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close this book Handbook for building homes of earth
View the document Table of contents
View the document Foreword
View the document Chapter 1: Introduction - Types of earth houses
View the document Chapter 2: Soils - And what can be done with them
View the document Chapter 3: Stabilization of soils
View the document Chapter 4: Where to build
View the document Chapter 5: Foundations
View the document Chapter 6: Lightweight roofs
View the document Chapter 7: Preparing the soil
View the document Chapter 8: Making adobe blocks
View the document Chapter 9: Making pressed earth blocks
View the document Chapter 10: Making walls with earth blocks
View the document Chapter 11: Making rammed earth walls
View the document Chapter 12: Roofs for earth houses
View the document Chapter 13: Floors
View the document Chapter 14: Surface coatings
View the document Suggested references
Open this folder and view contents Appendix

Chapter 3: Stabilization of soils

Many kinds of soils can be used for earth walls by adding substances known as stabilizers. Nearly any soil can be made into a better building material with the addition of the CORRECT stabilizer.

This is what stabilizers do:

1. They cement the particles of soil together so the block or wall will be stronger.

2. They can "waterproof" the soil so that it won't absorb water.

3. They can keep the soil from shrinking and swelling.

Adding stabilizers - even cheap ones - to your soil means that your house will cost more. But the natural "enemy" of earth walls is water in one form or another. Stabilizers fight that enemy. There is less need for stabilizers in very dry climates. Builders in arid areas protect against the slow weathering caused by winds and blowing sands by making the walls a little bit thicker. Some walls like this have lasted well over 100 years. It is important to know the experience of other builders before deciding on this method of building.

Because of the many different kinds of soils and the many types of stabilizers, there is no one answer that is best in all cases.

All this manual can do is tell you what stabilizers can be used, which ones work best on different kinds of soils, and approximately how much stabilizer seems to work best. It is up to the builder to make trial blocks with various kinds and amounts of stabilizers and then, test them as described in Chapter 2.

Kinds of Stabilizers

Here are the more commonly used stabilizers:

1. Sand and clay - Usually we think of soil stabilizers as something unusual and different, but ordinary sand and clay can also be used as stabilizers. If your soil is too sandy, then add a little clay to it, or add sand to a clayey soil. It's true with all stabilizers - and sand and clay are no different - that they must be mixed thoroughly into the soil before they can do the job. If you have one soil that is very sandy, and another that is very clayey, they probably won't mix very well because the clay lumps cannot be easily pulverized. The only way to find out whether two soils will mix well is to try it. It's easier to mix a small amount than a large amount, so try to do the mixing just as you would when building a house.

2. Portland Cement - The same kind of portland cement used in concrete is also one of the best soil stabilizers. The mixture is often called soil-cement. Cement works best with the sandier soils. Table 1 shows you which soils are stabilized best with cement. (If your soil has been checked by the laboratory tests described in Appendix A, you can use portland cement with any soil that has a plasticity index from 0 to about 12.)

Some stabilizers mix easily with soil but this is not true of portland cement. It must be very thoroughly mixed and the soil clods should be broken down so the cement comes in contact with all of the soil. (This is one reason why cement is not recommended for clayey soils.)

Cement starts to react as soon as it touches water, so do not mix it into wet soils. Mix it completely into dry soils before adding water. Then, the moist soil-cement mixture should be formed into blocks or rammed in the wall quickly. If you wait too long before doing this, the soil-cement will harden and it must be thrown away. Don't mix more than you intend to use.

Cement needs water to get hard. Since it gains most of its hardness or strength in 7 days, you need to keep it moist this long. One way to do this is to put a watertight cover over the blocks or walls. If you cannot do this, cover them with wet burlap sacks and sprinkle the sacks often. After 7 days of this moist curing you can take the covering off but it is still a good idea to keep the blocks in the shade for another 7 days before you let them dry in the sun. The longer you keep your soil-cement blocks or walk moist, the stronger they will be.

Using cement has two disadvantages: it is expensive and it may be hard to get. So try to find out first how little you have to use.

You can make a portland cement yourself. To do it, though, takes a lot of heat, some crushing facilities, a source of clay, and a source of limey material such as &hells, limestone, caliche, etc.

