flationfil Bureau of Standards

Library, N. W. Bidg,

AUG 1 9 1952

Investigation of Failures of White-Coat

Plaster

United States Department of Commerce

National Bureau of Standards

Building Materials and Structures Report 121
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UNITED STATES DEPARTMENT OF COMMERCE • Charles Sawyer, Secretary NATIONAL BUREAU OF STANDARDS • E. U. Condon, Dirtclor

Investigation of Failures of White-Coat

Plaster

Lansing S. Wells, Walter F. Clarke, Edwin S. Newman, and Dana L. Bishop

Building Materials and Structures Report 121

Issued February 1, 1951

For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C.

Price 25 cents

Foreword

The cost to the Nation of continual repairs of plastered surfaces is difficult to appraise, but it runs into millions of dollars annually. The cost to the

Government of plaster repairs to only four buildings, for example, has been

over $158,000. The chief cause of these plaster failm-es has been a recurrent type that appears as a bulge, or blister, in the white coat of plaster, which may

or may not affect the underlying base coat. This report presents the results

of tests made on material taken from a large number of plaster failures of the

blister type, scattered over the eastern United States. Detailed discussion is given of the preparation and properties of white-coat plaster and of the investigation leading to the conclusion that delayed hydration and expansion of the

magnesia in the hydrated dolomitic lime used is responsible for the blister type

of failure. The Bmeau has cooperated in developing methods for the preparation of white-coat plaster that is not subject to these changes, and these methods

have been introduced into actual production.

E. U. Condon, Director.
Contents

Page Foreword . iii I. Introduction 1

II. Plaster and plastering 2

1. Definition of plaster 2

2. Definition of plastering 2

3. Furring 2

4. Base coat 2

5. Finish coat 3

III. Description of failures 3

1. Principal characteristics 3

2. Building components and plastering 3

3. Typical illustrations 4

(a) On concrete beams and pilasters 4

(b) On ceilings 5

(c) On side walls 7

IV. Discussion of the formation and subsequent history of white-coat plaster 8

1. Materials used in preparing the white coat 8

(a) Quicklime 8

(b) Hydrated lime 10

(c) Gypsum gaging plaster ^ 13

2. Preparation of the lime putty 13

(a) Putty from quicklime 13

(b) Putty from hydrated lime 13

3. Application of the white coat 14

4. Chemical constituents of freshly set white-coat plaster 14

5. Drying and carbonation of the white coat 14

6. Chemical reactions involving volume changes 15

V. Investigation of white-coat-plaster samples taken after failure 16

1. Chemical analyses 16

2. Computed compound composition 17

3. Differential thermal analyses 21

4. Extent of hydration of magnesia in the aged white coat 22

5. X-ray analyses 22

6. Heat of solution 22

7. Composition of lime used in preparing the white coat 25

8. Ratio of lime putty to gaging plaster 25

9. Extent of carbonation of the lime 27

10. Expansion in autoclave 27

VI. Investigation of some base coats 28

VII. Possible causes of failure of white-coat plaster other than hydration of magnesia 33

1. Shrinkage of concrete 33

2. Differences in thermal expansion 33

3. Poor workmanship 35

4. Improper carbonation 35

5. Improper gaging 35

6. Painting plaster before it is sufficiently dry 35

7. Paint shrinkage 35

8. Vibration 35

9. Movements in the structural frame 36

VIII. Preventing failures of white-coat plaster in new buildings 36 IX. Methods of repairing damaged plaster . 39 X. Summary and conclusions 41 XI. References 42

iv
Investigation of Failures of White-Coat

Plaster

Lansing S. Wells, Walter F. Clarke, Edwin S. Newman, and Dana L. Bishop

An investigation was made of a particular kind of plaster failure that is characterized by

the formation of blisters or bulges in the finish coat of plaster. Extensive surveys revealed

that the failures are widespread; that several years elapse before the bulges appear and

thereafter the number and extent of failures increase as the age of the buildings increases; that the failures occur more extensively and rapidly during warm humid weather; and that

there is a marked similarity in the failures regardless of the kind of plaster base coats, type

of construction base, varying job conditions, and type of workmanship. The usual white coat

is prepared from a lime putty gaged with plaster of paris (gypsum gaging plaster) . Chemical

analysis of 88 samples of white-coat plaster that had failed showed that in every instance a

dolomitic lime had been used in preparing the white coat. Regularly hydrated dolomitic

limes contain about 32 percent of total MgO (by weight), of which only about 5 percent is hydrated and the remaining 27 percent is still present as unhydrated MgO. This highly

incomplete hydration is due to the fact that the magnesia has been badly overburned in the production of dolomitic quicklimes (which contain CaO and MgO in nearly equal molecular

proportions) and thus is inactive toward hydration in the usual hydrators. Furthermore,

only about 20 percent of the hydration of the magnesia is completed at the end of the cus- tomary 1-day soaking period. The amounts of CaS04.2H20, CaCOs, Ca(0H)2, Mg(0H)2, and

MgO in the hardened plasters were calculated from the chemical analysis and closely checked

by the method of heat of solution. The presence of CaS04.2H20, Mg(0H)2, Ca(0H)2, and

CaCOa was confirmed by thermal analysis. The average amount of hydration of the MgO

for 88 samples of failed white coat was 59.0 percent—much in excess of the 20 percent at- tained after the customary 1 day of soaking the lime as a putty. Hydration of magnesia

must, therefore, occur on the wall. The hydration of MgO is attended by marked expansion.

Failures were also observed in base coats where dolomitic lime containing considerable un- hydrated MgO was used in their preparation. It was shown that suggested causes other than hydration of MgO with its attendant expansion cannot account for the bulges that occur in plaster several years after a building has been erected. The precautions that should be exercised in specifying a proper lime in order to prevent future failures are discussed.

I. Introduction

Because it describes so clearly the type of

plaster failure that is the subject of this investigation, the following excerpt is quoted from a

letter wi-itten by the manager of a building

to "Building and Management", a magazine

published principally for building owners and

managers.

The Provident Building, of which I am manager,

has had trouble with falling plaster for the past ten

years and I really expect it to continue until the

building has been entirely replastered. The build- ing was completed in 1924 and it was not xmtil three

or four years later that plaster trouble began to develop, but it has gradually increased until the condition has become really serious for the past several

years. The plaster in this building has been applied di- rectly to concrete beams, to combination tile and concrete ceilings, and to hollow tile which forms the partitions. We have had no trouble at all with the brown coat of plaster but altogether with the white

or skim coat of plaster. It manifests itself first in a

slight bulge or blister which gradually increases until

it breaks away from the brown coat and falls off the

wall; the size of the places varies from a silver dollar to two or three square feet and in some instances whole sheets of white coat fall from ceilings . . .

In conclusion I would refer vou to Research Paper

RP 1022, U. S. Department of Commerce, National Bureau of Standards, which has just come to my at- tention and which may have some bearing on the

question . . . [1] i Numerous letters from widely scattered localities have been received at the National Bureau

of Standards, describing similar plaster failures on various backings and seeking advice either as

to their cause, or remedy, or both.

Publication of National Bureau of Standards

Research Paper RP1022, "Hydration of Magnesia

in Dolimitic Hydrated Limes and Putties" [2], re- ferred to in the letter to "Builduag and Building

Management", soon led to consideration of the

actual effect (if any) of unh^'drated magnesia

(MgO) in partially hydrated dolomitic limes on

the performance of mortars and plasters containing such limes, and the National Bm-eau of Standards soon became interested in this problem as it relates to plaster.^

1 Figures in brackets indicate tlie literature references at the end of this paper.

2 The contention that the pi esence of unhydrated MgO in the white coat might be responsible for failure because of the expansion attending its delayed hydration is by no means as recent as 1937 with the publication of RP1022.

In 1922 J. W. McBurney ascribed the plaster failures at Mt. Sinai Hospital, Cleveland, Ohio, to an expansion attending the delayed hydration of the magnesia in the lime used in preparing the plaster.

1
It was decided to undertake surveys of plaster

in buildings to determine the prevalence of failures

and to seek as much information as possible as to their causes. The first of these surveys was made

in Washington, D. C. This was followed by limited

inspections in Baltimore, Md., and in some other

localities in the Middle Atlantic States. Finally, an extensive survey was made of buildings in numerous towns and cities in Ohio, Michigan,

Indiana, Illinois, and Kentucky, with the cooperation of representatives of the Public Buildings

Administration of the Federal Works Agency, the

Lime, Gypsum, and Portland Cement Associations, the Contracting Plasterers' International

Association, the Operative Plasterers' and Cement

Finishers' International Association, and others.

For the most part, the surveys were confined to public buildings of the monumental type, first- class office buildings, hotels and apartment houses,

department stores, etc., where the walls are generally of masonry construction. Although only

limited inspections have been made of residential

buildings, the knowledge gained from the larger

buildings, where appearance and durability are given prime emphasis, has important bearing on

the smaller buildings.

Throughout the inspections attention was paid

not only to the materials used but also to the

structural frame of the building, the construction

bases, the kind of base coats, the job conditions

or type of workmanship, the extent of failures

and the age at which they first appeared, the

explanations advanced for their occurrence, the

methods of repairing the damage, and other

factors that might have a bearing on the problem.

Numerous samples of plaster were removed from

the walls and ceilings when feasible and taken to the Bureau for chemical analyses and other

determinations which will be described later. It was felt that only by these methods of considering

the assembly as a whole and then resolving it into its individual components and finally corre- lating these to each other and to the whole could

any definite decision be reached as to the cause

of failure. The purpose of this report is twofold. First, to give the results of the investigation of a particular

kind of plaster failure that is characterized by the

formation of blisters or bulges in the finish surface

of plaster; and, second, to explain to the architect,

engineer, plastering contractor, plasterer, and

others concerned with plastering, the precautions

that should be exercised in zaew buildings to pre- vent such failures and in old buildings to repair

the damage.

II. Plaster and Plastering

Before undertaking a description and discussion

of the failures encountered, it seems advisable to explain some of the terms relating to plaster and

plastering, especially for those not too familiar

with building construction.

1. Definition of Plaster

When a dry material is mixed with water into a

plastic mass that subsequently hardens to form the

surface of interior walls or ceilings, the term plaster

is applied to the material in any of its three

stages—dry powder, plastic mass, or hardened

surfacing.

2. Definition of Plastering

The word "plastering" (used as a noun) is given

a broader definition than the word "plaster."

Plaster is usually confined to the material. Plastering includes the plaster, the backing to which it

is applied, the workmanship used in the application, and the subsequent history of the completed

structure. Plastering may be divided into two

kinds, depending upon whether the plaster is applied to a masonry backing or to a lath backing.

3. Furring

"Furring" is a trade term applied to a type of

construction wherein there is provided an air space between the solid surface of a wall or ceiling

and the plaster. The furring of a wall may be

accomplished by the use of one of three different

types of material—furring tile, furring strips and

lath, and self-furring lath. It is sometimes desirable to make use of the principle of furring in

the construction of ceilings. If the ceiling is the

underside of the roof, or where excessive condensation may be expected, the insulation value of

furring is desirable. If the floor is constructed

in such a way that the beams project below it, and it is desired that the ceiling shall be plane,

the plaster must be placed on lath suspended

from the masonry.

4. Base Coat

The base coat may be defined as that portion of

the plaster which is applied to the masonry or lathing base and which supports the finish coat. In three-coat plastering work the plaster is applied

in three coats. They are designated "scratch,"

"brown," and "finish," respectively.

The scratch coat is applied first. Its primary

function is to act as a bonding coat to attach the

body of the plaster securely to the lath or masonry.

The properties required of a scratch coat are evidently dependent upon the nature of the backing

to which it is to be applied. After it is applied, but

before it hardens, this coat is scratched with a

suitable tool. Thus the name, scratch coat. After it has hardened, it is ready to receive the

brown coat. The first coat is usually sanded.

However, when the base is concrete, a plaster

laiown to the trade as "bond plaster" is generally

used as the first coat. It is essentially neat gypsum plaster and is recommended to be used without the addition of sand.

The brown, or second, coat forms the main body

of plaster on lath. This coat also is generally

2
sanded. The brown coat must be sufficiently

hard to form either the wearing surface of the wall

(if a sand-float finish is specified) or a solid backing

for the finish coat. In two-coat plastering work, the base (first coat)

is apphed with sufficient material and pressure to cover well the lath or masonry, as the case may be,

and then doubled back with the same mix to bring

the plaster out to the desired thiclmess. The surface is then straightened to a true surface and left rough to receive the finish (second coat).

5. Finish Coat

The finish coat is usually applied when a smooth

white finish is desired. This coat must be free from

cracks, it must be hard enough to withstand

reasonable abuse, and, above all, must present' a

pleasing appearance. These requirements limit the

composition. In order to produce the required

hardness and the smooth white appearance, no

ordinary sanded material can be used. Although

there are several white-coat plasters, the one most

generally applied is made from a lime putty mixed

with calcined gypsum (also known as gypsum

gaging plaster or plaster of paris). The lime used

in preparing the putty, the application of the

white coat, and its subsequent behavior will be

taken up in detail in this paper; and, in fact, will be

the main theme of this publication.

III. Description of Failures

1. Principal Characteristics

The type of failure that was foimd again and

again thj'oughout the inspection trips is separate

and distinct from cracks resulting from move- ment of or within the structural frame, damage

from leaks, and similar difficulties that always

have been and always will be encountered in plaster work.

The principal characteristics of this particular

kind of failure are as follows:

(a) Bulges (large blisters) are formed with the

finish surface convex. This may be the result of

(1) separation of the smooth or white skim coat from an underlying layer of white coat, (2) separation of the entire white coat from either the

sanded brown coat or coat of bond plaster, (3) in a few instances, separation of sand-lime-plaster

base coat from the construction base.

•(b) Several years (from about 5 to 15, but

usually from 8 to 9) elapse before the blisters or bulges begin to appear, and thereafter the number

and extent of failures increase as the age of the

building increases.

(c) The failures occur more extensively and

rapidly during the warm weather of summer and

during periods of high relative humidity.

(d) There is a marked similarity in the failures,

regardless of the kind of plaster base coat, construction base, varying job conditions, and type

of workmanship.

2. Building Components and Plastering

It seems desirable to group together a brief

description of the various structural members of

the numerous buildings inspected and of the

different kinds of plastering where failures oc- curred. This has been done in table 1 where the

descriptions of the structural members of the building have been subdivided as to the framing, floor

slabs, outside walls, and interior partitions; and

the kinds of plastering, as to the plaster bases and

plaster work. It should be emphasized that the

surveys were confined primarily to public buildings

Table 1. Descri-ption of the various structural members of the buildings inspected and of the different kinds of plastering where

failures occurred

Structural members ol the buildings

Framing Floor slabs Outside walls Interior partitions

Structural steel, fireproofed. Structural steel and reinforced concrete. Reinforced concrete.Reinforced concrete. Concrete joists with tile fillers. Concrete joists with sheet-metal-lath fillers. Concrete joists with expanded-metal-lath

fillers. Reinforced concrete, flat slabs.

Brick.

Brick, furred with tile. Brick and terra cotta, furred with tile. Stone and brick, furred with tile. Stone and terra cotta, furred with tile, stone and brick, furred with metal lath.

Hollow tile. Gypsum block. Metal lath plastered on both

sides, 2 inches thick.

Kinds of plastering

Plaster bases Plaster work

Ceilings, determined by construction of floor slabs: One-coat work:

Monolithic concrete. White coat applied directly to concrete. Concrete joists with tile fillers. Two-coat work:

Concrete joists with sheet-metal-lath fillers. First coat of sanded plaster, doubled back; second coat of white-coat finish Concrete joists with expanded-metal-lath fillers. (mostly over masonry-finish construction). Suspended metal lath attached close to bottom of concreted joist (furred Same, but sand-float. type). Gypsum bond plaster over concrete, finished with white coat. Ceilings—suspended sheet or expanded metal lath. Three-coat work:

Outside walls and interior partitions: Scratch, brown, and finish coats (mostly over metal lath but also over

Brick. masonry)

.

Hollow tile. Scratch, brown, and sand-float finish coats. Furred metal lath. Gypsum block. Gypsum plaster (2-inch solid partitions).

3
of the monumental type where walls are generally

of masonry construction. Therefore, the woodframe type of construction is not included in table 1. Although only limited inspections have

been made of residential buildings, they have,

nevertheless, revealed that white-coat-plaster fail- ures occur in such buildings; for example, within

the white coat applied to gypsum plaster on gypsum lath. Failures have also been noted both

where the plastering was over wood lath and

where it was on masonry construction.

3. Typical Illustrations

Figures 1 to 13, 22 to 24, and 27 to 29 have been

selected that illustrate typical ways in which the

plaster was failing. These will be described and

interpreted in sufficient detail to bring out certain

of the salient factors involved.

(a) On Concrete Beams and Pilasters

Failures in the white coat of plaster over concrete bases were of frequent occurrence. Figure 1 shows a series of bulges on the side of a concrete beam. In this case, the bulging was confined entirely to the white coat. On breaking these bulges

it was found that the underlying coat of gypsum

bond plaster (which had been applied directly over the concrete) had been well scratched, was

hard, and still attached firmly to the beams.

Figure 2 shows bulging white coat along the side

of a concrete beam located above a door in an

inside stairway of an eight-story fireproofed apartment house built in 1924. Here, again, the failure was confined to the white coat; the underlying

gypsum bond plaster was stUl adhering tightly to the concrete base. Failures are also taking place

in the white coat on the ceilings and side walls. There has been no trouble with the underlying

base coats of sanded g^^psum plaster. Failures

started in this building about 7 years after the

building was erected.

When figure 3 was taken, new bowling alleys were being installed, and, consequently, no effort

Figure 1. A series of bulges in white coat along the side of a

concrete beam, caused by the expansion attending the slow hydration of free MgO in the hydrated lime used in preparing the white coat.

Figure 2. Bulging white coat along the side of a concrete beam located above the door of an inside stairway in an

eight-story fireproofed apartment house. The underlying gypsum bond plaster was still adhering tightly to the

concrete base.

was being made to keep in repair the white-coat

finish. In fact, the failures had occurred so fre- quently that the owner of the bowling alleys found

it cheaper and more advantageous to hang velvet

drapes in front of the concrete beams to hide their

unsightly appearance rather than to replaster the

beams repeatedly. Inasmuch as the bulged and

loose plaster had been removed before the concrete beams were photographed, the picture re- veals where the failures had occurred. For the

most part they were at the bond between the successive layers of white coat, applied to straighten

up the beam, rather than between the white coat and scratch coat of neat gypsum bond plaster.

Only in very limited areas was there any separation of the bond plaster from the concrete. Figures 1, 2, 3, and 4 are typical of those

failures on concrete beams that were observed

throughout the surveys. Troubles of a similar

nature occurred in instances where no bond plaster was used and the white coat was applied directly

to the concrete, even where the concrete appeared

to be sufficiently rough for satisfactory bonding.

Inquiry invariably revealed that the bonding had

been entirely adequate for 5 to 15 years prior to the formation of bulges and the shearing of the

white coat from the underlying base. Bulges

in the white coat were also observed where a sanded plaster was used in lieu of bond plaster;

and, as with the bond plaster, the failures were

almost invariably in the white coat and not in the bond between the underlying sanded plaster

and the concrete base. In those instances where

the bond plaster had been apphed to a smooth

dense concrete, and especially where the oil from

the forms had not been removed, the failm-es were

mostly between the bond plaster and the con- 4
Figure 3. Failures in white coat along sides of concrete beams over bowling alleys. The failures caused by the expansion of the white coat were confined almost entirely to loosening of the bond either between successive layers of white coat

or between the white coat and the gypsum bond plaster. Only in very limited areas was there any separation of the bond plaster from the concrete.

Crete. In these cases, where the bond between

the plaster and the concrete was not so strong

as that between the bond plaster and the white

coat, the bond plaster was pulled from the concrete by the expanding white coat. Thus, there was a marked similarity in the

failures, regardless of the type of plastering over

the concrete. It is believed that such whitecoat-plaster faihnes are largely responsible for

the conception held by architects and others that

plaster does not adhere satisfactorily to concrete. Too often the fact is overlooked that the failure

is primarily in the white coat and that the underlying bond coat is still adhering firmly to the

concrete. Without careful observation, the bond

plaster, because of its color, may be mistaken

for the concrete base.

Figure 5 shows a large bulge of the white

coat on a concrete pilaster.

(b) On Ceilings

Typical defects of bulging, separation of the

final white coat from an undercoat of white coat,

and separation of the entire white coat from the

brown or bond coat were also observed on ceilings.

Figure 6 shows a white-coat blister, cracked

and bulging from an underlying sanded gypsum

brown coat on a plastered ceiling, but still held

Figure 4. Bulging white coat along the edge of the soffit and

side of a concrete beam in an office building. Note the old patch in the white coat in the ceiling.

909872—51 2 5
FiGUEE 5. Large bulge of white coat on a concrete 'pilaster in a school building.

