Question? about refactory cement.

Erica is talking about the refactory cement. She said that they tried Heat riser and core and all and lit it up and it thur chunks out from the concrete underneath it and blew it through the heat riser Into the crowd..
I am wondering if you could tell me how thick it was?
Also, I'm wondering if you made it thicker if it wouldn't do what she said in the video?.
Or do you think I should just use Fire brick on the inside of it?
The reason why I ask is because I have refactory cement that I got free from a friend I was wanting to incorporate it into the rocket mass heater

I thought I would give your topic a bump in case someone knows more about refactory cement than I do.

I would love to hear more about your plans so I hope you will make a project thread for your build.

It depends on what exactly you are calling refactory cement?
Is it just cement powder or ready mixed with crushed fire brick or something else?

Tammy Lull wrote:
The reason why I ask is because I have refractory cement that I got free from a friend I was wanting to incorporate it into the rocket mass heater

in my local, this could mean:

Castable refractory  dry mix
or
Refractory mortar used thinly between fire bricks
or
bricks or already casted with above material, intended to be used like fire bricks

You mention that the concrete beneath the core exploded. This is a big part of the answer - any core needs to be isolated from ordinary concrete so the concrete doesn't overheat. Even just insulation will only delay the explosion, not prevent it. You need some air circulation space between core and concrete to allow air currents to carry heat away.

My understanding of castable refractory cement is that they have very specific instructions on how to cure it as far as temperature and time. A rocket mass heater might not be the best device for curing it. I'd imagine it would be possible to get it to blow apart in a similar fashion to portland cement if heated too quickly among other factors, and a thicker cross section would increase that possibility.

If you couldn't cast it as a separate core that you could try to find other methods to either more closely replicate the manufacturers curing instructions, possibly mitigate catastrophe by adding grog (very small broken up bits of cured refractory or high fire clay) or thinner cross sections backed by ceramic fiber blanket. I learned a lot about this from a metal casting forum over a decade ago. People with experience doing that and using refractory in atypical scenarios would probably have more specific instructions and tips from experience. Knowing the exact brand and product would be helpful.

I also know that people have had problems if the refractory is old and has been sitting around a long time collecting moisture from the atmosphere. If the brand or how long it has sat around is unknown, it might not be worth the free price if it wastes your time and resources.

Refractory cement is special. It comes rated in different grades (typically based on max service temperature), and I'd be super cautious about freebies, old mix, or anything not fresh from the maker/vendor. Remember that a wood fire gets hot, like 2000ish F flame temperature.

One excellent page is here: https://www.sheffield-pottery.com/LOUCAST-3000-CASTABLE-MORTAR-p/lvclc.htm

Read the mixing and working and curing and first-heat instructions to get the full story. It is all about water. Portland explodes not because you degenerate the bonds (that requires the long hot furnace at the plant) but because the free water in the material cannot escape fast enough, and you flash it to steam inside. Portland is a sponge with teeny pores (see also why efflorescence happens on walls exposed to groundwater flows). Water can move through those pores only so fast, and Portland is hygroscopic, meaning there is always water inside. So when you heat it up, water tries to get out, and the more heat, the harder it tries to migrate, until bam, you flash boil and spall.

(I have been near a steam explosion, I worked at a foundry, the melt lead ordered wet scrap dumped in a hot furnace. Bang. Thank God nobody died, but a LOT of glass shattered and the furnace refractory lining needed rebuilding.)

So the moral of the story, in my view, is know where to be obsessive about the right materials. Refractories are one of those. (So, too, is supply side plumbing, I think.)

Happy homesteading!
Mark