[Stoves] calcium aluminate cement

Norbert Senf norbert.senf at gmail.com
Wed Dec 13 15:13:14 MST 2017


Andrew Heggie wrote:
Date: Mon, 11 Dec 2017 21:01:43 +0000

Thanks for your input Norbert, some comments and questions below:

On 10 December 2017 at 20:40, Norbert Senf <norbert.senf at gmail.com> wrote:

> The castable refractories to which Crispin refers mostly use calcium
> aluminate cement as a binder.
> There is also a phosphate binder and sodium and calcium silicate, but
fondue
> (calcium aluminate)
> is by far the most common.

>>>I understand that concrete setting is very complicates but it firstly
involved the anhydrous silicates or aluminates re-ydrating the water
of crystallisation. Crispin gave a breakdown temperature of 400C for
portland cement, which is calcium silicate and other salts, the
greyness coming from a ferrous salt [1], my guess was the reason it
failed at a lowish temperature was that the heat drove of this water.
How does ciment fondue differ, is the initial binding by hydration of
the salt and then some other bonding which is more heat tolerant
develop? Or does it sinter in sme way as it is exposed to high
temperature?

Hi Andrew:
I don't know the chemistry very well. What I have learned is that
calcium aluminate concrete goes through a "twilight zone", where the water
of hydration is driven off, reducing strength, but it has not yet reached a
high
enough temperature to achieve a ceramic bond. Therefore, for heavy duty
industrial
applications (electrodes for electric arc steel smelting for example),
the pieces are fired in a kiln, which achieves substantial additional
strength.

We use it for casting components for wood burning masonry heaters. Some
people
use it in the firebox, but we don't. I much prefer firebricks in the
firebox. There is a spalling test,
where you heat the brick up red hot and quench it in water, and note how
many cycles
it can take. Here is the rundown: soapstone: 0 - 2 cycles, castable
refractory: 5 - 10 cycles,
firebricks 10 - 30 cycles.

It is good to have a rough test like this for testing castable mixes, etc.
We cast 1" thick
test pieces and put them on a high BTU propane barbeque burner for an hour
(during shop
heating season when we can make use of the heat), with the cold side
insulated with ceramic blanket.
Dip them in water, and check them with a 30 power microscope/magnifier.

Calcium aluminate castable is very sensitive to water (water/cement ratio).
Like portland
cement concrete, but much more so. We use a very stiff mix and a vibrating
table.
For high stess pieces, we will incorporate some reinforcement such as steel
diamond
lath, used for plastering.

There are specialty formulations such as low cement and ultra low cement,
which use
very particular grading of the aggregates and super low water ratios. You
need a special
high shear mixer for these, and they are impossible to mix by hand. You can
get around
300% the normal strength.

There are other formulations for high flow, requiring no vibration.
Nothing beats testing in your particular application.



>>>>>>[1]In UK some of the iron is provided by firing the chalk with vehicle
tires, the iron reinforcing then being incorporated in the cement
clinker.
(snip)

>>> I didn't realise adding portland cement speeds up the setting. We used
to add high alumina to portland (premixed with aggregate in the bag)
to fix fence posts, it was stiff enough for the post to self support
within 15 minutes.

Yes, it works both ways. You can use calcium aluminate as an accelerator
for portland as you describe. When you use calcium aluminate, it is very
important to have clean equipment that is not contaminated with portland.

Norbert

-- 
Norbert Senf
Masonry Stove Builders
25 Brouse Road, RR 5
Shawville Québec J0X 2Y0
819.647.5092
www.heatkit.com
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