Upon completing this section, you should be able to identify plaster ingredients, state the principles of mix design, and describe common types and uses of gypsum plaster.

A plaster mix, like a concrete mix, is made plastic by the addition of water to dry ingredients (binders and aggregates). Also, like concrete, a chemical reaction of the binder and the water, called hydration, causes the mix to harden.

The binders most commonly used in plaster are gypsum, lime, and portland cement. Because gypsum plaster should not be exposed to water or severe moisture conditions, it is usually restricted to interior use. Lime and portland cement plaster maybe used both internally and externally. The most commonly used aggregates are sand, vermiculite, and perlite.


Gypsum is a naturally occurring sedimentary gray, white, or pink rock. The natural rock is crushed, then heated to a high temperature. This process (known as calcining) drives off about three-quarters of the water of crystallization, which forms about 20 percent of the weight of the rock in its natural state. The calcined

There are four common types of gypsum base coat plasters. Gypsum neat plaster is gypsum plaster without aggregate, intended for mixing with aggregate and water on the job. Gypsum ready-mixed plaster consists of gypsum and ordinary mineral aggregate. On the job, you just add water. Gypsum wood-fibered plaster consists of calcined gypsum combined with at least 0.75 percent by weight of nonstaining wood fibers. It maybe used as is or mixed with one part sand to produce base coats of superior strength and hardness. Gypsum bond plaster is designed to bond to properly prepared monolithic concrete. This type of plaster is basically calcined gypsum mixed with from 2-to 5-percent lime by weight.

Finish Coats

There are five common types of gypsum-finish coat plasters.

  • Ready-mix gypsum-finish plasters are designed for use over gypsum-plaster base coats. They consist of finely ground calcined gypsum, some with aggregate and others without. On the job, just add water.
  • Gypsum acoustical plasters are designed to reduce sound reverberation. Gypsum gauging plasters contain lime putty. The putty provides desirable setting properties, increases dimensional stability during drying, and provides initial surface hardness.
  • Gauging plasters are obtainable in slow-set, quick-set, and special high-strength mixtures.
  • Gypsum molding plaster is used primarily in casting and ornamental plasterwork. It is available neat (that is, without admixtures) or with lime. As with Portland cement mortar, the addition of lime to a plaster mix makes the mix more "buttery."
  • Keene’s cement is a fine, high-density plaster capable of a highly polished surface. It is customarily used with fine sand, which provides crack resistance.


Lime is obtained principally from the calcining of limestone, a very common mineral. Chemical changes occur that transform the limestone into quicklime, a very caustic material. When it comes in contact with water, a violent reaction, hot enough to boil the water, occurs.

Today, the lime manufacturers slake the lime as part of the process of producing lime for mortar. Slaking is done in large tanks where water is added to convert the quicklime to hydrated lime without saturating it with water. The hydrated lime is a dry powder with just enough water added to supply the chemical reaction. Hydration is usually a continuous process and is done in equipment similar to that used in calcining. After the hydrating process, the lime is pulverized and bagged. When received by the plasterer, hydrated lime still requires soaking with water.

In mixing medium-slaking and slow-slaking limes, you should add the water to the lime. Slow-slaking lime must be mixed under ideal conditions. It is necessary to heat the water in cold Magnesium lime is easily drowned, so be careful you don’t add too much water to quick-slaking calcium lime. When too little water is added to calcium and magnesium limes, they can be burned. Whenever lime is burned or drowned, a part of it is spoiled It will not harden and the paste will not be as viscous and plastic as it should be. To produce plastic lime putty, soak the quicklime for an extended period, as much as 21 days.

Because of the delays involved in the slaking process of quicklime, most building lime is the hydrated type. Normal hydrated lime is converted into lime putty by soaking it for at least 16 hours. Special hydrated lime develops immediate plasticity when mixed with water and may be used right after mixing. Like calcined gypsum, lime plaster tends to return to its original rock-like state after application.

For interior base coat work, lime plaster has been largely replaced by gypsum plaster. Lime plaster is now used mainly for interior finish coats. Because lime putty is the most plastic and workable of the cementitious materials used in plaster, it is often added to other less workable plaster materials to improve plasticity. For lime plaster, lime (in the form of either dry hydrate or lime putty) is mixed with sand, water, and a gauging material. The gauging material is intended to produce early strength and to counteract shrinkage tendencies. It can be either gypsum gauging plaster or Keene’s cement for interior work or portland cement for exterior work. When using gauging plaster or Keene’s cement, mix only the amount you can apply within the initial set time of the material.