FiddRJi 6. Criii-I;( il liiilgi nj irhiii ciiiit an a. ceiling, still held suspvndid by a jiliii oj paint.

Figure 7. All that remained after a bulge of while coat fell from a ceiling of an office building. Note the firm underlying brown coat of sanded gypsum plaster and a thin

layer of white coat still intact.

6

IKiL KE 8. A kitchen ceiling in an apartment house; reveal- ing pronounced bulging in the white coat, caused by ex- pansion.

Note the small patches previously applied.

suspended by a film of paint. The scratch and

brown coats remained firmly bonded to the base

of concrete joists with hollow-ceramic-tile fillers. Figure 7 shows the loosely held remains of a shattered blister after the missing portion had

fallen from the ceiling, thus revealing the firm

underlying brown coat of sanded gypsum plaster

and a thin layer of adhering white coat still intact.

The plaster base of the ceiling was composed of concrete joists with hollow-ceramic-tile fillers. The surveys disclosed that separation of the

white coat occurs irrespective of whether the

plaster is applied over the concrete joists or the

hollow-tile fillers. An example of pronounced bulging of white

coat is illustrated in the photograph of a kitchen

ceUing in an apartment house (fig. 8). Figure 9

Figure 9. A more detailed photograph of the bulges shown

in the background of figure 8.
presents in greater detail the bulge in the background of figure 8. Trouble had started about 7 years after the building was erected and had con- tinued for 18 years up to the time of the ins])ec- tion. White-coat-plaster failures had occurred on tiie side walls also. There had been very little trouble with the sanded gypsum base coats on

ceilings and side walls.

(c) On Side Walls

Figure 10 shows a large bulge of white coat protruding from a sanded base coat on a side wall

and extending downward from the plaster moulding for about 2)^ feet. There was no evidence of

poor Avorkmanship in the plastering. The 12- story building was of reinforced-concrete construction with holloAV-ceramic-tile partitions. No

details were obtained as to when failures began

nor as to their extent each year, but at the time of

the inspection, 27 or 28 years after the building was completed, a great deal of trouble was being

experienced with the white-coat plaster throughout the building. Wliite-coat-plaster failures were

as extensive on the ceilings as on the side walls. The gypsum sanded base coats on the side walls were in good condition.

Figure 11 shows the bulging of white coat,

involving separation of this coat from the gypsum

sanded base coat within an area of about 1 square

yard. This is a photograph of the side wall of an

inside stairway of an apartment house where there were several similar failures in the white coat;

Fl(_; 1 KE 10. .1 lanjr bulge i)f irliitc coat cxlcndiiij Lboiit 2]2 feet downward from a cast gypsum plaster moulding.

The photograph was taken in a corridor of a 12-story building of reit^forced- concrete construction with ceramic-tile partitions. The building, originally designed and occupied as a hotel, was later converted to an office building. No expense had been spared in the plastering work.

Fna KK I 1. liulgiiig of white coat on a side wall, involving

separation from the sanded gypsum base coat within an

area of about 1 square yard.

Figure 12 shows a bulge of white coat on the side

wall of another building. In this case the backing was wood lath. An example of extensive white-coat failure is shown in figure 13. This is a side-wall panel in a shop of a store and office building. The gypsum

brown coat underneath this white coat was firm

and well bonded. The white coat in an adjoining

panel had been replaced, and the members of the

inspection committee were told that the bulges in

this panel were even more pronounced than those

shown in figure 13. Up to this point, with the exception of figure 12,

the illustrations are confined to white-coat failm-es

where the plaster bases were of masonry construction. This should not be construed to indicate

that faiku'es were not also occurring on other bases,

Figure 12. Bulge of white coat on the side wall next to a

door jamb.

The firm base coat of sanded gypsum plaster was "applied over wood lath.

7
FiGURK ]3. Scries of white-coat bulges along the side wall

of a plastered panel in a shop of a store and office building.

such as metal lath. In fact, numerous failures were observed where the base-coat plasters had

been applied over metal lath on ceilings where the

floor slabs were of concrete joists with both sheetand expanded-metal-lath fillers. Failures also oc- curred where the metal lath was of the furred type

and attached close to the bottom of the concrete

joists. In several instances, failures started in

such ceilings prior to those on the side walls where

the sanded plaster had been applied to ceramic

fm'ring tile. The manager of a building where

there were two types of construction stated that

there was no appreciable difference between the extent of white-coat-plaster failures on ceilings

having ceramic-tile fillers and on those with metallath fillers.

IV. Discussion of the Formation and

Subsequent History of White-Coat

Plaster

The formation of a bulge in the coating on a

rigid nonyielding backing must be accompanied by

an increase in the surface area of the coating. In

white-coat plaster, a rigid brittle substance, such an increase can be accomplished only by expansion

of the material in the bulge. It is shown later that

plaster backings are incapable of contracting suffi- ciently to cause bulge-type white-coat-plaster fail- ures. As expansions are often the result of chemical reactions, it is proposed (1) to discuss the

composition, preparation, and properties of the

materials used in the white coat, (2) to describe

briefly the preparation and application of the

white coat, (3) to trace the chemical reactions that

take place from the time the white coat is prepared

and applied until long after it has hardened, (4) to point out which of these reactions most definitely causes an expansion, and (5) to correlate the field observations with these various factors. Such

considerations should clarify the reasons for making certain chemical analyses, computations, and

auxiliary experiments—such as heat-of-solution

tests and thermal and X-ray analyses—on samples

of white-coat plaster.

1. Materials Used in Preparing the White Coat

The usual white coat is prepared from a lime

putty gaged with plaster of paris (gypsimi gaging

plaster). Lime putty may be prepared either by

slaking quicklime on the job with an excess of

water or by soaking a dry commercial finishing

hydrated lime with water.

(a) Quicklime

Lime, chemically speaking, is calcium oxide

(CaO), but the commercial article may differ widely from this composition. The burning of

lime, which dates back to early antiquity, may be

defined as the process of converting limestone into

lime through the agency of heat. Carbon dioxide

(CO2) is liberated during the process. The term "limestone" is used to describe a class of sedimentary rocks varying in composition from

almost pm-e calcimn carbonate to a mixture of

calcium and magnesium carbonates in equal

molecular proportions (dolomite). Any gradation between these limits may be found; and, in

addition, all limestones contain impurities. The

chemical composition of a lime depends on that

of the limestone from which it was made. Consequently, only those limestones may be used

from which a marketable lime can be produced.

Because of the loss of about half the weight of the

stone as carbon dioxide during the burning, the

proportion of every other constituent will be nearly

doubled in the lime. The amount of impurities

(primarily silica and oxides of iron or aluminum)

permissible in a hydrated lime is small, certainly

not over 5 percent (2% percent in the stone).

Lime may contain from 0 to 42 percent of magnesium oxide (MgO), depending on whether the

lime was prepared from pure calcium carbonate

or pure dolomite. When properly burned and

fresh from the kiln, lime should contain no water

and less than 1 percent of carbon dioxide.^

The material as it comes from the kiln is fre- quently called "quicklime" to distinguish it from

"slaked lime."

There are three factors essential to the process

of lime burning (1) the limestone must be heated

to the temperature of dissociation of the carbonates, (2) this temperature must be maintained for a certain length of time, and (3) the liberated car- bon dioxide must be removed.

2 Both Federal Specification SS-Q-.351 for quiclclime (for) structural pur- poses (1930) [3] and American Society for Testing Materials, ASTM C5-26

[4], specify that the total CaO+MgO in quicklime shall be not less than 95

percent, calculated on the nonvolatile basis. This purity is, in turn, re- flected in the purity, calculated on the nonvolatile basis, of hydrated lime as given in Federal Specification SS-I--351 for hydrated lime (for) structural purposes (1930) [5] and ASTM Specifications C6-46T [6], C206-46T [7], and

C207-46T [8].

< Although the carbon dioxide is usually less than 1 percent as the lime comes from the kiln, specifications such as SS-Q-351 [3] and ASTM C5-26T

[4] allow as much as 5 percent of CO2 if the sample be taken at the plant and

7 percent if taken elsewhere. This is because quicklime readily reacts with

the COi of the atmosphere. 8
Many experiments have been made to determine

the dissociation temperature of calcium carbonate,

and the Hme-burning reaction has become the

classical example of a gas-solid equilibrium involving a distinct chemical reaction:

CaC03;=^CaO+C02.

The system consists of two components in three

phases (two solid and one gaseous) and is therefore

monovariant. It can be shown readily that the

dissociation pressure at equilibrium is independent

of the amounts of the separate phases and depends

solely on the temperature. Thus, when calcium

carbonate is heated, the dissociation pressure in- creases. The thermal dissociation of calcium

carbonate has been studied by numerous investigators, and Kelley and Anderson [9] have shown

that the better results of the dissociation-pressure

data may be divided into two groups. They

state that those of Johnston [10], Smyth and

Adams [11], and Dutoit [12] are in good agreement

and indicate that in the dissociation of CaCOj

(solid) into CaO (solid) and CO2 (gas), the dis- sociation pressure of CO2 reaches 760 mm of Hg

(1 atm) at a temperature of 898° C (1,648° F).

The data of Andrussow [13], von Kohner [14], and

Tamaru, Siomi, and Adati [15] are likewise in good

agreement, but consistently show higher pressures

for a given temperature than do those of the former

group of workers. Kelley and Anderson have

shown that if the average of both groups is taken,

the calculated value for the pressure of CO2 is

1 atmosphere at 884° C (1,623° F).

The corresponding temperature for magnesium

carbonate (MgCOa) is very much lower, but the

thermal dissociation values are in considerable

disagreement, largely because of the difficulty in

attaining equilibriiun conditions. Kelley and

Anderson [9] have also critically analyzed the data

for MgCOa and show that the equilibrium decomposition pressure of MgCOa apparently reaches

1 atm at 408° C (766° F), which is low compared

with 884° C (1,623° F) for CaCOa. It has been

shown, however, in studies by Ralston, Pike, and

Duschak [16] that at 1 atm an appreciable rate of decomposition is not reached below 555° C (1,031°

F). In other words, because of the slowness of decomposition, MgCOsmay be consideraby superheated.

Kelley and Anderson [9] state "It it possible then

vu'tually to complete the reaction MgCOs.CaCOs =MgO+C02+CaC03 at temperatures below

those where the reaction CaC03=CaO+C02 takes

place to an appreciable extent. "In the case of mixed compounds such as dolomite the decomposition temperatures of the

less stable constituent should be somewhat higher

than those in the table. The increase in temperature may not be expected to be large, however, since the free energy of formation at 298.1°

of such compounds from the carbonates, that is,

i MgC03+CaC03= MgCOs.CaCOs is small with

respect to the free energies of decomposition of the

individual carbonates ... it would seem that

100° or 200° increase in temperature is necessary

to overcome the increased stability due to com- pound formation in decomposing the less stable

constituent of a mixed carbonate." The table

referred to gives the decomposition temperatures

of MgCOj and CaC03 as 681 and 1,157° K,

respectively. These temperatures correspond to 408° and 884° C (766° and 1,623° F), respectively.

It is evident that in the lime-burning of dolomite,

the magnesium carbonate is decomposed into

MgO-|-C02 before the calcium carbonate has been

completely decomposed.

Decomposition temperatures, melting points,

and transition temperatures may be approximately

located by differential thermal analysis. This is done by heating at a uniform rate the material

under investigation side by side with a similar

body of inert material and contmually measuring

the temperature differences between the two substances. When a reaction occurs which absorbs

or evolves heat, the measured temperature differ- ence will deviate sharply from that previously

established by the differences in thermal conductivity, heat capacity, and density of the two

substances. This technique has frec^uently been

described in detail [17]. As it is a dynamic process, decomposition temperatures, etc., determined

by differential thermal analysis agree only approximately with data obtained under equilibrium

conditions. However, much valuable information can be extracted from the heating curves so obtained. The technique of thermal analysis has

been applied, in this laboratory, to many of the

substances discussed in this paper. Figure 14

shows heating curves pertinent at this point in the

discussion. The decomposition temperature of

calcite (CaCOs), taken as the lowest point of the

differential heating curve (1), is 917° C (1,683° F),

33° C (59.4° F) higher than the average equilibrium value of 884° C (1,623° F).

The existence of the superheating of magnesium

carbonate is demonstrated m figure 14 where its heating curve (2) shows a (nonequilibrium) decomposition temperature of 635° C (1,175° F) for

magnesite (MgCOs) . The temperature is changed

only slightly by the presence of calcite. The heating curve of an equimolar mixture (curve 3) shows

decomposition temperatures of 625° C (1,157° F)

and 935° C (1,715° F) for the magnesite and calcite,

respective!}^.

The fourth heating curve in figure 14 demonstrates the increase in the decomposition temperature of dolomite over that of magnesite. In this

test, dolomite (NBS Standard Sample 88) was

decomposed in two steps, showmg decomposition

temperatures of 778° C (1,432° F) and 935° C

(1,715° F), respectively.

It should be necessary to heat a limestone only

slightly above 884° C (1,623° F), the decomposition temperature of CaCOs, to convert both the

magnesium and calcium carbonates completely to

oxides. However, after the dissociation tempera9
T I r

0 400 800 1200 TEMPERATURE, °C Figure 14. Four differential heating curves showing the dynamic decomposition temperatures of (1) calcite, {2)

magnesite, (3) equimolar mixture of calcite and magnesite,

and (4) dolomite.

ture has been reached, it must be maintained for some time in order to transfer the required amount

of heat to the stone. The time requhed to transfer a given amount of heat, roughly speaking,

varies iaversely with the temperature difference.

Consequently, it has been found economical to use

as high a temperature as feasible in order to reduce

the time requhed for burning, and the temperature of the ordinary lime kiln is usually somewhat

in excess of 1,100° C (2,012° F) or even 1,200° C

(2,192° F). Increasing the temperature is also economically advantageous, because the dissociation pressure of calcium carbonate is an exponential

function of the temperature. Consequently, an

increase of temperature will cause a decided in- crease in the dissociation pressure and a corre- spondingly large reduction in the time required to produce a given quantity of lime.

When, however, the temperatures are raised too high, a phenomenon known technically as overburning occurs. The product becomes denser, is not as white as properly burned lime, but,

most important of all, its activity toward water is reduced. These changes, in part, are caused by

the chemical combination of the CaO with Si02^

AI2O3, and FcoOs to form a melt (somewhat

similar to the clinker formed in the burning of

cement) which reduces the pore space in the

quicldime and surrounds particles of the oxides.

In general, for high-calcium limes, the smaller the

percentage of impurities, the higher can be the

temperature of burning without the formation

of overburned lime. But even with rather pure

limestones, the activity of the resulting CaO

toward water to form calcium hydroxide, Ca(0H)2,

is reduced somewhat as the temperature of burning is increased. Lime may be- overburned by

being heated for too long a time as well as at too high a temperature.

Magnesium oxide is far more susceptible to over- burning than calcium oxide, and its reactivity

may be decreased enormously thereby. For

example, if overburning has not occurred, magnesium oxide, like calcium oxide, unites rapidly

with water to form the hydroxide in the process

known as slaking. Campbell [18] found that the

hydration of MgO prepared by heating magnesite

(MgCOs) to a temperature of 800° C (1,472° F) is practically complete in 3 days. He found that a change in the reactivity of the magnesium oxide

sets in between 1,000° and 1,100° C (1,832° and

2,012° F) and that MgO heated somewhat below

1,100° C (2,012° F) required nearly 3 months for

complete hydration. Samples burned at 1,200°

C (2,192° F) required 3 years to become completely hydrated, and samples burned at 1,300° C

(2,372° F) and above were not completely hydrated even in 6 years.

It should be obvious, therefore, that by the time

dolomite or a magnesium limestone has been

heated to the temperature necessary to convert

the CaCOs economically into CaO and CO2,

roughly 1,100° to 1,200° C (2,012° to 2,192° F),

the MgO, formed at a much lower temperature,

has been considerably overheated and its activity

toward water greatly reduced. Also, the over- burning of MgO increases with the time of heating

and to a gi'eater degi'ee than that of CaO. In

addition, it would appear logical to assume that

the MgO in the outer zones of a lump of the

calcined dolomitic limestone would be over- burned to a gi-eater extent than the MgO in the

inner zones, because of the temperature gradient,

and consequently each lump of quicklime would

be composed of MgO of varying degrees of reactivity toward water.

(b) Hydrated Lime

Dry hydrated lime is a product of comparatively recent origin. In the normal ^ "manufacture of diy hydrated lime, starting with the

burnt quicklime as it comes from the Idlns, the

following operations must be performed: crushing, hydrating, screening, and packing. The

object of crushing the quicldime is to produce

a large surface for the action of water and to

facilitate handling in the slaking process. The

crushed quicldime is fed into a hydrator. Here

it is mixed with water and agitated. In the

hydration, the quicklime is broken down and

slaking to a dry powder is accomplished by

' Lime produced in this manner is designated "normal hydrate" by the American Society for Testing Materials.

10
adding sufRcient water to hydrate the active

constituents. (If an excess of water is used in

the normal hydration, an undesirably moist

powder is produced). The fine, dry powder that comes from the hydrator must be classified either

by screens or air separators to remove any core

or other hard, coarse, particles of overburned

lime that did not slake. Chemical analysis of hydrated high-calcium lime

as commercially produced shows that the combined

water is almost sufficient for complete hydration.

The CaO, imless "dead-bm'ned", is a very active

constituent of quicldime and reacts so rapidly

with water to form Ca(0H)2 that its hydration

is complete or very nearly complete in the few

minutes the material is in the hydrator.

On the other hand, chemical analysis of regularh^

hydrated dolomitic lime shows that ordinariT\^ only

55 to 65 percent of the water required for complete

hydration is present. This large deficiency in the

combined water is due to the fact that the inactive

j\IgO of dolomitic quicklime (which contains CaO

and MgO in a nearly equimolar ratio) has resisted

hydration in the manufacture of the dry hych-ate

powder.

In 1927 Kichardson [19] described a thermochemical method for determining the degree of

hydration of magnesia and calcium oxide in commercially hydrated dolomitic limes. The method

was based upon the fact that the two hydroxides

have appreciably different dissociation temperatures. Thus, the percentages of magnesia and

calcium oxide hydrated could be calculated from

the weight losses when the temperatiu-e was

raised just high enough to decompose first the

magnesium hydroxide (AIg(0H)2) and then the

calcium hydroxide (Ca(0H)2). By this method,

Richardson analyzed 10 representative commercial

dolomitic iiAxlrates and concluded that in such

hydrated limes the calcium oxide is largely hydrated, whereas the magnesia is only slightly

hydrated.

Fox and Mathers [20] in 1934 determined by a

similar method the amount of hydrated magnesia

in nine hydrated dolomitic limes. Then* data in- dicated that onh^ from 2.8 to 14. .3 percent of the

total magnesia was hydrated.

Wells and Taylor [2] in 1937 developed heat-ofsolution and ignition-loss methods for determining

the degree of hydration of magnesia in hydrated

dolomitic limes and in hydrated dolomitic lime

putties. The heat-of-solution method was based on the fact that the components of hydrated limes

have substantiall}' different heats of solution.

The observed heats of solution of the limes agreed

closeh^ with those calculated from the algebraic sums of the heats of solution of the individual

constituents as computed from the chemical

analyses. It was sho^^^l that in hydrated dolomitic

lime CaO is generally completely hydrated, whereas

the hydration of the MgO is far from complete.

The degree of hydration of the magnesia in six hvdrated domolitic limes as received from the

50

40

30

20

10

0 10 20 30 40 UNHYDRATED OXIDES , %

FiGUEE 15. Distribution of the percentage of unhydraied

oxides in 185 (N) samples of regularly hydrated dolomitic

lime, submitted to the Bureau for test from 1923 to 1940,

showing that the average amount (X) was 26.98 percent with a standard deviation (cr) of 3.04 percent. manufacturers averaged 12.5 percent (ranging from

3.3 to 22.8 percent).

More recently, in a stud}^ of the expansive

characteristics of hydi-ated limes, Wells, Clarke,

and Levin [21] showed that the amount of un- hydrated MgO in 27 regularly hydrated dolomitic

limes averaged 27.7 percent, with the range ex- tending from 16.1 to 34.3 percent. Considering

the total amount of magnesia in the sample, this means that on the average only 15.7 percent of

the total magnesia was hydrated.

Calculations based on the analyses of the 185

regidarly hydrated dolomitic limes submitted to the Bureau for test for compliance with Federal

Specification SS-L-351 [5] from 1923 to 1940 « indicated that the average amount of unhydrated

MgO was 27.0 percent, by weight, and that the

percentage of the total magnesia that was hydrated was 17.0 percent.^

Figm-e 15 shows the distribution of the per8 Since 1940 most ot" the Government agencies have included the require- ment of the proposed tentative amendment to Federal Specification SS-L351 [5] that, "The total free (unhydrated) calcium oxide (CaO) and magne- sium oxide (MgO) in the hydrated product shall not exceed 8 percent by

weight (calculated on the 'as received basis')."

