Furnace floor insulation

[bencuch]

Can vermiculite be used in the production of concrete for floor insulation instead of Portland cement - refractory concrete?

[david s]

Portland cement does not handle heat as well as refractory cement. Vermiculite (or perlite) is mixed with cement to create an insulating concrete called vermicrete (or perlcrete) Refractory cement is very expensive and unnecessary for an under floor insulation slab IMO as the temperature drops off really quickly from the surface exposed to the heat as the heat travels deeper. This means that there’s little degradation of the material evidenced by its integrity if an oven is deconstructed. Perhaps a thin layer of vermicrete of around 1” made with refractory cement over a standard vermicrete is a possibility, but you’d need to use a fine grade of vermiculite to layer a thin mix. Personally I don’t think it’s worth the extra trouble, even though I have various grades of both perlite and vermiculite as well as both Portland and refractory cement.

[bencuch]

Hello,
Do you think that the vermiculite grain size of 2-8 mm is sufficient for the production of vermiculite concrete?
Or is it necessary to have a larger grain size.

[bencuch]

Would you use vermiculite or perlite when making insulating concrete?
Which is better in terms of strength and insulation while maintaining the same mixing ratio.

[david s]

While loose perlite is a better insulator than loose vermiculite, as soon as you add some cement to it, even a lean brew, you are adding more mass from the cement than is contained by the aggregate. In addition the water added of which some is used up in the hydration process as it reacts with the cement also adds additional mass. This means that the insulation value for a perlcrete mix is effectively the same as a vermicrete mix. I find that vermiculite is friendlier to use because it doesn’t have the fine dust like perlite that is irritating to inhale (but not toxic). It also seems to pick up the cement slightly better than perlite. I now use a medium grade perlite and a fine grade vermiculite in equal proportions as the variation of grain sizes makes a better mix (but requires more water).

A 2-8mm vermiculite is pretty coarse, but for a 5:1 underfloor mix is quite suitable as an insulating slab under a pizza oven floor. If you are making an insulating slab for a furnace, its service operating temperature will be an important factor. If it's over 800C you'd be better to use calcium silicate board between the vermicrete slab and the floor bricks. Also, if it's a furnace are the floor bricks dense or insulating firebricks? Both vermiculite and perlite are only suitable for temperatures under 1000C, They both begin to degrade at temperatures north of that. So ok for pizza ovens but not the hot face of a kiln or furnace.

[bencuch]

I have already glued an insulating board - SUPERIZOL (5 cm) on the supporting board.
Next, I want to put insulating concrete (vermiculite concrete or perlite concrete) 5 cm and then brick the pizza oven.
Instead of Portland cement, I want to use refractory concrete for the insulation board.
I just have to decide whether to use vermiculite or perlite for the insulating board. (What would you use?)

[david s]

I have already glued an insulating board - SUPERIZOL (5 cm) on the supporting board.
Next, I want to put insulating concrete (vermiculite concrete or perlite concrete) 5 cm and then brick the pizza oven.
Instead of Portland cement, I want to use refractory concrete for the insulation board.
I just have to decide whether to use vermiculite or perlite for the insulating board. (What would you use?)

Ok, so it’s not a furnace, but an oven. Refractory concrete is an aggregate (sand, grog vermiculite,perlite or quarried stone) mixed with refractory cement. Do not use any heavy aggregate in the mix if you want to make it insulating. Generally a 5:1 vermiculite or perlite: cement is used for oven underfloor insulation. As previously explained refractory cement for this insulating slab for an oven is expensive and overkill. Standard Portland cement is sufficient. If it were mine I would have placed the vermicrete on the bottom and the calcium silicate board on the top, but as you’ve already glued it down just lay up the vermicrete slab on top of the calcium silicate board. Give it a minimum of a week to dry before laying the floor bricks over it.

[bencuch]

I will come back to refractory concrete.
- I already have it at home, so if I use it on vermiculite concrete instead of Portland cement, it will be OK

Well thank you.

[david s]

Be careful. If what you have is refractory concrete or refractory mortar, it is a product that already has heavy aggregate mixed with refractory cement. You need to use pure refractory cement (CAC calcium aluminate cement) only. Then mix that with your vermiculite or perlite.

[bencuch]

I have refractory concrete, so I prefer to use portland cement.
It is better to use Portland cement with a strength of 42.5 MPa or 32.5 MPa.

[david s]

The strength of concrete is largely determined by how much cement is added. Just add (by volume) one part of cement to every 5 parts of vermiculite or perlite.
The attached table explains the cement to strength relationship.The more cement you add the stronger it becomes, but also the less insulating. A 5:1 mix is the recommendation for underfloor insulation, but over the dome can be a lot leaner because it's not weight bearing.

[bencuch]

It is necessary to put an aluminum foil on the SUPERIZOL insulation board before pouring the vermiculite concrete (see the picture below from youtube).

I don't think it's important, but I want to get some advice.

Well thank you.

[SableSprings]

As David S recommended in an earlier response, you'd do yourself and your oven a favor by putting your insulated cement mix on your hearth slab and then laying the ceramic board on top. Perlcrete or vermicrete contains a lot of water initially and it takes a long time to drive out. That water needs a path to exit, which is why we recommend weep holes for drainage and mosaic (or broken tiles) between the insulating cement (IC) & the hearth to provide pathways/channels to the weep holes.Besides, the ceramic board is a much better insulation so it makes sense to have it directly under your cooking floor bricks.

Also, by using the weep hole/tile layer on top of the slab, you'll have an effectve barrier fron water that may seep in from the oven's perimeter if water gets onto the top of the slab (rain & sprinklers are common culprits). Putting IC on top of the board means that water will drain down into the ceramic and if you put a layer of foil between them, you'll trap the moisture & make it even more difficult to drive out during your drying/curing fires.

Hope that helps explain this preferred method of oven base design & construction.

[david s]

As you’ve already glued down the insulating board, you’d not want to pull it up, so swapping layers is out. A better solution would be to simply place a second layer of board over the first. Being already dry, it won’t create any issues with having to allow it to dry.

[bencuch]

That's why I want to put vermiculite concrete on SUPERIZOL to make it a solid layer under the fireclay floor.
I think vermiculite concrete is stronger than SUPERIZOL.

If I were to leave vermiculite concrete over SUPERIZOL, aluminum foil is probably needed between SUPERIZOL and vermiculite concrete.

That's why I think so, because the fireclay brick will be firmly glued to the vermiculite concrete, and during expansion - thermal expansion and contraction, the vermiculite concrete will also expand, and it would not be good if the vermiculite concrete was also firmly glued to the SUPERIZOL.

If I were to replace the SUPERIZOL and the vermiculite concrete as you write, all the layers can stick firmly together.

I appreciate all your advice.

well thank you