Home made castable concrete recipe?

[Stone Mason1]

Hi all, im venturing into the world of pizza oven construction and fire places/BBQ's as a part of my small stone masonry buisness.

Im looking for a recipe for casting the inner layer of the pizza ovens.

Heres what i know... A standard fire brick i can purchase contains this:

Al2O3 (alumina)
SiO2 (Silicon dioxide, Silica, Quartz)
FE2O3 - (Iron Oxide)

And the extra ingredient in a refractory cement is:
CaO (Calcium oxide,quicklime or burnt lime)

I have sources for Iron oxide, silica sand and calcium oxide.

Im just not to sure about alumina and the drying time of using calcium oxide.

Is it possible to make a mix using:

Portland cement 30%
Silica sand 40%
Quartz 10mm pebbles 28%
Iron oxide 2%

What would the cons be not having the alumina content?
Also info on where alumina would be able to be sourced would be nice.
Any help on this would be great...

[wotavidone]

Hi,
Just noticed your post. I'm no expert, but I think you will find the iron oxide is there as an impurity, not because its needed to make the brick refractory. Essentially, the fireclay the fire brick was made from had a bit of FeO in it. But it would be the amount of aluminium silicate that determines how refractory the brick is.
As far as homebrew goes, the CaO will turn into CaOH as soon as you mix it with water, and get rather dangerously hot while you do it. So you may as well use ordinary slaked/hydrated lime to start with. In any case, your portland cement, sand and quartz pebbles are simply going to make an ordinary concrete, nothing particularly refractory about it. I suggest you read the Fornobravo high heat mortar primer available for free right here on the FB website. I also recommend a few searches on wikipedia. I've found that very illuminating. There is an interesting article on slaking burnt lime in a pit and allowing it to mature for months to make "hydraulic lime putty". As well, try reading the data sheets for a few commercially available castables. they give you some clues.
Remember that the assay of a material does not tell you the whole story.You can't just mix a couple of materials together and get a refractory material. For example,I can dry mix lime and silica sand together, and when that mix is assayed, I will get a result of say 50% lime, 50% silica. But clearly, the two would not be chemically combined in any way, and therefore, there would be no refractory calcium silicate present.

The forno bravo homebrew using sand, portland cement, lime and fireclay is described by FB as not a true refractory. This statement would be correct, I think, because the components are not chemically combined.

I've been thinking, reading, and applying as much of the knowledge I gained in 19 years in an industrial analytical laboratory as I can. Here is how I think the FB homebrew mortar would work.
It simply is a portland cement and sand mortar that has been provided with lime and clay to hold it together after the fire has burned away the portland cement. The lime CaO will eventually form limestone, calcium carbonate, CaCO3, by reacting with carbon dioxide in the air. This has crackfilling/mending properties, as CaO does not take up as much space as CaCO3. If your pizza oven gets hot enough some of this limestone will be converted to burnt lime, CaO, which will react with water in the air to form CaOH, hydrated or slaked lime, and eventually this will react with carbon dioxide to form limestone again. If your oven gets hot enough. Otherwise, the lime will just plain harden to calcium carbonate over time (weeks or even months) so as to take the place of the portland cement.
If the oven gets hot enough to actually start burning the lime back to CaO, the clay will have been fired hot enough to go through the silica inversion that occurs, at about 500degrees centigrade. I think that's the approximate temperature. This is an irreversible change in the structure of the clay, and it becomes a ceramic, a porcelain or fired terracotta, depending on the clay used. At this stage, your homebrew mortar, if dried carefully and slowly, so that the portland cement has reached near full strength before the clay dries and tries to shrink and crack the mortar, will convert into one monolithic piece, as strong and fire proof as it is ever going to get.
A few people on this forum have used the FB homebrew as a castable to make the dome. Unfortunately, I can't seem to find many reports on longterm durability.
People who have used it as a mortar for brickwork swear by it.
Probably the best source of alumina is ciment fondue, also known as calcium aluminate cement. Local building supplier can actually get that stuff for me. At five times the price of ordinary Portland cement.
Anyway, hope I've given you a some food for thought.
Regards,
Mick

[dmun]

Portland cement 30%
Silica sand 40%
Quartz 10mm pebbles 28%
Iron oxide 2%

This looks like ordinary concrete, with fancy aggregates, and a bit of red colorant. I'm certain that this will fall right apart in contact with direct flame.

[david s]

Alumina is not like a material you buy. It is found mostly in clays which are composed of varying proportions of alumina and silica along with all kinds of other, sometimes undesirable, fluxing materials like iron oxide. You can trial lots of materials and combinations, but I think you are better off sticking to a proprietary castable.