3. Lime - Lime, either slaked or unslaked, makes one of the best stabilizers for clays. Lime reacts with the clay in the soil to form a binder. Unslaked lime is harmful to a person's &kin and vital parts and must be used with great care. It is much safer to first slake the unslaked lime before wing it. Table I shows the soils which work best with lime. If you use the more exact laboratory tests described in Appendix A, you can try lime with nearly any soil having a plasticity index greater than about 12.

Lime makes most clays less sticky, but it doesn't make all of them stronger. It will usually strengthen volcanic clays, but with any other clayey &oils, the only thing to do is try the lime out and see how it works. Use the tats in Chapter 2.

Soils containing a lot of clay are usually fairly lumpy. But lime breaks the lumps down and makes the soil easier to mix. In fact, lime even makes the soil look and feel different. If your soil has a lot of clay in it, here's what you should do:

Add the lime to dry soil and mix with sufficient water to dampen entire mixture, then cover it for a day or two but keep it wet. After you do this mix the soil again to break down any remaining lumps and use it right away.

Lime also needs to be kept moist to gain its strength but it takes much longer than cement to harden. Keep lime-stabilized blocks covered and moist at least 7 days, 14 days if possible. Then keep them in the shade at least 7 more stays before exposing them to the sun. When making trial blocks with lime-stabilized soils, try to make them early enough that they will have plenty of time to cure before testing. At least one month of curing is necessary, two months are better.

Lime is not as expensive as cement and you can get it nearly any place in the world. You can make lime yourself but it's not an easy job. You'll need heat and a material such as limestone, seashells or caliche, and finally a way to grind up the burnt limestone.

It takes lime-stabilized soils about 6 times as long to get their full strength as it does soil-cement. Remember this when you are trying to compare lime and cement stabilized soils.

4. Combinations of Lime and Cement - Sometimes you'll run into a situation like this:

The soil has a little too much clay in it for cement to do a good job of stabilizing.

Lime will make the soil easy to work, but it won't react enough with the soil to waterproof it or make it strong.

When this happens, you can use both lime and cement. It will cost more and take more time to add the two stabilizers, hut it may be the only way to build your house.

Usually, equal parts of lime and cement are used. The lime is always added first. Then, add enough water to make the mix moist. Cover the mix and let it stand for I to 2 days. -After this mix the soil well to break up any lumps and immediately add the cement plus any water necessary to bring the soil to its correct water content. After thorough mixing, use the stabilized soil immediately, before the cement hardens. Cure it as you would cement.

5. Asphalt - Another stabilizer that has worked out well for earth houses is asphalt. Asphalts made especially for use in earth houses are made in plants in the United States, but they don't have to be a special kind. Natural asphalts were used thousands of years ago to stabilize earth blocks in Babylon. Asphalt is usually restricted to those soils that are mixed by "puddling," such as adobe. It is harder to mix into moist soils used for pressed blocks or rammed earth. It won't work on clayey soils because it won't mix with them.

Asphalt in its natural form is too thick to be added to soils without heating, so they are often mixed with other materials to make them thinner and easier to mix. If they are mixed with water they are called asphalt emulsions. These are the best to use in earth walls because there is no danger in handling them and they mix easily into the soil. After asphalt emulsions have been added to the soil they will separate back into pure asphalt and water - leaving the asphalt as a film on the soil grains. One that goes beck' to asphalt and water quickly is called a "fast-breaking" or "fast-setting" emulsion. These are not good for earth houses because they may separate before they are completely mixed into the soil. "Slow-setting" or "slow-breaking" types are ideal for earth houses. (If you cannot find an emulsion made especially for earth houses, then get a regular emulsion, but make sure it's the "slow-setting" or "slow-breaking" type.)

Other types of asphalt that have been used are called "cutback" asphalts. These are asphalts that have been mixed with gasoline. kerosene, etc., to make them thinner so they can be mixed without heating them. They can be used with soil but they are not as good as emulsions. After a soil is treated with a cutback asphalt, it must be spread out to allow most of the gasoline or kerosene to evaporate before it can be made into blocks. Cutback asphalts can catch fire if you get them near an open flame.

Since asphalt is really a very thick oil, it will "grease" the soil grains and cause the soil to lose some of its dry strength, at least until the stabilized soil becomes a few years old and the asphalt hardens. Asphalts do a good job of waterproofing the grains, and they keep the soil from losing strength when wet.

