High Heat Refractory Mortar for building the Dome

[pocahontius]

Hello,

in the next weeks I’ll start to build the dome of my wood fired oven. I’ve three questions, that remain:

Question 1: I’ve read a lot about the refractory mortar but the more I read, the more I get confused. I also asked three suppliers of mortar what is the right product, as the translation to German is not clearly. Could someone give me an advice, which is the best product to use for building the dome?

Supplier A:

I should use „Schamottemörtel“ mixed with 10 % portland cement at the inner joints where they are not larger than 3 mm. The „Schamottemörtel“ only sets ceramic at about 800 C, but with the 10% Portland cement it sets hydraulic at normal temperatures. The „Schamottemörtel“ has the following specs: Al2O3 (38 %) / SiO2 (2,3 %) / no Fe2O3 / no CaO. Temp: 1450 C.
(Google Translator: Schamottemörtel = fireclay mortar)

For the outer joints larger than 3 mm I should use „Feuerfeste Reparaturmasse“. It sets hydraulic at normal temperatures. The specs are: Al2O3 (50 %) / SiO2 (41 %) / Fe2O3 (1,8 %) / CaO (4,9 %). Temp: 1450 C.
(Google Translator: feuerfeste Reparaturmasse = refractory repair compound)

Supplier B:

I should use „Feuerzement hydraulisch“ which sets hydraulic at normal temperatures as the only mortar. The specs are: Al2O3 (48 % - min 43 %) / Fe2O3 (3,6 % - max 4 %) / SiO2 (40 % - min 38 %) / CaO (9 % - max 11 %). Temp: 1350 C.
(Google Translator: Feuerzement hydraulisch - fire cement hydraulic).

Supplier C:

I should use „Schamottemörtel Monolith E“ which sets hydraulic at normal temperatures and ceramic at about 800 C as the only mortar. The specs are: Al2O3 (35 %) / the supplier does not know the other ingredients. Temp: 1250 C.
(Google Translator: Schamottemörtel Monolith E = fireclay mortar Monolith E)

Homebrew:

Or is it better to use the homebrew recipe? I’m very unsure there about the 3:1:1:1 mixture. Is it volume or weight based? What lime to use? What exactly is fireclay? In Germany firebricks are called „Schamottesteine“ and there is a product called „Schamottemehl“ which seems to be the powder of these bricks…

Question 2: Supplier A stated that I should should use a product called Ölpapier (= oilpaper) in Germany. I should use it as a separating layer between the ceramic blanket and the vermiculite/Portland cement layer in my build. It should protect the blanket from the humidity of the cement. Does that make sense or is it a wrong information?

Question 3: I read somewhere that I do not have to mortar the first circle of bricks of the dome to the calcium silicate board. Only brick to brick (left-right). The second circle and the following circles are then mortared to the one below. Is that right? I can’t find that information again…

Thanks for your help! It’s so confusing :-)
Jens

[david s]

The refractory mortars from all those suppliers contains materials designed to withstand temperatures that your oven will never see. They also recommend a maximum joint thickness that is far smaller than you will require in your build. The homebrew has proved time and again to be the most suitable and cost effective mortar for building ovens.
The recipe is 3:1:1:1 by volume not weight of sand, Portland cement, clay, lime. The sand type doesn’t matter much but needs to be fine enough to be able to create thin mortar joints. Bricklayers sand is good. Standard Portland cement. Any powdered clay should be ok, but avoid Bentonite as it’s so fine it has excessive shrinkage. Try to obtain Bricklayers clay, If you can’t get that go for Ball clay from a pottery supplier. The lime should be hydrated lime obtained from building suppliers.
Provided your fire bricks are an even thickness they can be laid dry on the cal sil insulation. Because it is so absorbent it is extremely difficult to lay mortar on it. You could seal it first to overcome this problem but the usual method is to lay the first course dry.

[pocahontius]

Hello David,

thanks a lot, that helped me! But I'm still unsure about the clay. In Germany we have "Ton" (Google translates it to clay) and "Lehm" (Google translates it to clay and loam).

I found "Tonerde" which seems to be the name for pure aluminum oxide.
And I find "Lehmpulver".

Perhaps all these things are more or less the same, I'm not sure about that. I cannot find bricklayers clay in our masonry supplies here.... Perhaps someone who speaks German can give me a hint here?

Is this clay powder the same I should use under the cooking floor ih the CaSil board is not laying flat?

Greetings Jens

[Gulf]

Hi Jens,

Welcome to the forum. I'm not familiar with the German language and the multiple terms for clay are confusing to me also. You may want to go to or search the website of a masionry supplier or a brick yard. Here in the states it is called brick layers clay or fireclay. It is usually sold in 50 lb bags. I'm not sure what the metric equivelant would be.

[Yokosuka dweller]

Hello David,

And I find "Lehmpulver".


Greetings Jens

Hi Jens, I used to live in Germany many years ago, so I can understand. Lehmpulver as far as I can imagine is just dry powdered clay. As DavidS recommended that unless it is bentonite clay, this powder should be good to use as mix in the homebrew. Chamottenerde would just be another aggregate like sand, so if you'd use that you would have to reduce the amount to sand used in the mix; cannot be used instead of clay.

As for lime, it can be really tricky, but try to read up on hydrated and hydraulic lime. It is supposedly common in Europe since it was used historically for building in the past and has had a renaissance due to it being more environment friendly than cement. Should be possible to find it. Do not use lime which is used for fertilizer though.