DIY Refractory Cement Formula Decision - Couscous

The science content here leaves a lot to be desired. Doing the best I can not to screw this up too much.

I've been stewing over what recipe to use for the insulating refractory panels that will replace the fire brick. After many hours spent reading and dredging up what I remember of chemistry I've come up with some conclusions:

• The chemistry of how alumina (aluminum oxide) behaves when fired is a bit of a mystery even to those who know chemistry really well it seems.
• Aluminum oxide absolutely raises the working temperature of the ceramic.
• There is a clay called Kaolin that has somewhere from 28% - 40% aluminum oxide (alumina) in it that is used in most firebrick recipes and I think many "fire clay" recipes.

Here's the analysis for Lincoln 60 Fire clay

K2O - 2.00%
Al2O3 - 29.00%
SiO2 - 52.00%
Fe2O3 - 2.00%
LOI - 12.40%

Here's the analysis for EPK Kaolin

CaO - 0.18%
K2O - 0.33%
MgO - 0.10%
Na2O - 0.06%
TiO2 - 0.37%
Al2O3 - 37.36%
P2O5 - 0.24%
SiO2 - 45.73%
Fe2O3 - 0.79%
LOI - 13.91%

LOI = "Loss On Ignition" I assume these are organics or hydrocarbons with flash points well below the temperature needed for vitrification. Vitrification is generally used to describe the point at which some of the silicon oxide begins to melt.

The Lincoln 60 analysis very likely omits trace elements below 2%. So if you look at the big hitters here they are aluminum oxide and silicon oxide (silica). The organics are important as they likely provide some of the binding action when water is added. 

The aluminum oxide and the silicon oxide have an affinity for water. I haven't tried it yet, but it must have a role on creating the sticky consistency of the clay. We all know how granular sand tends to stick together when wet, the powdered silica has to be doing nearly the same thing. This affinity for water is called hydrogen bonding. It's a fairly weak bond. It can be broken with a little bit of heat of even just having an imbalance of water in one place as compared to another (air). The water will evaporate off the bond is so weak. 

This affinity for water gives the silica and the alumina the ability to stick together so you can mold or shape the mix. 

Oh, This is NOT cement.

Cement and mortar are based on a combination of calcium oxide reacting with carbon dioxide and other components like aluminum oxide being converted to hydrates like aluminum hydroxide. See this article.

As far as I can tell, this hydration process is not associated with the making of clay based refractories. 

Ok great, so what's the recipe going to be?

• EPK - Kaolin
• Aluminum Oxide (enough by weight to add 3% - 4% to the mix)
• Couscous

The "normal" recipes use saw dust. The saw dust burns out of the ceramic leaving voids that create the insulating properties. The gap don't transmit energy well, and the sheer reduction of mass reduces the thermal conductivity of the ceramic. So why am I using couscous?

If you don't know couscous is a wheat based pasta that is round.

Here's why I think it will work:

• It will burn out like sawdust.
• It is more consistently sized and fairly large so the gaps should be bigger than with sawdust.
• It's round. Round gaps don't have sharp corners that tend to promote crack propagation. So, the resulting part should be stronger. 

So, this week I will make a stop at Leslie Ceramics and Safeway! Maybe this week I'll get to fire some panels. 

Wish me luck!