Pure Clay as a building material?

Hey there permies!

No sure if anyone can shed some light here…. We’re looking to build a small cob structure. We’re located in the wet tropics close to the equator. Our land is almost 100% clay under the few inches of top soil, the clay is very pure with almost no impurities. We have a lot of the common red (terra-cota) clay in the higher parts of the land, but we also have a clay that’s grey and one that’s blueish in the low swampy areas.

From my understanding reading several books, internet research and attending a workshop on cob building… the cob mix always contains sand. Right??

Over the several months we have experimented with different mixes, ratios, etc. Including:

CLAY + SAND + STRAW
CLAY + SAND + STRAW + CEMENT
CLAY + SAND + CEMENT
CLAY + STRAW
ONLY CLAY

Out of this different mixes we experimented with different ratios and sample sizes from a few inches to full brick sizes.

After testing several combos and to our surprise the samples that were the strongest were the Grey and Blue clays in pure form. When mixed with sand and or straw the samples would crumble and break much easier. The ones with the pure grey and blue clay harden into a rock! Incredibly strong, and showed no cracks at all in small samples and some very minor cracks on the brick size samples.

Not sure what’s going on here… is possible this clay is non-expansive? Can we just use this pure clay to build with?

Any advise is much appreciated!

Best to everyone!

It could be that you are just very lucky to have a clay that doesn't crack readily upon drying out.  Most clays do, and this is one reason why some sort of other ingredients like sand or straw are added to it to make cob, adobe, etc.  Even for stucco I found that I needed to actually have more than half sand by volume to not have the stucco just crack and flake off.  I guess one way to proceed, if you have the time and energy, is to do one or more small building projects with it, and then see how they perform for say a year or so, before doing anything really important with it.

Hi Emy,    Its possible that you have the perfect 'ready mix'. Have you done a jar test to see the ratios of what is in your soil? To me, pure clay is like pure cement, very hard but not a lot of tensile strength which is why rebar is added or in the case of clay some kind of fiber. With this in mind, I can't see how your test with clay and straw was less strong than just the clay by itself. Its a mystery to me too....

What you report is unusual.
You may just be lucky, but one thing to do is wait 12 months.
I made some compressed earth bricks which seemed perfect, but after 12 montsh they fell apart.
We found that they stayed together because of the way we pressed them together and the normal osmosis that occurs with moisture and clay did not take place.
In your case, you may just be lucky but I would make a few more and see if the luck is replicated.

Clay shrinks as it dries out. As long as your sample dries out fairly uniformly, all parts shrink at the same rate, and the overall geometry stays the same (apart from being a bit smaller).
However when some parts dry faster than others, they get smaller than the surrounding parts and you will get cracks at the boundary (or in the faster shrinking part).

I am also sitting on pretty much pure clay. What works is to make green bricks from pure clay and then drying them. That way only the mortar needs sand.

As a potter and a builder, I would strongly advise not using pure clay for anything structural, unless maybe you are able to bake bricks. Another way is adding sand, and using earth bag walls reenforced with rebar, or solid cob walls (not necessary for tropical climates). If you have access to bamboo, using bamboo for structural would work. Email me if you have any questions.