Castable refractory

[wotavidone]

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 100?C 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 600?C 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 1000?C 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 1000?C 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 1000?C.

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]

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]

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]

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]

Re: Castable refractory

Yep, good info.

[stonecutter]

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]

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]

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]

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]

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.

[wotavidone]

Re: Castable refractory

The problem with silica is that it wants to turn into glass in the presence of fluxes.

Yep, if you mix about 0.73 parts of lime, 1.3 parts of iron oxide, 1 part of zinc oxide with 1 part of silica, you can reduce the melting point to 1080 degrees C, give or take twenty degrees.

Leastways, that's what we do with our Blast Furnace. Used to be my job for 7 years, manipulating the feed stocks to achieve those ratios.
BTW, mullite is porcelain clay.
I really must post that phase diagram.

[stonecutter]

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.

Probably because there isn't much that can be improved on...at least, nothing that would substantially improve the performance or extend the service life in a major way...mainly because an oven is such a dynamic structure. I would guess, that a well built occasional use oven, kept dry and fired properly, will last at least 20 years before any maintenance work needs to be done.

Now, understand I'm not saying it isn't worth trying to find a better refractory mortar for lower temps. I would think that research needs to be done with material that has refractory quality by chemically bonding, and not ceramic, like the high temp linings.

[wotavidone]

Re: Castable refractory

Now, understand I'm not saying it isn't worth trying to find a better refractory mortar for lower temps. I would think that research needs to be done with material that has refractory quality by chemically bonding, and not ceramic, like the high temp linings.

Reckon you are right. That was what I was alluding to with my tongue-in-cheek comment regarding it being a terribly neglected area.
(The romans managed without the science, of course.)

When the specialists talk about refractories, they talk about furnaces for smelting (ours runs a slag temperature of about 1180 degrees C), or settings for refining pots that have to be able to withstand gas flames constantly impinging on them, or handle molten metal, etc.

The trouble is, then we find that refractory that can withstand 1200 degrees C won't actually help our oven much, unless we can heat it to 1000 degrees C to "cure" it.

Much like making a brick - until it's actually fired, it's just a block of dry clay with some filler in it.

So yes, given that the cements that harden by hydration are not real good in the temperature range we are talking about, it would be cool find a new mechanism for binding the aggregates in the mortar or castable together.

Given that there are some commercial mortars available, then the scope of the challenge needs to be narrowed even further.

It has to be cheap to do a homemade version, otherwise we just buy the commercial stuff.

So there is the challenge, come up with a binder that doesn't work by hydration like the cements, loves 500C and costs bugger all.

The knowledge base I was perusing today spoke of low cement refractories that used three different mechanisms to harden. One by hydration, one by other chemical reaction, one by drying.

The FB Homebrew actually fits all selection criteria.
The ingredients are cheap and there are, arguably, three binders that work in different ways.
You could argue that the OPC hydrates, the lime has a chemical reaction with the CO2 in the atmosphere, and the clay hardens when it dries, and while the mortar cannot really be classified as a true refractory, it handles the temperature ranges we want to use it in.
Now that I have proved beyond all doubt that the FB Homebrew is the best mortar for pizza oven conditions, I'm going to bed.

[stonecutter]

Re: Castable refractory

It's like the saying goes... If it ain't broke, don't fix it.


Given the wide range of material ratios different builders use, and the success rate they have, ' improvement' on a mix design is very subtle oven to oven. For me, when I make mortar, there is no one type fits all. That carries into oven building too, because there are several different areas in the oven that benefit from slight changes in the component ratios...like aggregate size. I still do that dispute knowing that a generic mortar will work for every part of the construction.

If I get a chance I'll try to find some links that might be of interest.

[david s]

Re: Castable refractory

BTW, mullite is porcelain clay.

Not quite. Mullite is the major and most important component of a porcelain body.
Mullite is the crystalline structure that is formed from Kaollin over 1200 C.
A porcelain body contains kaolin, feldspar and silica.