my first cast of 12cm batchbox

[grga]

I found time to make my first test cast ob 12 cm batch box rocket. I used styropor (EPS) for the mould as this is just for the test. Later I would probably use XPS or wood mould. I used CAC (Calcium Aluminate Cement with 40% alumina) cement and crushed firebricks (parts of maximum size of cca. 10mm) mixed 1 part cement and 4 part crashed fire bricks... For much stronger cast and higher temperatures you could use brown Corundum but it is hard to get and expensive. I also added some glass fibers for strength.
Although I use EPS the casting is quite ok - use of ordinary tape makes the casting much easier, the mould can even be reused a few times if unmounted carefully. For cutting EPS I used simple home made hot wire cutter (e.g. www.taurusarmed.net/forums/firing-line/62547-pictoral-guide-home-made-hot-wire-foam-cutter.html).

I hope the core will stand burn temperature... I build it yesterday I will test it in a few days and report. How long I should wait before making first fire in batch box?





The inside of the cast is very smooth (looks like plastic) - this is because I used tape.

and the floor channel

[peterberg]

Hi Grga, welcome to the boards.

Your links to the pictures don't work for me, could you host those somewhere else?

I would say let the casting dry for at least a week and keep it close to a heat source during that time.

[grga]

Peter thank you for reply. I would have fire it tomorrow without this information - now I will wait 1 week as you suggested.  

[firewatcher]

Jan 20, 2017 8:35:59 GMT -8 grga said:

I found time to make my first test cast ob 12 cm batch box rocket. I used styropor (EPS) for the mould as this is just for the test. Later I would probably use XPS or wood mould. I used CAC cement and crushed firebricks (parts of maximum size of cca. 10mm) mixed 1 part cement and 4 part crashed fire bricks... For much stronger cast and higher temperatures you could use brown Corundum but it is hard to get and expensive.   I also added some glass fibers for strength.
Although I use EPS the casting is quite ok - use of ordinary tape makes the casting much easier, the mould can even be reused a few times if unmounted carefully. For cutting EPS I used simple home made hot wire cutter (e.g. www.taurusarmed.net/forums/firing-line/62547-pictoral-guide-home-made-hot-wire-foam-cutter.html).

Your mould and resulting casting both look fantastic! Great job with that!

[peterberg]

Firewatcher, it's clear you've seen those pictures but I still can't. Any others who can't see those or is it just me?

[peterberg]

Yes, I can see the pictures now. Looks very smart, exactly according to drawing or so it seems. The 5-part design looks a bit more complicated but is a very stable one, mine is still without any visible crack. So this is a 120 mm sized core, and what about the riser? When you want to fire it, you inevitably will need one. For first fire a simple stove pipe will do, although in a real installation inside a heater a steel pipe, especially when insulated, will be destroyed in no time.

[coisinger]

Very nice casting.

[grga]

Done the test firing… I start with a couple of small fires, wait between them for the core to calm down. Than half fill it and run it was all fine, I was impressed by the fire but not so much with the two cracks which appear symmetrically on the left and the right side (cca 8 cm hair thin crack). When the core cools down the crack is not visible but appears visible at the next fire. Today I first warm the core and then fully load it one additional crack appear on the rear of the riser. I hope there will be no more cracks in future.



Maybe I should have wait a few days more, instead of 7 days maybe 14 days – the core was all the time next to a heat source.

Could be that the cement (CAC, 40% alumina) I used is not sufficient (should be good to 1250 C) I will later try with better one. Or could be that crushed firebricks is not a good option... But overall, the core behaves fantastically and if the cracking would stop there is no serious damage done…

Some images follows:



I would appreciate any ideas on the following:

- Now I added to the concreate some glass fibers (they are not very heat resistant, they slowly melts if I put them in firelighter fire). Do these fibers help or even makes it worse?

- I could also add some thin wire (maybe 0,6mm INOX) for armature either like chopped fibers cca 3 cm long or longer parts inserted during casting. Would this be problematic due to different temperature stretch coefficient (iron, concrete)

- I could also add parts of the iron mesh in the core, e.g. chicken mesh

- Would adding water glass instead of regular water (or only part of it) to the concrete improve the cast properties

- and for the riser, would chimney pipes e.g. Schidel ceramic (UNI plus) be good enough. In thermal shock it could crack, ones it does it should stay this way?





image of the 3 cracks which are visible only when the core is hot.

and short video during full fire

[peterberg]

Jan 29, 2017 12:55:36 GMT -8 grga said:

Maybe I should have wait a few days more, instead of 7 days maybe 14 days – the core was all the time next to a heat source.