" In the calculation of the percentage of unhydrated oxide, the percentage

of "free or extraneous water" should be ascertained according to the proposed amendment to Federal Specification SS-1^351 [22] or according to the "Tentative revision of standard methods of chemical analysis of limestone, quicklime, and hydrated lime." ASTM Designation: C25-44, submitted June

1945; see ASTM Standards, part II, 1701 (1946) [23]. However, inasmuch

as the amount of "free or extraneous water" is usually but a few tenths of

1 percent, failme to determine and exclude this water in the calculation of the percentage of unliydrated oxides will make but slight difference in the value obtained—usually less than 1 percent, but always on the low side. Although this correction is important when a limit for unhydrated oxides is set in a specification, nevertheless, it can be disregarded for a general survey

of the approximate degree of hydration of magnesia in regularly hvdrated

dolomitic lime produced commercially.

11
30

10 -

10 20 30 40 50 PERCENTAGE OF HYDRATION OF MAGNESIA

60

Figure 16. Distribution of the percentage of hydration of

the magnesia in 185 (N) samples of regularly hydrated

dolomitic lime, submitted to the Bureau for test from 1923

to 1940, showing that the average amount (X) was 17.29

percent with a standard deviation (cr) of 8.4O percent.

centage of unhydrated oxides for these 185 samples

of regularly hydrated dolomitic lime; figure 16,

the distribution of the percentage of the total magnesia that was hydrated. In this figure,

N represents the number of samples; X, the

average percentage of unhydrated oxides (actually

MgO); and a, the standard deviation calculated

from the grouped data.

Differential thermal analysis also shows that

the MgO of regularly hydrated dolomitic lime is only partially hydrated. In figure 17 are shown

the heating curves of two hydrated limes and that

of the partially dry material recovered from a sample of putty prepared from one of them.

Curve 2 represents a hydrated dolomitic lime of

the regular (or normal) type. The presence of

Mg(0H)2, Ca(0H)2, and small amounts of CaCOs

is shown by characteristic deflections (C, D, and

E, respectively) in the heatmg curve. The same

lime after 1 day of soaking as a putty is represented

by curve 2II.

" The presence of uncombined water

not removed by the drying process is indicated by

the first break in this differential heating curve. The small increase in the amount of Mg(0H)2

caused by hydraition as a putty for 24 hours can

be observed by comparing the relative sizes of the characteristic deflections of Mg(0H)2 and

Ca(0H)2 in these curves. That only a smafl

part of the potential Mg(0H)2 has been formed

can be seen by comparing these curves with

curve 1 which represents a completely hydrated

dolomitic lime. In curve 1 the deflections characteristic of Mg(0H)2 and Ca(0H)2 are of approximately the same size. It has been pointed out that a well-burned

quicklime contains but a small quantity of

unburned limestone, and in the usual lime plant

this unburned material or core is discarded by the

"pickers" or "sorters". With liigh-calcium limes

the remaining carbonate in the core would be

essentially CaCOs; with dolomitic limes it would

also be largely CaCOs, because calcium carbonate

has a much higher dissociation temperature than

magnesium carbonate. In the handling of the

quicklime, during the slaking process, and subsequently, the hydrated lime picks up additional

carbon dioxide from the air. Usually, however,

the quantity is small.

In a recent investigation of the expansive

characteristics of hydrated limes, Wells, Clarke,

and Levin [21] showed from analyses of eighty

samples of hydrated lime that the content of CO2

averaged 1.42 percent. For the 27 regularly

hydrated dolomitic limes, the average was 1.34 percent. The quantity of carbonate is roughly

0 200 400 600 800 1000 TEMPERATURE , °C Figure 17. Differential heating curves of a completely

hydrated dolomitic lime (1) in contrast to a regularly

hydrated dolomitic lime {2) and a putty made therefrom {2H).

The small increase in the amount of Mg(0H)2 caused by hydration of the lime as a putty can be observed by comparing the relative sizes of the characteristic deflections, C, of Mg(0H)2 at 400°C and, D, of Ca(0H)2 at 580°C

in these curves (2 and 2H) and in curve 1. Small quantities of CaCOs are indicated by E.

12
twice that of the CO2. Even so, the percentage

of carbonate in freslily prepared hydrated lime is small, and, as stated previously, the carbonate

that is present is primarily CaCOs.

Thus, by independent methods, the following

facts in regard to hydrated lime appear to be well

established: (1) The combined water in commercial hydrated high-calcium lime is sufficient or almost sufficient for complete hydration because

CaO reacts rapidly and practically completely to form Ca(0H)2, (2) only 55 to 65 percent of the

water required for complete hydration of regularly

hydrated dolomitic lime is present, because of the

slowness of hydration of MgO in the ordinary

hydrator, (3) the calcium oxide of hydrated

dolomitic lime, like the CaO in hydrated highcalcium lime, is generally completely hydrated,

but the magnesia is only slightly hydrated, (4) all commercial hydrated limes contain some calcium

carbonate with little or no magnesium carbonate,

and (5) all commercial hydrated limes contain

certain other impurities, such as silica and oxides

of iron and aluminum.

(c) Gypsum Gaging Plaster Gypsum gaging plaster is variously known as

plaster of paris or as calcined gypsum. Calcined

gypsum when pure is essentially hemihydrate and

has the formula CaS04.J^H20. The article of. commerce, however, may and usually does vary

in composition from chemically pure hemihydrate.

Inasmuch as the gypsum, CaS04.2H20, from which

it is made (by heating and driving off 1 }^ molecules

of water) may contain impurities, such as limestone, iron oxide, clay, and silica, it is to be ex- pected that these materials will be found in the

calcined gypsum. Some gaging plasters may also contain a small percentage of anyhdrous CaS04,

present either as the mineral, natural anhydrite, in

the gypsum, or formed when the gypsum was

dehydrated beyond CaS04.}^H20.

2. Preparation of the Lime Putty

To prepare lime for use in the white-coat

plastering operation, it must fii'st be made into a

putty. Because the finish coat must be applied

in a thin layer over a dry brown coat, it is essential

that the wet mix be sufficiently plastic; otherwise,

it is almost physically impossible to use the mix

for finish-coat work. Plasticity, therefore, has an

important bearing on labor cost. Since the usual

gypsum gaging plaster is not plastic, the plasticity

of the white-coat mix must be derived primarily

from the lime. But different limes have different

degrees of plasticity.^

(a) Putty From Quicklime

Almost any kind of quicklime, when properly

slaked with an excess of water, will produce a

" The Emley plasticimeter is the instrument in general use in the United

States for measuring the plasticity of "finishing" limes to determine whether

they meet the requirement that the plasticity figure of the putty shall be not

less than 200 [.'i, 6, and 7].

putty of high plasticity. Prior to about 1900,

lump quicklime was generally used to prepare

putty for plastering, and it was the practice to age the putty for at least 2 weeks before use.^ More recently, whenever quicklime is used, it is generally of the pulverized type, which was introduced to facilitate slaking and to obtain complete

hydration in a shorter time.''' The slaking of quicldime directly to a putty in

large batches is conducive to complete hydration

because of the presence of an excess of water, the

boiling temperature attained, and the slow rate of

cooling. Laboratory studies, made at the Bureau,

of the hydration of dolomitic lime putties at dif- ferent temperatures [2] indicated that even the

MgO would be hydrated under these conditions.

A subsequent study of the progress of hydration

of MgO in a large batch of putty prepared by

slaking pulverized dolomitic quicldime on the job

showed that the MgO had become completely

hydrated by the time the putty had cooled to room temperature.

The importance of the completeness of hydration will be recognized when it is realized that the

surveys revealed no failure of the type heretofore

described whenever the lime putty of the white

coat had been prepared by slaking quicklime on

the job.

(b) Putty From Hydrated Lime

Lime putty is made from hydrated lime simply

by adding water, but not all hydrated limes will

produce a plastic putty. Many quicklimes that

will themselves produce a putty of great plasticity

will not yield a dry, hydrated lime possessing

highly plastic properties. At the time the present

investigation was undertaken, hydrated limes were divided rather sharply into two classes on

the basis of their plasticity: (1) the "finishing"

hydrated limes made from a dolomitic limestone

peculiar to a small district in northwestern Ohio,

and (2) all other hydrates. Plasterers were quick

to discover the difference between the two classes

of hydrates. The result was that putty made

from either quicklime or Ohio finishing hydrate was specified to be mixed with the gypsum gaging

plaster in the preparation of the white coat, with

the tendency very strongly in favor of the hydrate

because of its greater convenience.

Manufacturers of finishing hydrated lime al- most invariably recommended that the putty be

used only following a 12- to 24-hom- soaking

period. An erroneous belief seemed to be prevalent that such a soaking period was sufficient to provide a plastic putty and at the same time

hydrate the product completely. The manufactm-ers of finishing hydrated lime were disposed

to disclaim any responsibility for subsequent

trouble if it could be proved that their dnections

• Directions for slalting lump quicklime and for screening and aging the putty are given in Standard Specifications for Gypsum Plastering issued by

the American Standards Association A42.1-1946 [24]. See also, ASTM

C5-26. Appendix.

Directions for slaking pulverized quicklime are also included in

A42.1-194fi.

909872—51 3 13
as to the time of this soaking period had not been

followed.

Actually, however, the 12- to 24-hour soaking

period hydrates but little of the large percentage of

unhydrated MgO in regularly hydrated dolomitic

lime. Wells and Taylor [2] showed that for six regularly hydrated dolomitic limes the average

quantity of MgO hydrated at the end of the

customary 1 day of soaking as a putty was only

19.1 percent. A more recent study of the progress

of hydration of 18 such limes as putties [25] revealed that after soaking for 1 day, the average

percentage of hydration of the MgO was 22.6 percent, in fably close agreement with the average

value of Wells and Taylor. At 3 and 7 days, the

percentage hydration for the 18 hydrates increased

to 29.2 and 41.0, respectively, in closer agreement

with the 30.2 and 40.9 percent found by Wells and

Taylor. Thus it can be seen that, rougWy, but

20, 30, and 40 percent of the MgO in regularly

hydrated dolomitic lime is hydrated at the end of

thesoaldng periods of 1, 3, and 7 days, respectively.

Inasmuch as most white coats are applied when

the hydrates are soaked as putties within these

time periods (usually 1 day), it is evident that the

bulk of the MgO is stUl unhydrated at the time the

white coats are applied. Although a small additional percentage of MgO may be hydrated while

the white coat is setting, moist, and plastic, it cannot be great in view of the fact that it requii-es

3 to 5 months or more [25] to hydrate the MgO

completely at room temperature, even as a wet

putty.

3. Application of the White Coat

Generally the white coat is mixed by hand.

When ready to apply, some of the putty is circled

out on a mixing board. The circular space sur- rounded by the putty is then filled with water to about one-third its depth, the desired amoimt of

retarder is mixed in the water, and the calcined

gypsum is sifted into the water. After the water

has wetted the calcmed gypsum, the putty and

gypsum are mixed thoroughly with a trowel and

applied immediately over a base coat which has set and is surface-dry. The mix should be scratched

in thorouglily, laid on well, doubled back, and

filled out to a true even surface. The thickness

should be from Ke to Ys inch. The finish should

then be allowed to "draw" for a few minutes. At

a certain stage, the glaze caused by the water on

the surface will suddenly disappear and the surface

wUl become dull, as the gypsum begms to set. The

chemical reaction at this stage is CaS04.mO+ lKH20^CaS04.2H20

(Calcined gypsum) (water) (gypsum)

This is the proper time to trowel the finish. The

plasterer brushes the surface with water, holding

the brush in one hand and the trowel in the other

so that the trowel may follow the brush immediately. It is essential that the plasterer use all the

pressure he can apply to the trowel during this

operation, and that the whole surface be gone over

as rapidly as possible, without interruption.

As the finish-coat mix is composed largely of

lime putty, there is a decided tendency for this coat to sin-ink and crack. This tendency can be overcome in two ways—by making the coat as thin as possible and by troweling it at exactly

the proper time. A thin coat contains less volume

than a thick coat and permits troweling with

greater facility. With a thin coat the cracks can be closed, or rather prevented, by troweling

just as the cementitious material is s'etting and

hardening. If it is troweled before set has oc- curred, the plaster will continue to shrink (and

crack) after the troweling. It cannot be troweled

after set has occurred, for the material is then too

rigid to yield to the trowel.

4. Chemical Constituents of Freshly Set

White-Coat Plaster

The principal constituents of freshly set whitecoat finish are (1) gypsum (CaS04.2ll20), formed

from the hemihydrate during the application of

the white coat, and (2) the constituents of the lime

putty. If a high-calcium lime has been used,

Ca(0H)2 will be the only other main constituent.

But if a regularly hydrated domolitic lime has been

used, the principal constituents other than

.CaS04.2H20 will be not only Ca(0H)2, but also some Mg (011)2 and considerable MgO. A small

quantity of anhydrite (CaSO^) in the gaging plaster and impurities, such as Si02, AI2O3, and Fe203,

which occur in both the lime and the gaging plaster, will be in the white-coat plaster, regardless of

the kind of lime used. In addition, there will be a small quantity of calcium carbonate, CaCOa; that

originally present as core, that formed in the lime

putty during soaking, and that occurring as an

impurity in the gaging plaster.

5. Drying and Carbonation of the White Coat

After the plasterer has completed his work, the

plaster will normally be exposed to air that is above freezing and is not saturated with moisture.

Water immediately starts to evaporate from the

plaster. The amount of water that will evaporate

in a given time depends not only on the temperature and relative humidity of the air, but also on the quantity of air coming in contact with the

plaster—that is, the circulation. The amount of

water that must be evaporated wUl depend upon

the amount put into the plaster and the amount

either used up or liberated by the chemical

reactions involved in the setting and hardening

processes. During the hardening of a white-coat

plaster, the conversion of the hemihydrate to gypsum uses up nearly half of the mixing water. Additional hardening takes place as the constituents of the lime putty combine with the

carbon dioxide of the air. This hardening, however,

differs from that resulting in the conversion of

hemihydrate to gypsum in that no water is actually

used in this process. Not only the free water con14
tained in the lime putty must be evaporated, but even that which is chemically combined with the

lime will be replaced by carbon dioxide (Ca(0H)2 +

C02->CaC03+H20) and evaporated as the hardening continues. A peculiarity of this reaction is, that it will not occur unless there is a small amount

of water present, so that the lime will not continue

to harden properly under excessively dry conditions. On the other hand, the presence of too much

water will also prevent the proper carbonation of

Ca(0H)2.

Inasmuch as the combination of Ca(0H)2 with

CO2 from the air, to form CaCOs, proceeds slowly

in normally dry plaster, but more rapidly when the

optimum amount of moisture is present, it can be

predicted that the extent of carbonation will vary

greatly.

6. Chemical Reactions Involving Volume

Changes

To determine which of the reactions is the most

likely to cause expansion of the aged white coat,

consideration will now be given to the volume

changes involved in the chemical reactions of the

setting and hardening of the white coat as well as

in those occurring subsequently.

If the reactions involving increase in volume

occur prior to the setting and hardening of a

plaster, the danger of subsequent expansion from

this source is eliminated. On the other hand, if these reactions occur after the plaster has hardened

the likelihood of trouble ensuing because of volume

change is greatly augmented. A full consideration

of the possibility of damaging expansion from

chemical reaction therefore resolves itself into the

study of not only those reactions that are attended

by an increase in volume, but also of the magnitude

of the volume change and whether the increase occurs before or after hardening.

The computed increases in net volume that may

occur are given in table 2. Column 4 gives the

molecular volume, obtained by dividing the molecular weight by the density. The percentage increase

in volume is given in columns 5, 6, 7, 8, and 9.

Table 2. Molecular weight, density,^ and molecular volume of compounds that may occur in white-coat -plaster; and the calculated increases in volume in the formation of hydrates, carbonates, and sulfates

Compound Molecular weight Density Molecular volume

Increase in volume in the formation of the compound from

Oxide Hydroxide MgCOs-SHjO CaS04 CaS04.J4H20

1 2 3 4 5 6 7 8 9

CaSOi (natural anhydrite) _ _ 136. 14

145. 15

172. 17

56. OS 74.10

100. 09

40. 32

5S. 34

84. 33

138. 38

246. 49

g/ml

2. 96

b 2. 76

2. 32

3. 40

2. 34

2. 71 3.7

2. 38 3.04

1.85

1.64

ml

46.0

52.6

74.2

16.5

31. 7 36.9

10.9 24.5

27. 7 74.8

150.3

% % % % %

CaS04.i/2H20 CaS04.2H20 (gypsum) 61 41 CaO -- Ca(0H)2 92 CaCOs (ealoite) _-_ _-_ _-_ 124 16 MgO - Mg(0H)2 125

154 586 1,280MgCOa (magnesite) , 13 205

613 MgC03.3H20 (nesquehonite) _. MgS04.7H20 101

" Handbook of Chemistry and Physics. Chemical Rubber Publishing Co., Cleveland, Ohio (1939).

i> J. H.van't Hoff, E. Armstrong, W. Hinrichsen, F. Weigert, and G. Just. Z. phys. chem. 45, 272 (1903).

The first reactions to be considered will be those

involving the constituents of the gaging plaster. It can be seen (column 9) that the computed net

increase in volume of CaS04.KH20 to CaS04.2H20

is 41 percent. Inasmuch as the bulk of the hydration of CaS04.MH20 to CaS04.2HoO takes place

during the setting process while the white coat is soft and plastic, the expansion is small, because

the stresses set up by this increase are relieved.

In fact, the setting process depends on the occur- rence of this reaction. The small net expansion

is of advantage to the plasterer as it occurs at a time when he is completing the troweling of the

white coat and thus facilitates the closing of the

cracks caused by the shrinkage of the lime. It is this expansion during setting that produces such

excellent detail in plaster-of-paris casts. That the

reaction does not proceed subsequent to the setting

and hardening is amply demonstrated by the lack

of any report of bulging of plaster-of-paris casts

in the centuries during which plaster of paris has

been used to produce cast objects.

As mentioned previously, some gaging plasters

may contain a small amount of anhydrous CaS04,

present either as natural anhydrite or formed in

the calciners when gypsum is dehydrated beyond

CaS04.KH20. Anhydrous CaS04 can hydrate

to CaS04.2ll20 at room temperature. The computed net increase in volume is 61 percent (column

8). However, the amount of CaS04 in gaging

plaster is small, and if the CaS04 is formed in the

calciners, it hydrates readily. It, therefore, is converted to gypsum during the setting process. That the small quantity of natural anhydrite

in the calcined gypsum is very unlikely to cause a damaging expansion after the white coat has set and hardened is also amply demonstrated by the

lack of any report of subsequent expansion and

bulging of plaster-of-paris casts. Even with these very strong arguments against

15
the possibility of CaS04.}^H20 or CaS04 causing

any trouble, the probability of the presence of

these constituents in the hardened white coat should and wUl be given consideration in seeking

the cause of white-coat trouble.

The hydration of both CaO and MgO is ac- companied by marked expansion. Thus, it can be seen (column 5) that the h5^dration of CaO

to Ca(0H)2 is accompanied by an increase in

volume of 92 percent; and that of MgO to Mg(0H)2, hy an increase of 125 percent. As

stated previously, if the hydration occurs prior

to the settmg and hardening of the plaster, the

danger of any subsequent expansion from these sources is eliminated. As calcium oxide reacts

so rapidly with water to form calcium hydroxide

that hydration is completed in a very short time,

there appears to be little or no likelihood of any

danger of expansion because of this reaction.

Magnesia, on the other hand, as has been

pointed out, is very subject to overburning, does

not hj^drate rapidly, is present in large quantities

in regularly hydrated dolomitic finishing limes,

and is hydrated but little during the soaking

period prior to application of the finish coat. Therefore, it is apparent that the probability of

damaging expansion resulting from the subsequent hydration of the MgO is a factor of moment

and must be given prime consideration.

The reactions of the carbon dioxide of the air with the constituents of lime are also accompanied

by an increase in volume. It can be seen from

table 2 that when CaO or MgO carbonates directly

to calcite (CaCOs) or magnesite (MgCOs), re- spectively, there is a marked increase in volume.

The increase in volume for CaO is 124 percent and

for MgO 154 percent. In the first place, it has

been shown that there is little or no CaO in lime

putties, and in the second place, the oxides are usually converted to the hydrates before carbonation occurs. The possibility of any damaging

expansion occurring as a result of the direct

carbonation of CaO to CaCOs can, therefore, be

dismissed. It has been suggested that if the

MgO in the set and hardened white coat had

carbonated rather than hydrated, there would

have been no expansion. This is not true. The

volume increase in the direct carbonation of MgO

to magnesite is 154 percent, an increase that is greater than that accompanying hydration of

MgO to Mg(0H)2, which is 125 percent. The

carbonation of the hydrates to the normal car- bonates is attended by only a small increase in volume; for Ca(0H)2 to calcite, it' is 16 percent

and for Mg(0H)2 to magnesite, but 13 percent.