Remember, asphalt will be very difficult to use when the soil has a lot of clay in it. It works out best with soils suitable for adobe blocks.

6. Straw - A material that has often been used in adobe blocks is straw. In the same manner, materials such as tree bark, wood shavings, hemp and other tough fibers have been used. The only on of these that has appeared to be of much use is straw - although some people have had fair success with wood shavings.

Straw doesn't react with the soil in any manner. If anything, it will make the dry block a little weaker and it will let it absorb water a little easier. Straw does provide "pipes" or exits from the inside of the block so the water can get out easier during the curing period. In clayey soils especially, this causes less cracking during curing. Straw or other fibers also give added strength to wet adobe blocks during the curing period.

Although most old adobe houses contain straw in the blocks, modern builders do not use it. It may have some value when your soil is a little too clayey and you have no other way of stabilizing it.

7. Fly-Ash and Lime Combinations - Fly-ash is the fine dust that is given off during the burning of coal, coke, lignite, and some other solid fuels. If you live near a plant that burns these fuels and saves the fly-ash, you have a very good cheap stabilizer if you have lime to mix with it. The lime and fly-ash together will make a cement almost as good as portland cement. It can be used on both sandy and clayey soils.

When using lime and fly-ash together, use about 2 to 4 times as much fly-ash as lime. For example, for every bucket of lime used, add between 2 to 4 buckets of fly-ash. The only way to find out whether you should use 2 buckets or 4 buckets, or something in between, is to make some trial blocks and test them. You'll probably find that lime is more expensive than fly-ash so try to use as much fly-ash as possible and still make a satisfactory earth wall.

8. Sodium silicate - This is-sometimes called "water-glass." It is available in many parts of the world and not expensive when bought in large quantities. It works best on sandy soils such as clayey sands and silty sands. Clays do not stabilize well with it.

The best way to use sodium silicate is to coat the outside of earth blocks with it so it makes a thin "skin" of hard, stabilized soil around the blocks. (For use on walls that are not made of blocks - such as rammed earth - see Chapter 14.) To use it, mix one part of commercial sodium silicate with three parts of clean water. Dip the earth blocks in the solution for about one minute. When you remove them there will be a little solution left on the blocks. Use a stiff brush to brush this into the block. Repeat this treatment a second-time before the blocks dry. Then, let the blocks air-dry in a protected place at least 7 days before using them.

The thick solution of water and sodium silicate can be made to penetrate deeper into the blocks if you add a very small amount of a group of chemicals known as surfactants (surface active agent). These chemicals numbering over 1500 are sometimes classed into four groups called amphoterics, anionics, cationics and non-tonics. These groups include such chemicals as polyoxyethylenealkylarylether and dodecylbenzenesulfonic, amines, etc.; however, many common detergents will work as well as these chemicals.

There are many other stabilizers that have been used successfully with some soils. Because there has been little written about their use in earth houses they are discussed only briefly below. But, don't be afraid to try them out if they are available. In fact, if you have something else that you think will be a good stabilizer, try it. Sometimes a waste product that other people are throwing away may be a good stabilizer.

9. Sulfite Liquor - This is a waste product from certain paper mills. It is primarily a waterproofing agent and you shouldn't expect it to increase the dry strength of the soil. Some types of sulfite liquors react very favorably with soils, others have a very harmful effect on them. You won't know until you try it out on your soil. The amount of sulfite liquor you should use depends on the particular paper mill you get it from, so you will have to try different amounts and select the best one yourself.

10. Aliquat H226 - A special chemical, (quaternary amine), made in the United States but available in other parts of the world. It is shipped as a very thick liquid and it must be mixed with warm water before it can be added to soils. The manufacturer will give you instructions on how to use it. Although it is expensive, it is a good waterproofer for silts and clays once the treated soil has been allowed to air-dry. It would be very good for use in the first few layers of earth blocks closest to the ground if these blocks didn't pass the requirements of the absorption test.

11. Wood ashes - In some countries, wood ashes have made very successful soil stabilizers. Probably it is the lime or calcium in them that actually does the stabilizing. Like sulfite liquor, certain wood ashes can actually be harmful instead of stabilizing the soil, so you will want to try it out on your soil. The correct amount to use will vary depending on the type of wood and how well it has been burned. The fine, white ashes from fully burned hardwood seem to work best.

