Adobe
Adobe is one of the oldest building materials in use. It is basically just dirt that has been moistened with water, sometimes with chopped straw or other fibers added for strength, and then allowed to dry in the desired shape. Commonly adobe is shaped into uniform blocks that can be stacked like bricks to form walls, but it can also be simply piled up over time to create a structure. The best adobe soil will have between 15% and 30% clay in it to bind the material together, with the rest being mostly sand or larger aggregate. Too much clay will shrink and crack excessively; too little will allow fragmentation. Sometimes adobe is stabilized with a small amount of cement or asphalt emulsion added to keep it intact where it will be subject to excessive weather. Adobe blocks can be formed either by pouring it into molds and allowing it to dry, or it can pressed into blocks with a hydraulic or leverage press. Adobe can also be used for floors that have resilience and beauty, colored with a thin slip of clay and polished with natural oil.
Adobe buildings that have substantial eaves to protect the walls and foundations to keep the adobe off the ground will require less maintenance than if the walls are left unprotected. Some adobe buildings have been plastered with Portland cement on the outside in an attempt to protect the adobe, but this practice has led to failures when moisture finds a way through a crack in the cement and then can't readily evaporate. When adobe is used as an exterior plaster it is either stabilized or replastered on a regular basis.
Adobe is a good thermal mass material, holding heat and cool well. It does not insulate very well, so walls made of adobe need some means of providing insulation to maintain comfort in the building. Sometimes this is accomplished by creating a double wall, with an air space, or some other insulation in between. Another approach is placing insulating materials on the outside.Rammed Earth
Ramming earth to create walls is at least as old as the Great Wall of China. It is really quite similar to adobe and cob techniques, in that the soil is mostly clay and sand. The difference is that the material is compressed or tamped into place, usually with forms that create very flat vertical surfaces. Modern rammed earth typically utilizes heavy mechanized equipment to move and compress the material . The walls are normally at least a foot thick to give enough bulk to be stable and provide the thermal mass for comfort. It is best to apply insulating material to the outside of the building to keep the interior temperature constant. The interior surface can be finished in a variety of ways, from natural oil to more standard surfaces. Rammed earth houses tend to have a very solid, quiet feeling inside.Cob
Cob is a very old method of building with earth and straw or other fibers. It is quite similar to adobe in that the basic mix of clay and sand is the same, but it usually has a higher percentage of long straw fibers mixed in. Instead of creating uniform blocks to build with, cob is normally applied by hand in large gobs (or cobs) which can be tossed from one person to another during the building process. The traditional way of mixing the clay/sand/straw is with the bare feet; for this reason, it is fairly labor intensive. Some of the process can be mechanized by using a backhoe to do the mixing, but that diminishes the organic nature of it. Because of all the straw, cob can be slightly more insulating than adobe, but it still would not make a very comfortable house in a climate of extreme temperatures. The wonderful thing about cob construction is that it can be a wildly freeform, sculptural affair. I've seen some very charming homes made this way. Cob was a common building material in England in the nineteenth century, and many of those buildings are still standing.
A variant of cob is what is commonly called "light straw". This is made with the same long fibers of straw which is tossed like spagetti with a sauce of clay slip. The idea is to coat the straw fibers with enough of the clay to get them to stick together, but not so much that it makes a gummy clump. This material is then tamped into a form and left to set up enough to remove the form. Light straw walls could be useful for interior partitions and even exterior walls if it is thick enough. Such walls would be quite a bit more insulating than cob, but they require a timber frame of some sort because the straw itself would not be load bearing.