NZ build

[JSeward]

Thanks again, much appreciated

[david s]

Provided the surface is smooth and flat you'll get good contact and heat flow by conduction.

[JSeward]

Cheers for all the tips, I think I'll spend the cash on fire brick/ tiles, or maybe finish the cooking surface with tiles, 25mm ones are easy to get, with the refractory concrete beneath.

This wouldn't cause a heat break would it, i.e. due to two differing materials ?

[david s]

I use a proprietary castable refractory, but if I were making my own brew I'd add a little powdered clay in the mix to make it more workable and also add some fine polypropylene fibres which will disperse when mixed well and burn out at 160C leaving a network of fine pipes to assist water removal and reduce any steam spalling tendency. Ask your supplier if they have them or get them from specialist concrete supplies.

[JSeward]

Sorry I probably wasn't too clear. I have calcium alumina cement, they told me it's the cement they use in their fire bricks and refractory tiles. I was told a 4:1 ratio of sand -cement concrete would work in place of fire bricks. Cracking aside, I am a little concerned the surface will degrade faster than firebrick

[david s]

I'm not clear on what they sold you. If it is just the calcium aluminate cement then it's probably Ciment Fondu, there are other brands. You may also have been sold a premixed castable (just add water) Contact your supplier to clarify exactly what it is that they sold you.

[JSeward]

Thanks David, but I am a little confused, is it ok to cast an unfired floor by myself or not ? It is sounding like I was sold a line by the guys who sold me the cement

Cheers

[david s]

So I need to fire the floor ?..., I cant just pour it like normal concrete, what would happen e.g. cracking or spalling ? cracking is fine, the gaps between brick are, after all cracks, but as long as the performance isn't to badly affected I am fine with that

Ok. In that case get a proprietary dense castable refractory that uses calcium aluminate as the cement component in the mix. The floor takes a real beating and the home brew, while OK as a mortar for the dome is not suitable as a castable to use as the oven floor going to pizza temperatures. If you can, although not absolutely necessary, add some stainless steel needles (melt extract fibres) at the rate of at least 2% of the weight of the dry mix.Both castable and needles should be available from refractory suppliers.

[JSeward]

Thanks David, yeah, doing some more reading I realised the 25mm was too thin. I have 50mm of calsil board and was planning on 50mm of floor now.

So I need to fire the floor ?..., I cant just pour it like normal concrete, what would happen e.g. cracking or spalling ? cracking is fine, the gaps between brick are, after all cracks, but as long as the performance isn't to badly affected I am fine with that.

The problem is it hard to get 50mm fire brick brick here in NZ. Certec have them but they aren't standard, and getting 500mm square 50mm thick tiles is $70 per tile, otherwise, 75mm is a bit too thick for a floor no ? or at least too high for my current plans....

Thanks
John

[david s]

Hi All, I am trying to keep costs down and make things as easy as feasible with my build . Is it feasible to cast my own cooking floor i.e. rather than use bricks ?? there must be a reason more people don't do this ???

Some manufacturers do, but the problem with calcium aluminate castables is that any large castings are subject to cracking because of the uneven heating. This is why many builders lay the floor bricks loose to allow for differing temperatures and expansion. The larger the floor bricks are the more likely they ar to crack. A cast floor made with Portland cement won't last too well but if made with a more refractory cement (calcium aluminate) it can take the heat. Ideally these castings should be fired slowly to at least 1000C when it will be properly sintered. In practice no one does that because you need a large kiln and plenty of time and money spent on kiln fuel to take the material up to temperature slowly and progressively. If done this way the material is essentially the same as a firebrick. Unfired it only relies on the chemical bond of the cement. A cast floor does work, but needs to be done in a few sections unless you don't mind some floor cracks.
25 mm for the floor is way too thin IMO, it won't hold enough heat. You really need 50 mm. The usual under floor insulation is 100 mm of 5:1 vermicrete or 50 mm of cal sil board. You can use both but your plan looks a bit too thin to me.

[JSeward]

Many thanks

[UtahBeehiver]

Contact David S. He is our resident cast concrete expert.

[JSeward]

Hi All, I am trying to keep costs down and make things as easy as feasible with my build . Is it feasible to cast my own cooking floor i.e. rather than use bricks ?? there must be a reason more people don't do this ???

[JSeward]

So, I got back to the oven after a short hiatus. So the dome , do I build it on the hearth or outside, are there any pros and cons ? I am thinking 25mm vermicrete, 25mm ceramic board then 25mm floor tiles. 50mm vermicrete under the dome walls. Do I mortar the first soldier row down? Thanks for any help

[JSeward]

Ahh I see, great analogy thank you.