Quick Setting Gypsum Plaster
Sometimes, gypsum plaster is made to set more quickly by adding some materials, as follows.
- If you add common salt to plaster by 0.5% of plaster weight, plaster will set more quickly up to 5minutes.
- If you add common salt to plaster by 1-4% of plaster weight, setting time will reduce to 2.5 minutes. If you add some alum to plaster by 5%, the gypsum mortar will set more quickly. and setting will begin after 7 minutes.
Retarded Gypsum Plaster.
Plaster coatings for facades.
The gypsum is cooked and milled to produce a powder. The powder is then mixed with alum solution to make a paste for plasterwork (alum is aluminum potassium sulfate). The paste is white and its strength is greater than quick setting plaster. It is used for plastering bathhouses, damp places, and facades.
Gypsum plaster turns retarded using some additives.
- As mentioned earlier, if you increase the percentage of common salt, the setting time will be delayed. and the plaster will turn more retarded. Adding 10 percent of common salt to plaster makes it start setting after 12.5 minutes.
- Alum turns gypsum plaster retarded.
- Glue makes plaster retarded too. If you add glue to gypsum mortar by 1-6%of its weight, setting time will delay by 12 to 28 minutes. (a kind of vegetable glue used in bookbinding industry).
As people did not usually use to paint rooms in the past, they added some glue to gypsum mortar water. so that their clothes would not become dirty when they sat leaning back againt the wall.
- Adding borax to plaster by 0.5-1% of plaster weight will result in 1.5 minutes to 1.5hours delay in setting time.
- Warm water does not affect the plaster setting time.
- If you add glue to gypsum mortar by 0.1-0.5% of plaster weight, will postpone plaster setting for 1minute to 2hours.
- If you add alum to plaster up to 1%t of plaster weight, it’ll postpone the setting up to 15 minutes.
