The Properties of Clay in association with the Manufacture of Compressed Earth Block

By Dan Powell

It has long been known that Clay soils have been needed for the casting of adobe block, Emperically, most other soils without clay would not maintain the desired shape and through trial and error mankind, for the past several millennia, has found that this material is, if not superior for the task, at least adequate.

          Modern science has given us an insight into the behavior of clay by describing not only the behavior of this remarkably usable material but also describing the structure so that we can understand the why of this behavior and target our process to take advantage as well as modify the process.

          One of the distinct advantages of the CEB system is the abundance of clay throughout the world. This allows us to make block nearly everywhere and by using some very primitive methods determine the clay content of most soils. This allows for a touchy feely method that has confused many folks.

So for those of us who want to know why clay works and for those of you who are just now being initiated into the knowledge I will endeavor to shed some light on the behavior of clay soils as they pertain to the manufacture of Compressed Earth Block.

          What is clay? For a brief explanation click here: http://www.essortment.com/all/propertiesof_rfwy.htm. In my own words clay is a mineral association that forms in fine layers. Whether in ionic or covalent bonding I’m not certain maybe both depending on the minerals involved. I do know that clay is hydrophyllic unless it is completely dry then requires shearing to wet. It was explained to me once back in my oilfield days that Bentonite a montmorolinite (see here: http://en.wikipedia.org/wiki/Montmorillonite ) requires shearing by water under pressure to yield. As further explained the yielding process occurred by jetting (mixing with a pressured water jet) expanding the clay particles making then string like thereby enabling the drilling mud to carry the rock particles/chunks up the hole. All this may sound irrelevant but hang with me. I found a really great explanation for why clay requires shearing. Check out this link: http://pubpages.unh.edu/~harter/crystal.htm. When you see all those layers in ionic linkage you can see why water would have such a hard time penetrating, yet each layer wants water which will slip into the bond if even temporarily.

          Ok, so what’s the big deal? Adobe traditionally requires taking the clays to mud so that the clay would yield and surround the non clay particles. This works fine if you have lots of land and sun to allow time for the block to dry. When you make a compressed earth block you only need a little moisture, 6 to 10%, to make a block that is ready to place in a wall immediately. So we can make hundreds and thousands of block daily on a small piece of real estate and build at the same time.

          So with this understanding we can answer lots of other questions such as:

          Why do the blocks maintain their shape?

The clay is formed with a low moisture yet yields surrounding the non clay particles with enough strength to maintain its’ shape

          Why do I get shrinkage cracks?

With a high clay and high water content the water is compressed in the layers and when pressure is released the excess water is released causing a separation in the layers

          What happens if the blocks get wet?

Because clay is hydrophyllic the blocks will absorb moisture. Too much moisture will relax the clay from around the nonclay particles with a resulting disintegration. This is a slow process as the clays when dry are so tight that they actually resist moisture penetration.

          What does sand and gravel do.?

We call those “non clay” particles and give clay something to grab. Even though a pure clay block will be extremely strong it will also shrink and crack, sand and gravel mixed will minimize this quirk.

          Can I find the right soil locally?

Every where there has been volcanism, granite, sedimentation you will find clay, in decomposed choral deposits, in lime stone deposits, pretty much every where except on a sandbar. Clay is every where. Usually you will be looking for the nonclay.

          Will every clay work?

Yes, but some better than others.

          How much clay, sand and gravel?

We successfully make block with as little as 20% clay, we try to keep the larger aggregate (gravel) to around 10%. We’ve made good block with as much as 70% clay but to do that we need to keep the moisture down to close to 5%.

          I hope this helps, is you find errors in this work please email me @ dan@adobemachine.com .