TECIDTTCAL INFORMATION ON BUILDING MTERIALS

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FOR USS IN THE DESIGN OF LOW-COST HOUSING

THE NATIONAL BUREAU OF STAITDARDS

LDHTED STATES DEPARTlJ-ENT OF COMl.IERCE

WASHINGTON, D. C.

November 3 . 1936 .

WALL PLASTER, HIEING. AND APPLICATION;

This is the second in a series of three sections of a digest of

Circular CI 5I. "Wall Plaster: Its Ingredients, Preparation and

Properties", (January 9 , 192^),^ issued hy the Bureau of Standards.

This section deals with ingredients and apnlication.

Materials

me uroperties of any plastering material should he considered

frcm the following standpoints in order to determine its value. When

in the plastic state, can it he spread into position without requiring

an imuracticel amo^unt of care and effort? Does it harden qumicly

enouyh to prevent undue delay in the building operation snd is me

shrinkage, which normally accompanies the hardening, negligible How

closely do the properties of the hardened material approach the desired standards?

All plasters must contain at least one cementitious material,

generallv lime, aoq^sum, or Portland cement. Sand is used to increase

capacity and to reduce shrinkage. Water is necessary to develop the

plasticitv of the cementitious material, and to assist in the chemical

reactions involved in the hardening process.

Lime: Q,uicklime is made hy heating limestone to a teinperature

of over 900°C. It consists essentially of calcium oxid.e or of calcium

and magnesium oxides. L'amp lime will deteriorate when exposed to air

and, therefore, must he kept in air and m.oisture-proof containers, or slaked immediately/ upon receipt.

Tien water is added to quicklime, a chemical reaction takes

place. The cafLcium oxide is converted to calcium hydroxide, and the

^Available from Super in ten r?.ent of documents, Washington, D. C.

(Price 10 cents).

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roagnesiurn o:cide may or may not be converted into magne sium hydroxide

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denending unon how hard it was burned. This reaction is known as slaking. If an excess cf water is used, lime putty results. In this form,

lime may be stored indefinitely, provided only that it is protected

against drying out. The value of the quicklime depends very largely

uuon its being properly slalced, different kinds of lime requiring

radically different treatments to obtain the best results.

Hydrated lime is made from, quicklime by slaking it with just

enough water to corrrolete the chemical reaction. It comes in the form

of dry, white uowder, and is sold in paner bags containing fifty

pounds each.

Lime nutty is made from hydrated lime by adding water. IVhen

exncsed to the air, it loses waiter by evanoration , causing a f^ecided

decrease in volume. It then com.bines with the carbon dioxide of the

air, which unites with both the ca.lcium and magnesium to form the

respective carbcnates. Unon the interlocking of these carbonate

crystals deuefids the strength of the hardened material. A peculiarity

cf this reaction is that it will not occur unless there is a smallamount of water present. In other words, lime plaster will not harden

properly under exceedingly dry conditions, lleither will it harden

successfully wlien excessively and continuously damp.

Both quicklimes and hydrates are divided commercially into two

grades on the basis of nlasticit?/—masons' and finishing-. Only

plastic limes should be used in the finish coat o:*^ plaster.

Gypsum: Gj’psum is a native rock, composed essentially of

calcium sukohate , crystallized with about 20 percent of water, ’7b.en heated to about llO^C , three-fourths of the water content is driven

off producing that product knov.n as "stucco" at the mill; "plaster

of Paris" by the trade; and according to the most modem designation

"dfilcined gqq:sum".

Vhien water is ad-ded to calcined gpDs'im, the solid material

dissolves and crystallizes in the form of the original gj'psum, using

some of the water in the process. Tiie strength of the set material

depends upon the effectual interlocking cf these crystals of gypsum,

The comparatiyely great solubilitv of calcined gypsum in water

accelerates the hardeninrg reaction which usually requires about ten

minutes. One of the most iroportant properties of gypsum is that the

time of set may be controlled by the use of accelerators and retarders.

For the purpose of retarding tiie time of set, an organic compound

similar to glue is sold under the trade name of "commercial retarder".

