Mixing refractory concrete

[Fox junior]

Hi I have just joined the forum to ask some questions.
I have already read about casting pizza ovens and about the home brew mix but I would like to use fondue cement mix in a mold that I am going to build.
I just want to cast the entry bit and the oven floor (in four pieces) not the dome, I already have a 32” pre cast dome.
I have read many negative responses to previous questions about this subject on this forum, however I am a shuttering carpenter and I feel I can both build the mold and cast the component, I also have a vibrating table and a vibrating poker to hand.
My question is …. I have easy access to Fondue hi alumina cement but, I dont know what to add to the cement to make a good suitable mix?
Can I just use corse sand at 4-1?

[Gulf]

Hi Fox junior, welcome to the forum. I have no firsthand experience with Fondue. Over the years, I have followed discussions of those that did use it. My take from reading about it on the forum is that most folks opted for firebrick for the oven floor. Here is a search of the forum using the key word "fondue". It brings up several old discussions as well as this current one..

[david s]

Hi I have just joined the forum to ask some questions.
I have already read about casting pizza ovens and about the home brew mix but I would like to use fondue cement mix in a mold that I am going to build.
I just want to cast the entry bit and the oven floor (in four pieces) not the dome, I already have a 32” pre cast dome.
I have read many negative responses to previous questions about this subject on this forum, however I am a shuttering carpenter and I feel I can both build the mold and cast the component, I also have a vibrating table and a vibrating poker to hand.
My question is …. I have easy access to Fondue hi alumina cement but, I dont know what to add to the cement to make a good suitable mix?
Can I just use corse sand at 4-1?

Just to establish the correct terms to reduce confusion, especially when researching, "fondue" is a dish that uses various foods dipped into melted cheese. "Ciment Fondu" is the name of a Frech brand of Calcium Aluminate cement. It is mixed with an aggregate depending on its application. For the lining of chimneys a mix of cracker dust (crushed granite) is used in the proportion of 5:1. For high temperature stuff like kilns that see service temps up to 1300C, crushed fire brick (sieved so grain size is no greater than 3mm, discard fine dust) to in the proportions of 4-5:1 For a WFO the service temp is up to 450C use either grog or silica sand 4:1 Do not use it in conjunction with hydrated lime because it will act as an accelerant to the already short working time of the calcium aluminate cement.
Cast floor sections are likely to crack, particularly if the castings are large and unfired. If you have the facility to get them kiln fired to at least 1100C this will increase their density as well as their durability. Fired firebrick is a better choice, being both more durable and denser.The bricks are generally laid loose so replacement is easier and expansion of individual tiles can take place. Also the fire directly on an oven floor is rather uneven, typically way hotter in the centre. This leads to both uneven temperature rise and in turn uneven expansion causing stress.

[Fox junior]

OK thank you for your replies, yes Fondu (auto spell check) we use it to accelerate concrete set times and seal large gaps or holes before a big pour.
Unfortunately the Fondu comes in re packed plain bags with no instructions, we use basalt fibres all the time but I have never seen basalt aggregate.
Yes I can see why bricks would be a better choice, I will try casting a 10” central circle and four surrounding cake slice shaped bits.
I guess there is a reason to use such small aggregate with the fondu, we have 3mm sharp sand to hand so I will try that at 5-1.
Thank you.

[david s]

I have never seen basalt aggregate.
.

My apologies, cracker dust is granite not basalt. It is the waste fines, from dust to about 3mm, when the aggregate is crushed. Because it is generally waste, it’s pretty cheap.

Here is a useful link on CAC for use with refractory applications.

https://www.calucem.com/wp-content/u...plications.pdf

[Fox junior]

Thanks you for the link (the second one seems to relate to something political) it would appear that crushed fire brick would be a good option rather than sand.
I dont have much experience with high temp castings but I see the link mentions 4-1 vermiculite mix, although we do sometimes use vermiculite dust in expansion floor joints. I have never though about it being fire proof.
What I have bought is a pre cast 32” dome, it came in 5 parts from an eBay seller and is said to be a high quality, dome made from high temperature cement.
This is not a new dome direct from a manufacturer but it has not be used, just kept outside for two years as the seller said he has not the time to build it.
My main issue is,is does not have the entrance bit, I have looked at many pictures and I think I can easily build something similar.
I will look into buying the proper fire bricks for the floor and finding some fire brick dust to mix with the fondu.
After looking around the forum a bit, I can see there is quite a lot of effort goes into building a good oven!

[david s]

Thanks you for the link (the second one seems to relate to something political) it would appear that crushed fire brick would be a good option rather than sand.
I dont have much experience with high temp castings but I see the link mentions 4-1 vermiculite mix, although we do sometimes use vermiculite dust in expansion floor joints. I have never though about it being fire proof.
What I have bought is a pre cast 32” dome, it came in 5 parts from an eBay seller and is said to be a high quality, dome made from high temperature cement.
This is not a new dome direct from a manufacturer but it has not be used, just kept outside for two years as the seller said he has not the time to build it.
My main issue is,is does not have the entrance bit, I have looked at many pictures and I think I can easily build something similar.
I will look into buying the proper fire bricks for the floor and finding some fire brick dust to mix with the fondu.
After looking around the forum a bit, I can see there is quite a lot of effort goes into building a good oven!

Sorry, one of those links was the wrong one (working on too many tasks at the same time) This is the other one I meant to post.


https://cimsa.com.tr/en/what-is-calc...minate-cement/

Here's another very useful site

http://www.sparref.com/uploadedFiles..._castables.pdf

Crushing dense fire brick is hard work. It would be far easier to get a bag of castable refractory (obtained from a refractory supplier and probably the material used to cast the dome you bought) which contains CAC as well as the high temperature aggregates and the required burnout fibres. But frankly it is overkill for a casting that only gets to around 300C because it doesn't receive much intense flame impingement.If you still have your heart set on the CAC and crushed firebrick approach you should also be able to purchase "grog"(crushed firebrick) from a refractory supplier, but you will need to add some burnout fibres. pp fibres, well dispersed, do a good job for this application.

Vermiculite, being extremely light weight results in lowering strength drastically so don't use it for a casting that requires strength.

CAC are far more sensitive to spoilage, temperature and working time, so for these reasons and for reduced cost the homebrew 3:1:1;0.5 (sand, OPC, hydrated lime, brick layers clay) proves quite adequate with good workability. While OPC begins to fail at around 300C the lime takes over as the cementious binder. The lime, which is good to around 500C also imparts some crack healing properties.