very low clay content, sandy soil

Hello I did the subsoil & water in a jar test and can see no clay in my soil. It crumbles if I try to make sausages.

I need a cob bench for a rocket mass heater. I have a free supply of local chalky rock.

Any advice about best way to fill my bench and make cob mortar v. Much appreciated.

I have some cement left from a previous project so I was thinking of replacing the clay with the cement. To clarify. I am not discussing  the stove parts. Just the bench which will store the heat from the hot air vented.

You can't very well do without clay if you want a proper cob, from what I read on the subject. I think some methods use animal manure, specifically cow, I think, but some in your situation who need more clay in their soil makeup sometimes bring it in from outside, bought or dug up from a clay-rich area, and mix it in.

-CK

When we starrted building our school out rammed earth, we used a seam of what we thought was clay very nearby, to mix with the sandy onsite soil. We made a bunch of adobe bricks with different ratios, let them dry, then tested them in various ways: rubbing, kicking corners, dropping, etc. Then we chose the ratio of the bricks that did best, and built our whole school, two story buildings. Years later we learned that technically that's not clay, it's fine slt. But it works fine in earth building: the walls are strong and durable.

The proof of the pudding is n the eating -- so try making some bricks with materials you have on hand in different ratios, and see if any are good enough to use.

The materials I have to hand are rock and sand.
No clay. Or so little clay as to render all attempts at cob useless. It just crumbles.

I'm pretty sure that's all there is nearby me. Well. Anywhere for free, that is.

I've tested the soil twice from several areas and there's no clay or very little in it. Just sand very little silt and humous

So I need to add clay, I think.

I wonder if anyone can say what type of clay to look for in what proportion to very high sandy soil and from what type of source?

In your case, I think clay is clay, and any type you can find will work. Using it in small proportions, like around 15%, even expansive clay like bentonite will probably not have any bad effect, especially as you will be keeping it dry so swelling will not be an issue.

You might be able to find dry bagged ground clay at a masonry supplier; I have no idea what is available in France. The cheapest clay you can find should work fine.

I bought some fireclay from the UK in the end...I was on holiday so it wasn't off my travel route.
not being able to source anything nearby in France it seemed like the best option

It seems a bit unusual not to find any clay at all in the soil but I'm not much of a soil expert so I assume there's no point keeping looking. I live right by a river just next to two sand quarries. So I guess it makes sense.

it was £14 for 25kg from Bath Potters supplies which turns out to be not in Bath but in Mid Sommer Norton


The shop was great. They were very helpful.

https://www.bathpotters.co.uk/fireclay-dried-amp-pulverised/p5968

I didn't spot Inspector Barnaby.🤪🤩

I've been wondering if wood ash from the stove would help to hold it together in the absence of clay. I have read that very fine (fly) ash can be used in place of part of Portland cement for concrete - .

Alot of my soil is this way and so when I build with cob I often need to buy in clay.  Be careful that the local fireclay you buy at the building store is just clay and does not have any cement in it.   I have the best luck buying clay from a pottery supply house and yes it is still bags of dry powdered clay.  I have the best luck when mixing the dry clay to add the clay to the water, not the other way around.  SO, you put the water in a bucket or old bathtub or what have you and add the pwdered clay on top and mix and then when that is mixed, add more, etc... You can even use a mixing rod on your drill to help with this, you can buy these mixing tools at the hardware store.  Donot breath in the powder.  Look for instruction for making clay slip for light straw clay construction for tips on making a clay slip.

Then, you use this wet clay slip with your native soil to make your cob mix instead of some or all of the water.  you will need to experiment a bit for your ratio

'Primitive Technology' on YouTube has a couple of really good videos on making lime from ash.  In one of his videos he fired bark (I think species matters), then took clean white ash, mixed it with sand, and formed a brick.  He noted that it warmed as he was mixing in the water, which means it was slaking without the need for any extra processing.  After the brick had set up he immersed it for 24 hrs with no degradation!  How nice and simple.  As I understand it, hardwood ash tends to have a higher calcium content.  The specific use of bark in this video makes me wonder if maybe bark contains the best concentration in the tree.  That would be great because I am busy stripping red and white oak logs and I don't want to sacrifice any of the wood itself on the off chance that I'll wind up with lime.

To see the video, search YouTube for 'Primitive Technology Pot Made of Wood Ash'.

He has lots of other good videos!