Calcium aluminate home-brew castable

[totalnewbie]

Hey all. What a great resource. It's great you all share your knowledge and experience. I have a question on home-brew castable.

I don't have the skill to build a cut brick oven, or the budget to buy a kit. But I read about the 3:1:1:1 home-brew recipe using Portland cracking and I worry. And I see that HWI castable is expensive. If the proprietary castables use calcium aluminate, should I make a home-brew with a bag of calcium-aluminate at the same 3:1:1:1 ratio?

It's more expensive that Portland, but maybe not crazy.

What does everybody think? Has anyone done it?

Also, is it easy to find real calcium aluminate cement. I'm in VA. Thinking about a 2" dome around 32".

Thanks!

[totalnewbie]

One other thing. I want to buy insulating board under the oven. It seems easier than mixing vermiculite and portland. FB sell alu-sil board but here people are recommending calcium sil boards. Which is better? Does it matter? Where do you buy it? Thanks!

[david s]

I’ve only used calcium silicate board so can’t comment on calcium aluminate board. Regarding replacing calcium silicate cement withcacium aluminate cement in the homebrew recipe, the answer is definitely don’t. Firstly the calcium aluminate cement is around 10x the cost, secondly it has a very limited working time and you run the risk of it going off before it can be placed. Re-wetting it just reduces its strength significantly. Thirdly the combination of lime and calcium aluminate cement drastically reduces the already short working time of the mix. It will probably have gone off before you’ve finished mixing it. If you want to use a calcium aluminate mix then purchase a dense castable refractory which has the calcium aluminate, high temperature aggregates and the burnout fibres all premixed in the correct proportions, just add water.

[totalnewbie]

Thanks David! You are so helpful. I'm in Virginia and I can't find HWI materials anywhere locally. So the KS4 price is really high (high price plus high shipping). Do you know how many 55# bags you need for a 32" dome (2" thick?). Also, is it possible to buy "real" refractory aggregates to keep the cost down?

What would happen if I mixed the calcium aluminate with just sand and fireclay (and ceramic fibers) and left out the lime?

[david s]

Sorry, I’m not sure what you mean by 55# bags? Do you mean calcium aluminate cement, loose vermiculite or perlite?
We get 100 litre bags here. The stuff compacts a bit when mixed with cement and water, so a 100 litre bag of loose vermiculite or perlite will make about 80 litres of vermicrete or perlcrete.

Yes you should be able to buy high temperature aggregate, but it will be harder to obtain, enquire through refractory suppliers. It is really just crushed firebrick. IMO it is a waste of money because you would be adding a material that is designed to withstand temperatures exceeding 1400C or more. Our ovens will top out at around 500C. Sand is an adequate aggregate up to 500C. At higher temperatures (like double) it can, in the presence of fluxes decide to turn to glass, but this will never happen in a WFO.
However, care should be taken not to overfire your oven as the 500C - 650c range is where problems can occur. Even calcium aluminate cement begins to fail north of 500C and getting towards 600C the body begins to sinter (clay becoming permanent).
The other problem over 500C is that most materials undergo rapid expansion. Unfortunately the rate of expansion varies considerably from material to material which obviously creates some incompatibilities.This is why potters always make the temperature rise in this zone slow and controlled. The same applies to refractory as well as pottery wares. Unfortunately temperature control is very difficult when using wood in an oven with a large mouth. It’s easy with a gas kiln because careful control with gas delivery and air intake are easy to adjust minutely.
If you make up your own castable using calcium aluminate cement and other ingredients you will be trying an untested mix. You might get it right, you might get it wrong, but you will still have the problem of using an expensive cement that’s hard to control with its short working time. Proprietary castable manufacturers are very secretive about their recipes and carefully design their products for specific purposes and service temperatures.
The homebrew basically works well, is tried and tested. If the Portland cement fails the lime takes over as the cementious material (good for 500C when it starts to fail too) At 573C the clay sinters and the body gets harder and stronger as the temperature rises further.

[totalnewbie]

Cool!

I meant to say 55lb bags of HarbisonWalker KS4 Plus. The stuff everyone seems to recommend to self-casting real refractory ovens. I'm concerned about the cost and how many bags I will need.

[totalnewbie]

After lots of reading, I'm going to go with the flow and use 3:1:1:1 home-brew and a sand mold. I'm happy and relieved to not be doing something stupid.

One question. How much material do I need for the dome? I am considering 32" and 36", 2" thick coating.

How many pounds of dry material do I need to buy? In total weigh, I can calculate how much of each component to buy!

Thanks all!

[david s]

For a hemisphere inner diam 32” (40cm) and outer diam 36” (45cm) I calculate 57 litres. Be careful though because the finer material fills the spaces between sand grains and there is also a reduction in volume when you add water to the dry ingredients. Also don’t forget to get the polypropylene burnout fibres.

[totalnewbie]

Would you calculate in kg/lbs, rather than liters, because it's dry weight?

[david s]

If you want to do that you’ll need to know the densities of each ingredient. The 3:1:1:1 recipe is based on volume not weight. Dry sand is around 1.4 kg/litre. When I buy sand it is usually damp and will be heavier than that so it’s sold by volume. You should get 80 kg of sand if it’s dry.
Australian cement comes in 20kg bags. You would need one and a half of these. Hydrated lime is less dense, one 20kg bag should be sufficient. Bricklayers clay here comes in 25kg bags. One bag should be sufficient. If you are also going to cast a flue gallery in the same mix you’ll need proportionally more. Also better to have some left over than get 3/4 done and run out.

Sorry, we’re metric in Australia, you’ll have to do the conversions to imperial.

[totalnewbie]

I get it. You mix by volume (shovel or bucket), and you can't really do weight for sand because it might be wet or dry, and the other ingredient are more and less dense than the others. I'm basically trying to figure out how much to buy.

So very roughly I'll need to buy 30kg/66lbs of Portland cement, 20kg/44lb of hydrate line, 25kg/55lbs of fire clay and 80kg/170lbs of dry clay. Which comes out to between 300-350 lbs on the sand mold. Then I need to work out bag size in the US to figure out what to buy.

Does that make sense?

[david s]

Yes, but you don’t need two types of clay
If you are using sand to create the mould then you will need an additional 270 litres. So that would be a total of 330 litres or half a cubic yard.

Interestingly, one of our local landscape suppliers began charging sand by weight rather than volume recently. It is common to wet down the surface of the sand to reduce dust and of course being outside it is also in the weather. Wet sand obviously weighs considerably more than dry sand so the supplier was making a bigger profit the wetter the sand was. They must have received lots of complaints because they quickly went back to charging by volume rather than weight.

[totalnewbie]

:-)

I see I said dry "clay" instead of dry "sand".

But I just realized I need to buy sand for the mold and for the 3:1:1:1. That's a lot of sand.

[david s]

You can reduce the required volume considerably by placing something like polystyrene fruit boxes or the like in the centre of the sandcastle. But it’s got to be something you can remove easily from the oven mouth.

[bamforp]

If you cast the dome and flue gallery separately (strongly recommended) you can reuse the sharp sand from the dome mould in the homebrew mix for the flue. Not a big saving in the scheme of things but reduces amount of excess sand to get rid of at the end.