CEB Stabilization Questions

We are getting much closer to full-scale building with the CEB – with September 15 as our starting date. Another collaborator will arrive here around that date, Alex Rollin – which is extremely exciting becuase he is also an information architect, organizer and integral thinker – so we’ll be likely to double the organizational efforts at Factor e Farm. The pen is mightier than the sword, especially when the other hand is in the dirt.

We are currently discussing the issue of stabilizing compressed earth blocks with Class C flyash (cementitious flyash), which is a byproduct of coal-burning power plants and can be obtained for free or near free. As far as I know, its performance is similar to Portland cement, and I have hearsay evidence that it actually performs better than Portland in the long term.

We are looking at building structures with stabilized CEBs below-grade – 3 feet down to the frost line, on a gravel foundation. As such, the CEB blocks are part of the foundation. To do this, we need to stabilize the bricks to make them moisture resistant. Each 4x6x12 inch brick weighs about 15 lb. We are considering 25% weight ratio of fly ash cement to soil. These are the options that we are considering for a 600 square foot addition, with 18″ thick walls with insulation between the two layers of brick:

  1. Using stabilized bricks down to 3 feet below grade. Disadvantage: requires 6 tons of flyash to stabilize the bricks – and flyash must be transported to our site. Advantage – seems like a sure bet.
  2. Using unstabilized bricks, with either cement stucco or tar for the water resistance. Advantage – allows us to use local soils for the foundation, with only a thin layer of cement or tar for the required stabilization. Disadvantage – effectiveness of this stabilization method is in question.
  3. Start the wall at ground-level, with gravel foundation to below the frost line, and insulation extending 3 feet down. Advantage – the goal of retaining warmth is accomplished – and issues of brick stabilization are avoided or minimized. Disadvantage – 18 tons of transported gravel are necessary to fill the foundation.

We need your help on these issues. What are the suggestions from any builders out there? Please post this question to other forums as well.


  1. George Fryer

    Where are you located?

    Kansas City area, Missouri.

  2. Tim Krahn

    Hello. I am a civil engineer from Canada. I am currently working on an inter-disciplinary PhD in Architecture & Engineering specializing in earthen construction. I have some experience with foundations and alternative construction methods.

    Do you have access to used tires near your site? These can make excellent permanent forms that allow low quality soils to be used to make very stable rammed earth wall systems. Alternatively, I am working on a system to use geosynthetic fabric (landscape cloth) to make a fabric formed rammed earth wall system that could really reduce the amount of cement (fly-ash or portland) necessary for a below grade application.

    I strongly recommend that your final solution include insulation to make use of thermal mass, and that you think about drainage all the way along. It is much better to guide water to where you want it on your site (both above and below grade) than to try to make your building elements water-proof.

    Thanks for the comments. We have access to tires – but we’re shying away from that due to the significantly increased manual labor required. If you have any suggestions that would make tires anywhere close to CEB in terms of time-effectiveness – let us know. My wrist is not in great shape from the various manual tasks here – so packing tires with dirt by hand is out of the realm of options for me even if I had the time – Marcin

  3. Mark Hamilton

    Okay, I am an over 55 year old, reclining chair dreamer; stuck in suburbia till I fully retire. On the other hand, I am more than happy to share my ‘book learn’in’ in some areas – in particular, earth and cement materials. Be forewarned, I have read lots and experimented little.

    The standard ‘stabilization’ is 10% lime or portland cement. While both work for rammed earth, the type of soil can sometimes suggest one type for another. When I get home tonight, I’ll look it up and give a little more infor on that consideration.

    Fly Ash, Type C, may have much to like as a stabilizer. It requires less moisture than PC, is easily worked by itself, and is cheaper than PC. It also reacts as a hydraulic (aside from its possalanic potential). However, it also hardens slower than PC. While I think 15% would be acceptable purely as a stabilizer, I would be careful with higher portions in the mix. In particular, fly ash may not be nearly as sticky as your rammed blocks and it may decrease cohesiveness. Its counteracting ‘cementing’ of soil value may be too low to add cementing strength – even at 25%. And because it is very slow to harden, you may not see the real benefits for a month or two.

    Also it is worth noting a few facts:

    High F.A. cements have been made, but usually at a 50:50 mix with P.C.
    More usual cements have been made with a 25:75 FA:PC ratio. I don’t believe anyone has made 100% FA concrete due to low strength and slow cement hardening. FA value in concrete is (usually) its ability to pack more fine particles into voids (lowering permeability), its low heat of hydration, and lower cost.

    A more robust approach would be to analyize your soil mixes. Rammed earth book(s) usually start with a 70% Sand-silt content to 30 percent clay. Also, they avoid top soil and use sub soil (avoiding the organics). I can provide more ref information on that.

    Once a well packed and cohesive soil recipe is used, then consider FA. You might try making several blocks using various ratios of both FA and PC. I suspect, for example, that a 50:50 mix of FA/PC which contributes to 15 percent of the total block will be useful for waterproofing (not cementing) the block. Any amounts over 25% of a block suggests you might as well make soil crete – which should have LITTLE clay and a different soil composition.

    It is a matter of design approach – the soil for making CEB’s and rammed earth is NOT the sort of soil you would want for cement. CEB’s depend on cohesion and clay – and clay usually weakens (for various reasons) cement mixtures.

    Hope this helps – if you need more info – just ask.

    Thanks – good notes on soil block stabilization vs. pressed block stabilization. Any thoughts on external protection of the bricks with tar or stucco?