One-fourth of one percent of this material added to calcined gypsum is

enou^ to retard the hardening for about two hours. If this is net

available, ordinary glue will give the same results.

Gypsum neat plaster- may be produced by adding tp calcined ;;^mrpsura

about one-sixth of its weight of hydrated lime and enpu^.h retarder to

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mal-ce it set in atout two hours. This is the chief gypsum product used

hy the plastering trade. It may he had either fihered or plain and is

sold in hags containing one hundred pounds each. Gypsum ready-sanded

plasters contain all the necessary ingredients, except water, for either g scratch coat or hrown coat work. Sometimes a shredded wood fiber is [ used instead of sand, the product being designated gypsum wcod-fihered

plaster . .1

Portland Cement ; Portland cement is made hy heating a natural or artificial mixture of limestone and clay to above 1,200 C, or until it,

begins to melt. The partially vitrified material is then ground to a

fine powder and mixed with a small amount of gypsiom to retard its set.

It is shipped in cloth or paper bags, containing ninety-four pounds each.

Portland cement requires water in its hardening process and for

complete hydration a sufficient suuply of water must be maintained for - many days. The conditions to which wall plaster is ordinarily subjected

are not those best adauted to developing the full strength of Portland

cement. On the other hand, Portland cement is peculiarly adapted for use as a plaster under continuously and excessively damp conditions.

Since one of the products of the hydration of Portland cement is

hydrated lime, the hardened material will have an alkaline nature.

Keene * s Cement ; Keene's cement is made from gypsum heated to such

a temperature that its water content is driven off. This product when

mixed with water will re-crystalli ze , developing hardness. The reaction

takes place so slowly, however, that a small amount of some chemical,

such as alum, is used to accelerate its hardening. Keene's cement is

used for plasters , requiring a hard, smooth surface.

Sand ; Care should be exercised in the selection of sand, the

principal aggregate in plaster. Both the angular and globular types

are satisfactory and may be cither natural .sand or ground rock. It

should be composed of clean, hard, durable stone particles, free from

objectionable matter, with an allowance of not more than 5 uercent of

loan, silt and clay. Sand for plaster should be graded to pass a To,

3 or No. U sieve and at least S5 percent should be retained on a No.

100 sieve.

Tiber : The only function of fiber is to enable the plaster to

be spread in place without losing too much of it through "dropping".

Tne fiber may be of animal or vegetable origin and should be long

enou^ to serve its purpose (one-half inch), but not so long that it

will tangle (2 inches). It should be flexible enough to stay buried

in the plaster, and should be clean, for dust in the fiber is as objectionable as dust is in the sand.. It should be evenly distributed

throughout the plaster.

Water ; Sea water, or water know.n as brackish, sulphur, chalybeate or carbonated, should not be used for plastering. With the

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above excer)tions, ordinary drinking T.’^ater ..should prove satisfactory.

Heady-nixed Plaster ; Ready-mixed ulasters may he obtained, manufactured from either gypsum or lime and containing all the ingredients,

except water, necessary for either scratch or brown coat. 'iThere delivery

cost is not excessive, they offer many advantages. Mixed by machinery

under exuert supervision, they are more nearly uniform and homogeneous

than plasters mixed by hand at the work. The great drawback being cost

of transuortation, if a reasonably good sand can be had locally, it is

poor economy to pay freight on a ready-mixed plaster.

Ready-mixed Mortar ; Ready-mixed lime plaster to which water has

been added is now available in some cities. It is sold as "ready-mixed

mortar" and is equally serviceable as mortar, plaster, or stucco, ^There

available, it has many advantages. The architect and contractor are re- lieved from all resuonsibility in regard to the purchase, storing, proportioning, and mixing of the various ingredients. The lime putty is more thoroughly aged, and the mixing is more uniform than could be ac- complished by hand. Its value is finally determined by whether or not

the delivered cost is lower than the cost of buying the ingredients and

mixing them on the job.

