Recycled Calcium Sulfate
Recycled calcium sulfate, CaSO4 (1 H 2O) is the formula for recycled wallboard. You will note that it is missing one water molecule. This is the result of the original material being subjected to excessive heat in the process of making wallboard. Technically it can’t be referred to as gypsum as defined by the California Department of Food and Agriculture, but must be referred to as calcium sulfate hemi-hydrate, (gypsum equivalent).
- Recycled wallboard in the marketplace has an origin of fresh material as a result of rejects, trimmings, and broken panels at the factory for the wallboard.
- The other source is recovered wallboard from building remodel and of removal. This is currently being evaluated as a hazardous waste material by the government, and should be avoided at the commercial level at all times until regulations are well defined.
Factory Reject Material
This material has been subjected to very high temperatures as noted above, but has had a combination of compounds added to the original material as binders and fire retardant agents. The fire retardant chemicals that are used contain very high levels of boron and the binding chemicals contain high levels of formaldehyde, both toxic to humans.
The other thing to consider is the presence of paper, which is what keeps the wallboard together. It is almost impossible to remove all of the paper. The more paper the lower the bulk density of the material. Most wallboard sources of calcium sulfate have an analysis of between 80% and 90% gypsum equivalent.
The material is less costly on a delivered basis over virgin mined calcium sulfate. It does have a place in the early stages of soil reclamation in extreme sodic soils of little productive value.
However, when applied to very high land values and high value crops the potential long term risks are simply not worth it.