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Diatomaceous earth for cob

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Hi there, Long time reader, first time poster here.
Has anyone heard of Diatomaceous Earth (DE) being used in cob or earthen plasters? It's often referred to as "clay", but I'm not sure how/if it would work well as a binding agent. I know it's too cost prohibitive for most, but we own 40 acres of desert, of which, about half is DE. I know it's very light with good insulative value, and has MANY uses. We are going to be building there soonish (less than 2 years) and if DE would work, it would save a lot of time, effort, and expense. I plan on taking soil samples when we go there this summer... but I'm curious and impatient. So I figured I'd run it past some of the cob gurus here.


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Hello Lew,

Welcome to Permies, Yes it should work fine. do your testing and remember it is an expansive clay and is worse that ice for heavy a foundation when it gets wet and expands. Very plastic and sticky in my experience. If you have any Adobe workers in your area, ask them.


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Location: Okanogan Highlands, Washington
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Thanks for pointing me toward this post, Lew - I don't spend enough time on the cob forums.

Like you say, most places diatomaceous earth is not common enough to be cheap building material ... I haven't worked with it. Marvelous luck to have so much, though I imagine it makes a rather sterile soil.

When the ground gets damp in the springtime, and you walk or drive on it, does it become rock-hard and stay that way after it dries out? that can be a good indicator of cob-ready soils.
If it gets sticky and slick when wet, then remains in hard little pieces as it cracks and dries, it can probably work as the binder. If it returns to soft dust when dry, probably not. Some loose minerals do work as a whiting/filler for paints, or aggregate, depending on the particle size.

The best way to find out (short of finding someone local who knows from experience) ...
... mix up some test bricks, with different proportions of binder, aggregate, and fiber.
Dry them thoroughly - in the sun, in a kitchen oven, don't coddle them because after all you want to stress-test them. (On your finish wall, you can work a gentler drying cycle if needed.)
You want something that doesn't crack, dust off, or otherwise betray its origins, but remains a hard surface that can take being dropped on one corner or end with minimal damage.

The thing I'd watch out for is that its other uses suggest that dry DE is a sharp, fine dust that might be hazardous to breathe. Of course there is no dust that is really 'good' to breathe, but fine particulate can be worse because it gets deeper in the lungs.
To limit dust exposure, we keep the mineral materials damp during building, trimming, and restoration. If the plaster seems to dust off too easily, I'd finish the walls off with a paint or thin plaster with a little extra binder like wheat paste, casein, or Elmer's glue. I've heard yucca juice or some cactus juices can also work great; any starch paste is likely to be useful. These materials can be used maybe 1 part goop to 4 or 8 (or more) parts water when making your plaster or paint mix. For clay plasters the clay is the main binder; the organic binder is just there for extra boost and hardening.

If you give it a go, I'd love to see photos of the test bricks.

Erica W
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