my first cast of 12cm batchbox

[pigbuttons]

Feb 1, 2017 2:27:55 GMT -8 grga said:

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.

I just re-read your instructions and that is some complicated curing process. I'm going to have to think long and hard to figure something out. If there is anything similar to what happens in the Portland cement world, then the curing can happen in a more irregular fashion, but it just takes somewhat longer; like when Portland Cement freezes the curing process just stops and then resumes when it thaws. It sounds like the CAC "kicks" at about 120C and then requires more heat to complete the curing.

When I talked with the engineer where I bought my CAC, he recommended a 2 day wet curing for the calcium to set up, 5 day drying, then a few tiny fires, then work up to a full load over a few days. Since we aren't looking for complete compressive strength it may not be as critical to get the precise curing exactly right.

But thank you so much for sharing what you've learned. I look forward to seeing more of what you are doing.

[grga]

Pigbuttons thanks for your advices and comments. Next cast I will make with inox needles but they are difficult to get so I might take paper clips, small nails or will chop thin inox wire. I will also used CAC with more alumina. I still think glass fibbers should be fine if more temperature resistant are used.

Your curing- drying-fire is a good info and this is what can easily be done. I now know that I was impatient and make the full fire too soon. After a few more burns the core seems to be fine although it is too soon to tell. So I will extremely test the core for a few weeks before going on with next cast.



What are your experiences with CAC, what do you mix to it?

[pigbuttons]

Feb 6, 2017 2:04:29 GMT -8 grga said:

Pigbuttons thanks for your advices and comments. Next cast I will make with inox needles but they are difficult to get so I might take paper clips, small nails or will chop thin inox wire. I will also used CAC with more alumina. I still think glass fibbers should be fine if more temperature resistant are used.

Your curing- drying-fire is a good info and this is what can easily be done. I now know that I was impatient and make the full fire too soon. After a few more burns the core seems to be fine although it is too soon to tell. So I will extremely test the core for a few weeks before going on with next cast.



What are your experiences with CAC, what do you mix to it?

You are welcome. The thin inox wire sounds like the best alternative of those mentioned. The "needles" are very thin, light weight, and slightly curly. I think this helps prevent settling during vibrating. So far I have not seen any "drying cracks" in my CA casts, and no shrinkage to cause them, so in my opinion the fiberglass is added cost with no benefit, but it's your project.;

I'm just getting into the CAC casting so not much experience. I did find out that plasticizer/air entrainment that I used extensively for Portland Cement will make the CAC crumble, even with just a few drops of the liquid. Most plasticizers do three things, make the slurry slicker by lubricating between molecules and therefore leave some barrier to crystalization, They are also retarders, slowing the onset of crystalization, and they entrain air making longer crystals more difficult to form. None of these are good for our refractory use.

For my needs I'm using CAC 70% and vermiculite, with just enough water to get to a plastic state after about 5 minutes of slow mixing with a large kitchen spoon; at this point it still looks slightly dry and crumbly, but when pressed into the form and vibrated it does liquify quite nicely. I'm still working in very small batches though and have yet to use a tiny mixer I bought for when I step up to a full size batch. Like Peter said, these refractory products are not Portland Cement and the chemistry is totally different, so additives are not likely to be useful in a CAC world. Even the ratios, the cement's reaction to water, and how it vibrates out are all very different, so I'm having to experiment just like you to find things that work.