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Author: Subject: Aerated refractory concrete
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[*] posted on 21-9-2007 at 18:11
Aerated refractory concrete


Inspired by the method to make Ytong, the aerated concrete blocks for building houses of and that is not very resistant to high temperatures, I tried to do this aeration to my calcium aluminate cement/chamotte mix that I previously used to encase some heating wire for a small tube furnace.

I mixed 3 spatulas of dry powdered cement/chamotte mix (1:1 by volume) with a spatula tip of calcium hydroxide and another spatula tip of aluminium powder (75 micron, spherical, epoxy resin filler grade).
About two minutes after adding water and mixing to a thin runny consistency, the mixture started to increase in volume like bread dough! With its dark color and peculiar consistency, it reminded me very much of mousse au chocolat :D
The problem was that the tiny bubbles partially escaped upon further stirring and pouring of the mix.

I quickly found the solution to this problem: Adding a drop of dishwashing detergent to the mixing water. This stabilizes the foam and allows scooping and pouring without volume loss.

This mixture roughly triples in size after adding water, and if the aluminium and calcium hydroxide content of the mixture is increased will no doubt expand even more. I will start doing a few experiments with different amounts of added aluminium and compare the properties of the cured concrete (hardness, insulating properties and resistance to fire).

The cement is quick-setting, and is completely hard after 24 hours, so tomorrow I can see how the test batch came out.

This aerated refractory concrete should make a very high quality insulating castable for electric furnaces, being devoid of perlite which is not compatible with Kanthal wire and softens already at 800°C.
Hydrogen peroxide would be an alternative "expander", which I have not tried yet. This might also be subject to experiments.




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[*] posted on 22-9-2007 at 13:42


I'm interested, even if no one else is. What sort of results did you get? I was thinking of doing something similar, maybe with chopped fiberfrax mixed in for strength.
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[*] posted on 22-9-2007 at 14:38


Today afternoon I checked the samples, they unfortunately were very fragile and crumbled easily. However, their strength improved markedly over the course of the day, seemingly due to drying (I used much more water for mixing that I should have used). I'm going to leave them alone further to see if they become decently hard and resistant.
Interestingly, the sample with no detergent added was by far the hardest one, probably due to its lower gas content. It seems like the aluminium powder content should rather be reduced instead of increased in order to achieve better strength.
The dtergent-free sample was still quite light, so I still think the idea has potential even without any further additives (refractory fibers are WAY too expensive for me).




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[*] posted on 22-9-2007 at 15:46


I think the Ytong is cured in a pressure cooker after it has done it's initial set. If portland cement is mixed with too much water, it looses strength. Probably calcium aluminate cement is the same. Something as stiff as bread dough rises from CO2 bubbles, so you can probably mix the cement to it's normal consistancy.
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[*] posted on 23-9-2007 at 15:50


Can you get Rockwool where you are? It is supposed to be good to 1000C - might make a good reinforcing fibre for you. Should be pretty cheap, as it is used for insulating houses.

I would expect smaller bubbles to result in better strength, so you might try finer Al powder to get better dispersion.

[Edited on 24-9-2007 by Twospoons]




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[*] posted on 24-9-2007 at 04:23


I did a few experiments with portland cement some years ago and here are a few things I remember:

-All the soaps and detergents I tried decreased the final resistance steeply. Some completely prevented hardening.

-Zinc powder completely prevents hardening.

-Sodium silicate solutions promote instant hardening.

-Adding styrofoam balls (1-2mm diameter) makes a simple light concrete. Kind of tricky to work with though, the balls tend to float, so little water should be used. I always wondered what would happen if I heated this concrete to the point where styrofoam would pyrolize...

- Some commercial aerated concretes are made by adding a powerful foaming agent and mixing it hard, so that air gets mechanically trapped. I dont't think it releases any gas, just traps it. I don't know what this agent is, but I know it stinks like hell. I believe is the same foaming agent used in firefighting foams.



[Edited on 24-9-2007 by Tacho]
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[*] posted on 26-9-2007 at 07:09


I Googled "cement foaming-agent" and found things like polyvinyl alcohol, hydrolyzed protein and glue-colophony used as foaming agents and stabilizers. Something you could make with readily available materials would be 1 part liquid dishwashing detergent with 2 parts white glue (PVA). This you would use in your cement at about 1%.
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[*] posted on 26-9-2007 at 11:45


What did you set it in? I'm guessing that if it was that crumbly it would have been hard to get out of something unless you could peel it off?
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[*] posted on 26-9-2007 at 11:54


All the samples have now become hard and resistant. They resist fire (blue bunsen burner flame, the concrete glowed red) very well and insulate so good that one inch from the hot zone they could be held in hand.
It seems like they just needed more time than usual to cure.

What remains to be done is to determine the weight of aluminium per volume of concrete powder that is needed to properly aerate it. Maybe calculate the amount that would theoretically be required to generate e.g. the five-fold volume of hydrogen of the mixed concrete.

The samples were set on a flat plastic surface.

[Edited on 26-9-2007 by garage chemist]




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[*] posted on 26-9-2007 at 13:37


Quote:
Originally posted by garage chemist
one inch from the hot zone they could be held in hand.


I always find this claim humorous. Lots of people state insulation in terms like this, with absolutely no quantitative* basis for it. I can hold a bar of steel in my hand, strike an arc and melt it completely in half before my hand, one inch away, feels any conducted heat. After a few seconds, it will get noticably, painfully and sizzlingly hot, yes, so the question is, how hot does it ultimately get, and how long does it take?

*Incidentially, touch tests are surely some of the most qualitative, but some suprisingly precise assumptions can be made, like a heat dissipation (into a tightly gripped palm) of perhaps 1W/in^2 without becoming uncomfortable, and a maximum temperature of perhaps 150-160°F (65-70°C). The "ten second test" (hold your finger on the object for more than ten seconds) finds results in this range.

Heavy, well-insulating refractory will take ages for the heat to traverse the thickness, while lightweight, equally well-insulating refractory might reach equilibrium in tens of seconds. Care must be taken with such rough guessing!

Tim




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