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.
Keenes 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 west.kr. Magnesium lime is easily
drowned, so be careful you dont 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 Keenes cement
for interior work or portland cement for exterior work.
When using gauging plaster or Keenes cement, mix
only the amount you can apply within the initial set time
of the material.
PORTLAND CEMENT PLASTER
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
As we mentioned earlier, there are three main
aggregates used in plaster: sand, vermiculite, and
perlite. Less frequently used aggregates are wood fiber
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