Cement board (hardie backer) with water glass

Has anyone ever experimented with cement board coated with water glass to create a "batch box" for a rocket stove? These materials are very cheap and it seems to me if you place fire brick splits at the burn tunnel end of the chamber and just coat the rest with water glass it should be able to withstand prolonged use.

I'm thinking about trying this, creating a box, coating the inside with fireglass, wrapping the outside with ceramic fiber blanket and then covering that with another layer of cement board for finishing. Any reason this wouldn't work?

Thanks,
Wileythenord

Hi Willie;
Well we call ourselves rocket scientists and experimenting is what we do!
So that said, if you have all the materials then give it a try.
I do think you have your terminology confused. You are talking like you want a batchbox but then you mention the burn tunnel.   A burn tunnel is in a J tube rocket not a batchbox.
Quite a few of us have tried waterglass. It has  proven to not hold up very long.  One tap on it with a piece of wood and it flakes off.
If it falls off, your hardy board will disintegrate from the heat very quickly.
Putting hardy board outside of your ceramic blanket is not necessary, it will not serve any purpose.

Cement board and hardibacker are waterproof and fireproof, but not suitable for sustained high heat use. I have used cement board as the floor for test fireboxes, and have a piece of it as a cover for my J-tube feed. It has lasted several years, but has cracked and disintegrated at the hottest parts, and needs to be replaced now. I tried a piece of hardibacker as a cover, and that broke down within a few firings. This is just at the top of the feed tube, never mind in the burn tunnel.

I think I understand what you are thinking about, water glass seems to work very well on a more porous  surface that allows it to soak in a little.
There are many people who use it on ceramic fibre board to great effect.
There may be different grades of water glass (I don’t know).
I think you also intend to use the cement board as an exterior finish over the insulation and I think that would work fine.
There are quite a few fire rated cement board products available in the U.K. some are rated  to extremely high temperature but only for a limited time like a few hours.

Sorry when I said burn tunnel I meant the hallway leading from the burn chamber to the heat riser.

Yes scots John, thats the gist of what I'm shooting for. I'll use firebrick in the areas that get hot and just paint the top area with water glass to keep it from burning. I'm going to try to mock it up this weekend using a 60" 5 minute riser and 2 55 gallon barrels stacked.

Thanks,

In a batch box system, the part between firebox and heat riser is called the port. It is only 2 or 3 inches long, and a relatively narrow slit compared to firebox and riser. This part needs to be strong and resistant to long-term high temperature. No kind of cement board will work for long.

A J-tube burn tunnel is around 6 to 10" long between vertical feed tube and vertical riser, and is about the same cross section as both of those. It can be softer than the feed tube needs to be, but must stand extreme temperatures for hours on end.

Thank you, I appreciate your comments and your help.

If you go back and read my post I talked about using fire brick splits in the burn tunnel, which I now know is called the port. I wouldn't use bare cement board at the port area.

Anyway, I was really just asking to see if anyone has tried it, I guess no one has. I'll try it out when I get time to mock it up and report back.

While I haven't tried your exact suggestion, I have seen that hardi-backer board doesn't tolerate fire well.

I use water glass when sand casting an object that has a hollow core and the core is very solid prior to casting but once the heat hits it  at the end the core is not a cohesive mass like it was before the heat. so I'm not sure how repetitive heats would affect water glass.