Portland cement plaster is similar to the Portland cement mortar used in masonry. Although it may contain only cement, sand, and water, lime or some other plasterizing material is usually added for "butteriness."

Portland cement plaster can be applied directly to exterior and interior masonry walls and over metal lath. Never apply portland cement plaster over gypsum plasterboard or over gypsum tile. Portland cement plaster is recommended for use in plastering walls and ceilings of large walk-in refrigerators and cold-storage spaces, basements, toilets, showers, and similar areas where an extra hard or highly water-resistant surface is required.


As we mentioned earlier, there are three main aggregates used in plaster: sand, vermiculite, and perlite. Less frequently used aggregates are wood fiber and pumice.


Sand for plaster, like sand for concrete, must contain no more than specified amounts of organic impurities and harmful chemicals. Tests for these impurities and chemicals are conducted by Engineering Aids.

Proper aggregate gradation influences plaster strength and workability. It also has an effect on the tendency of the material to shrink or expand while setting. Plaster strength is reduced if excessive fine aggregate material is present in a mix. The greater quantity of mixing water required raises the water-cement ratio, thereby reducing the dry-set density. The cementitious material becomes over-extended since it must coat a relatively larger overall aggregate surface. An excess of coarse aggregate adversely affects workability-the mix becomes harsh working and difficult to apply.

Plaster shrinkage during drying can be caused by an excess of either fine or coarse aggregate. You can minimize this problem by properly proportioning the raw material, and using good, sharp, properly size-graded sand.

Generally, any sand retained on a No. 4 sieve is too coarse to use in plaster. Only a small percentage of the material (about 5 percent) should pass the No. 200 sieve.


Vermiculite is a micaceous mineral (that is, each particle is laminated or made up of adjoining layers). When vermiculite particles are exposed to intense heat, steam forms between the layers, forcing them apart. Each particle increases from 6 to 20 times in volume. The expanded material is soft and pliable with a color varying between silver and gold.

For ordinary plasterwork vermiculite is used only with gypsum plaster; therefore, its use is generally restricted to interior applications. For acoustical plaster, vermiculite is combined with a special acoustical binder.

The approximate dry weight of a cubic foot of 1:2 gypsum-vermiculite plaster is 50 to 55 pounds. The dry weight of a cubic foot of comparable sand plaster is 104 to 120 pounds.


Raw perlite is a volcanic glass that, when flash-roasted, expands to form irregularly shaped frothy particles containing innumerable minute air cells. The mass is 4 to 20 times the volume of the raw partlicles. The color of expanded perlite ranges from pearly white to grayish white.

Perlite is used with calcined gypsum or portland cement for interior plastering. It is also used with special binders for acoustical plaster. The approximate dry weight of a cubic foot of 1:2 gypsum-perlite plaster is 50 to 55 pounds, or about half the weight or a cubic foot of sand plaster.

Wood Fiber and Pumice

Although sand, vermiculite, and perlite makeup the great majority of plaster aggregate, other materials, such as wood fiber and pumice, are also used. Wood fiber may be added to neat gypsum plaster, at the time of manufacture, to improve its working qualities. Pumice is a naturally formed volcanic glass similar to perlite, but heavier (28 to 32 pounds per cubic foot versus 7.5 to 15 pounds for perlite). The weight differential gives perlite an economic advantage and limits the use of pumice to localities near where it is produced.


In plaster, mixing water performs two functions. First, it transforms the dry ingredients into a plastic, workable mass. Second, it combines with the binder to induce hardening. As with concrete, there is a maximum quantity of water per unit of binder required for complete hydration; an excess over this amount reduces the plaster strength.

In all plaster mixing, though, more water is added than is necessary for complete hydration of the binder. The excess is necessary to bring the mix to workable consistency. The amount to be added for workability depends on several factors: the characteristics and age of the binder, application method, drying conditions, and the tendency of the base to absorb water. A porous masonry base, for example, draws a good deal of water out of a plaster mix. If this reduces the water content of the mix below the maximum required for hydration, incomplete curing will result.

As a general rule, only the amount of water required to attain workability is added to a mix. The water should be potable and contain no dissolved chemicals that might accelerate or retard the set. Never use water previously used to wash plastering tools for mixing plaster. It may contain particles of set plaster that may accelerate setting. Also avoid stagnant water; it may contain organic material that can retard setting and possibly cause staining.