Calcium Aluminate — This is the most important ingredient. Alumina cement generally makes up anywhere from 15 — 40% of the mixtures we use (though higher and lower than that is possible). Theoretically it should be possible to lower the cement portion to 10% or even a little lower, but I find that depends on all conditions being perfect, so it does not survive the test of real conditions very well — for us at least.
There are numerous types of calcium aluminate, with varying degrees of purity and alumina content, ranging from about 40 to 80% alumina, with the ones at the higher end usually having less iron content. These also vary in other aspects, and sometimes come with additives premixed. I choose ones that are low in iron, require lower amounts of cement to set properly, that contain about 70% alumina, and which are most readily available.
Shelf life can range from 6 month to two years or more. Based on my research, short shelf life is often the result of pre-mixed additives, so it is better to purchase pure calcium aluminate and introduce additives yourself. There are two companies I know of with products that will last for 2 years if properly stored — Almatis and Gorka, both of whom supplied me with enough sample material to do most of our testing so far.
Unfortunately, calcium aluminate can be difficult to obtain, and expensive per pound. Suppliers are geared almost exclusively toward bulk quantities, and shelf life is often too short to make it feasible to buy large quantities and then use it over a longer period. I am hopeful that might change if enough of us want to use this process, and perhaps ceramic suppliers might offer it.
Here is a list of suitable calcium aluminate products for making ceramic concrete mixtures :
Aggregate — Avoiding shrinkage is crucial with this process, at least up to the point when the mold is burned away. Hydraulic cement (water-set cement) helps with this by hardening before water is removed, but there is usually still some shrinkage despite that. Potential shrinkage can be further reduced by ensuring there is a range of particle sizes including larger particles of ceramic aggregate materials like grog, molochite, pyrophyllite, mullite, and sand. The strategy with the best results is to have some particles that are relatively large, and then particles sized to fit the space between those, and so on down. Such a mix will also limit the amount of shrinkage due to sintering and vitrification, which happens after the mold has melted away (we have not been able to do away with that, having reached a limit of about 1.5% so far). The size of the largest particles in this mix is determined by the thickness and size of the form. The larger and thicker the form, the larger the largest particles must be, as with any concrete. However this need for a range of sizes of aggregate must be balanced by other needs. If aggregate sizes are too large, the mix will not flow easily into the mold, and details will be lost, or whole sections if there is a bottleneck. Also, the higher the proportion of aggregate, the lower the proportion of concrete and other materials.
Flux — One of our goals has been to fuse the material sufficiently for good glaze results. This is where things get tricky, because when you look at the mixture from the perspective of a unity formula, the calcium aluminate introduces a considerable proportion of alumina. I had hoped to come close to matching the unity formula for typical clays, but I have found that more challenging than expected (that might be easier with more calcium aluminate options, because lower alumina cement would be easier to balance). When I tried adding enough silica and flux to counterbalance the alumina, while also controlling shrinkage and including sufficient cement for setting, I was not able to find a way to do that yet. Eventually I set aside that goal in favor of a more empirical approach of adding varying amounts of frits and feldspars and seeing what gave the best results. This is an area where there is hopefully room for a lot more progress to be made.
Plasticizers — Next I want to discuss additives that impact the concrete pour itself. These are a minor proportion, but crucial. Without them, the material does not flow well enough or have enough consistency. The most important of these is “superplasticizer”, which provides good flow with less water. Superplasticizer is added in relation to the weight of the cement alone rather than the overall mixture, and should be about ½ to 1% of that. So for an overall mix containing 20% calcium aluminate, one would use 0.1 or 0.2% superplasticizer.
Set Retardant — Another additive we have found to be important is a “retardant” to slow down the casting process, without which the set can happen too quickly. For this, we have found citric acid to be a good option, using the same amount or a little less than the plasticizer. Pure sodium citrate can also work for this purpose.
Clay — One other additive that impacts flow into the mold is clay. Too much clay causes shrinkage, but without it, the top of the cast is often watery and weak. This situation was improved by adding 1-2% bentonite, and/or by including 5-15% clay in the mixture (fireclay or kaolin are good for this).
Fibers — Another important additive category is fiber. Fibers add tensile strength, limiting cracking and warping. Some fibers such as nylon only add strength before firing, while fiberglass or stainless steel also add strength after firing. Nylon is the one we have had the most consistent results with, even though it burns away. Steel is used in refractory applications, so it seems to have more potential when working with very large forms, but with smaller forms it has caused cracking in our tests. Fiberglass has shown promise, but is harder to work with than nylon because of its stiffness. All fibers will impede flow in some situations, so they must be used in moderation unless your mold is very open. Generally, we have been using between about 0.1% and .5% fiber by weight when we test it.
Water — Finally, perhaps the most important ingredient: water. The amount of water to use is quite important, because with too little, the mix will not flow well, and with too much, there will be cracking, or a weak top layer. As a percentage of the total for dry ingredients, we have found that between 22 and 25% of the dry ingredient total is a good range, although it can vary above or below that.