Could be that the cement (CAC, 40% alumina) I used is not sufficient (should be good to 1250 C) I will later try with better one. Or could be that crushed firebricks is not a good option... But overall, the core behaves fantastically and if the cracking would stop there is no serious damage done…

Grga, you are using a home made concoction and it cracks, this is to be expected. What I did is using a proper castable refractory, right specifications. Those castables are designed materials, not thrown together just like that.

Jan 29, 2017 12:55:36 GMT -8 grga said:

-Now I added to the concreate some glass fibers (they are not very heat resistant, they slowly melts if I put them in firelighter fire). Do these fibers help or even makes it worse?

-I could also add some thin wire (maybe 0,6mm INOX) for armature either like chopped fibers cca 3 cm long or longer parts inserted during casting. Would this be problematic due to different temperature stretch coefficient (iron, concrete)

-I could also add parts of the iron mesh in the core, e.g. chicken mesh

-Would adding water glass instead of regular water (or only part of it) to the concrete improve the cast properties

- and for the riser, would chimney pipes e.g. Schidel ceramic (UNI plus) be good enough. In thermal shock it could crack, ones it does it should stay this way?

Whether or not the glass fibres are helping is hard to say, in essence you are using untested materials.

The Inox needles would be beneficial, these are tested by most of the suppliers and it works.

Please, no chicken mesh.

Buy proper castable and please, no waterglass in the concrete. You'll find out the hard way what it does. I don't know either, by the way.

Those Schiedel ceramic pipes are good, although you are running the risk of cracking. To circumvent that, one slit lengthwise in the pipe would do the trick. Actually, by doing that the pipe is already cracked and probably will stay forever that way.

One other thing: the floor channel looks good, but just a month ago I found out it would be better to bend that little 45 degree plate up to vertical. The corners are too close to the port walls and as it is, there's a slow orange flame behind it in full burn. Bending it up, which happened with mine accidentally made the whole of the port flame much more uniform.

[grga]

Peter thank you for all your useful comments. Inox needles then definitely. And I will modify channel - should I only bend the part of this plate up (part which is not welded, so from the middle of the vertical pipe, so no additional cutting) or remove entire plate and weld it vertically?



I know it is experimenting with materials and it costs some money and a lot more time. But the cement and the mix I did should be fine for this purposes. I believe that ready mixes have some additional magic additives added. I will try also with more powerful cement for my target cast for indoor, but only if this test core will survive some more burns without serious damage.

Glass fibres in ordinary concrete are definitely fine; their temperature expansion coefficient is very similar to the concrete. For combustion core the problem is their melting point which probably is somewhere around 700 -800 C (the version I have). I should find better ones.


I am really curious to know what would happen with water glass in the concrete mix. Probably I will try with a small brick made and put in the batch box to see. After some thinking: cement needs H20 for its reaction. While waterglass would react with parts of firebricks (Silica). So it could be a good idea to use only use part of water glass in the mix.


Another idea I got is to add plastificator to the mix – this would lower the need for water and the resulting cast will be more strong and easier to vibrate. Because less water in the mix means less free space in concrete after cement reaction. But if there is not enough water for the cement to react then some of it could be added in the cement reaction time (moisture the surface) after an hour or so. But this is probably to tricky and calls for problems. However adding only small amount of plastificator should be fine.


Too much ideas one could try to learn from its mistakes, therefore I prefer additional suggestions to lower the risk of producing crap.


The other thing is that such a big irregular surfaces is hard to make resistant to cracks at locally very hot temperatures and extensions. Therefore cracks are to be expected? I would say bravo to all of you who have no visible cracks!!

[peterberg]

Jan 30, 2017 7:09:02 GMT -8 grga said:

Peter thank you for all your useful comments. Inox needles then definitely. And I will modify channel - should I only bend the part of this plate up (part which is not welded, so from the middle of the vertical pipe, so no additional cutting) or remove entire plate and weld it vertically?

