[Stoves] High alumina cement

[Andrew Heggie]

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?

[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.
>
> It is a "high early" cement and achieves 90% full strength in 24 hrs.
> Normally it takes several
> hours for initial set. You can use portland cement as an accelerator, and
> get a virtually instant
> set for certain specialty structural applications, but you would never ever
> do this in a refractory
> application.

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.

Andrew