It will be shown later that carbonation in a white coat, even in the case of Ca(0H)2, is far

from complete and, in general, the carbonation of

the Mg(0H)2 does not occur until the more easily

carbonated Ca(0H)2 has been completely changed

to CaCO.,. These conditions require the presence

of moisture. If carbonation then proceeds with

the conversion of Mg(0H)2 to a carbonate, the

hydrated mineral nesquehonite (MgCOs.SH^O) is formed and not magnesite (MgCOs). The conversion of Mg(OII)i to MgC03.3ll20 is accompanied by an increase in volume of 205 percent.

The net increase of MgO to MgCOj.SHsO through

the hydroxide (Mg(0H)2) is still larger—586 percent. Bishop [26] has shown that the somewhat

soluble nesquehonite in turn reacts with gypsum,

CaS04.2H20, to form less soluble CaCOa and

magnesium sulfate. The crystallization of

MgS04.7H20 results in a further increase in volume: 101 percent over MgCOs.SHaO, 513 per- cent over Mg(0H)2, and 1,280 percent over MgO.

This results in marked efflorescence and eventually

the complete disintegration of the white coat.

It is most fortunate, therefore, that the Ca(0H)2

is not completely carbonated to CaCOs, and that

little or no carbonation of the Mg(0H)2 takes

place. However, plaster failure characterized by

efflorescence and ultimate disintegration is not the

subject of this paper; it has already been dis- cussed by Bishop. It has been mentioned at this

time to show the very great magnitude of the

expansion when magnesia is hydrated, carbonated,

and then converted to magnesium sulfate.

V. Investigation of White-Coat-Plaster

Samples Taken After Failure

A large number of samples of failed white-coat

plaster collected in the eastern half of the United

States were assembled, and an intensive study of

their constitution was undertaken.

1. Chemical Analyses

Any paint on the fragments of white-coat

plaster was carefully removed, and representative

samples were then ground in a mortar and placed

in airtight containers.

Because a white coat may contain a small

amount of free, or uncombined, water, samples were dried to remove this water and to determine

its quantity. This drying temperature should be

such that the free water is completely removed

without decomposing hydrated compounds that

may be present. In the usual chemical analysis a temperature of 105° to 110° C (221° to 230° F)

is used for the determination of free, or uncombined, water. However, such a temperature

for wliite-coat samples is too high because it results in a partial dehydration of gypsum. Consequently, the samples were dried at 45° C (113°

F), the temperature given in both the Federal

Specifications [27] and those of the American

Society for Testing Materials for drying prior to the analysis and testing of gypsum proclucts [28]. It has been found that when gypsum is heated

for 1 hour at 110° C (130° F) it is converted

primarily to hemihydrate, CaS04.}^H20. There

is always a possibility, however, that a portion of

the hemihydrate formed may, in turn, be dehydrated to anhydrous calcium sulfate. The loss

16
at 110° C (230° F) can, nevertheless, be used to obtain a fair approximation of the quantity of

gypsum present. This holds true provided other

hydrated materials are not decomposed by this heat treatment. The Ca(0H)2 and Mg(0H)2 are not decomposed to any significant extent w len

heated to 110° C (230° F). Consequently, the

loss on heating for 1 hour at 110° C (230° F) was

determined for many of these samples, and the

CaS04.2H20 content of the white coat was cal- culated from this loss on the assumption that it represented the water evolved in the reaction

CaS04.2H20-^CaS04.}^H20+ D^HaO.

It was found, however, to be difficult to obtain

good checks by this method, because the procedure

only partially dehydrated CaS04.2H20.

It has also been found that when the gypsum

is heated for 1 hour at 200° C (392° F) it is converted for the most part to anhydrous CaSOi.

At this more elevated temperature there is, however, the added possibility that a small amount of

Mg(0H)2 or Ca(0H)2 may be decomposed. Such

a procedure, nevertheless, gives a better indication

of the content of CaS04.2H20 in the set white

coat than heating at 110° C (230° F). The loss on heating for 1 hour at 200° C (392° F) (column

2, table 3) was determined for the bulk of the

samples, and the CaS04.2H20 content (column

11) was calculated from this loss on the assumption that it represents the water evolved in the

reaction

CaS04.2H20->CaS04 +2H2O

As the decomposition pressure of calcium car- bonate is 1 atm at 884° C (1,623° F), the samples were next ignited for 1 hour at a temperature

ranging between 900° and 925° C (1,652° and

1,697° F). This ignition temperature converts

the constituents of white coat to an anhydrous

and a carbon-dioxide-free basis without dissociating the anhydrous CaS04 to CaO+SOj. The

loss at 900° to 925° C (1,652° to 1,697° F) includes

therefore, the sum of the carbon dioxide and water

of hydration and is given in column 3. Samples of the white coats were decomposed

with hydrochloric acid, and the carbon dioxide

evolved was collected in a gas-absorbing train. The percentage of CO2 thus obtamed is given in

column 4. By subtracting this from the per- centage of loss on ignition at 900° to 925° C

(1,652° to 1,697° F), one obtains the combined

water of hydration in the white coat. This is recorded in column 5. Another sample of each

white coat was then analyzed for Si02, ^^Ox,

CaO, and MgO. As the quantities of SiO, and

R2O3 (Fe203, AI2O3, Ti02, P2O5, etc.) were small in

nearly all cases, only the combined percentage of Si02 and II2O3 (column 6) is given. The per- centages of CaO and MgO are given in columns

7 and 8, respectively. The percentage of SO3,

given in column 9, was determined on stUl another

sample.

The percentage of calcium hydroxide was

obtained directly by the glycerol-ammonium

acetate method [29] for determhiing "free lime"

in Portland cement. This method for determining

uncombined CaO is also applicable for Ca(0H)2.

Consequently, it can be used for determining

Ca(0H)2 in white-coat samples, since it was found

that CaS04.2H20, CaC03, Mg(0H)2, and MgO

do not interfere with the determination. The

possibility of any CaO being present in aged white

coat is too remote to warrant consideration of any

interference by CaO. The reaction of Ca(0H)2

with the glycerol-ammonium acetate solution is even slower than that of CaO ("free lime") and,

consequently, the sample must be boiled for some

time after each addition of the titrating solution.

When the crystals of calcium hydroxide are coated

with a film of calcium carbonate, the extraction

is still slower, and it is very difficult to determine

when the reaction has reached completion. There

is a tendency, therefore, to obtain values which

may be slightly low with carbonated material.

Nevertheless, the percentages of Ca(0H)2 obtained

by this direct method are given in column 10.

2. Computed Compound Composition

On the assumption that the SO3 (column 9) is present as gypsum, the percentage of CaS04.2H20

is given in column 12. The percentage of CaS04.-

2H2O thus obtained can be compared with that

calculated from the loss at 200° C (392° F) (column

11). The percentage of CaS04.2H20 cannot be

higher than that calculated in column 12, which

is based on the percentage of SO3. It can, however, be lower if some of the SO3 is present as natural anhydrite (CaS04), but only slightly

lower, because the quantity of natural anhydrite

in gaging plaster is generally small.

It was assumed that the CO2 was combined with

the CaO as calcium carbonate, and the percentage

of CaC03 calculated from the CO2 (column 4) is given in column 13. Although some of the

carbon dioxide may have been combined with the

MgO, it seems probable that it was almost

entuely combined with the calcium oxide, as the

latter is much more active, and as calcium car- bonate has a higher dissociation temperatm-e than

magnesium carbonate.

The CaO other than that calculated to be combined as CaS04.2H20 (column 12) and CaCOa

(column 13), was assumed to be present as Ca(0H)2

and the calculated percentage is given in column

14. If the assumptions up to this point are

correct, then the percentage of Ca(0H)2 obtained

directly by the glycerol-ammonium acetate method

(column 10) should agree with that computed

from the chemical analyses (column 14). This

would be true, since, as stated previously, CaS04.-

2H2O, CaCOs, Mg(0H)2, and MgO do not inter- fere with the direct determination of Ca(0H)2.

Considering the tendency of the glycerol-ammonium acetate method to give a slightly low value,

the percentage of Ca(OH)o obtained by the du'ect

17
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20
method agrees well with the computed value.

If the magnesia had carbonated to any appreciable

extent, the calculated percentage of Ca(0H)2

would be much higher, and this good agTeement

would no longer prevail. The close agreement is therefore added proof that there is normally little or no magnesium carbonate present in aged whitecoat samples.

It should be emphasized that all of the samples

of defective white coat contained from 9.6 to 25.3

percent of magnesia (see column 8). In other

words, it seems clear that all of these samples of

bulging plaster were made from mixtures of dolomitic lime and plaster of paris.

The quantity of water that remained unac- counted for after calculating that required by the

CaS04.2H20 and Ca(0H)2 was assumed to be

combined as magnesium hydroxide, and the

calculated percentage of Mg(0H)2 is given in

column 15. Rarely was there sufficient water to combine with all of the magnesia as given in

column 8. Consequently, in nearly every case, there was some free or uncombined MgO; its percentage is given in column 16. On whatever

basis the computations are made, it will be found

that there was seldom enough H2O present to hydrate completely all of the magnesia, even assuming that the SO3 was present as CaS04./2H20

rather than gypsum, CaS04.2H20. Such an

assumption, however, is entirely without justification, considering (1) that rapid conversion of

CaS04.}^H20 to CaS04.2H20 is an essential

feature in the initial setting of white coat, and

(2) that there is an approximate agreement between the contents of CaS04.2H20 given in

columns 11 and 12. It should be noted that no consideration has

been given to the small percentage of Si02 plus

R2O3, shown in column 6. Although the Si02

and II2O3 may have been present in part as calcium silicates, aluminates, or ferrites, the per- centages of the various constituents were calculated by ignoring them. It should be noted that

in a few instances the Si02 plus II2O3 was slightly over 1 percent. Generally the silica in these in- stances was present as grains of quartz and no

doubt consisted of particles of sand that had been

troweled into the white coat. To summarize, then, in this method of calculating the compound composition of the white coat,

it was assumed that the SO3 was present as gypsum, CaS04.2Il20; the CO2, as CaCOs; and

the lime in excess of that combined as CaS04.2HoO

and CaCOs, as Ca(OH)o. The H2O, other than

that combined in the gypsum and calcium

hydroxide, was assumed to be present in Mg(0H)2

and the magnesia in excess of magnesium hydroxide, as MgO. In 1922 J. W. McBurney

made similar assumptions in his study of the

plaster failures at Mt. Sinai Hospital, Cleveland,

Ohio.

3. Differential Thermal Analyses

The results of differential thermal analyses

verify the assumptions made in calculating the

constitution of the white-coat plasters. In figure

18 are shown the heating curves of four mixtures

of CaS04.2H20, Mg(0H)2, MgO, Ca(0H)2, and

CaCOs. These mixtures were prepared to repre- sent the composition of white-coat plasters in various stages in the hydration of the MgO.

Beside each curve in figure 18 is sho\vn the per- centage of Mg (0H)2 contained in the sample, 27

percent representing complete hydration of the

MgO.

Five distinct regions of heat absorption are apparent on the curves. These absorptions, in

the order of increasing temperature, are caused b}^

(A) the dehydration of CaS04.2H20 to CaS04.-

KH2O, (B) the dehvdration of CaS04.}^H20 to CaS04, (C) the dehydration of Mg(0H)2 to MgO,

(D) the dehydration of Ca(0II)2 to CaO, and,

finally (E) the decarbonation of CaCOs to CaO.

These five reactions are represented on the heating

curves (fig. 18) of all the samples containing

Mg(0H)2.

O 200 400 600 800 1000 TEMPERATURE , °C Figure 18. Differential heating curves of four mixtures of CaS04.2H20, Mg(0H)2, MgO, CaCOH)., and CaCOj.

"

The peroontage of Mg(0H)2 iffindicatod beside each curve. Five regions

of heat absorption, in the order of increasing temperature, are A, dehydration

of gypsum to hemihydrate, B, dehydration of hemihydrate, C, dehydration

of magnesium hydroxide, D, dehydration of calcium hydroxide," and E, decomposition of calcium carbonate.

21
0 200 400 600 800 1000 TEMPERATURE, °C Figure 19. Differential heating curves of three samples of

white-coat -plaster that had failed by expansion. The five major regions of heat absorption, in the order of increasing temperature, are A, dehydration of .eypsum to hemihydrate, B, dehydration of hemihydrate, C, dehydration of magnesium hydroxide, D, dehydration of calcium hydroxide, and E, decomposition of calcium carbonate. The five major heat efi^ects are shown on the

heating curves of samples from three plaster

failiu-es, figm-e 19. Particularly to be noted is the

almost complete absence in curve 1 of the deflec- tion characteristic of the dehydration of Ca(0H)2.

This sample, in which almost complete carbonation

of Ca(0H)2 had occurred, contained no detectable

magnesium carbonate. The composition of the

plasters represented by the curves in figure 19 can be found in table 3, where samples numbered

42, 32, and 6 correspond to curves 1, 2, and 3, respectively. The areas of the magnesium-hydroxide deflections on the heating curves are roughly proportional to the calculated Mg(0H)2

in these plasters. It is apparent that the assumptions made in calculating the compound composition of the plasters must be approximately correct.

4. Extent of Hydration of Magnesia in the

Aged White Coat

The percentage hydration of the total magnesia in the white coats is recorded in column 17

of table 3. This percentage was calculated from

the total amount of magnesia in the samples

(column 8) and the percentage of unhydrated

MgO (coliunn 16). The average amount of

hydration of the MgO in these plaster-failure

samples is 59.0 percent, with a standard deviation

of 17.2 percent and extreme values of 27 percent

and 100 percent. This 59.0 percent is much in

_ excess of the average of 19.1'^ percent hydration

found by Wells and Taylor [2] for six regularly

hydrated dolomitic limes after the customary 1 day of soaking of the lime as a putty. It is also

much in excess of the average of 22.6-percent

hydration more recently obtained [25] for 18 additional samples of normally hydrated dolomitic

lime at the end of the customary soaking of 1 day as a putty. In agreement with the contention

of Newman [30], hydration of the magnesia must,

therefore, have occurred on the wall.

5. X-ray Analyses

Although in many materials the presence of a suspected material can be shown by the X-ray

pattern, the presence of MgO in hardened plaster

is not easy to prove. In table 4 are given the

interplanar spacings and estimated intensities of

the lines of the X-ray patterns of three plaster

samples. Also tabulated from the literature are the patterns of gypsum, plaster of paris, anhydrite, calcium hydroxide, calcium oxide, calcium

carbonate, magnesium oxide, magnesium hydroxide, magnesium carbonate (magnesite), and nes- quehonite (MgC03.3H20). All these compounds

must be considered potential constituents of a hardened plaster. Simplifying assumptions, such . as have been made in this paper in connection

with the chemical analyses, would eliminate

plaster of paris, calcium oxide, and magnesite,

but evidence has been observed of the presence of

each of the other compounds in hardened plaster.

Many lines are identical in the patterns of two

or more compounds as well as in the patterns of the

hardened plasters. Study of the X-ray spectrograms of mixtures of CaCOa, Mg(0H)2, MgO,

Ca(0H)2, and CaS04.2H20 has led to the conclusion that not less than 5 percent of Mg(0H)2

must be present to permit its detection by means

of X-rays. The detection of MgO is less easy as the strong lines of MgO coincide with moderately

strong lines of other constituents of hardened

plaster. The presence of MgO must be established

by intercomparison with other lines of the interfering substance. It is probable that not less than 10 percent of MgO must occur in a sample of

hardened plaster for its presence to be established

with certainty. This quantity, 10 percent, is more than half the average amount of total MgO

present in all forms in hardened plaster, as shown

by the chemical analyses in table 3.

6. Heat of Solution

It was shown by Wells and Taylor [2] that the

analytical determination of the degree of hydration of magnesia in hydrated dolomitic limes and

putties can be indu'ectly verified by means of heatof-solution determinations. It was decided,' there-

'1 Erroneously reported by Newman f30] as 12 percent, which is the average

value of the percentage hydration of these commercial hydrated limes just

prior to soaking them for 1 day as putties.

22
Table 4. X-ray patterns of plasters and of potential constituents

[Data for compounds from "X-ray diffraction data cards," American Society for Testing Materials, Philadelphia, Pa., 1945]

Sample F Sample D Sample B

CaS04.- 2H2O Ca(0H)2 CaCOj MgO Mg(0H)2 CaS04.- HH2O CaO CaSOi 3H2O Mg003

*3s

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3.01 2.85 s VW3.00

2.84 2.66 2.60

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M

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2.48 VW

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2. 19

2. 09

1.913

1.870 1.79

1.74

1.680

1.630

M

VW

w

M

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2. 35

2. 26

2. 22

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1.915

1.865 1.792

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M

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2.35 1.00 2. 34 .02 2.39 1.00 2.32

2.26 2.20

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1.99

1.93 1.86

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1.91 1.87

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1.64

1.56

1.51

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1.683 1.62

1.603 1.580

1.520

1.485

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1.66 VW 1.66

1.62

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1.59

1. 56 1.52

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1.595 M 1..60 .16

1.569 M 1. 58 .10 1. 55 .02 1.57 .33 1.55 . 12

1. 51 M 1.51 .12 1. 53 .04 1.472 M 1.485 .20 1.485 .75 1.490 .17 1.50 .08 1.470 VW 1.475

1.439

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1. 430 1.390

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1.410 1.357 M

VW

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1.435 .13 1.420

1.395

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1.382 1.369

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1.376 VW 1.370 . 13 1.382 .20 1.360 .10 1.350 .02

1.340 VW 1.350 .03

1.330 VW 1.325 .10 1.310 M 1.315

1.305 w

w

w

w

w

M

w

w

MW

VW

VWWM

W

M

W

VW

1.315 .16 1.318 1.296

1.275

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1.262

1.243 1.210

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.04

.04

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1.267 VW 1.270 1. 270 .03 1.266 .06

1.24S VW 1. 248 1.238 1.219

1. 182

1. 158

1. 145

1. 132 1.107

1.088 1.063 1.055

1.044

1.038

1. 015

1.001 .996

1.243 .03 1.248 1.235

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.15 .10 1.213 .15 1.215 .05

1.209

1.180 Ww

1.200 .10

1.188

1. 160

1. 145

1. 128

M

M

VW

VW

1.178 .02 1. 179

1. 150

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1.163

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1.150 .04

1.143

1.125 MW

1.145 .15

1.138 .08 «- 1.125 .02 1.100 .07 1. 103 .09

1.0S9 1.060 VWM

1.083 .05

1. 068

1.050 VWM

1.063 .10 1.070 .02 1.071 .25 1.065

1.048 .2

1.050 .04 .5

1. 045 .06

1.038

1.012

1.000 W

M

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1.035

1.015 WW

1.037 .03 1.035

1.012 .05

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1.031 .03 1.038 1.012

1. 000

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1. 005 .04

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VW

VWW

VW

w

w

M

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VW

VWMM

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w

s

.972 W .963 .01 .968 .957 .2

.955 .05 .958 .02 .2

.941 .919

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.886 .860

.846

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VW

VW

VW

VW

VW

VW

VW

.940 .10 .945 .04

.920 .02 .922 .03 .913 .4

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.862

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w

VW

sw

. 882

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. I .860 ..04 .862 .01 .1 .850 .02 .847 .03

.815 w .812 .790

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VW

VW

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.800

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.671 .666

.03 .06

.02

.01 .02

.01

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S, strong; M, medium; W, weak; V, very.

I/Ii is the ratio of intensity to that of the strongest line.

23
fore, to use the heat-of-solution calorimeter as a

further check upon the compound composition of

the samples of white-coat plaster. From the

chemical analyses for the oxide constituents and

on the assumptions that have been discussed, the

compositions of the aged white coats have been

computed as mixtures of CaS04.2H.>0, CaCOs,

Ca(0H)2, -Mg(0H)2, and MgO. The percentage

of Ca(0H)2 has been verified by an independent

method—the glycerol-ammonium acetate method.

The content of CaS04.2H20 calculated from SO3

has been fairly well checked from the loss on ignition at 200° C (392° F).

Although differential thermal analyses were

used to show the presence of CaS04.2H90 and

Ca(0H)2, as well as Mg(0H)2 and CaCOs, the

method, as here applied, does not determine the

actual percentages of these constituents. There

does not seem to be available any adequate independent method for checking the percentage of

MgO. However, if the constituents of the white

coats were present in the proportions as given in table 3, the observed heats of solution should

agree with those calculated from the algebraic sums of the heats of solution of the individual

constituents. The heat of solution of CaS04.2H20

in 2.00 A'^ hydrochloric acid has been reported by

Newman and Wells [31] and the heats of solution

of CaCOs, Ca(0H)2, Mg(0H)2, and MgO by

Wells and Taylor [2]. For convenience, the

values used in calculating heat of solution are:

Compound

CaS04.2H,0

CaCOs

Ca(0H)2__.

