Castable refractory

[Les]

Re: Castable refractory

I need to jump in here. I want to build a very small oven for tailgating. I'm thinking 24 inches max and I don't want it to be brick, obviously cast. Fire it up and cook a few simple pies, bake something for 45 minutes and I'm golden. What would you guy's suggest using for a simple home rolled mix?

[david s]

Re: Castable refractory

If you are doing it yourself and want it cheap, use the home brew keeping the mix pretty stiff. If you want it stronger and more permanent use castable refractory with 2% (by weight) SS needles.

I make my 21" ovens 2" thick.

[wotavidone]

Re: Castable refractory

Use the homebrew.
I suggest that you pack a mould very tight with a stiff mix. Ram it tight as it sets.
It is very cheap, it should not be expensive to make a mould from medium density fibre board, and if it doesn't work you've only lost a few hours of interesting hobby time.

[wotavidone]

Re: Castable refractory

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.

Let me rephrase that:
"mullite or porcelainite is a rare silicate mineral of post-clay genesis"
Mullite - Wikipedia, the free encyclopedia

So, at temperatures below 1595, for alumina contents of 10 to 60%, alumino silicate bricks (the white blocks that everyone in the smelting business calls firebricks) consist of silica reinforced with needles of the same stuff that gives porcelain its strength.

Note that alumina silicate bricks are "short" at about 12% alumina. The way I read this diagram, at that point if you heat them above 1600 degrees they go straight from solid to liquid.

[wotavidone]

Re: Castable refractory

I've just found another version of this phase diagram. Apparently people make use of this diagram for firing ceramics in the first place. They aim for the temperatures where some liquid is formed, when firing clay, as it means they will vitrify the clay , but hopefully have enough mullite to prevent the piece from sagging.

Once upon a time I cared not one jot for the details of refractories, as long as the engineers gave me a liner that would survive whatever temperature molten material I wanted to put in it.

Now, I find it all downright fascinating.
That's what building pizza ovens does for me - gets me interested in the science behind it.
The Romans did it all without a materials research lab, an assay lab, and a crew of trained refractory masons. Which just goes to show, the Romans were smarter than the average bear.

[Saovicente]

Re: Castable refractory

I have to say this has been one the best threads for information on the hunt for concrete info on a cost effective, efficient homebrew mix.

After reading various threads and discussions with David S, Stonecutter: I will make a homebrew recipe of 3:1:1:1 (maybe split the fireclay (1/2)with grog) and the recommendations of David S to add stainless steel needles, polypropylene fibres (AR fiber glass) into the mix. These combinations will integrate all components onto one dense, well bonded unit.

Great thread....feed me more

[david s]

Re: Castable refractory

Poly propylene fibres are not the same as alkaline resistant AR fibreglass fibres.

[wotavidone]

Re: Castable refractory

Use the homebrew.
I suggest that you pack a mould very tight with a stiff mix. Ram it tight as it sets.
It is very cheap, it should not be expensive to make a mould from medium density fibre board, and if it doesn't work you've only lost a few hours of interesting hobby time.

Quoting myself here (how egocentric).
I've realised that, just like everyone else on this forum, I'm just spouting opinions, even if those opinions are based on fairly extensive reading.
Time to back this up with proper research, I feel.
Put my money where my mouth is, so to speak.
So, I've decided to sacrifice an hour of TV watching and beer hoisting every week, and create a mold, then cast a dome based on my thoughts. It'll take a few months, but I reckon it'll be an interesting little project.
Cement, lime, clay and sand are so cheap, I reckon a little time and $100 should see if the homebrew can be used as a packed/poured castable in a mould. Bearing in mind that the "brickless Oven on a Shoestring" of michelevit has already proved it works, I'm going to try to try it too and see if I can replicate it with local Aussie materials.

I won't bother with insulation - the increased temperature differential will further test the "castable", I reckon.

[wotavidone]

Re: Castable refractory

Just had a very interesting conversation with an engineer.

(Who knew an engineer could be interesting, eh? )

Started out discussing pizza ovens. He mentioned his Italian neighbour has a stand with a concreted slab on which he laid a floor of bricks. Then he built a soil mound in the shape of a dome, and stacked some bricks over the mound.
Then he scraped up some clay soil from his backyard, mixed it up with water and a little cement. Smeared it all over the dome bricks, let it set for a while and dug out the soil mound. Instant oven.
It falls down every couple of years, no weather protection of any sort, and he rebuilds it in a weekend.

