For a trowel-finish coat using lime-Keenes cement
plaster, the recommended proportions are, for a
medium-hard finish, 50 pounds of hydrated lime or
100 pounds of lime putty to 100 pounds of Keenes
cement. For a hard finish, the recommended proportions
are 25 pounds of hydrated lime or 50 pounds of lime
putty to 100 pounds of Keenes cement.
For a trowel-finish coat using lime-portland cement
plaster, the recommended proportions are 200 prods
of hydrated lime or 5 cubic feet of lime putty to
94 pounds of Portland cement.
For a finish coat using portland cement-sand plaster,
the recommended proportions are 300 pounds of sand
to 94 pounds of Portland cement. This plaster may be
either troweled or floated. Hydrated lime up to
10 percent by weight of the portland cement, or lime
putty up to 24 percent of the volume of the portland
cement, may be added as a plasticizer.
For a trowel-finish coat using gypsum gauging or
gypsum neat plaster and vermiculite aggregate, the
recommended proportions are 1 cubic foot of
vermiculite to 100 pounds of plaster.
The total volume of plaster required for a job is the
product of the thickness of the plaster times the net area
to be covered. Plaster specifications state a minimum
thickness, which you must not go under. Also, you
should exceed the specs as little as possible due to the
increased tendency of plaster to crack with increased
The two basic operations in mixing plaster are
determining the correct proportions and the actual
mixing methods used.
PROPORTIONS. The proper proportions of the
raw ingredients required for any plastering job are found
in the job specifications. The specs also list the types of
materials to use and the type of finish required for each
area. Hardness and durability of the plaster surface
depend upon how accurately you follow the correct
proportions. Too much water gives you a fluid plaster
that is hard to apply. It also causes small holes to develop
in the finish mortar coat. Too much aggregate in the mix,
without sufficient binder to unite the mixture, causes
aggregate particles to crumble off. Without exception,
consult the specifications prior to the commencement
of any plaster job.
MIXING METHODS. As a Builder, you will be
mixing plaster either by hand or using a machine.
Hand Mixing. To hand-mix plaster, you will need
a flat, shallow mixing box and a hoe. The hoe usually
has one or more holes in the blade. Mixed plaster is
transferred from the mixing box to a mortar board,
similar to that used in bricklaying. Personnel applying
the plaster pick it up from the mortarboard.
In hand mixing, first place the dry ingredients in a
mixing box and thoroughly mix until a uniform color is
obtained. After thoroughly blending the dry ingredients,
you then cone the pile and add water to the mix. Begin
mixing by pulling the dry material into the water with
short strokes. Mixing is continued until the materials
have been thoroughly blended and proper consistency
has been attained. With experience, a person squires a
feel for proper consistency. Mixing should not be
continued for more than 10 to 15 minutes after the
materials have been thoroughly blended. Excessive
agitation may hasten the rate of solution of the
cementitious material and reduce initial set time.
Finish-coat lime plaster is usually hand-mixed on a
5- by 5-foot mortar board called a finishing board.
Hydrated lime is first converted to lime putty by soaking
in an equal amount of water for 16 hours. In mixing the
plaster, you first form the lime putty into a ring on the
finishing board. Next, pour water into the ring and sift
the gypsum or Keenes cement into the water to avoid
lumping. Last, allow the mix to stand for 1 minute, then
thoroughly blend the materials. Sand, if used, is then
added and mixed in,
Machine Mixing. For a quicker, more thorough
mix, use a plaster mixing machine. A typical plaster
mixing machine (shown in fig. 7-11) consists primarily
of a metal drum containing mixing blades, mounted on
Figure 7-11.Plaster mixing machine.