12. Resins - These are made from the sap of trees. They can make very good waterproofing agents, but they probably will not add much dry strength to the soil. Some are difficult to apply to the soil because they will not dissolve in water. It is best to ask the manufacturers of the various resins how they should be applied.

13. Coconut Oil - This has also been used as a waterproofing agent. There is no cementing action but it will probably increase the wet strength of the soil.

14. Tannic Acid.

15. Rotted Plantain Leaves.

16. Cattle Urine.

17. Cow Dung.

18. Molasses.

19. Gum Arabic.

20. Juice from Various Plants.

How do you know whether a stabilizer will work?

Because there are so many different kinds of soils, it is impossible to say whether any one stabilizer will work well on any particular soil. In Table I and in the early part of this chapter; you have been given some idea of what type of stabilizer to use with what type of soil. But the only way to find out what is best is to try them out on your soil. Make trial blocks and test them just as you were told to do in Chapter 2. Don't use expensive stabilizers if cheap ones are available and your tests show that they will do the job.

What can you expect stabilizers to do?

You want stabilizers to increase both the dry and wet strengths of your soil; to reduce the amount of water absorption, and to keep your soil from being "melted" by a water spray or rain. Some stabilizers will do only one or two of these things, a few may do all of them.

For example, portland cement or lime may not decrease the water absorption, but it wouldn't make much difference if the wet strength of the block was high.

Some waterproofing type stabilizers may actually decrease the dry strength of the soil, but will greatly increase the wet strength and decrease the amount of water absorption. Naturally, then, these might be best in wet climates.

Certain stabilizers - lime is a good example - may actually cause a pressed or rammed earth block to weigh a little less. Don't let this bother you though because it won't hurt the strength of the soil.

Some stabilizers - lime again is a good example - will change the best moisture content for compaction. Lime stabilized soils usually need more water than the same soil unstabilized. You probably wouldn't notice this unless you accurately measured the amount of water needed. You can still use the simple test described elsewhere to determine whether stabilized soils for rammed earth and pressed blocks have the right amount of water in them to make the best blocks.

Do stabilized soils require special handling?

Some types of stabilizers require special handling or they won't work at all. Remember it was mentioned earlier in this chapter that portland cement and lime needed to be moist-cured for at least 7 days or they wouldn't gain strength. This is true of nearly all cementing stabilizers.

Most waterproofing stabilizers only work after the soils have been dried out once. This won't require special attention because you dry the soils out when you cure them in the sun, anyway.

With many stabilizers, the curing takes place after the blocks have been molded. But with asphalt emulsions, at least some of the curing has to be done while the soil is still loose unless you are making adobe blocks. For rammed earth and pressed blocks, spread the stabilized soil out in the sun to cure. There is no good rule to tell you when you can start using these soils and you will have to find out yourself by experience. If you can ram or press the soil and it doesn't act spongy or heave around the sides of your rammed earth tamper, it should be ready to use.

Cutback asphalts - those containing kerosene, gasoline, etc., are slow in curing. It might take several days before they are ready to be made into blocks or an earth wall.


How much stabilizer should you use?

Again, it is impossible to bay with any accuracy just how much should be used. It depends on the type of soil you have and what you want the stabilizer to do.

Sometimes you will need a lot of stabilizer - particularly with very sandy or very clayey soils.

You might find a soil that is suitable in all ways except that it doesn't meet one requirement, for example, it absorbs too much water. Then, probably a very small amount of a stabilizer will be needed. A soil that is not good enough to meet any requirements or maybe only one of them will require more stabilizer.

So, you can see that the only way to find out how much stabilizer is necessary is to make some trial blocks and test them. You should make up three trial sets of seven blocks each. If you use portland cement, trial amounts should range from 4 to 12 percent and for dime 2 to 6 percent.


When you have finally selected the exact amount of stabilizer that you need, there is one more thing to consider. With trial blocks, you use a small amount of soil and it is easy to do a good mixing job. During the actual construction of your earth house, the workers will handle much larger quantities of soil and the mixing will not be as good. The easiest way to take care of this i. to add a little more stabilizer.

All of this sounds, we realize, like a great deal of preparation and testing that needs to be done. That's the way it is intended to sound. It will be well worth your time and effort to find out for yourself what the best answers will be for you. If it takes you a couple months of preparation, remember, it's worth it. If you do your job right, your grandchildren will also have a good house to live in.