I’m currently exploring how to get better performance from cast refractory.
 

Some notable things I’ve come across include calcining your own additives. I learned that if you burn epsom salt with a torch it releases some toxic gas that is basically sulphuric acid. What’s left over however is magnesium oxide, the highest melting temp oxide you can get. In small amounts it increases the melting point of refractory materials. 1/10% is enough to stabilize alumina and zirconia when brought up to full temp (3000f+)
 

Alumina hydroxide aka alumina hydrate is usually available as a pottery additive. If you calcine/burn the heck out of it you get alumina as a raw material. Great for refractory mixes. Don’t mix alumina hydrate into refractory mixes. It has made several of my tests fail. The hydrate vents a ton of water (roughly 1/3 by weight) when it heats up and it blows up and or crumbles to dust.
 

A fair few times I’ve read posts on websites about people trying to create a flame face coating using water glass/sodium silicate and a product called zircopax. The zircopax is never going to stick doing that, and due to the super fine particle size it will flake and crack and warp as it dries. Get calcium aluminate cement binder instead. When using the CAC, calculate the amount of water you need, and use the water glass instead of just water. Water glass is 40% solids, so that remaining 60% is available water for your cement to cure. The effect is a faster cure, and a much stronger binder. Sodium metasilicate is sold dry as deck cleaner. It’s cheaper than buying the pre diluted solution, just dissolve it in hot water. 

If youre trying to go the zircopax route like I am since its got a higher service temp than most other products, you will benefit from making frit out of it first. Mix a batch with 10% water glass and pack it into a sheet. Let it dry and then heat it up as much as you can. I ran it in my forge until it was white hot. Then smash it up into coarse sand size particles. Mix 4 parts sand size particles, one part fine particle as you bought it (should be 2-5 micron powder), and somewhere around 15% by weight of CAC. Casting this 1/2” thick should be a very durable material. As an optional note I got from one of the manufacturers of similar products, add a 1/4% by weight polyester fiber to the mix. Its job is to burn out and make vents for excess water vapor to get out as you cure and fire the mixture.

What specific properties or improvements are you trying to achieve?

Hi JHCC, I want the cast mixture to be more durable and already have the IR reflective properties of the available coatings. I have been experimenting with binders, instead of silicate binder or calcium aluminate, the next one I’ll be trying is alumina phosphate. Even if none of this works it’s still very fun. 

I have dabbled a bit in refractory experiments.  While MgO has a higher melting temperature, Zirconia is known for it's toughness and better insulating property.  In particular, Yttria stabilized Zirconia.  I got some waste powder from where I work.  I attempted the first ones using diHydrogen Alumuminum Phosphate as a binder.  Not especially successful.  My next run I will try aluminum phosphate as a binder.

Hi Lee, I continued my post in another thread under the everything else category. One test I did with the aluminum phosphate binder went very well. I’m attributing it to having a contamination of about .25% hydrated calcium aluminate cement. My mixture was  zircopax frit…I fired a mix of zircopax and sodium meta silicate to white hot and smashed it up through a screen until nothing was larger than 1/16”, a small amount of coal slag, perlite that I ground into the mix, and some alumina. Exact measurements I’m not sure about but call it 75% zircopax total. Turned out very hard and thermal shock resistant. Now I’m reading that to get real flux resistance from borax products I need to add chromium oxide since everything else either melts with it at low temps or gets stress cracks from it.

Hello to you as well.  I read that other thread too.  You could choose to not use Borax for flux obviously.  I don't think I would want to go messing around with any Chrome compounds except maybe Google Chrome.  The reason for using Yttria stabilized Zirconia is that the Cubic phase is desired, and if not stabilized it can more readily undergo a phase transformation into monoclinic resulting in a volume change which can cause failure. 

Just out of curiosity I do feel the need to mess with the chrome oxides haha. As I read more though it’s very important not to have it in a mix containing any calcium material. Turns out at medium temperatures (right around forging and welding temperature) it converts to hexavalent chromium. I’ve ordered some chromium trioxide to experiment with, I’ll be using a zero contact safety procedure. I refuse to accept that the inside of my forge is a consumable material.
 

what is your background if I may ask? You seem educated not just like someone who reads on these subjects. 
 

Important Update….Lee you were very much correct to say don’t mess with the chrome compounds. Apparently adding the chromia (Cr2O3) is great for a reducing environment, it’s the best binder and adhesive available for a lot of industrial settings, but in an oxidizing environment in a temperature range that includes the warming phase of gas forges, it can transform into pure solid hexavalent chromium. So I will still be performing my tests just because I have the safety equipment to do so, but definitely don’t try to replicate it without really understanding the hazard it presents. It will basically turn your forging area into a superfund site after it kills you. It’s far worse than welding stainless and huffing the fumes for a week straight through a straw, it’s as bad an idea as you can possibly have.
 

Mods or admin if there is any way you can delete the last post I made just in case someone reads it and doesn’t see this one I’d appreciate it. This is not suitable for anyone outside of a lab or very specialized industrial uses. 
 

Will update with test results. Maybe even a picture.

 

sorry about that

Ok pretty deep in the rabbit hole now. It seems we can swap the chromium oxide for zirconium hydroxide in the phosphate binder. This is currently being tested for ablative shielding on space shuttles and hypersonic vehicles. I’ll be ordering some to test shortly. Supposedly it’s rated for 2500C and is flux resistant. Borax will not win this.

16 hours ago, ZebraPaste said:

what is your background if I may ask? You seem educated not just like someone who reads on these subjects. 

Hey, I'll take that as a compliment.  I have a BA in Telecommunication and Film.  I have also taken a few Material sciences classes.  However, I program industrial robots.  I suppose that I could add that, as a kid, I was kind of a junior mad scientist, with like three chemistry sets, test tubes, flasks, etc.

Where I program robots we do thermal spray, a variety of processes that adds material on the surface of parts to enhance properties or otherwise protect from heat, wear, corrosion and so on.  A large part of the work is to apply TBC's, Thermal Barrier Coatings.  This would be mainly Yttria stabilized Zirconia.  That is how I have gained what I know about this material.  I also used to be a thermal spray operator at a couple different place.  That was how I got started in robot programming.