No additional cutting or welding, just bend the plate up.

[morticcio]

I admire you for making your own refractory but I've followed Peter's example and used ready-mixed castables - as long as you follow the instructions they just work! I've had a few hairline cracks but these I filled in with refractory cement. I was tempted to use stainless needles but so far haven't found I needed them.

With any refractory castable, how well the product is mixed and the amount of water you add is critical. Rule of thumb is the more water you use the weaker the end product. Here is an excerpt from the instructions:

Highest strength is obtained with castable refractory by using the least amount of clean mixing water that will allow thorough working of material into place by vibrating. A mechanical mixer is required for proper placement (paddle type mortar mixers are best suited). After adding the recommended water mix for at least 4 minutes, place the material within 20 minutes after mixing. For maximum strength cure 24 hours in a damp condition before initial heat-up.

But how much water do you use? This varies depending on the ingredients! On ready-mixed castables it will tell you on the product datasheet and can range from 1.25 litres to 4.75 litres per 25kg bag. Here are some examples:

Type	Description	Water addition litres per 25 kg	Maximum service temperature
Vitcas 1300 INS-V	High strength, medium weight insulating castable	4.25-4.75	1300°C
Vitcas 1600 STD-F	Fine grade low iron castable with good thermal shock resistance	3.00-4.00	1600°C
Morgan Morflo 165 AL	Low cement castable exhibiting ultra high strengths	1.25-1.6	1650°C

[grga]

For CA cement and grog mix I should add cca 14% water of solids weight. Probably I did it a bit more as I was afraid how to vibrate it. But after some experiments I found out that CAC mix vibrates better than ordinary cement. So the proper mix is when you can make ball in your hand.

- Adding water glass only a small amount (less than 10%) to regular water was a disaster. Concrete did not react properly and did not harden. Peter was right.
- Adding plaster to the mix (cca 1.5% of cement weight) and less water (cca 12%) seems to be fine – it was not worse at least. Although CA cement can be vibrate quite ok by itself. I need however to test it in fire, but first it must fully dry


After some reading about CA cement I found the following important things:
1)The CAC concrete should be left 24 hours to react properly at ambient temperature. If it is becoming to dry it need to be moisture or mould covered by wet clothes.
2)Drying phase:
Increasing temperature of the cast about 30C/hour to around 110C and let it dry there at least 15 hours. This allows all free water to evaporate.

After that increase from 120C to 240C (50C/h) and stay there cca 10 hours. Then go from 240C to 600C (50C/h) stay there cca 10 hours. This allows complete dehydration and crystal structure to be made (compressive strength). Drying at ambient temperature is not enough.


Drying process is very complicated and impossible to do it exactly but you can at least simulate it.

[peterberg]

I took on the habit of covering the casting by plastic, right after the vibration is done. It doesn't need to be sealed completely tight, as long as the moisture is staying in.
Covering by dry clothes doesn't seem to be right, this will extract moisture, I'd say.
Vibrating castable refractories is much easier than normal concrete, yes. In fact, it is a completely different material, all the components of concrete as we know it aren't there. It looks like it, but that's about all. So the rules are different too, plastificator might be weaken the casting in the long run. Learned all this the hard way too.

[pigbuttons]

Thanks for sharing your experiments, they are good ideas and some good info has come out, like the proper curing times and temps.

1 Fiberglass fibers are to prevent drying cracks only, they add no strength to cured concrete.
2 Stainless needles are the only way I know of to prevent heat cracks. I was told that they do not cause expansion problems.
3 Looking at the pictures of the cracks I notice a lot of air bubbles in your concrete. You may have over mixed it and trapped too much air. The CAC is slow to react to the water so you have to be patient, mix slowly and watch for it to turn plastic. Then you can add a layer of mix into your form, vibrate the bubbles out and then add another layer and repeat. With fine grain aggregate and a barely plastic mix you don't have to worry about your aggregate separating out.
4 I have tried concrete lathe like they use for when you want to put up a rock wall on a wood frame house. It does Cause cracking because of the expansion rates at high temps.
5 you used a 40% CAC. They also make a 70% that may hold up better to the higher heat.

I hope this helps. I'm just getting into this whole rocket stove thing but I'm pretty good at the concrete stuff.