On the subject of corroding reinforcing, this guy reckons that it is the alkalinity in the cement that protects the steel, and in normal conditions, i.e. ordinary atmosphere and ambient temperatures, steel protected by an inch of concrete should last 50 years.
However, over that time if you monitor the alkalinity of the concrete, you can see it disappearing from the outer surface in. Eventually the concrete is no longer alkaline and the steel corrodes. Apparently you can paint on stuff to restore the alkalinity to extend the life of your structure.
He's going to bring his "concrete book" in next week so I can educate myself some more.

[NCMan]

Re: Castable refractory

I agree w/him. That is why, especially on some larger commercial projects here they specify epoxy coated rebar. Also, any steel reinforcement, whether it be rebar or mesh should not be in contact w/the ground or open air. Most specifications require a minimum of two inches of concrete to cover all reinforcement. This applies to the sides of the concrete, also. When tying rebar, a minimum of two inches clearance should be left away from the forms. I shoot for three. In the FB instructions, they advise 6 inches from the forms and that is not the correct placement. I've seen it so many times where concrete is being poured and the steel and/or mesh is just left to lay on the ground. Not the proper way to pour concrete. It can't do it's job laying on the ground or exposed to air.

[david s]

Re: Castable refractory

Use the homebrew.
I suggest that you pack a mould very tight with a stiff mix. Ram it tight as it sets.
It is very cheap, it should not be expensive to make a mould from medium density fibre board, and if it doesn't work you've only lost a few hours of interesting hobby time.

The time and effort to make a mould in MDF is hardly worth the trouble for a one off. A sandcastle covered in wet newspaper is far quicker and works adequately if you get the mix fluid enough to be pretty much void free, but still stiff enough to stand up. Around 10% water added to dry sand produces a strong mould and some powdered clay added to the mix also helps produce a good mould.

[wotavidone]

Re: Castable refractory

Just watched a forklift unload a whole semi load of castflo 1600 AR castable in 25kg bags on pallets.
I got on the net to check the specs, and lo and behold the suppliers website has a link about pizza ovens.
Truckie said there were no spare pallets that could accidently fall off the truck at my place.

Sigh....

Dave, you are right about a sand dome working adequately, but I reckon a mould should be made for two reasons.

1) So the material can be rammed, to get all air out and uniform density
2) Looks. I suspect that the "home made adobe look" might put off some people who might otherwise have a go.

Regards,
Mick

[david s]

Re: Castable refractory

Mick,
If you are set on doing it this way then use inner and outer form work that is vertical up to about half way. Getting spherical forms with MDF would be difficult. An outer mould for the top section is even more difficult so I suggest you have this formed in sand ready to cover with the castable before the bottom section has hardened. You will get a surprisingly smooth finish if the sand is covered with strips of wet news paper avoiding folds. An orbital sander held to the outside of the formwork does a good job with the vibration if you load up the forms about 2" at a time. My moulds are fibreglass so I can use them again. The MDF will be a strictly one casting only.

[wotavidone]

The science behind corrosion

My engineer acquaintance keeps his word.
His "concrete book" is a joint publication from Standards Australia, Stds. NZ, the CSIRO, and the Australian Concrete Repair Association.

The first thing that jumped in my face was the discussion on carbonation of concrete. The mechanism that allows our ovens to be strong even though Portland Cement is not a refractory is also the mechanism that can reinforcing.

The carbonation of our lime rich mortar mix will be assisted by the fire - there must surely be more CO2 in the combustion gases than normal atmosphere.
The breaking down of the Portland cement by reaction of the Calcium Hydroxide with CO2 results in a strong layer of calcium carbonate, but also reduces the pH. If the carbonation goes all the way through the concrete to the steel reinforcing, the pH at the steel will be below 11, and the steel is no longer "passivated".
Fortunately the carbonation process leaves a less permeable layer that retards the penetration of CO2, but I reckon I'd leave the reinforcing out of the hot face if I was casting a dome.
To my mind, this also raises another argument for slow and careful cure. Getting rid of water is one thing, but gentle exposure of the mortar to increased levels of CO2 before getting it really hot must be a good thing too.

[wotavidone]

Re: Castable refractory

There is more than one sort of Portland Cement available in Oz.
Leaving out rapid-set, sulphate resistant, high early strength, the special white stuff for making nice white mortar, etc, there are two types that you might be offered if you roll up to the hardware store and say "give us a bag of cement, mate".

Type GP is the everday Ordinary Portland Cement.

Adelaide Brighton's FAQ page says Type GB is Portland Cement blended with 30% ground granulated blast furnace slag.

So, the question: is Type GB, containing a cementitious substance other than OPC, a slightly better choice for the cement portion of the homebrew mortar?
Opinions, anyone?