Mg(0H)2___

MgO

Heat of solution in 2 N HCl

cal/g -33

+ 58 + 422 +461 + 891

The heats of solution of the plaster samples were

calculated from the compound compositions given

in table 3. Each constituent was assumed to contribute its heat of solution in proportion to the

amount in the plaster sample, and the sums of

the contributions are given in column 1 9 of table 3 as the calculated heats of solution. The observed

values of the heats of solution, measured with a

precision corresponding to an estimated standard

deviation of ±2.1 cal/g, are given in column 18. The coefficient of correlation between the cal- culated and observed heats of solution is 0.993.

(A correlation coefficient of 1.000 represents

perfect correlation.)

24
Figure 20 is a scatter diagram of the 88 pairs of

observed and calculated heats of solution. The

line best representing these data, determined by

the method of least squares, is Ob ===5.4 +0.980 C,

reganiing the calculated values as the independent

variable, and Ob =2.6 +0.993 C, regarding the observed heats of solution as the indei^endent variable. In these equations Ob and C represent the

observed and calculated values, respectively. In

figure 20 is shown only the line representing equality of the calculated and observed values. The

two lines representing the equations determined

by the method of least squares are not shown in figure 20, because they would be so close to the

line plotted that the paths of the three separate

lines could scarcely be followed and the figure

would be confusing.

In addition, it might be mentioned that the

average percentages of CaS04.2H20, CaCOs,

Ca(0H)2, Mg(0H)2, and MgO, as given in col- umns 12, 13, 14, 15, and 16 of table 3, were as- certained, and that from these average values an

average value of the heats of solution was calculated. It was found to be 216 cal/g. The average

value of the observed heats of solution, as given

in column 18, was also ascertained. It was 217

cal/g.

The agreement of the calculated and observed

values of the heats of solution is strong evidence

of the correctness of the assumptions upon which

were based the calculations of the compound compositions of the plaster samples. The heats of

solution of the plasters were also calculated from

the constitutions obtained by making assumptions

as to the distribution of the oxide components

other than those which have been discussed. In

all cases the values so obtained differed from the

observed heats of solution much more than with

the present assumptions.

The differences between the observed and cal- culated values of the heats of solution of the

plasters were plotted against each of the oxide

and compound constituents shown in table 3. No

correlation was apparent.

It should be emphasized that the accuracy of

the chemical analysis and the validity of the as- sumptions are reflected most in the calculated per- centage of the unhydrated MgO—the last constituent determined. The MgO has the highest

heat of solution of any of the constituents, 891

cal/g. An error of 1 percent in the analytical

determination of MgO therefore makes a difference

of approximately 9 cal/g in the calculated value

of the heat of solution. Seldom is the difference

between the observed and calculated heats of

solution more than 9 cal/g. An error in the determination of the quantity of combined II2O will

make an even greater difference in the calculated

heat of solution, as the II2O, in turn, will be

reflected in the final percentage of the computed

MgO. One percent of H2O will be equivalent to 2.22 percent of MgO or 20 cal/g in the computed

heat of solution. In no case is there this large

difference between the observed and calculated

heats of solution. Considering all these factors,

the validity of the computed composition of the

white coat appears to be firmly established.

7. Composition of Lime Used in Preparing

the White Coat

It can be seen (table 3, column 8) that there was considerable magnesia in all the samples of

failed white-coat plaster. The values in column

8, however, do not give an indication of the actual

percentage of magnesia in the lime that had been

used in preparing the white coat. To obtain this,

it is necessary to consider the separate percentages

of CaO, MgO, and SO3, as given in columns 7, 8, and 9, respectively. Calculating the percentage

of CaO combined with the SO3 to form calcium

sulfate and subtracting that CaO from the total

CaO (column 7), one obtains the amount of CaO

in the lime used. As the MgO in the plaster came

from the lime and not from the gypsum, the per- centage of MgO in the lime (on the nonvolatile

basis) can readily be computed. This percentage,

disregarding minor constituents, is given in column

20. The range is from 39.0 to 42.1 percent, and

the mean is 41.1 percent. The analyses of 185

samples of dolomitic hydrated lime received at the Bureau for test from 1923 to 1940 showed

that the average percentage of MgO (on the nonvolatile basis) was 40.1, a value in remarkably

close agreement with the average of 41.1 percent

from the values of column 20. It can, therefore,

be concluded very definitely that in every instance

recorded in table 3 a dolomitic lime was used in

preparing the white coat—which failed eventually.

In not a single instance had a high-calcium lime

been used.

8. Ratio of Lime Putty to Gaging Plaster

Although many specifications call for the use

of three volumes of lime putty to one volume of

calcined gaging plaster in the white coat, the

proportions are often left to the discretion of the

plasterer. Inasmuch as incorrect proportioning

of lime putty to gaging plaster has been suggested

many times as the cause of plaster failures, the

proportions that had been used in preparing the

original white coats were calculated from the

analyses of the set and hardened white-coat

plasters.

As the analyses of the set white-coat plasters were made on a weight basis, the calculation of the proportion of lime putty to calcined gypsum

on a volume basis, in which they are mixed on the

job, involves several assumptions. Excepting

samples 40 and 46, table 3, the calcined g^-psum was assumed to be pure hemihydrate, CaS04.-

^^HoO, and 65 Ib/ft^ was taken as its bulk density.

A cubic foot of lime puttj^ was assumed to contain

45 lb of dry hydrated lime.

25
IS

R--

The volume ratio of lime putty to gaging plaster

volume of lime plaster

volume of gaging plaster mass of dry lime/45 mass of gaging plaster/65

weight percent of lime

: 1.444

weight percent of gaging plaster

As all the constituents of white-coat plaster,

except for hemihydrate, are supplied by the lime,

the ratio can be written

i?= 1.444 . . . 100 — weight percent of hemihydrate

weight percent of hemihydrate

A fair approximation of the percentage of hemihydrate used can be obtained from the quantity

of gypsum in the finish coat merely by determining the loss on heating at 200° C (392° F)

for 1 hour, as this ignition brings about the

approximate dehydration of gypsiun, CaS04.2H20,

to CaS04.

For a more exact determination it is necessary

to analyze the white coat for CaO, MgO, and SO3.

Neglecting the minor constituents (Si02, AI2O3,

Fe203, etc.) and assuming (1) that the calcined

gypsum was pure hemihydrate, (2) that the

lime was made up of Ca(0H)2, Mg(0H)2, and

MgO, and (3) that the magnesia was 20 percent

hydrated, the ratio (by volume) of lime putty to hemihydrate is

i?=1.444

' ^ MW CaO \ MW Ca(0H)2 , ^ , . ^ , . ^„ , , ^ MW Mg(0H)2 .^"Q- MW SO3 MWCaO MgO+0.20 MgO

SO;

MW CaSO^.^^HaO

MWSO3

where MW is the molecular weight of the compound in question and CaO, SO3, and MgO represent

the weight percent of these oxides in the analysis of the white-coat plaster. This relation becomes

„ ^ ^^^^ (CaO-0.7005 SO3) 1.3213 + 1.0894 MgO

xv =U.7yD7 gQ }

and this can be written

1.0527 CaO+0.8679 MgO

SO3

0.7374.

This formula was used in obtaining the values

recorded in column 21 of table 3. Plaster failures

from which these samples were taken occurred

with ratios of lime putty to CaS04.KH20 as low

as 0.8 and as high as 9.8. Figure 21 is a dis- tribution diagram of the ratios of lime putty to CaS04.}^H20 (by volume) for the 88 samples.

The average ratio, taken from the grouped data, was 4.0, and 90 percent of the samples were

scattered more or less symmetrically about that

figure. Only the samples with ratios of 7.0 and

higher (8 in all) appear to fall out of that dis- tribution.

At this point attention should be called to the

volume ratio of lime putty to gaging plaster in

separate sections of a white coat. Samples 24,

25, and 26 of table 3 give the analysis and other

properties of three sections of a white-coat plaster

that was originally about ]i inch in thickness.

The outside section next to the paint is sample 24,

that next to the base coat of plaster is 26, and the

intervening section is 25. From the table it can be seen that the percentage of gypsum,

CaS04.2H20, (column 12) increased from the

outside surface of the white coat inward. In other

words, the outside surface of the white coat was

richer in lime.

This condition is reflected in the calculated

volume ratios of lime putty to CaS04./^H20, as

given in column 21 for these three sections. It is also reflected in the decreases of the heats of solution attending a decrease in the percentage of

lime and a corresponding increase in the percentage of gypsum from the outside surface inward.

LIME PUTTY -.PLASTER , BY VOLUME Figure 21. Distribution diagram of the ratios (by volume)

of lime putty to gaging plaster (CaS04.1/2H20) for 88 {N)

samples of white coat. The 3ver,ige ratio (X) is 4.0, and thn standard deviation (<t) is 2.0. 26
It seems unlikely that the plasterer changed the

proportion of the white-coat mix dm'ing the

application of the successive layers of white coat. To determine the significance of the greater

proportion of lime in the outer surfaces, additional

samples were taken from some freshly plastered

walls where it was known definitely that the

separate layers of white coat as applied were of the same composition. Again this enrichment of lime in the outer surface was observed. This has

brought the suggestion that in the troweling

process while finishmg the application of the white

coat there is a tendency to "float" the lime to the

sm-face. This is akin to the foi'mation of "lait- ance" (or a separation of the cement paste)

accompanying excessive workmg, or trowehng, of concrete or stucco. Plastering experiments also revealed that this floating of the lime was greatei' where the white

coat was applied to a base coat that had not dried

sufficiently. In fact, it can be so aggravated

when the base coat is very "green" that the outside surface of the white coat is composed primarily of a very thin coating of hydrated lime.

Consequently, when the white coat dries, this

outside coating of "whitewash" with little or no gypsum can be easily rubbed off with the hand.

Such a siu-face is undesirable to receive oil paint,

and some few experiments at the Bureau showed

that such surfaces also requii-ed larger quaatities

of paint than did white coat that had been applied on a drier base and had not been troweled so much. The subject, however, needs further in- vestigation for its bearing on painting plastered

surfaces.

9. Extent of Carbonation of the Lime

It is pointed out in section IV, 5, page 15, that

inasmuch as the combination of Ca(0H)2 with

CO2 from the air, to form CaCOs, proceeds slowly

in normally dry plaster, but more rapidly when

the optimum amount of moisture is present, it can

be predicted that the extent of carbonation will vary greatly. To determine the degree of carbonation, calculations were made from the data pre- sented in table 3. Although it can be seen from

column 13 that there was considerable CaCOs in

all the samples of failed white-coat plaster, there

is no indication of what percentage of Ca(0H)2 in

the lime had been carbonated in the preparation,

application, and subsequent treatment of the

white coat. This percentage carbonation of the

Ca(0H)2 can be calculated from the data in table

3. Columns 13 and 14 contain the equivalent of

all the Ca(0II)2 derived from the lime putty. In

addition, there is included whatever CaCOs

existed as an impurity in the calcined gypsum,

which is assumed negligible in this calculation.

The percentage of CaCOs in column 13 is multiplied by 0.7403 to obtain the equivalent percentage, A, of Ca(0H)2. This, added to the per- centage of Ca(0H)2 in column 14, gives the total,

B, supplied by the lime. The value A is divided

by B and multiplied by 100. The result is recorded in column 22 of table 3 as the percentage

carbonation of Ca(0H)2. The range is from 9 to 100 percent and the mean is 60 percent.

10. Expansion in Autoclave

It is well known that if materials containing

free magnesia are steamed in an autoclave, forced

hydration of the MgO to Mg(0H)2 is brought

about rapidly.

Many of the jjlaster samples were cut into narrow slabs several inches long and steamed in a

suitable autoclave for 1 hour at a steam-gage

pressure of 120 lb/in. ^. In most instances the

specimens expanded markedly, in some cases as much as 10 to 15 percent. Because the thin

specimens warped badly and often cracked, no

satisfactory correlation could be obtained between the percentage of expansion and the per- centage of free MgO in the samples. Invariably

the buckling was convex in a direction indicating

that the material next to the troweled surface had

expanded more than that next to the base coat. The outer portion of the white coat was denser

than the interior, because the pressure of the

trowel more completely closed the pores of the

outer portion. In addition, because of this

troweling action, the magnesia is more concentrated in the outer region. Consequently, the

hydration of the magnesia caused a greater expansion in this region.

This convex expansion is well illustrated by

figures 22 and 23. Figui'e 22 is a cross section of

white coat adhering to a sanded gypsum-plaster

base coat. The sanded base coat had been applied over a concrete beam, and a thick white coat had been used to straighten up the surface. The

bond between the sanded gypsum base coat and

concrete beam had failed in service. Figm-e 22 was made prior to autoclavmg. Figure 23 shows

the same specimen after autoclaving. Note the

expansion and convex bending of the white coat,

which in turn caused a bending and deformation

of the base coat. It was the prelimmary very

slight and not readily discernible deformation of

the base coat that had resulted in a shearmg of

the sanded base coat from the concrete base. The

autoclaving merely completed the hydration of

the MgO remaining in the white coat, amplified

the expansion, and showed clearty the directions

of the forces being set up. Specimens of white coat prepared from highcalcium or dolomitic limes containing httle or no

imhydrated oxide showed little or no expansion

when autoclaved. This demonstrates that if the

oxide is hydrated before the white coat is fabricated

the expansion and attendant bulging does not

occur. Recently, Wells, Clarke, and Levin [21] published a paper on the expansive characteristics of

80 hydrated limes and proposed therein an autoclave test for soundness. They showed that speci- mens fabricated from high-calcium limes and from

27
Figure 22. A cross section of white coat adhering to a sanded gypsum plaster base coat that had been applied over a concrete beam.

Photograph made prior to autoclaving.

Figure 23. The specimen shown in figure 22, after autoclaving.

Note the expansion and bending of the white coat, which in turn caused a bending and deformation of the base coat. The autoclaving completed the hydration of the MgO remaining in the white coat, amplified the expansion, and showed clearly the direction of the forces being set up.

those highly hydrated dolomitic limes containing a small percentage of unhj^-drated oxides expanded

insignificantly, whereas specimens prepared from

the regularly hydrated dolomitic limes that contained considerable mrhydrated MgO gave large

expansions. Their results clearly demonstrate

that the marked expansion attencling the hydration of MgO to Mg(0H)2 must be considered of

prime importance in causing plaster failures of

the type studied in this investigation.

VI. Investigation of Some Base Coats

Up to this point the discussion of plaster failures

has been confined largely to the white coat. During the field surveys, however, due consideration was given to the condition of the base coats. In the majority of cases, those responsible for building maintenance concurred with the state- ment of the manager of the Providence Building,

mentioned in the introduction of this paper, to the effect that "We have had no trouble at all with the brown coat of plaster but altogether with

the white or skim coat of plaster." This, of course.

does not mean that all the base coats of plaster were perfect. In those instances where the base- coat plaster fell into the perfect or very satisfactory category, the failures in the white coat in- variably resulted in a bulging of the white coat and a shearing away from the base coat, with the

base coat remaining intact and firmly attached

to its base. On the other hand, where the base

coat was soft and weak the white coat in bulging

away from the base coat invariably pulled some

sanded plaster from the underlying base coat. In both cases, the characteristic faihu'es of the

white coat, with the bulging and shearing away

from the base coat, were identical. It should be emphasized that most of the buildings inspected were of the monumental type, or, at least, of the better type of construction. The

findings, therefore, are not necessarily representative of the type of plastei'ing and workmanship

that may be too frequently found in speculative

home construction, where too-thin coats of plaster are apt to be encountered, as well as oversanding

of the base coats. Nor can the results of these

surveys be interpreted as showing that oversand28
ing and apparently bad plastering practices are not to be found in what may be classified as the

better type of plastering.

Reviewing the knowledge gained from the sur- veys and investigation, it appears that it would

have been desirable to have obtained samples of

the base coat in every instance where white-coatplaster failure occurred. Had this been done,

loiowledge would have been gained of the compositions of the base coats of plaster as commonly

applied. Unfortunately this was not done. During the surveys, however, the question as to the

compositions of the base coats arose and samples

were taken for analysis. The compositions of

24 of these that are believed to be of special

interest are included in this study. The data are given in table 5. Inasmuch as it was not known in many cases whether the samples of sanded plaster were gypsum or lime plaster, the samples were treated

with hydrochloric acid rather than with the

ammonium acetate solution prescribed in the

determination of the sand content of set gypsum

plaster [28]. It should be noted from the chemical analyses

that there were appreciable percentages of MgO

(column 6) in most of the samples of sanded

plaster. Inasmuch as it was reported in many

instances that gypsum plaster was used in the

preparation of the base coat, it must be assumed

that either lime containing magnesia was used, or that the magnesia was derived from the sand.

Because reports of chemical analyses of sand,

given in U. S. Geological Survey Bulletins, indi- cate that most of the territory covered in the

survey in the Middle West States contains considerable quantities of dolomitic limestone, the

presence of dolomitic limestone is a reasonable

assumption as to the source of the MgO. Such

an assumption is indicated further by the rather

low Si024-R203 content of the sanded plaster.

Furthermore, the quantity of CO2 present was

sufficient to have carbonated all the CaO not

present as CaS04.}^H20 and, in addition, all the

magnesia. If the magnesia had been present as a

constituent of dolomitic lime in these instances,

it is doubtful that it would have been appreciably

carbonated because the magnesia in the adjacent

white coats showed no carbonation.

In many cases the base coats were apparently

sanded gypsum plaster. The proportion of plaster

to sand in column 18 is given as pure CaS04.}2H20

to the sum of Si02+R203+CaC03+MgC03.

In column 19 the proportion is calculated on the

basis that the plaster contained 85 percent of CaS04.)^H20.

Sample 1 represents a rather weak and what

appeared to be an oversanded base coat. Analysis showed that this base coat was composed of 1 part of gypsum plaster to 6.5 parts of sand

(column 19). It was, therefore, badly oversanded.

Such oversanding is to be condemned.

Base-coat sample 2 represents the composition

used in tlie annex of the building from which

sample 1 was taken and shows that in tliis case a

lime plaster was used. Again, failure of the white

coat resulted in the formation of bulges. But still worse, the presence of the unhydrated MgO in the lime used in the preparation of the lime-sand

base coat caused expansion; thus, in addition to the

expansion of the white coat, there was an expansion

of the base coat. Consequently, most of the

plaster, including white and base coat, had to be

replaced. With the gypsum base coat, only the

badly oversanded portions had to be replaced.

Samples 3 and 4 represent the compositions

of the base coats of sanded gypsum plaster taken

from the ceilings of the nurses' home of a large

hospital in which there have been extensive

failures of white coat. In general, the brown

coats were hard and firm and sufficiently rough

for good bond. Despite this, white-coat-plaster

failures started in this building 9 years after it was

erected and are continuing. They occur most

extensively during the spring and summer,

especially following periods of high temperature

accompanied by high relative humidity.

Sample 5 was taken from the sidewall of an

outside corridor of a large hotel. The frame is structural steel, fireproofed, with reinforced- concrete floor slabs. In the ceilings, suspended

metal lath was attached close to the bottom

of the concrete joists (furred type). The plaster on the ceilings was three-coat work; on waUs,

two-coat work. The finish coat was smooth and

hard. White-coat failures, characterized by the

formation of bulges, have occurred on ceilings

and on walls throughout the building, especially

in corridors. Failures began in 1935, 8 years

after the building was erected. They occur most

extensively during the summer. A plasterer is continuously engaged by this hotel to repair the

failures. In general, the brown coat was firm

and hard. However, a sample of a portion that

seemed to be somewhat weaker and possibly

oversanded was taken from the side wall of an

outside corridor (sample 5). Analysis verified

that this particular sample was somewhat over- sanded (columns 18 and 19).

Base-coat sample 6 was taken from a 12-story

office building where the frame was of structural

steel that was fireproofed. The floors were of concrete joists with sheet-metal-lath fillers. The

outside walls were of brick furred with ceramic

hollow tile. The interior partitions were of

ceramic hollow tile. Two-coat work was used on

ceilings and walls. The finish coat was smooth

and hard.

