Furnace floor insulation

[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]

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]

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 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]

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 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]

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]

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.

[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.

[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]

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