Are there any simple field tests I can perform on the soil I have?
How do the clay and sand work together to form a brick?
1. What is an adobe brick made of?
While many types of soil can attain the strength and durability required, the most workable or moldable raw material is a mix of about 75% sand and 25% clay for bricks, mortars and plasters. A small percentage of 5 to 7 percent cement may be added in rainy climates to improve the structural performance and wall surface durability, as well as improve the brick making process, as stabilized bricks can be handled the next day, whereas unstabilized bricks have to lay flat for 2-3 weeks before they can be lifted.
2. Can I use the earth from my site?
Most often the answer is yes. However, your material may benefit with the addition of clay, sand, aggregate, or stabilizer, depending upon what is available and what is needed to create a workable and suitable mix. A soil stabilization engineer will test the composition of your raw material and recommend a structurally stable mix design.
As you can see here, the clay needs to be screened so that there are no lumps in the mix that are greater than 1/2" in diameter for brick making.
3. Can I use straw in the mix?
In dry areas, straw is sometimes used as a structural reinforcement agent in the adobe mix. The addition of fibers, such as straw, can hinder cracking accelerate drying, lighten the material, and increase tensile strength. However, it is necessary to fully protect earthen walls when using straw in very wet climates, since the straw may act as a conduit for water and vermin, allowing moisture to seep into the walls.
Photo shows adobe bricks made with straw made for a home in Milyanfan village, Kyrgyzstan. Photo by Vmenkov.
4. Are there any simple field tests I can perform on the soil I have?
The jar test is a simple way to determine the proportions of clay, silt and sand in your soil. The first step is to collect 2/3 of a jarful of a representative sample on your land. Clear at least 1 sqft of topsoil before collecting and assure that the sample is free of any organic matter. Fill the jar with water and shake vigorously for 1 minute. As you allow the material to settle you will find that the sand will settle to the bottom, then the silt, then the clay, with the water above. Here you can measure your overall percentages to determine what you might need more or less of to make a workable mix. Other helpful tests can be found in the ASTM Standards for earth building at www.astm.org/Standards/E2392.htm.
5. How do the sand and clay work together to form a brick?
The sand and aggregate provide strength to the mix, whereas the clay acts as a binder and plastic medium to "glue" the other ingredients together. When using cement, the cement particles fill the smaller voids, which "chink" or lock the matrix together.