In most cases the failures were characterized

by a separation of the white coat from the brown

coat. Failures occurring before the time of

survey had been confined to ceUings, but were

starting on outside and inside walls. The base

coats were rather soft and weak. This was the reason for taking sample 6. Analvsis showed

that the gypsum : sand .ratio was 1:4.7 (column

29
Table 5. Constitution of base coats obtained from, various buildings, showing the chemical analysis, calculated compound

composition, proportion (by weight) of CaS04.y2H20 to Si02 + R2O3 + CaC03 + MgCOa and of gypsum plaster to sand,

or of lime to sand, or of lime to gypsum plaster to sand

Chemical analysis Computed compound

composition

13 14 17

0 +

0+8

CI „ + 2 S -

g "O

D. - 03

18

o W

20

•

fl

ciIS

0'. s «

CD il a

13 CI 2 .0 0 03 'go. 0 0 H

0. P.B 0 0 3

Ui (5

22

Remarks

23

5.3

3.5

8.8

3.9

16.8

5.3

4.1

6.6

10.8

2.8

8.0

5.7

7.8

10.1

6.3

16.4

17.4

16.4

11.7

17.5 17.9

13.1

14.4

1.9

2.9

6.3

4.2

%

75.8

70.8

57.4

64.7

70.9

72.6

46.2

68.6

3.7

40.9

44.5

39.8

27.1

39.1

31.9 40.8

35.7

76,

78.6

83,

81.5

%

8.1

12.1

16.0

11.7

10.2

8.3

17.3

11.9

31.8

21.1

20 3

20.4

26.7

21.7

24.9

22.1

25.0

10.2

11.6

6.0

10.1 2.9

8.0

16.5

.3

38.8

10.7

11.0

14.6

10.4

9.0 14.2

%

13.1

1.1

23.9

19.6

14.6

-16.7

35.5

.6

'80.3

23.0

17.4

23.7

31.4

22.4

19.4

6 30.1

31.6

%

6.8

18.7

14.6

4.8

10.2

20.9

8.2

24.3

26.

22.6

26.6

25.7

33.2

21.7

26.2

4.3

6.6

12.1

9.6

2.1

10.1

2.2

1.9

12.1

3.0

3.5

6.3

1.8

6.1

10.9

11.4

12.3

.0

11.9 6.3

6.1

.0

.0

100

83

91

1:7.8

1:3.7

1:4.9

1:6.8

1:5.7

1:2.1

1:6.5

cal/g

1:3.4

1:3.0

1:4.

1:5.

1:4.

1:1.

1:3.2

26 1:1.4:3.0

1:1.0:3.2

1:1.3:2.4

1:1.1:1.0

28

1:2.7

1:2.5

1:2.

1:2.

93

2:2.5

6:1.1

l:3.i

1:4. (

1:6.!

1:5.:

1" 1:3.7

•"1:3.

1" 1:2.

Over-sanded gypsum base coat; .see table 3, sample 32, for white coat. Sanded lime plaster in annex of same

building as 1; see table 3, sample

33, for white coat. Base coat expanding. Sanded gypsum base coat. See sample 34, table 3. Sanded gypsum base coat. Over-sanded gypsum base coat on

outside corridor; see sample 35, table 3, for white coat. Soft and weak over-sanded gyp- sum base coat; see sample 36, table 3, for white coat. Excellent sanded gypsum base

coat; see table 3, sample 37, for white coat. Sanded lime plaster; under water

for about 2 weeks (1937); see sample 42, table 3, for white coat. Bond plaster (lime to gypsum 1:6, by volume); see sample 43, table

3, for white coat. Lime-gypsum-sand plaster on metal

lath; under water 10-12 days, January 1937; see table 3, sample

45, for white coat. Lime-gypsum-sand plaster on hol- low tile, under water 10-12 days, January 1937; see table 3, sample

46. for white coat. Sanded lime-plaster base coat gaged with calcined gypsum applied on gypsum bond plaster on bitumi- nous coating on concrete. Base

coat expanding. Sanded lime-plaster base coat applied over gypsum bond plaster and brought to sandfloat finish. Base coat and finish expanding.

Similar to 12. Similar to 13. Excellent sanded gypsum plaster applied on ceramic hollow tile; see table 3, sample 49, for white coat. Similar to 14; see table 3, sample 50, for white coat. Expanding sanded lime plaster base

coat. See table 3, sample 55, for white coat. Satisfactory sanded lime plaster base coat prepared from lime

containing 11.8 percent of MgO; see table 3, sample 64, for white coat. Satisfactory high-calcium lime-sand

base coat; see table 3, sample 66, for white coat. Unsatisfactory expanding sanded

lime plaster base coat prepared from dolomitic lime containing

40.2 percent magnesia; see table 3, sample 67, for white coat. Satisfactory sanded gypsum base

coat; see table 3, sample 68, for expanding white coat. Similar to 22; see table 3, sample 69, for white coat. Satisfactory sanded gypsum base

coat; see table 3, sample 72, for white coat.

a Where MgCOa is derived by carbonation of the lime, the excess HjO is- assumed to be combined with MgCOa as a hydrous compound. Elsewhere

the excess CO2 and excess H2O are assumed to be present as some hydrous

carbonated material in the sand.

i> Sand, column 19, is the sum of SiOj, EjOs, CaCOa, MgCOs, excess CO2, and excess H2O, less the 15 percent insoluble in the calculated gypsum plaster.

= Calculated on the basis that the sand is the sum of Si02 and R2O3, that a hydrated lime was used in which all the CaO and one-fifth of the MgO

was hydrated, and that the SO3 (as CaSOi) was present as an impurity in the lime. •

d Calculated on the basis that the original lime was prepared from a dolo- mite and contained 60 percent of CaO and 40 percent of MgO and that in the hydrated lime all of the CaO and one-fifth of the MgO was hydrated. The remaining nongypsum CaO is considered to be present as calcite in the sand which is calculated to contain in addition the Si02, R2O3, and excess CO2.

e Contains in addition 0.4 percent of CaSOi.

' Contains in addition 2.5 percent of CaS04.

e Contains in addition 0.3 percent of CaSOt.

t Determined by the ammonium acetate method for determining the sand

content of set gypsum plaster. 30
19). Failures began about 6 years after the

completion of the building and are general

throughout all stories. They occur most rapidly

during the spring and summer. The building

superintendent stated that after a hot, humid

night he could be sure that plaster repairs would

be necessary the next morning. The superintendent stated that $10,000 has been expended

for plaster repairs in a period of 5 years. A building where the base coats were in very

good condition, but where, nevertheless, there were failures in the white coat is exemplified by

the data pertaining to sample 7. In this case the

analysis revealed that the proportion of gjrpsum

to sand (on an assumed purity of 85 percent for the gaging plaster, CaS04./2H20) was as rich as

1 : 1.7 (column 19).

One of the very interesting cases of plaster

failure involved expansion of both a sanded base

coat and a white coat where a partially hydrated

dolomitic lime had been used in their preparation.

These plaster failures occmTed in a six-story

building, with, basement, covering about a block.

The frame of the building was of structural steel that was fu-eproofed. The floor slabs were of

reinforced concrete and concrete joists with

ceramic-hollow-tile fillers; the outside walls were

of stone and brick with tile furring; the interior

partitions were of hollow tile. Except for a few

suspended metal-lath ceilings, the plaster bases were of masonry construction. Tliree-coat work

was used over metal lath and concrete. The

scratch coat on concrete was gypsum bond plaster.

From table 5, sample 8, column 21, it can be seen

that the base coat was a lime-sand plaster (1 : 3.2).

Failures consisted of separation of the white

coat from the brown coat, bulges, and a considerable amount of separation of the lime-plaster base

coat from the masonry backing. Failures began

in 1938, about 5 years after the plastering was

completed. They occur most rapidly during the sunamer and are general throughout all stories. During the flood of the Ohio River in 1937,

water rose to within 6 inches of the fu'st-floor slab. Plaster work in the basement was imder water for a period of about 2 weeks, and it did not completely

dry out for months. Such an exposure to water

should have rather completely hydrated the magnesia; from column 12 it can be seen that there was no free MgO left in the base coat at the time

of the survey in 1940. Not only was all of the

MgO hydrated, but a portion of the Mg(0H)2

was in turn converted to the carbonate, nesquehonite, MgC03.3H20.

The hydration of the magnesia attending this soaking was also reflected in the data on the white

coat (table 3, sample 42). From column 17 of table 3, it can be seen that 83 percent of the MgO

had been hydrated; from column 22, that 94 per- cent of the Ca(0H)2 had been carbonated.

From a consideration of the volume changes

accompanying the hydration and carbonation of lime products (table 2), it might be concluded that

the expansion should have been of such a magnitude that the base and white coats would have

failed throughout. A probable explanation as to why the failures were not extensive is that when

the plaster was wet it was also soft; and that,

therefore, stresses set up by the increase in volume

could be relieved. Failures characterized by

bulging of both the sand-lime base coat and limegypsum finish coat were prevalent in the stories

above the flooded area. Samples 9, 10, and 11 were taken, in another

city, from a building that was partially flooded

during the rise of the Ohio River in 1937. The

construction of this four-story building with its basement and subbasement was similar to the one just described.

The plaster bases were of masonry, except for a few suspended ceilings of metal lath. Some concrete columns were encased with metal lath. Plywood forms had been used for the concrete

work, and the concrete surfaces were exceptionally

smooth. Over most of the ceilings, the white

finish coat was applied directly to the concrete,

but in certain limited areas a thin brush coat of

bond plaster had been applied. In both cases

these bases were far from ideal for the application

of white coat. In only one other building was

there encountered this bad plastering practice of

merely brushing on the bond coat. Sample 9

represents bond plaster taken from a concrete column which was above the flood level. The

ratio of lime putty to gypsum (by volume) was

1:6. The analysis of the corresponding white

coat is given in table 3, sample 43. Samples 10 and 11 of table 5 pertain to the

sanded base coats, and it should be noted that

these coats were essentially lime-gypsum-sand

plaster. Inquiry revealed that the lime putty

used in preparing the base coat was obtained from

a lime-putty plant that slaked a dolomitic quicklime with an excess of water and then aged the

putty to assm-e complete hydration.

Both samples 10 and 11 were taken from the

subbasement and had been thoroughly soaked

with water as a result of the flood. It was stated

by the buflding manager that no extensive plaster

repau's were required after the flood water had

subsided. This is not surprising when it is realized that not only were the flooded plasters

soft, but also that only thoroughly hydrated and

aged lime putty was used originally m preparing

the base coats—thus additional wettmg could not

have been attended by any subsequent hydi-ation

and increase in volume. In contrast to the other

buflding that was also flooded, there was no

evidence of any base-coat faflure in the upper

floors. Base-coat samples 12, 13, 14, and 15 were all taken from the annex of a large bufldmg where the

type of faUures occurring in the sanded base coat were simflar to those noted elsewhere in whitefinish-coat work.

31
The structure in question was a 20-story office building about a block long and one-half block

wide. The original portion was built in 1910;

the annex, in 1920. The frames of both were of

structural steel and reinforced concrete. The

floor slabs in both instances were of reinforced

concrete. The outside walls were of stone and

brick, furred with ceramic tile. In the original

building, the failures consisted in the separation of

the white coat from the hard and firm gypsum

base coat. Inasmuch as these failures were

typical of those found repeatedly thi-oughout the

surveys, no sample was taken of either the base

or white coat. There was no white coat in the annex; instead

the base coats were brought to a sand-float finish. As shown in table 5, column 22, they were essentially lime base coats gaged with calcined gypsum.

In other words, the plaster might be termed a "sanded white-coat finish". In considerable areas, a bituminous coating was applied over concrete

surfaces. Over this bituminous coating, gypsum

bond plaster was applied and scratched to receive

the lime base coat gaged with calcined gypsum.

The ultimate expansion of the sanded coat, result- ing from the slow hydration of the MgO in the lime

used in its preparation, caused numerous separations of the plaster from the bituminous base

material. Invariably the bituminous material was

hard and brittle—it had lost all of its "tackiness".

Thus the inner surface of the bituminous coating

(the weakest bonding material in the plaster work) was at the interface where separation occurred.

Where no bituminous material was used, the

failures were characterized by the separation of

successive layers of the sand-float plaster.

If one were to select the building that showed the

greatest variety of white-coat-plaster failures, it would be a certain small Post Office of one story

and basement.

The frame was structural steel, fireproofed. The

floor slabs and first-story ceiling slabs were reinforced concrete. The outside walls were of brick

furred with ceramic hollow tile. The inside partitions were of hollow tile. Thus the plaster bases were of concrete and masonry, except for some

suspended ceilings of metal lath and metal lath over the lookouts. The application of the white

coat directly to the rough concrete was in accord- ance with the specifications. There was two-coat

work on the ceramic hollow tile and three-coat

work on the metal lath. The finish coat was smooth

and hard, and there was every evidence of good

workmanship. The ratio of lime putty to gaging

(by volume) was 3.6 (see sample 49, table 3, column 21).

The failures on the concrete ceiling in the work- room began about 5 years after the building was

constructed. They were progressive for the next

9 years; at the end of that time all the plaster on

the ceiling was removed, and the rough concrete

slab was painted.

Failures were found not only on this and other

ceilings but also on the furred outside walls, concrete and brick inside walls, hollow-tile interior

partitions, and metal lath and plaster lookout

walls. These failures all showed the typical

bulging and separation of the white coat from the

base coat. The gypsum base coats were very

hard and firm. The proportion of gypsum plaster

to sand was close to 1:2 (table 5, column 19,

samples 16 and 17). The sanded base coats had

been left sufficiently rough to permit a good bond

with the white coat. It was conceded that no

better base coats had been seen on the survey.

Nevertheless,, at the time of the survey in 1940

(26 years after the building had been erected), the

white coat was still bulging away from the walls,

and the plastering had become a network of patches. To those who believe that all concrete

surfaces should be covered with metal lath, it should be pointed out that, in this building, cover- ing the concrete lookouts with metal lath did not

prevent white-coat-plaster failures. Again, it should be emphasized that the failure of plaster over concrete surfaces was almost invariably

characterized by bulges in the white coat and only

secondarily by failures of the bond between the

bond plaster and the concrete. Provided the concrete surface is sufficiently rough, there seems no logical reason why the white coat should not

be applied directly to the concrete, granting, of

comse, that the materials used in the preparation of the white coat will not undergo subsequent

expansion. If the lime is unsound, covering the concrete surfaces with metal lath will not prevent

failure in the white coat. Four samples of base coats (18, 19, 20, and 21) were obtained from post offices in various parts

of the country. In each case, it can be seen from

table 5, column 21, that the base-coat plasters were lime-sand plasters. In two of these (18 and

21), there were not only failures in the white coat,

but also bulging of the lime-sand base coats. Apparently both of these were prepared from a

regularly hydrated dolomitic lime. In the other

cases (19 and 20), there were failures in the white

coats, but no trouble in the base coats. The

data indicate that the lime used in sample 19 was

not dolomitic as it contained only 12.4 percent of

total magnesia. The lime used in base-coat

sample 20 was definitely high-calcium.

The remaining samples in table 5 (22, 23, and

24) were taken from buildings where the plaster

failures were entirely confined to the white coat. Samples 22 and 23 were from a building where

the hard gypsum base coats were proportioned

(column 19) as 1:3.7, gypsum to sand, by weight.

The other (number 24) was from a building

where the hard gypsum-sand plaster was proportioned as 1 part of plaster to 2.6 parts of sand

(column 19).

For several of these samples, the heat of solution

(column 20) \vas determined. It should be noted

32
that the vahies obtained were veiy much lower

than those obtained for the vvhite coat. This is due to the fact that the base coats were comprised

largely of sand with a smaller quantity of cementitious material. The cementitious material in turn was composed to a large extent of gypsum (which

has a negative heat of solution) or (in the case of

lime base coats) of calcium and magnesium carbonates that have low positive heats of solution.

Neglecting the Si02-|-;R203 in the computed

heat of solution of the white coat did not introduce

any appreciable error, as these constituents, as has been pointed out previously, were usually less

less than 1 percent. On the other hand, when the

so-called sand content comprises the bulk of the

sample, its effect on the heat of solution cannot be

ignored. Evidently there were in the sands

certain mineral constituents that evolved a small

quantity of heat when treated with the 2 HCl

solution. The observed heats of solution were

invariably a little higher than those computed

from the calculated heats of solution based on the

calculated compound composition on the assumption that the heat evolved by the sand was zero. As the original sands used in the preparation of the

plasters were not available, no correction can be

made for the possible amount of this heat evolved.

VII. Possible Causes of Failure of

White-Coat Plaster Other Than Hydration of Magnesia

Many causes for the bulge type of plaster failure

have been suggested by architects, plasterers, lime

manufacturers, and others. None of these causes when considered in detail seems adequate for

explanation of the observed failures. Nevertheless, some of those more often suggested will be

discussed at some length to point out how they fail to account for the observed phenomena. For this

purpose will be considered drying shrinkage of

concrete, differences in thermal expansion between

the backing and the various layers of plaster,

workmanship, improper gaging, improper painting, paint shrinkage, vibration, and structural

movements.

1. Shrinkage of Concrete

As stated previously, failures in the white coat

of plaster over concrete bases were of frequent

occurrence. It is well known that concrete undergoes a slight shrinkage on drying after it has been

fabricated. This shrinkage has often been cited

as a cause of the formation of bulges in the white

coat. Figure 24 shows a marked failure that occurred

in white coat along the side of a concrete beam.

The movement, whether ascribed to an expansion

of the white coat or to a shrinkage of the concrete,

resulted in the separation of a little of the bond

plaster from the concrete base. However, over

the remaining area of the failure, the strong and

well-scratched coat of bond plaster j-emained

intact. Chemical and thermal analyses indicated that

the white coat (sample 10, table 3) had been

prepared fi'om a mixture of gypsum gaging plaster

and a dolomitic liuK; that had contained considerable unhydratcd MgO. Although 70 percent

of the magnesia had hych'ated at the time of the

failure, which was about 12 years after the building had been constructed, the sample still contained 5.2 percent of MgO to be hydrated, with

its certain attending expansion. Calculation

based on measurements of the bulge in the white

coat revealed that a linear expansion of approximately 0.5 percent appears necessary to account

for a separation of the magnitude shown in

figure 24. The maximum drying shrinkage of concrete is, in general, of the order of magnitude

of 0.05 percent—a value one-tenth of that neces- sary to account for the formation of the bulge in

question. It should be noted that not even this

total amount of shrinkage would be available to form a bulge as white-coat plaster also shrinks on

drying and would tend to offset any such effect. Furthermore, the bulk of the shrinkage of a concrete beam in a building should be completed at the end of a year, whereas the formation of this bulge in the white coat was delayed for 12 years.

Finally, like this one, the other failm-es over

concrete have been delayed for years and have

been very largely separations between the white

coat and the gypsum bond plaster—not between

the bond plaster and the concrete base.

Consequently, it can be concluded that the

shrinkage of concrete cannot account for the

bulging of the white coat.

2. Differences in Thermal Expansion

White-coat failures have often been ascribed to the differences in thermal expansion of white- and

base-coat plaster. A search of the literature at the time that the surveys were first made failed to reveal any published data on the coefficients of

thermal expansion of these materials other than

those determined at the Bureau by McMiurdie

and Marsh [32]. They reported (1932) for set gaging plaster coefficients ranging from 8.3 to 9.5X 10"*^/deg F. Consequently, determmations

were made of the coefficients of thermal expansion

of some specimens made from white-coat and

sanded base-coat materials, as well as one from

gaging plaster.

The coefficient of thermal expansion of the set gaging plaster was found to be 9.7X10~®. This

value is in close agreement with 9.5X10"^, the

upper limit of the values recorded by McMurdie

and Marsh.

The white-coat specimens were prepared from

gypsum gaging plaster with finishing hydrated

dolomitic lime or with high-calcium lime, by

mixing (by volume) one part of the gaging plaster

with three parts of the lime as putty. The mixtures were cast into bars, 10 by 1 by 1 mch. In

33
CEILING

SIDE OF BEAM

FiGUEE 24. Plaster failure involving an expansion of about

0.5 -percent in the white coat alonfj the side of a concrete heam. making preliminary measm'ements between 50°

F (10° C) and 80° F (27° C), coefficients of thermal

expansion of about 12X10~Vdeg F were obtained.

The specimens were then raised to 110° F (43° C)

in a drying oven and kept at this temperature for 24 hours, after which measurements indicated a

coefficient of only about 4X10"Vdeg F. This

difference was attributed to a moisture effect. Therefore, all the specimens were dried before

determinuig the coefficient of thermal expansion

by a method similar to that described by McMurdie and Marsh. The specimens, maintained

at low humidities, were measured at five temperatures between 12° F (-11° C) and 110° F (43° C).

The coefficients of thermal expansion of the

specimens prepared from the hydrated dolomitic

lime averaged 6.2X 10"'^/deg F, with a variation

of ±0.2X10"Vdeg F. The average coefficient for the high-calcium specimens was 6.5X10~Vdeg F,

with about the same variation.

A sample of finish coat (containing regularly

hydrated dolomitic lime) from the wall of a building gave a coefficient of 6.4X10"Vdeg F. The average of three samples of scratch coat composed of 1 part of gypsum plaster to 2 parts

of Potomac River sand, by weight, was

8.2X10-7deg F; for the brown coat (1:3), the

average was 8.1X10~Vdeg F. Specimens prepared from a neat gypsum plaster,

sold as bond plaster, gave a coefficient of

9.0X10^^/deg F; checking the values obtained by

McMurdie and Marsh for gaging plaster.

