Using Crusher As A Fireclay Subsitute?

[cobblerdave]

Gudday all
Spoke to a bloke last week by phone...Brother of a friend who is casting an oven shell over a sand mold. He is using a mix of Ciment fondu and crusher dust.
Crusher dust is basically the small washed particles.. dust and up to sand grain size left over from washing gravel for road ways. Road base is another term I heard used. I have used road base to pack down a base under pavers...add a litle cement powder water a wack down and its like cement..and very stable.
My question...could crusher dust be used as a substitute for the fired component...the fire clay in the home brew fire proof mortar mix. Its crushed basalt fired in the earth not just a kiln so on the temp stakes that should be enough?
Any thoughts

Regards Dave
PS this blokes ovens now has under hearth pealite insulation plans a fire brick hearth and is to have ceramic insulation over the dome so in all other aspects should be a worker

[david s]

My refractory supplier tells me it is quite common to line industrial chimneys with a mix of cracker dust and calcium aluminate cement (fondu) How well it would work for WFO and for how long is anyone's guess. It should also be noted that the temps we fire to are insufficient to convert the material to a permanent fired material. Any attempt to try to fire the material high enough with wood is counterproductive because of the lack of consistent temp rate rise.The 450-600C range must be slow and controlled and this is not possible with a WFO.The material is unstable in this temp range.

[shuboyje]

On the ciment fondue web page it lists aggregates and their service temperature ranges. One that jumped out to me was crushed house brick. Don't remember the rating but it was much higher then we need for pizza.

[david s]

You need to break them up until the bits are no bigger than about 1/4" That's a lot of work. I've just been through that process with insulating firebricks (about 20L) which are way easier to break up, but still an extremely arduous task.

[shuboyje]

Yeah I envisioned some form of home made hammer mill, but I know not everyone has the access to scrap and metal working skills I do.

[cobblerdave]

Gudday all thanks for your replies...
DaveS
"It should also be noted that the temps we fire to are insufficient to convert the material to a permanent fired material."
Could you explain about "permanent fired material" a bit more It seems I have never really understood the role of fireclay in the homebrew mix.
Understand the portland cement is the temp glue to set the mix till finally burnt away. The Lime supplies the long term "glue"at high temps and the sand...an agregate. The fireclay????

Thanks for you input as always

Regards Dave

[david s]

Clay only becomes permanent (won't turn back to mud if wet) when it reaches 573 C beyond that temp it becomes harder the higher it is fired. In the case of calcium aluminate cement ( fondue) it goes hard but it is still not properly fired until it is sintered which begins from 573 +

[wotavidone]

Crusher dust is certainly a valid ingredient. As a matter of fact, it is a major ingredient in the Better Homes and Gardens pizza oven plans. I seem to remember, their recommendation is 1:1:1 of crusher dust, "plastic" clay, and sand. Mix well, throw in a cup or two of chopped sisal rope fibres, and just enough water to make it possible to roll it into sausages to build the dome out of. They say you can add about 5% portland cement to bind it all.
If you look at all the myriad homebrew mixtures, they all seem to have a couple of things in common:
A small amount of cement, presumably for early structural strength
Clay, and sometimes, but not always, lime for longer term structural strength
A mix of various size "inert" aggregates such as fine sand and crusher dust, that, being much smaller than those in regular cement, should give a dense dome, with very little air space between particles. The clay would play a role here too.
In my opinion, the clay is one thing you can't really leave out.
I have bought terra cotta clay, but I also obtained a good supply of clay for nothing.
I found a bit of clay in a wetlands area and processed it into a very clean product very easily:
Fill about one third of a 2 gallon bucket with your raw clay. Add about half a bucket of water. Using a stirrer in an electric drill, mix it all up until you have a slurry called "slip".
Let it settle for a while, until there is a layer of clearish water. Pour off the clear liquor, which will have the leaves, sticks and dead bugs floating in it. It will also remove any soluble components, such as salt.
Make the volume up with clean water and mix again. Let it settle a while. Pour it through a screen made from fly wire into a fresh bucket. More rubbish will be screened out.
If you let it settle for the right amount of time, which will come easy with experience, you will have the slip, but the sand and gravel that was in the clay will have settled to the bottom of the bucket. You make sure that you don't pour the sand through the screen.
You now have a bucket of clean slip. Line yet another bucket with a piece of old bed sheet, with plenty hanging over the edge all round. Pour your slip into the lined bucket.
Gather up the edges of the bedsheet and make a bundle tied at the top.
When you lift this out of the bucket, slurry oozes through the cloth, but the clay soon blinds the pores of the cloth, and then only clean water drops out. Hang this bundle under a tree or your carport, and in a few weeks all the water has dripped out or evaporated, leaving a very clean fine clay. Dry it completely and crush it if you want dry stuff for mixing homebrew mortar, or unwrap it while still damp if you want wet clay for making cob.
If you do this early it can dry while you are finding the rest of your materials and building your base.

[wotavidone]

P.S. I have read that part of the definition of a clay is that it consists of particles that are in the shape of very fine "platelets". When dry, these layer and lock together into a solid mass held together by friction and other forces (such as charge on the surface). This rockhard clay can be broken apart by wetting it, which lubricates the platelets and allows them to slide and separate. Firing clay results in a certain amount of fusion, actually melting things together somewhat. The higher and longer you fire it, the more completely fused together the particles become, and the stronger the finisehd product.
Unlikely to fire the clay in a pizza oven dome, except perhaps for the surface exposed directly to the flames. Doesn't matter much, as long as you keep the dome dry. In any case, if you have lime as well, that'll take care of the moisture and the clay will continue to contribute strength.

[cobblerdave]

Thanks for the input fellas
Result to date on the cast oven is that he has used a mix of Ciment fondue crusha dust and fire clay. He built it on a base of firebrick with cement/vermicule insulation. He is to insulate with 2in ceramic insulation and cement/vermucile over that but as you can see there is not much room left on the slab. looks to me also that the oven was cast onto the support slab not the insulation.



Sorry for the scant detail I have only spoken to the bro and had these pic sent to me MMS.

Regards Dave