Stabilizers make earth walls stronger, more moisture resistant, and reduce shrinking and swelling. Water is the number one enemy of earth buildings and adding stabilizers (particularly in rainy climates) is one way to create more durable structures.
Lime (Type S – Hydrated Lime) is one of the best stabilizers for clay soils, and more environmentally friendly than cement. With the addition of fly ash to hydrated lime, the walls will be almost as strong as cement and can be used with clayey and sandy soils.
Wood ashes and other stabilizers and surface coatings are described in detail in Handbook for Building Homes of Earth. This Peace Corps handbook explains which stabilizers work best on different kinds of soils, how much is needed, as well as many other topics on building with earth. It has recently been added to our Articles page.