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what is the mix for using refactory cement to cast heating core?

Hello, I think you will find what you are looking for here:

You'll have to read through the page, but you will find some good information in the process.

Good luck!

Manny, why can't he use insulated castable core mix, like that on Matt's Walker Stove site?
http://walkerstoves.com/products.html
Or is this only for the heat riser and not as good for use in the core?

Absolute newbie here (and anywhere else to do with RMHs).

I have natural clay in abundance on my quinta (small farm), loads of ash from log burner. Been saving the ash without a clue as to what it might be useful for!!!
When you quote 1:1 mix am I right to assume the ratio is volume not weight?
Have dug clay and mixed it with water using paddle mixer, obviously too wet, so will let it dry out before adding ash.
If the final mix is similar to consistency of concrete, making it easier to fill mould, is there an inherent problem with curing and shrinking?
Martin.

Welcome Martin. Volume, not weight, and if it shrinks too much it needs more aggregate, in this case typically sand or perlite for insulated mixes.

Yes, the 1:1 ratio is by volume, but you should test it with your materials to see how it holds up, and even make some samples with different ratios some using more ash, some less to see what works the best.

Hopefully your ash has not been subject to rain and the elements as that is said to take away some of the properties that make it work so well.

I use the waterlogged clay as my unit to add the unit(s) of ash to, although if you have more of a clay slip you may want to let it sit for a while. Adding enough water to make a clay slip makes it easier to get the roots and rocks out, but will affect the ratios. If I had a slip, I would let it sit until there was a layer of clear water on top, then pour it off and use what remains as my clay unit. You'll want to make sure to mix it back up because if you made a slip the sands and heavier items will settle at the bottom and clay on top and you want it mixed back up.

Just be sure whatever method you use you are consistent so your final product won't have big variations in layers or sections. Depending on how much ash you are using you'll probably end up adding more water to make it workable. And it will shrink some when it dries, and will likely crack a bit if you don't add some sort of fiber to the mix (and even with fiber it will likely crack some).

I recently wrote a post called "Sap Warmer Rocket Stove Made from Ash and Clay" describing a recent build of mine using this mix and included pictures so you might check that one out.

Thank you Matt and hpmer for you advice. I latched onto the clay and ash mix, as in hpmer's "Sap Warmer...." and also a post about work in Africa, mainly because I have this stuff in abundance ie. it is cheap (well...free). I have perlite but hadn't intended to use it. I also have "Fibras de polipropileno multifilamento para o reforco de betao e argamassa" which loosly translates as "multifilament polypropylene fibres for reinforcing concrete and mortar" or "feathers" as it is affectionately known in UK. I don't know if this will serve the same function as glass fibre or straw. I also have some refractory cement which I was going to use.

As for the mould I was hoping to remove inner forms before firing, maybe by making these forms very slightly tapered, just enough to slide out. Do you think the cast material would be strong enough to be self supporting if left to dry out for a day or two. The other thing I was hoping to make is a vibrating table, logic being a more compact cast and also smoother finish. Light film of oil as release agent. What do you think?
Martin

Something else I forgot to mention, I have an amount of dried "mud slurry" from a local borehole drilling. This area has granite bedrock, but more likely to be schist as that is what is all over the place. This dried slurry is off white in colour and very fine. However, it is a bit gritty and I think from the glittery bits contains quartz. Might this have any value in the mix? If not I won't bother trialing it.
Martin.

Welcome Martin. Glad to have you here.

Don't overthink your build. I would just stick with ash and clay, and let your form burn away as it may. It's just such an easy method and I don't anything is gained by complicating it.

I can't wait to see what you come up with.