Plaster of Paris and its Applications Quiz

Types of Plaster and Their Uses

• Gypsum plaster is used for interior finish for smooth concrete walls or ceilings, mixed with ingredients that develop more adhesive strength.
• Lightweight gypsum plaster is used when weight is important, and for fire-proofing other materials such as steel.
• Gypsum plaster is mixed on the job with water, perlite, vermiculite, or other suitable mineral aggregate.
• Gypsum plaster is applied on lath and is for interior use only.

Gypsum Plaster

• Bonding plaster is a type of gypsum plaster used for interior finish.
• The words "gypsum" and "plaster" are often used interchangeably.
• Fiber or hair is sometimes added to the mixture to give increased strength.

Plastering Methods

• Plastering is done according to two basic methods: two-coat and three-coat.
• A three-coat job consists of: a first binding coat called scratch coat, a second straightening coat called the brown-coat, and a final coat called the finish coat.
• In the two-coat work, the scratch and brown coats are combined into one.

Lime Plaster

• Lime plaster is mixed on the job with sand and gypsum plaster.
• Lime putty (hydrated lime and water) is used for two and three-coat finish surfaces for interior walls and ceilings.
• The ratio of lime putty to Po land cement or Keene's cement to sand by weight is:
  • Scratch coat: 1 part lime putty, 1 part Po land cement or Keene's cement, 2-3/4 parts sand.
  • Brown coat: 1 part lime putty, 1 part Po land cement or Keene's cement, 3 parts sand.
  • Finish coat:
    • Hard finish: 1 part lime putty, 1/3 part gypsum plaster by volume.
    • Sand finish: 1 part lime putty, 1/4 part gypsum plaster, 2 parts sand by volume.

Other Types of Plaster

• Plaster of Paris is mixed with alum or borax or other materials and burned (calcined) at 932°F.
• Plaster of Paris is used for ornamental plaster work and castings.
• Molding plaster is used for ornamental plaster work and castings.
• Acoustics plaster is used for acoustic treatment of interior walls and ceilings, applied on gypsum plaster base coats.

Po land Cement Plaster

• Po land cement plaster is mixed with water, sand, and lime putty.
• It is used for two and three-coat finish surfaces for exterior and interior walls and ceilings.
• The ratio of cement to lime putty to sand is:
  • Scratch and brown coats: 1 part cement, 1/4 part lime putty, 3 parts sand.
  • Sand finish: 1 part cement, 1/4 part lime putty, 3 parts sand.
• The thickness of plaster coats depends on the type of material to which the plaster is applied.
• Generally, the total thickness is 5/8" on metal lath and 1/2" on lathing board and gypsum block.
• In three-coat plastering work, the scratch and brown coats are 1/4" thick at minimum; the finish coat is 1/8" with a minimum of 1/16" at any point.