Bentonite is a type of clay having the unusual characteristics of cohesion, binding, sealing, and thickening. It is usually gray in color and when processed has the consistency of fine powder, similar to cement or flour. When bentonite is installed below grade as a waterproofing membrane, it becomes hydrated with the moisture that is naturally present in the soil and forms an impermeable barrier that absorbs and expels water and most chemicals, such as acids and salts (sodium bentonite). Bentonite can expand and contract an infinite number of times and is capable of absorbing seven to 10 times its own weight in water, swelling up to 18 times its dry volume. However, for bentonite to function properly as a waterproofing barrier, it is extremely important that this barrier remain under a constant minimum pressure of 30 to 60 pounds per square foot (PSF).