Faulty Perlite??

Hello Permies!

So, I have a batch feed rocket stove roughed in:


I wanted to cast the burn tunnel with a insulated mortar, so I followed ABroAudio's batch feed idea and David Eak's recipe for the core (essentially a more scientific version of BroAudio’s mix):

- 15 parts Perlite
- 2 parts fireclay (I substituted Kaolin clay)
- 2 parts furnace cement (1/2 Gal)
- 1 part Water Glass (home made)
- about 40oz of water

(That’s a lot of perlite. )
After a week it was still crumbly and I realized I needed to fire it, which I did, and then was pleased to find it setting up nice and hard. I coated the inside with refractory for a smooth finish, lit a test burn for an hour or so and it worked!!! I was feeling quite positive.

THEN…
A few days ago I had my longest burn yet, maybe six hours. After a few hours, my refractory started lifting off the walls. I wasn’t too concerned—maybe my mix wasn’t as dry as I thought and moisture was still evaporating? By the time my fire was out and the box cooled, the refractory was falling off everywhere. I was bummed, but still thought it was structurally sound…



However, upon closer inspection, I’m not so sure. There's cracks and areas where decent chunks of wall came off as well.


It seems as if my mix is getting crumblier as it dries out. The perlite granules themselves pick out easily and crumble between your fingers.


Now I’m wondering if it’s the perlite itself that’s failing and not the mix??? My perlite was granular, but to my understanding, that’s not an issue for this type of building…? This is the stuff I got: http://www.hometownperlite.com/masonry_fill_details.html

Does anybody know where I went wrong and where this could possibly be heading?

I'm currently using an insulated chunk of stove pipe as my riser for these test burns because I'm afraid my perlite/clay slip riser is too heavy for this base. I expected it to continue getting stronger with burns and not weaker. The way it's looking, I won't be getting that riser on any time soon. And with another burn like that, I worry my walls might give out and the roof of my stove could come down.

I'm really surprised at the way this mix is acting. Thoughts??

An insulating mix only combustion chamber is tricky to get just right, not to mention it's still subject to wear and tear from loading in firewood, cleaning tool abrasion and etc. Personally, I'd patch the firebox with your insulating mix, then completely line it with fire brick splits, mortared in place (1:1 clay/sand).

For curing I like to keep my stoves warm with several small back to back kindling wood only fires, for the first few days, to drive out moisture slowly. A big raging hot fire in a wet freshly constructed combustion unit makes for steam that destroys the integrity of the slowly curing masonry.

I've been chatting with the great BroAudio himself over on his channel, and from what I gather, my dimensions are faulty as well as my mix. I should've referenced the batch feed specs before building (don'tcha love finding the perfect info you needed too late!). So for the benefit of others, check the specs before going and casting a batch feed opening to the convenient size of the fireplace door you got (*eye roll*): Peterberg Batch Box Dimensions

Live and learn. I hope I can downsize my opening and reinforce my walls at the same time with firebrick just to get me through this winter.

In case you haven't seen it, here's a complete build thread for a 6" batch-box using regular size fire brick:

https://permies.com/t/41202/rocket-stoves/Emergency-quick-small-batchbox-sq