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wotavidone 01-08-2014 04:19 PM

Castable refractory
Found this while looking for something else.
Interestingly, using calcium aluminate cement in a home brew may result in a poor strength.
There is a range of temperatures from 350C to 600C where the calcium aluminate weakens significantly. The low end, 350C, is acheivable in a pizza oven.

It appears that pizza ovens fall into a difficult range of temperature to deal with.

Dense Castables
These were the first castables to be developed and contain a high cement content of 10 to 20%. The cement used in refractory castables is not Portland cement but a high alumina cement based on calcium aluminate. They are user friendly in installation, have relatively fast set time, high green strength and relatively low cost.

There is a problem though...

After casting, the cement reacts with the water and forms a strong hydraulic bond. It takes time for this to occur so we generally leave the casting 24 hours undisturbed. When initial heat up is carried out the free excess moisture (that was not consumed in the cement reactions) is driven off. At 100C this occurs more rapidly due to boiling of the water and precautions must be taken to prevent "steam spalling".

As the temperature increases further the chemically combined water is driven off. This occurs from about 350 to 600C and results in the destruction of the hydrated cement paste that is binding the structure together. It manifests as a significant drop in strength which persists until the temperature reaches around 1000C where low melting point phases begin to soften and coalesce to form ceramic bonds between the aggregate grains, forming a complex ceramic body.

When the process temperature is below 1000C we have a real problem. Then alternative bonding systems have to be utilised. This lead to the development of low cement castables which have multiple bonding systems, thereby bridging the weak zone in conventional dense castables.

Low Cement Castables
Although these products were developed for improved slag and chemical resistance they utilise multiple bonding systems which overcomes the weak zone that occurs in conventional dense castables. The level of high alumina cement added is typically 4 - 8% with the other bonding systems typically being micro silica and reactive alumina. The micro silica provides two types of bonding mechanism. Initially, colloidal silica bonds begin to form when the hydraulic bonds of the high alumina cement are being destroyed then it reacts with the fine alumina to form mullite bonding as the temperature exceeds 1000C.

A small percentage of sodium phosphate may also be added to provide strength via a phosphate bond.
Problems with Low and Ultra Low Cement Castables
Because of the use of silica fume in the formulation of low and ultra low cement castables they exhibit significantly increased (~100% higher) thermal conductivity when compared to convention castables.

They are significantly more dense than convention castables and require increased tonnages for placement.

The increased density is accompanied by decreased porosity and lower permeability. Therefore, they are much more difficult to commission with a much greater risk of steam spalling. Initial heat up rates need to be slower and this means a longer firing schedule which impacts adversely on production.

Tscarborough 01-08-2014 05:32 PM

Re: Castable refractory
Calcium aluminate gives you more strength at higher temps than portland cement.

It seems from my reading on castables that they are primarily concerned with use as industrial sacrificial linings rather than as a concrete material to form a freestanding structure.

The idea with concrete, as opposed to monolithic castings, is to reduce the amount of cementious paste to the absolute minimum by volume and maximum amount by surface area. The aggregate has to be sized to the minimum thickness, but 25% of that value is conservative. It should be graded down from there, with the intent being to fill all spaces and the aggregate to have a coating of binder, but no large areas of homogenous material, binder or fines.

stonecutter 01-08-2014 05:55 PM

Re: Castable refractory
If you want some more information on castable refractory, talk to a ceramic or refractory engineer. I have talked to several that were willing to answer my questions and they have a lot of insight...obviously.

wotavidone 01-08-2014 06:24 PM

Re: Castable refractory
The info is an extract from my own company's refractory knowledge base - which is extensive, I assure you.

Believe me we don't want refractory liners to be "sacrificial", we want them to last as long as possible. Downtime to replace linings is income lost.
The reason I posted it was because of the info about the calcium aluminate cement weakening significantly when heated over 350 degrees centigrade.
My oven reaches over 400C regularly, sometimes reaches 500C, on the surface of the bricks at least. Unfortunately the graph accompanying the info wouldn't copy properly.

I'm just making the point that our company engineers reckon calcium aluminate cement needs to be first fired above 1000C to be an effective and stable binder in concretes and mortars used at the sorts of temperatures we can expect in a pizza oven.
They do not recommend CAC for process temepratures below 1000C.

Accordingly, there seems no gain to be had from making the homebrew with CAC rather than Portland cement.

As I said, it seems pizza ovens fall in that temperature range where nothing works quite as good as we'd like.

Just trying to add to the knowledge.

Tscarborough 01-08-2014 06:45 PM

Re: Castable refractory
Yep, good info.

stonecutter 01-08-2014 07:19 PM

Re: Castable refractory
Keep it coming, more the better.

I believe I read a paper that described the process as ceramic bonding....I'll have to look around my files on my laptop.. I'm mobile right now. Anyway it's fascinating stuff. I have plans to experiment with a pourable mortar soon.

wotavidone 01-08-2014 08:10 PM

Re: Castable refractory
Another interesting fact. Fire clay is defined in by our engineers as clay that has been calcined (fired), crushed and sized.

david s 01-08-2014 10:20 PM

Re: Castable refractory
Castable refractory is designed to be fired up to 1000 C or it's operating temperature which may be higher. This must be done in a controlled rate especially at the lower end to drive out mechanical water and the 500 -600 C range. Unfortunately you can't do this with a WFO firing with wood, because the temp rise cannot be controlled well enough. If you had a kiln large enough to fire castable sections it would work, but the extra expense of doing so would make the process prohibitive. Unfired castable works though and produces ovens that are quite serviceable, probably for way longer than we will ever know.

wotavidone 01-09-2014 01:37 AM

Re: Castable refractory
Funny isn't it?
The more I look at available knowledge, the more I realise that pizza ovens are a terribly neglected area of science.
It seems there just isn't a proper scientific assessment of the appropriate cement for operating at 4-500 degrees C.
Today, for instance, I came across a 1982 survey of the different concretes available for containing liquid metal cooled fast breeder nuclear reactors.
They reckoned Portland cement based concretes are OK up to 4-500 degrees C as long as they are not in direct contact with the molten sodium.
But we think twice about using Portland cement for an oven.
As the American cousins would say, "Go figure".
(In acknowledgement of Tscarborough, they reckoned proper selection of aggregates is vital.)

Take the oft repeated maxim that high alumina in alumina silicate bricks is essential.
According to the phase diagram I was reading today, it is true that, as the alumina content of a brick rises from 10% to 60%, the liquidus line rises.
i.e the amount of liquid phase in the brick reduces as the alumina rises, and occurs at a higher temperature, meaning the brick is more refractory.
As long as the temperature is above 1595 degrees C!
Since it is an exceptional thing that an oven rises above 550 degrees C, the alumina content is almost irrelevant. Below 1595 C, the brick will consist of silica and mullite, for alumina contents from 10 to 60%.
I'll try to post the phase diagram tomorrow.

david s 01-09-2014 03:39 AM

Re: Castable refractory
The problem with silica is that it wants to turn into glass in the presence of fluxes. You are correct in assuming that this will not occur at the lower temperatures we fire to, but I think the thermal shock characteristics of a higher alumina content also come into play. That may also be worth researching. It is true that fired clays with an open body (high grog content) have good thermal shock characteristics if only fired to around 600 C. As the fired temperature rises, the thermal shock resistance falls until it gets to around 1300 C when the thermal shock resistance again gets better. But then in that case you need to use a porcelain clay to avoid the clay melting.

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