Plaster

Summary

Plaster is a paste of gypsum, lime, cement, or clay with sand and water that sets to a hard surface on walls and ceilings. It gives a monolithic, fire-resistant finish, but each binder type differs in setting time, moisture tolerance, and where it may be used.

Cols

Image

Gypsum plaster: calcined gypsum (hemihydrate) mixed with water
Lime plaster: slaked lime and sand, hardening by carbonation
Cement plaster: Portland cement, lime, and sand, used outdoors as stucco
Clay plaster: clay and sand, historic and earthen walls
Lath and plaster: plaster keyed into lath

History and demand

First uses in construction

Plaster used in the Middle East at least 5,000 years ago
79 AD: plasterwork at Pompeii
Circa 1675: Massachusetts contract specifying clay and lime plaster
Circa 1909: cement plaster first introduced in America

Composition and production

Raw materials

Gypsum plaster: gypsum heated to about 120 to 180°C (250 to 355°F); hemihydrate forms at 130 to 180°C, gamma anhydrite at about 180°C, beta anhydrite above 250°C
Lime plaster: limestone heated above about 850°C (1,560°F) to produce quicklime

Properties

Physical

Gypsum setting: begins about 10 minutes after mixing, complete in about 45 minutes; crystal transformation continues about 72 hours
Gypsum heat of setting: slight volume expansion; mix can exceed 140°F (60°C)
Lime plaster cure time on lath: about one month

Fire behavior

Gypsum plaster releases water vapor in fire, slowing spread for up to 1 to 2 hours depending on thickness

Uses in buildings

Building elements

Interior Plaster: gypsum and lime plaster on lath or masonry
Exterior Plaster and Stucco: cement and lime plaster
Gypsum Board Ceilings: plaster ceilings

Pros and cons

Pros

Monolithic, jointless surface
Gypsum plaster sets in about 45 minutes
Fire-slowing water release from gypsum

Cons

Lime plaster needs about a month to cure
Wet application and long drying

Sustainability

Health

OSHA workplace limit 15 mg/m3 total and 5 mg/m3 respirable dust over 8 hours; NIOSH recommends 10 mg/m3 total and 5 mg/m3 respirable