Cast Earth

Cast earth is a modified (and patented) building material developed since the mid-1990s by Harris Lowenhaupt. Based on the earlier unpatented Turkish Alker, it is a concrete-like composite with soil (of a suitable composition) as its bulk component, but is set with about 15% calcined gypsum (plaster of Paris) instead of cement. It can be used to form solid walls that need not be reinforced with steel or timber framing, unless extra seismic reinforcement is necessary. Forms are set up and filled with cast earth, which sets quickly and solidly. Once the forms are removed the wall stays sound.

The calcined gypsum sets quickly, which is one quality that has historically made plaster of Paris so useful. When calcined gypsum is added to soil, however, the setting time is reduced even further, to mere minutes. Often this quick setting is too fast and a retardant must be added to the mix so it can be poured. In Alker, lime is added to extend working time to 20 minutes. Cast earth uses another retardant for an even greater working time. When the structure is dry, it is similar to adobe in various ways, outperforming it in tensile strength, hardness, and erosion resistance. It also has less tendency to crack and shrink. Cast earth walls do soak up water, however, if they are not sealed with a silicone coating or other waterproofer.

Using cast earth in place of other materials reduces the time and labor needed to complete a project. Machinery is used in place of much of the human labor during cast earth construction. It is often less costly, since earth and calcined gypsum are plentiful and cheap materials to acquire.

Few contractors are licensed in the use of cast earth today, but if public demand for structures made of the material grows, more builders will seek training in its use.