After the dry specimens were measured for thermal expansion, they were maintained at constant temperature, and the relative humidity was increased to 90 percent. Both the highcalcium specimens and the gypsum bond-plaster

specimens expanded 0.02 percent, because of the

absorption of moisture. These expansions are equivalent to those due to temperature changes of

30 and 22 deg F (16 and 12.2 deg C), respectively.

The specimen containing the hydrated dolomitic

lime expanded nearly 0.07 percent.

The coefficient of expansion of sanded lime

plaster was not determined, because the surveys

revealed that in recent years gypsum plasters

had almost universally replaced lime plasters in

the base coats. It is believed that the coefficient of thermal expansion of sanded lime plaster is about that of lime mortar.

The coefficients of thermal expansion determined

at the Bureau, together with some values taken

from the literature for clay brick, mortar, brick

masonry, concrete, and lime mortar, are given

in table 6.

Table 6. Coefficient of thermal expansion of hardened

plaster coats and of bases for plaster

Material

Clay brick Brick masonry.

Lime-sand mortar

Concrete. Mortar - White-coat finish (dolomitic hydrated lime) White-coat finish (dolomitic hydrated lime, from a wall) White-coat finish (high-calcium lime)-_ ___ Gypsum plaster—brown coat Gypsum plaster—scratch coat Gypsum gaging plaster Gypsum bond plaster ___

CoefEcient of thermal expansion (in millionths per deg F)

2. 0 to 4. 2

2. 5 to 4. 5

4. 0 to 5.

4. 0 to 6.

4. 5 to 6.

6.2

6.4 6.5

8.1

8 2

5. 3 to 9. 7 9.0

According to the values in table 6, there is only

a small difference between the coefficients of

thermal expansion of the white-coat finishes

(dolomitic and high-calcium) and those of the

sanded and bond plasters. It is doubtful if there

is any marked temperature gradient between the

inner face of the base coat and the white coat,

which is usually Ke to Ys inch in thickness.

Considering also the relatively small temperature

differences in the interior of a building, even with

seasonal changes, it is difficult to see how the

marked bulges in the white coat can be ascribed

to thermal changes. For a temperature rise of

50 deg F (27.8 deg C), calculations show that the

bond plaster, brown coat, and white coat expand

0.025, 0.020, and 0.012 percent, respectively, more than the concrete backing. If the ends were

restrained, this would place all of the plaster

layers in compression, the bond coat more than

either of the others, and any separation, although

improbable, would be expected to occur between

the concrete and the bond plaster. Actually,

the maximum relative expansion for a 50 deg F

(27.8 deg C) temperature rise is only one-twentieth

of that estimated for the bulge shown in figure

24. Similar relationships occur between the

coefficients of thermal expansion of brick masonry

and the plaster coats, and here again the plaster

would be expected to separate from the backing.

In general, brick and hollow-tile masonry af- fords an. excellent base for gypsum plaster, and

the survey has indicated that plaster failures are primarily in the bond between the dolomitic

white-coat finish and the base coat, rather tha,n

34
between the base coat and either the masonry or concrete construction.

FiDally, the coefficient of thermal expansion of

the white coat made with hydrated dolomitic

hme is almost identical with that of the finish made with high-calcium lime. Only finishes made

with hydrated dolomitic lime containing free MgO

were found to produce bulges.

Therefore, the explanation that differences in thermal cofiicients of expansion cause bulges is untenable.

3. Poor Workmanship

Three factors must be considered in ascertaining

the cause or causes of troubles that develop in a

building: (1) the design of the building, (2) the

materials used, and (3) the workmanship. Up to

this point little has been said about workmanship,

but the following comments seem pertinent for those who have been inclined to blame poor workmanship for white-coat-plaster failures. First, the same workmen, or even different workmen with the same degree of skill, could not possibly have plastered all of the many buildings in

which typical white-coat-plaster failures have

occurred. As a matter of fact, it was the consensus of opinion of those on the surveys having

first-hand acquaintance with the plastering contractors that the plaster work had been done by

plastering contractors of excellent reputation, and

that in very few cases was there evidence of poor

workmanship in the monumental type of buildings

inspected. This, of course, does not mean that

there is no poor workmanship, especially in some

of the speculative buildings.

Second, there has never been a satisfactory

explanation advanced that could explain what a

skilled workman did, or did not do, in the mechanism of his application of the white coat that

could account for the formation of a bulge some

5 to 15 years after he had finished his work.

Third, a survey of buildings, in the area of

Washuigton and Baltimore, plastered by a contractor who used putties prepared in some cases from high-calcium quicklime and in others from

regularly hydrated dolomitic finishing lime did not

reveal any trouble where the high-calcium lime

had been used, but invariably showed faihu'es

with dolomitic finishing limes. On many of these jobs the same foreman and identical crews were employed.

4. Improper Carbonation

During the course of the surveys, the contention was often made that the plaster failures occurred

because the magnesia had not carbonated suffi- ciently. Although the volume increase resulting

from carbonation has already been discussed on

the basis of molecular volumes, it may be well

to point out once more that carbonation of Mg(0H)2 to MgCOs.SHsO (nesquehonite) effects a larger increase than does hydration of MgO to

Mg(0H)2. Consequently, were it not for the fact

that there is little or no evidence that the magnesium is carbonated to MgC03.3H20, except in

extreme conditions, it would have to be concluded

that carbonation actually accelerates the expansion and the formation of bulges. In fact, the

carbonation to MgC03.3H20 and the subsequent

reaction of this nesquehonite with the gypsum

to form MgS04.7H20 eventually causes marked

efflorescence and the complete disintegi'ation of

the white coat.

5. Improper Gaging

Gaging refers to the relative amounts of plaster

of paris and lime in the white coat. Individuals

encountered on the survey of plaster failures fre- quently blamed them on improper gaging. In some instances, it was contended that there was

too much gaging plaster; in others, too little. For these reasons the probable proportions of lime putty to gaging plaster were determined as described previously. The analyses showed that

wide variations seemed to be without measurable

eft'ect on the occurrence of failures. Analyses of

the many finish coats (table 3, column 21) showed

that failures have occurred where the proportion

of lime putty to plaster of paris (by volume)

ranged from 0.8 to 9.8. Nonuniform gaging can

therefore be dismissed as a cause for bulges in

the white coat.

6. Painting Plaster Before It Is Sufficiently

Dry

Failures that occur from painting too soon

usually show up within a few weeks and are characterized by the formation of small paint blisters

containing water. Failures in the white coat take

from 5 to 15 or more years to develop. On some

of the Federal buildings where white-coat-plaster

failures were observed, the paint had not been

applied until 3)^ to 4 years after the plastering

had been completed.

7. Paint Shrinkage

Paint shi-inks as it dries and ages. In any

plaster failure caused by paint shrinkage, the

paint would be on the concave side of the plaster

fragments separating from the base coat. The

most extensive white-coat-plaster failm'es are convex in the direction indicating expansion rather

than shrinkage at the painted surface.

8. Vibration

The authors are unable to imagine, and no one

has suggested, a mechanism whereby vibration of

a building could be transmitted from the structural frame in such a way as to form bulges in

the rigid, hard, and brittle white-coat plaster.

A mechanism that will distinguish between highcalcium and dolomitic limes in the white coat seems

particularly difficult to describe.

35
9. Movements in the Structural Frame

The movements caused by settling, deflection,

expansion, etc. in the frame of a building can be

transmitted to the wall and ceiling surfaces only

through the intervening structures. It is diffi- cult to see how such movements, characterized by

cracks, could cause bulges in the white coat, yet

most failures were found where there was no evidence of cracks in beams, slabs, walls, or columns.

VIII. Preventing Failures of WhiteCoat Plaster in New Buildings

In the introduction of this paper it is stated that

its purpose is twofold: first, to give the results of

the investigation of a particular kind of plaster

failure that is characterized by the formation of

blisters or bulges in the finish surface of plaster;

and, second, to explain to the architect, engineer,

plastering contractor, plasterer, and others con- cerned with plastering, the precautions that should

be exercised in new buildings to prevent such

failures. Up to this point, the paper has been confined

largely to the first of these objectives. It should

be evident that the white-coat-plaster failures

xlescribed were caused by slow hydration and ex- pansion of the MgO in the partially hydrated

dolomitic lime used in the preparation of the white

coat. In some instances similar failures occurred

in a sand-lime-plaster base coat. When the results of the investigation were first made known, other Government agencies re- quested assistance of the Bureau in formulating

a specification that would exclude partially hydrated limes containing an undesirably high per- centage of unhydrated oxides. Accordingly, the

following tentative amendment to Federal Specification SS-Lr-351 for hydrated lime [22] was

prepared: "The total free (unhydrated) calcium

oxide (CaO) and magnesium oxide (MgO) in the

hydrated product shall not exceed 8 percent by

weight (calculated on the 'as received' basis)." A

method was given for determining the quantity of

unhydrated oxides.

Shortly thereafter certain of the manufacturers

of regularly hydrated dolomitic lime started producing more completely hydrated lime. In most

instances this was accomplished by using autoclaves to hydrate the lime at elevated temperatures and pressures. The 8-percent limit of the unhydrated oxides

recently has been included in specifications of the

American Society for Testing Materials [7 and 8], as well as in those of the American Standards

Association [24], for hydrated lime to be used in

white-coat plaster.

Nevertheless, a limitation on the percentage of

unhydrated oxides has been criticized on the score that the percentage by itself may not be a complete indication of the potential expansion of a hydrated lime. This objection would be obviated

by a performance test. Since unsoundness, as has been pointed out in this investigation, does

not exhibit itself for several years, a feasible per- formance test must also be an accelerated one. Consequently, an investigation was made at the

Bureau to study more completely the expansive

characteristics of hydrated limes and to obtain

data and information that might be used in formulating an accelerated test acceptable for a specification for the soundness of hydrated lime.

Because there is no satisfactory test procedui'e

for ascertaining volume-change characteristics of

hydrated lime by itself, it is necessary to gage the

lime with some cementitious material having

hydraulic properties. Portland cement was se- lected as the most suitable hydraulic material for

gaging hydrated lime in the preparation of test specimens suitable for autoclaving. The linear

expansion caused by the autoclaving of cementlime bars thus provided a measure of the expansive

properties of the limes from which they were made.

Eighty commercial limes were subjected to the

autoclave treatmeot and to other tests for determining the effect of variables, such as the proportions of cement to lime and the use of different

cements in the bars.

From the techniques developed and the data

obtained, details of the test procedure have been

worked out [21]. It was proposed that hydrated

lime be tested in the proportion of 1 cement to

1 lime (by weight) ; that the cement used have an

expansion of between 0.05 and 0.15 percent; that

the autoclaving be done at 295 lb/in. ^ gage pres- sure for 1 hour according to a controlled schedule;

and, finally, that the expansion obtained after

subtracting the percentage of expansion of the

neat-cement bar from the total expansion of the

cement-lime bar be limited to not more than 1 percent.

Figure 25 shows the relations of the linear ex- pansions of some autoclaved cement-lime bars to the rmhydrated oxides contents of the limes used

therein. Note the high expansivity of those

limes containing a high percentage of unhydrated

oxides, especially of the regularly hydrated dolomitic limes—those limes that have caused and are causing such extensive white-coat failures. Bearing in mind that the test for soundness

should differentiate hydrated limes having low

potential expansions from all others, and that

"Not a single instance has been found where

bulging occurred in a white-coat plaster made

with high-calcium lime" [30], it would appear

logical to limit the percentage of expansion to that shown by the high-calcium limes. The highcalcium limes, characterized, in general, by the

lowest percentages of unhydrated oxide, gave the

lowest percentages of autoclave expansion. Most

of the highly hydrated dolomitic limes (known to the trade as autoclaved, pressure-hydrated, etc.)

had percentages of unhydrated oxides and expansions comparable with those of the high-calcium

limes. Some of the new limes did not meet this

36
requirement, but they are not truly representative of this type of lime as now produced. Several

of the limes tested were experimental products,

and the process of manufacture has been so modified that there is now good reason to believe that

the percentages of unhydrated oxides and ex- pansions of the highly hydrated limes now available are of the same order of magnitude as those

of the high-calcium limes.

A limit of 1.0 percent expansion should afford

adequate protection for the consumer and not

work an undue hardship on the producer. Furthermore, since an accelerated performance test should be preferable to a limitation of the chemical

composition, it is believed that a limit of 1 per- cent (net) on the autoclave expansion of bars

made in the proportion of 1 cement: 1 lime (by

weight) is better than the 8-percent limit on the

unhydrated oxides. An autoclave test for soundness, with this limit, is being submitted to the

Federal Specification Board for a proposed revision

of Federal Specification SS-L-351 for lime;

hydrated (for) structural purposes [5].

Until such a soundness test, or modification

thereof, is adopted in specifications, it is recom- mended that the consumer insist that hydrated

lime contain not more than 8 percent of unhydrated oxides (calculated on the received basis).

A statement to the eft'ect that the contents meet

this requirement is generally printed on the bags

of this new type of lime.

A satisfactory lime putty that is completely

hydrated can be obtained from quicklime. It is recommended that the directions for slaking as given in Standard specifications for gypsum

plastering, issued by the American Standards Association A42. 1-1946 [24], be followed. See, also,

ASTM C5-26 Appendix.

It has been shown in this investigation of whitecoat-plaster failures that methods have been developed for determining the extent of hydration

of MgO. These methods can be used even after

lime has been mixed with gaging plaster and applied to the wall or ceiling. To date, most of the

data presented relate to white coats that have

reached such a state of hydration of AlgO that

18

15

a. 12 <

I

I I I I I I I I I I I

I

* I ^ ^ I I I I I I I

I I I

J. J__L I I I I I. I I I

I I I I 0 5 10 15 20 25 30 35 UNHYDRATED OXIDES IN HYDRATED LIME , "/o Figure 25. Linear expansion of cement-lime bars prepared with hydrated limes containing various amounts of unhydrated

oxides, in the proportion of 1 cement to 1 lime (by weight), and subsequently autoclaved for Shrata gage pressure of 295 Iblin-. X, High-ealcium; %, regularly hydrated dolomitic; O. highly hydrated dolomitie; A, magnesian.

37
the expansion has resulted in failure. The data

in table 3 shows that the average amount of hydration of the MgO in these plaster-failure samples was 59.0 percent. It is pointed out furthermore

that 59.0 percent is much in excess of the average

of 19.1 -percent hydration found by Wells and

Taylor [2] for six normally hydrated dolomitic

limes after the customary 1 day of soaking as

putties. It is also far in excess of the average of

22.6-percent hydration more recently obtained

[25] for 18 additional samples of normally (regularly) hydrated dolomitic limes at the end of the

custom<ary soaking for 1 day as putties.

No data have been submitted previously in this

paper on the extent of hydration of the MgO in

samples taken shortly after the white coat has

been appHed. Table 7, however, contains data

for six samples of freshly apphed white coat—three

prepared from regularly hydrated dolomitic hmes

and three from the new highly hydrated dolomitic

limes (autoclaved products).

Samples 1 and 3 pertain to white-coat plasters

that were prepared from regularly hydrated limes

that had been soaked 1 day before application.

Analysis of these plaster samples showed that the extent of hydration of the MgO was close to 20

percent and, therefore, in agreement with the per- centage that should have been hydrated according

to values shown by Wells and Taylor [2] and in

unpublished data f25] for lime putties which have

been aged for 1 day.

It was known that the regularly hydrated

dolomitic lime used in preparing white-coat

sample 2 had been soaked 3 days before plastering.

According to these previous studies of the extent

of hydration of MgO in putties on aging, about 30

percent of the magnesia should have been hydrated. The analysis of the hardened white coat showed that this is the extent actually found.

Likewise, it was known that highly hydrated

dolomitic limes had been used in preparing the

white-coat plasters represented by samples 4, 5, and 6. Consequently, the percentage hydration

of the MgO should be high. Analysis of the set and hardened plaster samples, taken shortly after the white coat had been applied, showed this to be true. Figure 26 shows the heating curves of the six samples of freshly applied plaster. The samples

bear the same numbers in the figure as in table 7. Note that the heat effect corresponding to the

decomposition of Mg(0H)2 is distinctly larger for samples 4, 5, and 6, those containing highly hydrated dolomitic limes, than for samples 1, 2, and

3 in which regularly hydrated dolomitic limes were

used. The reader's attention is invited to the

fact that in the heating curves of these freshly

applied plasters, the two separate breaks for the

decomposition of gypsum and of hemihydrate are

less well developed than in the curves for the

older failed samples shown in figure 19.

Table 7. Chemical analysis; calculated compound composition; degree of hydration of the magnesia, together with the heats of solution (observed and calculated) ; percentage of magnesia in the limes; ratio of lime putty to gaging; and extent of car- honation of the Ca{0H)2 in freshly applied white coats '

Loss on ig- nition at

Chemical analysis (oxide composition)

O+

o

St; O GJ O ^

o o-s ^ 9 S o

Calculated compound composition

CaSO(.2H20

from

o ^

So

J3 tuO

Heat of solution

Ratio: lime putty CaS04.!^H!0

(by volume)

SERIES A. REGULARLY HYDRATED DOLOMITIC LIMES

% % % % % % % % % % % % % % % callg callg %

1 9.1 21.2.9 18.2 0.7 39.9 17.1 21.2 24.8 43.5 45.fi 6.6 28.2 5.8 13.1 « 23. 6 258 251 40.6 2.0 15 2 10.0 22.0 3.7 18.3 .8 38.9 15.7 23- 2 19.

47.8 49.9 8.4 23.7 6.8 11.0 b 29.9 215 218 40.8 1.6 21 3 .5.7 23.0 8.

14.9 .5 41.0 22.5 13.3 23.0 27.2 28.6 18.4 28.2 6.7 17.9 a 20.4 304 311 41.5 4.0 33

SERIES B. HIGHLY HYDRATED DOLOMITIC LIMES

4 6.1 25.7 .3.1 22.6 1.4 39.0 20.5 13.5 32.7 29.1 29.0 7.1 33.8 26.9 1.9 = "90.6 283 278 41.0 3.6 13

5 7.7 31.6 15.4 16. 2 . 7 35.0 16.0 16.8 2.2 36.8 36.1 35.0 4.8 23. 2 •0.0 1 100.

141 136 40.8 2.3 <i85 6 7.2 27.1 5.5 21.6 .5 37.9 18.6 15.8 22.6 34.4 34.0 12.5 26.2 26.3 .4 "97.8 238 231 40.9 2.8 26

» Hydrated lime soaked 1 day as putty before being used.

•> Hydrated lime soaked 3 days as putty before being used.

« Hydration of MgO to 90.6 percent corresponds to 96.2-percent hydration

for the lime.

^ Sanded gypsum base coat applied over bituminous coating on outside

wall, with result that the plaster remained damp for some time and conse- quently was rather weak. Dampness also probably accounts for high per- centage of carbonation of Ca(0H)2. Sample taken 6 weeks after plastering.

» Contains in addition 0.3 percent of excess H2O.

It should be apparent from these studies that

it is now possible to determine from an analysis

of the hardened white coat whether a regularly

hydrated or a highly hydrated dolomitic lime was used on the job.

It has often been suggested that after slight

additional aging beyond the customary 1-day

soaking, regularly hydrated dolomitic lime would

perhaps be satisfactory for the white coat. AIthougla the data of Wells and Taylor [2] indicated

38
T I I

} \ 1 r

FiGUHE 26. Differential heating curves of six freshly

applied white coats. The chemical analyses are given in table 7. The five major regions of heat

absorption in the order of increasing temperature are A, dehydration of gyp- sum, B, dehydration of hemihydrate, C, dehydration of magnesium hydroxide, D, dehydration of calcium hydroxide, and E, decomposition of calcium

carbonate.

that several months might be required to hydrate

the MgO completely, nevertheless, the data per- tained to but six limes, and there is the possibility

that recent technical advances in the lime industry

may have since produced more reactive hydrates.

Consequently, an investigation, previously mentioned, was undertaken at the Bureau to study

the rate of hydration of the present-day regularly

hydrated dolomitic limes and to ascertain the

time required to age such limes as putties so that the percentages of expansion of cement-lime

bars would be reduced to 1 percent or less [25]. Eighteen representative limes of tlie regularly

hydrated dolomitic type were chosen for that

study. A few of the findings of that investigation

have been mentioned in this paper. A complete

report of that work is being prepared for publication. Nevertheless, for the beai'ing which those

findings have on this problem of preventing whitecoat-plaster failures in the future, some of the

salient factors should be presented at this time.

As has already been pointed out, roughly but 20,

30, and 40 percent of the MgO in the regularly

hydrated dolomitic finishing limes is hydrated at the end of soaking periods of 1, 3, and 7 days,

respectively. The average expansions of the

cement-lime bars were 11.0, 10.6, and 9.7 percent,

respectively, at these time periods. The shortest

time required for aging the putties in order to reduce the expansion to 1 percent was 3 weeks,

and for three of the limes the expansion exceeded

1 percent, even after 32 weeks of soaldng. It should be evident, therefore, that it would be

most difficult to specify the time of aging required

to hydrate satisfactorily the magnesia in the

ordinary dolomitic finishing lime. To include all of the slower hydrating products would require a soaking period of at least 7 to 8 months. But

whether the period was 3 weeks or 7 to 8 months,

the time would be altogether too long from a

practical standpoint.