  4. Marcin

    Comment from Ben:


    Not sure about the block stabilisation, but make sure to put a geothermal array under the foundation (since you already have a big hole dug below the frost line). It will save you a lot of energy in the future. 4″ corrugated tubing works. With sufficient insulation, it’s free air conditioning in summer, and a good preheat in winter. Winter use might require a small fan (like from a computer) and thermometer (to indicate when your preheated air is done). Really fancy might use a thermostat switch to shut the fan off.

    Ben de Vries
    Certified Permaculture Designer


    Jeff Buderer
    to globalvillages

    My understanding is that there are downsides to using air as a thermal mass exchange mechanism to cool or heat buildings. However it might work starting as a simple example of the proces
    A water based system might be superior in terms of the efficiency in which it absorbs the heat or coolness from the area below the building.

    However, in terms of stabilization question why not just use concrete unless you want to prove how you can do it without any concrete.

    We use class f flyash (which is non cementacious flyash) at Arcosanti and so I am familiar with the use of flyash.

    My understanding is that it is not free because there are costs associated with transporting and packing it however, we did get our flyash free from the local cement company.


  5. Mark Hamilton

    Also, I found this:

    After digesting the Class C Fly Ash results on a spreadsheet, the general idea is…

    1) Without soil clay but with small aggregate (pea gravel and sand) obtaining up to 1500 PSI is possible for very low flyash/cement combo recipes.

    2) The mixes have only 7% cement total (1/3rd Portland, 2/3rds Fly Ash) in the mix (excluding water).

    3) Zero or near zero slump gives the best results. Note that the water required is only about 7 percent of the mix as well, very simiiar to that of a CEB.

    Although the clay in a CEB will be a factor (it will require additional water, which will weaken the cement), I suspect that you are on to something.

    And finally, when I use percents I am speaking of weight, not volume.

    Mark, if one uses portland cement only, what percent total (as compared to 7% for portland-flyash mix) is needed?


  6. Sasha

    Hi Marcin, wjile I am no way a builder, I can share my ideas here. I have little expereince with clay, I build clay stoves. Dry clay wals can be dry and hard as concrete. Ad them just a little water and they will fall like house of cards. Oldt traditional cob buildings around here as my father told me have a gravel (actually the remnants of fire , not sure what the English word is) hidro insulation layer (I mean athe wall foundations are made from the gravel. I would speculate that using earth blocks, even stabilised ones wouldn’t be a very smart idea. I sugest use proven methods (concrete) and experiment on a small scale (make a workshop or something like this).

  7. Mark Hamilton


    Gravel is very small rocks. What is left over from a fire are embers and ash.

    Here is a website that shows a rubble trench foundation. Rock and gravel are beneath the walls in a trench.

  8. Sasha

    Mark when you fire low quality coal, how do you call the remnants – its not ash but other impurities, thats what I meant.

  9. Mark Hamilton

    Sasha, oh I see what you mean. The left over is usually called “bottom ash” or “clinker” (or perhaps bottom slag). I have read of bottom ash being used as an aggregate in road building – especially in soil stabilization.

    Sounds like a good idea, here is the wiki entry (in part):

    “Bottom ash may be used as an aggregate in road construction and concrete, where it is known as furnace bottom ash (FBA), to distinguish it from incinerator bottom ash (IBA), the non-combustible elements remaining after incineration. It was also used in the making of the concrete blocks used to construct many high-rise flats in London in the 1960s…”

  10. Mark Hamilton

    Sasha, et al.

    This blog is one of the reasons I’ve become more inspired about researching CEB and masonry type materials. I’ve had a life long interest, and done tons of reading. So I think its about time to actually start some experiements – I’m setting up a small workshop in my garage to start some serious testing of various mixes, admixtures, and compressions. Hopefully, for a given recipe, I can develop a ‘perfect’ aggregate grading (as measured by compression MPI), compression, and cementing ratio.

    And I am still interested in creating light block with insulating properties. I tried papercrete and it is way to slow in setting, very water absorbtive, and prone to mildew. I’d like to use rice hulls (cheap around here) or even straw. Alternatively, I have ideas about makeing a layered block (or panel) with insulation.

    I’ve seen or read just about every idea published on the matter – so its time to experiement. Who knows, I might come up with something novel and practical.

    Yes, low-cost insulating block would be a great addition to sustainable building. I hope you’ll place your inventions in the public domain. – Marcin

  11. japhy

    a couple thoughts:

    A single _continuous_ coil of Pex tubing below the frost line would let you do some very very efficient heating & cooling. If all the connections are above ground you won’t have to deal with leaks. More initial cost but seriously worth it long term.

    Unless the site is really wet, perforated PVC next to the footers and a proper roof overhang should handle flooding. Also you could try extruded styrofoam insulation with building wrap or tar at the seams.

    The point of getting below the frost line is so your foundation doesn’t shift if the ground under it freezes. It actually doesn’t affect temps inside much.

    So also.. Since your sheds are so light, you could use a much smaller foundation- which could help the budget.

  12. Wes

    If the goal is stable foundation blocks with minimum outside inputs, and you already have a block maker, why not make enough blocks to stack into a temporary kiln/ beehive shape, fill it with brush, and fire them. Check out the book Ceramic Houses by Khalil Gibran (?) for more details on this. He used an oil drum heater to get the temp up above 2000 F, I think. I think bread ovens may only get to 700 F and I don’t know if this is hot enough to stabilize your block permanently — you may need some kind of blower.
    You can always soak a block and see if you can get it to dissolve after firing it.
    Worst case is your blocks shatter in the experiment and you have some rubble to use for drainage.
    I assume your location is pretty high in clay subsoil?

    Wes, yes, that may work. It does require some more infrastructure. Are you able by any chance to help us build the kiln? – Marcin