for the great help. I have actually acquired some perlite and vermiculite and will be drawing the form in which I will pour the perlite/vermiculite. I am slightly lost in dimensions though, I will be building a 42 inch internal diameter igloo which basically is a 51 inch exterior diameter (before insulation). I cannot find neither in the guide nor the forum the lenght of the vent/arch section. Can you please point to some drawings or dimensions?