IX. Methods of Repairing Damaged

Plaster

The cause of white-coat-plaster failures and the

precautions that should be exercised in the erection

of new buildings to prevent such failures have been

discussed. Two questions have often been presented to the Bm-eau: (1) Is there any known

method of stopping or minimizing destructive

expansion if a defective lime has been used in

preparing the white coat? and (2) what are the

best methods of repairing the damage? The

answers to these questions are still matters of

conjectm'e.

Frank Mm-phy, of the Krafft-Murphy Co.

(plastering contractors of Washington, D. C.) has

conducted some limited experiments on potentially

defective areas by applying thereon two or more

coats of aluminum paint made with a spar-varnish

vehicle. Such a coating may act as a sufficiently

effective vapor barrier to prevent the cotidensation of much moistm-e within the plaster dm^ing

periods of high temperatui'e and high relative

humidity. To date, his experiments seem en- couraging, but several seasons must elapse before

the eftectiveness of this method can be evaluated.

More extensive experiments are needed.

It is often difficult to decide upon the best procedure to follow when faced with the problem of

repau'ing defective white coat. Usually the o\\aier

39
Figure 27. A ceiling in an office building where whitecoat plaster failures are of frequent occurrence and where

repatching has been going on for years. The loose white coat has been scaled off, revealing the sound condition of the sanded base coat.

or building manager employs a plasterer to scale

off the loose white-coat plaster and patch the

damaged places. This plastering operation, necessitates redecorating the wall or ceiling involved,

thus adding to the cost of the repair. Almost

invariably the owner will discover that while the

fresh patch may stay on, other old areas of white

coat will break loose, until in many cases there will

be numerous patches in the same ceiling or side wall. Each of the plaster repairs will entail redecorating,

with resultant additional cost. Figure 27 shows a ceiling in an office building

where white-coat-plaster failmes are of frequent occurrence and where repatching has been going on

for years. The bulged and loose white coat has

been scaled off. Note the sound condition of the

sanded base coat. The patches in this building

have now become so extensive that they cover from one-half to two-thirds of the area of the

plastered surfaces. The old patches are now

showing signs of failure characterized by the

typical bulges and loosening of the white coat. Unsound regularly hydrated dolomitic lime, with

its high percentage of unhydrated MgO, was used

in the original white coat and all subsequent patching. After spending thousands of dollars, the owners have little to show for their efforts. The

walls and ceilings are a network of patches, and

the entu-e surface is still potentially unsound.

Figure 28 shows a plastered ceiling w'th an ex- tensive area that had been patched because of the

loosening of the white coat. But, more important,

it shows additional white-coat-plaster failures that

occurred subsequent to the patching but before the

painter got around to redecorating the ceiling.

Faced with the continuous reappearance of bulged

and defective areas, it was finally decided that the

Figure 28. An extensive area of patched white coat and

additional areas of delaminated white coat that occurred subsequent to patching.

best policy in this parochial school would be to cover the ceilings with some material that would

not undergo excessive expansion with seasonal

changes. Hence, the selection of pressed metal as a ceiling material. Note in figure 29, however,

that bulges are still appearing in the white coat on

the concrete beams.

Figure 29. Faced with the condition illustrated in figure

28, the ceilings in this parochial school are being replaced with pressed metal. Note that plaster failures arc still occurring on the concrete beams.

40
I It should be obvious, therefore, that owners and

building managers are continuously faced with the

] problem of whether it is more economical to patch

I and repatch or to remove the entire white coat.

It should be equally obvious that the probability

of future trouble is not eliminated if the regularly

hydrated type of dolomitic lime, with its high per- centage of free MgO, is used in making the repairs.

Another problem is the selection of the best

method of removing the white coat without unduly damaging the base coat. Some plasterers

prefer to heat the white coat with a blow torch;

others find it easier to saturate the white coat with

a fine spray of water. The water hastens the hydration of MgO and brings about an expansion that

loosens the white coat from the underlying base

coat. In either case it is necessary to chop off certain areas with hatchets.

Only lime putty prepared from a highly hydrated

finishing hme or a completely slaked quicklime

! should be used for the new white coat,

j In many instances it has been found to be more

! advantageous and economical not to attempt to remove the defective white coat but rather to cover

the ceilings with wall board or acoustic tile. As

it would be unwise to attach these materials

directly to unsound white coat, it is necessary to

nail furring strips' to the plaster backing and then

to attach the new covering to these strips.

X. Summary and Conclusions

As a result of this survey and investigation of

white-coat-plaster failures, the following state- ments and conclusions are justified:

1. Extensive surveys showed that plaster fail- ures characterized by the formation of bulges (large

blisters) in the white coat with its delamination

are widespread where ordinary hydrated dolomitic

finishing lime has been used in preparing the white

coat.

2. Several years (from about 5 to 15, but

usually 8 to 9) elapse before the blisters, or bulges,

appear, and thereafter the number and extent of

failures increase as the age of the buildings

increases.

3. The failures occur more extensively and

rapidly during the warm weather of summer and

during periods of high relative humidity.

4. There is a marked similarity in the failures,

regardless of the kind of plaster base coats, construction bases, job conditions, and type of workmanship.

5. The usual white coat is prepared from a lime

putty gaged with plaster of paris (gypsum gaging

plaster). Lime putty is prepared on the job

either by slaking quicklime with an excess of water,

or by soaking a dry commercial finishing hydrated

lime with water. The tendency is very strongly

in favor of the hydrate because of its greater

convenience.

6. Nearly all finishing hydrated limes are made

from a dolomitic limestone peculiar to a small

district in northwestern Ohio, because such

hydrated limes yield plastic putties desired by the

plasterer.

7. In the lime-burning of dolomite (MgCOs.-

CaCOs), the magnesium carbonate is decomposed

into MgO-|-C02 long before the calcium carbonate

is completely decomposed. Thus the MgO be- comes overburned and its reactivity toward water

is greatly reduced.

8. Only 55 to 65 percent of the water required

for complete hydration of regularly hydrated

dolomitic finishing lime is present, because of the

slowness of hydration of the badly overburned

MgO in the normal hydration process.

9. The calcium oxide of hydrated dolomitic

limes is generally completely hydrated, but only

a relatively small percentage of the magnesia is hydrated.

10. The 12- to 24-hour soaking hydrates but

little of the large percentage of unhydrated MgO;

thus, about 75 to 80 percent remains unhydrated

when the white coat is applied.

11. The presence of CaS04.2H20, Mg(0H)2,

Ca(0H)2, and CaCOs in the white coat was con- firmed by differential thermal analysis.

12. The observed heats of solution of the whitecoat plasters agree closely with those calculated

from the computed compound composition. This

agreement is strong evidence of the correctness of

the assumptions upon which are based the calculations of the constitution of the plasters.

13. The average amount of hydration of the

MgO for 88 samples of failed white coat was 59

percent. This is much in excess of the average

20 percent found after the customary 1 day of

soaking of the lime as a putty. Hydration of

magnesia must, therefore, have occurred on the

wall.

14. In every instance of failure, a dolomitic

lime had been used in preparing the white coat. That is, in not a single instance had a highcalcium lime been used.

15. Plaster failures from which the 88 samples were taken occurred with volume ratios of lime

putty to calcined gypsum (gaging plaster) as low

as 0.8 and as high as 9.8. The average ratio was 3.9. 16. The extent of carbonation of the lime in the

white coat at the time of failm-e ranged from 9 to 100 percent, and the mean was 60 percent.

17. Samples of white coat expanded markedly

on steaming in an autoclave for one hour at a

pressure of 120 Ib/in.^ when the MgO was con- verted to Mg (0H)2, with the attending increase

in volume.

18. Although the investigation was confined

primarily to white-coat plaster, nevertheless,

failures were also observed in base coats m which

dolomitic lime containing considerable miliydrated

MgO was used.

19. Proposed explanations of the cause of

failure of white-coat plaster, other than hydration

of magnesia, were given consideration. They

41
included the drying shrinkage of concrete, dif- ferences in thermal expansion between the backing

and the various coats of plaster, poor workmanship, improper carbon ation, improper gaging,

painting plaster before it is sufficiently dry, paint

shrinkage, vibration, and movements in the

structmal frame. It was shown that these

factors are inadequate to account for the bulges

in the plaster that occur several years after a

building has been erected.

20. It was concluded that the one factor offer- ing a reasonable explanation for the failm"es is the

gradual expansion attending the hydration of

free MgO in the set and hardened white coat.

21. The precautions that should be exercised

in specifying the proper lime in order to prevent

failures in new buddings are discussed.

22. Some of the methods that have been used

in repairing damaged plaster are discussed.

The authors express thanks and appreciation to E. P. Flmt for analyzing many of the samples of faded white coat, to H. F. McMurdie for obtaining

X-ray diffraction patterns of the samples, and to

S. W. Benedict and J. M. Cameron for their

statistical analysis of the coiTelation between

the observed and calculated heats of solution of the 88 samples of failed white-coat plaster. The

authors are indebted to Frank Murphy for his

great interest in this mvestigation and for the

many hours of his time that he so wdhngly gave

in the surveys in Wasliington, D. C, and Baltimore, Md.

XI. References

[1] E. D. Raht, Remedy for falling plaster, from section "Letters to the Editor", Building and Building

Management 39, No. 11, p. 44 (October 1939).

[2] Lansing S. Wells and Kenneth Taylor, Hydration of magnesia in dolomitic hydrated limes and putties,

J. Research NBS 19, 215 (1937) RP1022.

[3] Federal Specification SS-Q-351 for cjuicklime; (for) structural purposes (1930).

[4] Standard specifications for quicklime for structural purposes. ASTM Designation: C5-26, ASTM

Standards, part II, 90 (1946).

[5] Federal Specification SS-L-351 for lime; hydrated

(for) structural purposes (1930).

[6] Tentative specifications for normal finishing hydrated

lime. ASTM Designation: C6-46T, ASTM Standards, part II, 1304 (1946).

[7] Tentative specifications for special finishing hydrated

lime. ASTM Designation: C206-46T, ASTM

Standards, part II, 1306 (1946).

[8] Tentative specifications for hydrated lime for masonry

purposes. ASTM Designation: C207-46T, ASTM

Standards, part II, 1308 (1946).

[9] K. K. Kelley and C. T. Anderson, Contributions to the data on theoretical metallurgy, IV. Metal car- bonates—Correlations and applications of thermodynamic properties, U. S. Bureau of Mines Bui. 384

(1936).

[10] J. Johnston, The thermal dissociation of calcium car- bonate, J. Am. Chem. Soc. 32, 938 (1910).

[11] F. H. Smyth and L. H. Adams, The system, calcium

oxide-carbon dioxide, J. Am. Chem. Soc. 45, 1167

(1923).

[12] W. Dutoit, Tensions de dissociation des carbonates de

calcium, de strontium et de barium, J. chim. phys.

24, 110 (1927)... [13] L. Andrussow, Uber die thermische Dissoziation des Calciumkarbonats und die Differentialmethode, Z. phy. Chem. 116, 81 (1925).

[14] Karl v. Kohner, Ueber die Dissoziationsspannung des Calciumcarbonates, Inaugural-Dissertation, Friedrich- Wilhelms-Universitat zu Berlin, 1914.

[15] S. Tamaru, K. Siomi, and M. Adati, Neubestimmung

thermischer Dissoziationsgleichgewichte von anor- ganischen Verbindungen. I., Z. phys. Chem. ^-157,

447 (1931).

[16] 0. C. Ralston, R. D. Pike, and L. H. Duschak, Plastic magnesia, U. S. Bureau of Mines Bui. 236 (1925).

[17] Sidney Speil, Louis H. Berkelhamer, Joseph A. Pask,

and Ben Davies, Differential thermal analysis. Its application to clays and other aluminous minerals, U. S. Bureau of Mines Tech. Paper .664 (1945).

[18] Edward DeMille Campbell, On the influence of the temperature of burning on the rate of hydration of magnesium oxide, Ind. Eng. Chem. 1, 665 (1909);

10, 595 (1918).

[19] D. F. Richardson, Analysis of hydrated lime by a thermochemical method, Ind. Eng. Chem. 19, 625

(1927).

[20] E. L. Fox and F. C. Mathers, Determination of the degree of hydration of magnesia in dolomitic limes, Proc. Indiana Acad. Sci. 43, 113 (1934).

[21] Lansing S. Wells, Walter F. Clarke, and Ernest M.

Levin, Expansive characteristffcs of hydrated limes and the development of an autoclave test for soundness, J. Research NBS 41, 179 (1948) RP1917.

[22] Proposed amendment to Federal Specification SS-L351 for lime; hydrated (for) structural purposes. Date of amendment, Feb. 2, 1940.

[23] Tentative revision of standard methods of chemical

analysis of limestone, quicklime, and hydrated lime. ASTM Designation: C25-44, submitted June 1945;

see ASTM Standards, part II, 1701 (1946).

[24] Standard specifications for gypsum plastering; includ- ing requirements for lathing and furring. Am.

Standards Assn. A42. 1-1946.

[25] Unpublished data.

[26] Dana L. Bishop, Function of carbon dioxide in pro- ducing efflorescence on plaster and cement products,

J. Research NBS 30, 361 (1943) RP1538.

[27] Federal Specification SS-P-402 for plaster; gypsum

(1945).

[28] Standard methods of testing gypsum and gypsum

products. ASTM Designation: C26-42, ASTM

Standards, part II, 149 (1946).

[29] William Lerch and R. H. Bogue, Revised procedure

for the determination of uncombined lime in port- land cement, Ind. Eng. Chem., analytical edition,

2, 296 (1930).

[30] E. S. Newman, Investigation of plaster failure, Am.

Ceramic Soc. Bui. 26, No. 4, 117-118 (L947).

[31] Edwin S. Newman and Lansing S. Wells, Heats of hydration and transition of calcium sulfate, J. Research NBS 20, 825 (1938) RP1107.

[32] "H. F. McMurdie and F. L. Marsh, Volume changes

of gvpsum fiber concrete, Rock Products 35, 20 (March 26, 1932).

Washington, June 19, 1950.

42 U. S. GOVERNMENT PRINTING OFFICE! 19S1

I

I

1
BUILDING MATERIALS AND STRUCTURES REPORTS

[Continued from cover page ii] BMS32 Structural Properties of Two Brick-Concrete-Block Wall Constructions and a Concrete-Block Wall Construction Sponsored by the National Concrete MasonryAssociation 15{S

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Sponsored by the Munlock Engineering Co 100 BMS54 Effect of Soot on the Rating of an Oil-Fired Heating Boiler 100 BMS55 Effects of Wetting and Drying on the Permeability of Masonry Walls 100 BMS56 A Survey of Humidities in Residences 100 BMS57 Roofing in the United States—Results of a Questionnaire * BMS58 Strength of Soft-Soldered Joints in Copper Tubing 100 BMS59 Properties of Adhesives for Floor Coverings 150 BMS60 Strength, Absorption, and Resistance to Laboratory Freezing and Thawing of Building

Bricks Produced in the United States 300 BMS61 Structural Properties of Two Nonreinforced Monolithic Concrete Wall Constructions.- 100 BMS62 Structural Properties of a Precast Joist Concrete Floor Construction Sponsored by

the Portland Cement Association 100 BMS63 Moisture Condensation in Building Walls 150 BMS64 Solar Heating of Various Surfaces 100 BMS65 Methods of Estimating Loads in Plumbing Systems 150 BMS66 Plumbing Manual 350 BMS67 Structural Properties of "Mu-Steel" Prefabricated Sheet-Steel Constructions for Walls, Partitions, Floors, and Roofs, Sponsored by Herman A. Mugler 150 BMS68 Performance Test for Floor Coverings for Use in Low-Cost Housing: Part 3 200 BMS69 Stability of Fiber Sheathing Boards as Determined by Accelerated Aging 100 BMS70 Asphalt-Prepared Roll Roofings and Shingles 200 BMS71 Fire Tests of Wood- and Metal-Framed Partitions 200 BMS72 Structural Properties of "Precision-Built. Jr." Prefabricated Wood-Frame Wall

Construction Sponsored by the Homasote Co 100 BMS73 Indentation Characteristics of Floor Coverings 100 BMS74 Structural and Heat-Transfer Properties of "U. S. S. Panelbilt" Prefabricated SheetSteel Constructions for Walls, Partitions, and Roofs Sponsored bv the Tennessee

Coal, Iron & Railroad Co 1 200

BMS75 Survey of Roofing Materials in the North Central States 150 BMS76 Effect of Outdoor Exposure on the Water Permeability of Masonry Walls 150 BMS77 Properties and Performance of Fiber Tile Boards 100 BMS78 Structural, Heat-Transfer, and Water-Permeability Properties of Five Earth-Wall

Constructions 250 BMS79 Water-Distributing Systems for Buildings 200 BMS80 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 4 150 BMS81 Field Inspectors' Check List for Building Constructions (cloth cover, 5 x 7}^ inches) 300

•Out of print. [List continued on cover page iv]
BUILDING MATEKIALS AND STRUCTURES REPORTS

[Continued from cover page in] BMS82 Water Permeability of Walls Built of Masonry Units 25^

BMS83 Strength of Sleeve Joints in Copper Tubing Made With Various Lead-Base Solders 15)6 BMS84 Survey of Roofing Materials in the South Central States 15^ BMS85 Dimensional Changes of Floor Coverings With Changes in Relative Humidity and

Temperature lOji BMS86 Structural, Heat-Transfer, and Water-Permeability Properties of "Speedbrik" Wall

Construction Sponsored by the General Shale Products Corporation 15{5

BMS87 A Method for Developing Specifications for Building Construction—Report of Subcommittee on Specifications of the Central Housing Committee on Research,

Design, and Construction 15(t BMS88 Recommended Building Code Requirements for New Dwelling Construction With

Special Reference to War Housing * BMS89 Structural Properties of "Precision-Built, Jr." (Second Construction) Prefabricated

Wood-Frame Wall Construction Sponsored by the Homasote Co 15(i BMS90 Structural Properties of "PHC" Prefabricated Wood-Frame Constructions for Walls,

Floors, and Roofs Sponsored by the PHC Housing Corporation 15^ BMS91 A Glossary of Housing Terms 15^ BMS92 Fire-Resistance Classifications of Building Constructions 30^ BMS93 Accumulation of Moisture in Walls of Frame Construction During Winter Exposure 10^ BMS94 Water Permeability and Weathering Resistance of Stucco-Faced, Gunite-Faced, and

"Knap Concrete-Unit" Walls 15^ BMS95 Tests of Cement-Water Paints and Other Waterproofings for Unit-Masonry Walls 25^ BMS96 Properties of a Porous Concrete of Cement and Uniform-Sized Gravel 10^ BMS97 Experimental Dry-Wall Construction With Fiber Insulating Board 10^ BMS98 Physical Properties of Terrazzo Aggregates 15^ BMS99 Structural and Heat-Transfer Properties of "Multiple Box-Girder Plywood Panels" for Walls, Floors, and Roofs 15(4 BMSlOO Relative Slipperiness of Floor and Deck Surfaces 10^ BMSlOl Strength and Resistance to Corrosion of Ties for Cavity Walls 10^ BMS102 Painting Steel 10»5 BMS103 Measurements of Heat Losses From Slab Floors 15^ BMS104 Structural Properties of Prefabricated Plywood Lightweight Constructions for Walls,

Partitions, Floors, and Roofs Sponsored by the Douglas Fir Plywood Association 30^ BMS105 Paint Manual with particular reference to Federal Specifications $1. 25

BMS106 Laboratory Observations of Condensation in Wall Specimens * B MS107 Building Code Requirements for New Dwelling Construction * BMS108 Temperature Distribution in a Test Bungalow With Various Heating Devices lOfS BMS109 Strength of Houses: Application of Engineering Principles to Structural Design $1. 50 BMSllO Paints for Exterior Masonry Walls 15ji BMSlll Performance of a Coal-Fired Boiler Converted to Oil 15^ BMS112 Properties of Some Lightweight-Aggregate Concretes With and Without an Air- entraining Admixture 10f5 BMS113 Fire Resistance of Structural Clay Tile Partitions 15jS BMS114 Temperature in a Test Bungalow With Some Radiant and Jacketed Space Heaters 25^ BMS115 A Study of a Baseboard Convector Heating System in a Test Bungalow 15^ BMS116 Preparation and Revision of Building Codes 15fi BMS117 Fire Resistance of Walls of Lightweight Aggregate Concrete Masonry Units 20^ BMS118 Stack Venting of Plumbing Fixtures 15j5 BMS119 Wet Venting of Plumbing Fixtures 20f5 BMS120 Fire Resistance of Walls of Gravel-Aggregate Concrete Masonry Units in press BMS121 Investigation of Failures of White-Coat Plasters 25^

•Out of print.