Properties of the Wet Mix

Plasticity’' ; The ulasticity of the wet mix is determined by the

physico-chemical urouerties of the cementitious material used. A

finishing hydrate has considerably more plasticity than a mason's

hydrate. Consequently, its commercial advantages are apparent, as the more plastic a plaster, the easier its application.

Shrinkage and Sand Carrying Capacity ; ’%lle sand reduces shrinkage in plaster, it also decreases the strength of the material. Therefore, its proportion should be commensurate with the purpose of the

plaster. Oversanding of plaster should be avoided as such plaster lacks

strength (See Page 6 "Proportions"). The more plastic a material, the

greater its sand-carrying capacity,’’, as far as workability is concerned.

A certain amount of sand may be added to a cementitious material without

seriously affecting its working quality, while a small additional quantity of sand may change the character of the mixture from that of

cementitious material to sand.

Time of Set ; The time of set of a plaster has a bearing on the

cost of a finished wall. Quick- setting plaster should be mixed in such

quantities as can be applied without loss of material. By the use of

accelerators and retarders the time of set of gjrpsum plaster may be

regulated.

Mixing and Application

Storage ; All cementitious materials should be kept dry until

used.

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Weather Conditions- ; Por plastering the ideal atmospheric condi-

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tion-is warm and hnmid.p. Plaster shonli, not he applied to a masonry | wall containing' frpst, nor under such conditions that the plaster may - i freeze '^he fore ithas a chance to harden. Very damp and cool conditions, I may interfere, hut will -not perm^anently affect the hardening of lime ,.'i and gyp'sum unless these conditions persist. The rapid evaporation of -.-.y water may cause excessive shrinkage. It is possible that enough moisture may he lost in this way to prevent the hardening' reaction. . |

Mixing :' IThen the ingredients of plaster do not require prelimina- I ry soaking, the dry m.ate rials should he mixed thoroughly to a uniform | colhr. They -should then -he added to the water ('water should not he -I added to the dry materials .unless a mixing machine is used) and mixed ti thoroughly to a uniform consistency. As little wa,ter as possible should ;! he used to:' bring the plaster to such consistency that it may he properly ; Worked. Excess water may cause, trouble,; due to segregation, difficulty ' of application, loss of material as, drqppings , or excessive shrinkage. : [

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I Sometimes one of the ingredients must he soaked before using, , as

in the case of quicklime, or in order to develop the full plasticity of

a finishing hydrate.

Grounds : Ground strips, are narrow striris of wood or metal placed

arcund and parallel to the edges of the surface to he plastered and all ‘ openings therein, and are -attached to the hacking. They were originally i; designed to he used as -guides for a straightedge ,' to hrinm the brown coat'; to a true and even surface. The thickness of these ground strips, should,

therefore, govern the thickness of the plaster. Spot grounds are also I sometimes used, being pieces o,f wood attached to the hacking at various '? intervals .-

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' f Eunctions'of the Different Coats ; Plaster is apulied in not more t than three coats,- designated in order of a'oplication as "scratch”,

^ "brown" , and "finish". The primary function of the first or scratch { cs>at.'‘'is to act as a bond, attaching the body of the plaster to the j base. It should have plasticity, cohesion, and strength when set.

Tne brown coat formiS the main body of the plaster . /Quick- harden---. [I ing is not so necessary as for the scratch coat, it' being customary to ... a-, allow ample time for this coat to dry completely before the finish coat |:1? is apulied. It is important, however, that the" brown coat have suf- ff ficient hardness to provide a good backing for the finish coat, so that

in case the finish coat is .omitted, the decorative finish may be applied [ directly to the bro^wi coat. It is also essential that, in order to ( avoid cracks and differential expansion, the brow, coat be not richer in ,-if cementitious material than the scratch coat. I

A finish coat is used where a smooth white surface is required.

This coat should be free from crack's, hard enough 'to withstand r,easonable i: abuse, and present a pleasing appeararce. This limits its comuositi.cn.. |

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In order to avoid shrinkage cracks, the coat should he as thin as possible and he troweled at exactly the proper time. Since this coat

must he applied in a very thin layer over a dry, hrown coat, its plasticity is exceedingly important.

