What is "gypsum plaster" how does it differ from "cement plaster" and what does one use for backing?

Question

"Gypsm plaster is recommended in Christopher Alexander's Pattern Language (#235 Soft Inside Walls) If I were to go to a building supply store and ask for "gypsum plaster would they know what I was talking about? ...Is it a universal term? Can you give me a chemical formula?

Answer 1

Gypsm is hydradated pulverized limestone. You would ask for Gypsym topcoat or Gypsm tape adhesive in the Drywall department of Lowes. It costs today $9.95 for a 5 gallon pail of the stuff. Gypsm is resoluable in water after drying. In other words you can redissolve gypsm in water. Gypsm is mined in the US in states like Michigan, Indiana, Ohio and so on. It is a naturally occuring substance.

Plaster of Paris is unhydrated form of lime. When wetted with water it changes chemically and remains hard no matter if submerged into water. The process is irreversible.

Cement is made from clay and limestone fired to high temperatures, then ground and pulverized. When water is added to cement it changes its chemistry and the water reacts with cement forming a new compound It is irreversible The Romans invented Cement.

Common Drywall is gypsm between 2 layers of paper enclosure. There are also metal lath in 26x84 inch lenghts and plaster board lath in 16x36 inch lengths Those all above are nailed or glued to the house studs.

Answer 2

Gypsum plaster (drywall) is made from gypsum and would be used in most rooms of a house that do not have water in them (gypsum is very much like chalk and water will damage it and make it crumble away), like living rooms, bedrooms, halls, closets, etc... Can be used in kitchens, but cement board is recommended behind sinks.

Cement plaster (cement board) is made from cement and is used in high moisture areas especially bathrooms and behind kitchen sinks, also as a substrate for tile installation. It is much ore water-proof than gypsum.

Here's a couple of sites that explain the differences and the best uses for each.
http://www.nationalgypsum.com/
http://www.rinkermaterials.com/ProdsServices/DrywallAcoustical/DA_CementBoard.shtml