New build - Help needed with recipe

[Jaycee]

Hi all.....

I'm about to start my first pizza oven build, based on the gym ball form.

Quite a while back i purchased a few bits to start the build, but then time and the house put it all on hold. Now i'm gonna have another crack at it!

Now the more i read on here, i think i could of got away with basic homebrew recipes for the concrete and sourced it all from the DIY store. I initially bought a few bits from victas, these items were -

20kg High alumina cement
Bag of SS needles
25kg Fire clay
Several bags of perlite
25kg Silica sand

My thoughts are to cast the first couple of inches of the dome in a mortar that includes the High Alumina cement, sand, fire clay and SS Needles. Then 2 more inches on the top with perlite and cement.
Or combine the lot and make a cast with the perlite included

Does anyone have any thoughts on a better approach, design or better ingredients please? I don't mind going out to buy other bits and bobs if needed, but would like to use up the cement if its beneficial to the mix.
I've spent the last couple of night trawling through threads, on other peoples opinions with castable and i know the different firing temps seem to be an issue with using the H A cement at lower temps, i just need to know if it will hold up for a period of time, or it would be a waste of time.

Thanks......Any help would be appreciated.

[david s]

A gym ball has the advantage of being adjustable in size depending on degree of inflation, also assisting in its removal. However it is a complete sphere not a hemisphere so it requires either digging a hole in the ground to support the bottom half of the sphere or some extensive formwork raised up to the halfway mark on the ball. Added to this is that the resulting casting has to be moved into position on top of the stand, insulating slab and floor. As the casting is heavy, this is not an easy operation especially if it’s a single one piece casting. It is far easier IMO to lay the insulating slab on the stand, lay the floor in fire brick over it then make a sand form directly on top of the oven floor. This way the casting does not have to move as it’s cast in situ and the sand can be dug out once the casting has set. A good finished surface can be obtained if you cover the sand form with wet newspaper strips around 2” wide.
You mention various materials including ss needles. Burn out fibres are more important IMO as they provide some assistance in reducing steam spalling during the drying fires which are difficult to control. Proprietary dense castables usually already have these added in the mix. I’m also not a fan of making an insulating casting for the inner dome as apart from reducing thermal mass, important to maintain heat, it reduces strength leaving it vulnerable to abrasion damage.

[Jaycee]

Thanks for the response David s.

I've just ordered up a bag of fibres to add to the mix.
I'm casting over the ball because I plan to make another for a family member, if it turns out ok.
Here are a couple of pics if anyone's interested, I'll add more as it develops.
The board I'm using is called ecosheet. It's made from recycled building plastics.

[Jaycee]

After a bit of a nightmare with a burst gym ball from the SS needles, I've finally finished the cast. I ended up just forming a bottom ring of the cast after the ball burst. It made the dome easy to form the next day.

My plan is to insulate with a ceramic blanket, not perlite. Then render over that . Here's a couple of pics .