Frouortions : It is important to measure all ingredients used

in plaster. Although proportioning may he on a weight or volume basis,

the following uroportions by volume are recommended for different kinds

of plaster, urimarily since contractors find volume measurements more convenient

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ProToortions for Scratch Coat on Lath

1 volume lime to 1 l/2 volimes sand

1 volume gypsum plaster to 2 volumes sand

1 volume Portland cement to 3 volumes sand

Hair or fiber should not exceed 3 bushels per cubic yards of sand.

Proucrtions for Scratch Coat on Masonry and for Brown Coat

1 volume lime to 3 volurues sand

1 volume g^/psum plaster to 3 volumes s'and

1 volume Portland cement to 3 volumes sand

If finish coat is used, hair or fiber should not exceed 1 l/2 bushels

per cubic yards of sand.

Explanation of Tables ;' "One volume of lime” represents one volume of either lime putty or dry hydrated lime. Lime may be mixed

with Portland or ICeene’s cement in any desired proportions. Portland

cement shpuld not be mixed with either Keene's cement or gypsum

plaster. The quantities of sand suggested are accepted generally as good practice, but slight variations may be necessary because of

differing qualities of materials used,

For the finish coat of lime, enough calcined gypsum should be

.ydded bo maice the material set within the desired time. The proportions used depend upon the area to be covered and weather conditions.

The amount of water used should be such as to provide the desired

consistency. Tnite sand or marble dust is sometimes used in the

finish coat.

Apulication ; The scratch coat should be thoroughly troweled to

key the plaster to the backing and built up to about one-fourth inch

uniform thickness. Before hardening, its surface should be "scratched”

with a suitable tool ma!:ing horizontal V-shaped grooves one-eighth inch

deep, about one inch apart. On masonry wralls, the decorative finish

may be applied to the scratch coat which should be plane and true. If

a finish coat is used, however, the scratch coat should be allowed to

harden thoroughly before the finish coat is applied.

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The brown coat should be thoroughly troweled, pushing the material

into the depressions in the scratch coat. It should then be built up to

the desired thicknes.s., and rodded" to produce a plane true surface, flush

with the grounds, It should then be darbied to take^ out the irregularities left by the rod., Finally, it should be floated to take out the last

of the irregularities and produce sufficient uorosity to give the proper

suction for the application of the finish coat. It should then be permitted to set until thoroughly hand and dry, before auplying the finish

coat, or any. other decorative finish.

More care should, be used in applying the finish coat^ than the

previous coats. As large an area as possible of each wall surface should

be covered at one time in order to avoid ridges and joints, and the coat

should be as thin as uracticable but thick' enough to completely cover the

under surface. At the uroper time, the cca,t .should be troweled, applying

considerable pressure, in order to prevent shrinkage cracks. Brushing

the surface with water while troweling is the method generally, used to

obtain a smooth finish..

"Dcubled-up" or "laid-on" ulaster work (scratch and brown coats

apulied at one time) is usually not advisable. This uractice is not

recommended for wood lath, especially when the plaster is to be seven- eighths inch thick. It should never be applied on metal lath. Its use on masonry backings is largely a question of application. If the

plaster can be made to stay -in place with a true and plane surface, the

work should prove satisfactory.

Exposure : Lime has a high water-carrying capacity although very

little w^ater is required for its hardening. Consequently in its drying

process, a good circulation of warm dry air is necessary.

Gypsum plaster requires water in its hardening process and care should be tal:en to provide sufficient to prevent a "dry-out".

Portland cement requires the presence of water throughout its

hardening process. For gypsum, the hardening process, may be completed ' in a comparatively short time, whereas in the case of portland cement,

the urocess may be extended over a ueriod of several da.ys. This ac- counts for the practice of wetting a cement plaster wall once a day

until the plaster has completely hardened.

To prevent warping, wood trim should not be applied to plaster

walls Tintil they are sufficiently dry.

^Other finishes are described in TIBM - 27.

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