Hydration of Gypsum Plaster W. -4.CUR‘I\;IhiGHARI, R. RI. DUNHARII, AND L. L. ANTES The Cnicei*sity of Texas, Austin, Tex.
T
HOUGH there is some reason to believe otherwise, it is genbeen postulated, only the hemihydrate has had its identity established with any degree of certainty. Even the individuality erally considered that other than ice t’he solid phases of the of the hemihydrate as a definite compound has been questioned. system calcium sulfate-water are the dihydrate or gypsum ( CaS04.2H20), the hemihydrate, coininonly called plaster of Dunn (8) proposed a series of subhydrates ranging in water Paris or gypsum plaster ( 2CaS04.H20), and anhydrite (CaSO1). content from 3CaS04.2H20 to the anhydrous salt, but x-ray diffraction analysis lends little support t o this theory. Both the dihydrate and anhydrite occur in nature; the dihydrate provides the raw material for the manufacture of gypsum plaster On the basis of x-ray diffraction studies, Linck and Jung (I,$), Faivre and Chaudron ( 9 ) , and others deduced a zeolitic strucand gypsum wallboard. The commercial partial dehydration of gypsum t o plaster of ture of hemihydrate, since they reported that the powder diffraction patterns of hemihydrate and soluble anhydrite were essenParis is usually accomplished by heating t’he lump or pulverized tially identical. rock in rotary kilns or cast iron or steel kettles to temperatures ranging from 100”to 200 O C.; the exact temperature used is deterOn the other hand, Weiser, blilligan, and Ekholm ( 2 7 ) denied the zeolitic character of the hemihydrate and concluded that mined by the nature and purity of the raw gypsum, the propertieE water detected in “partially dehydrated hemihydrate” IS due to desired in the finished plaster, and plant economics. the adsorption of atmospheric moisture by completely dehydrated Commercial plasters are generally conceded to consist of soluble anhydrite. hemihydrate cont,aining a relatively small amount of an anhydrous There is, however, considerable justification in assuming two modification, spoken of as soluble anhydrite or, sometimes, deforms of hemihydrate. Kelley, Southard, and Anderson (19) hydrated hemihydrate. recognize “alpha hemihydrate,” produced by dehydration of Posnjak ( 1 8 ) states that the term “soluble anhydrite” is a gypsum by wet methods, and “beta hemihydrate,” obtained by misnomer and suggests the use of the term -y.CaSO, which, he dehydration of gypsum in vacuum or in the absence cf moisture. says, is a definite chemical compound which takes up moisture The alpha modification is distinguished by higher stability, lower readily to form 2CaS04.Hs0. The hemihydrate formed from gypsum at 9 7 . 5 ” i l o C., which is the only dissociation product solubility, and a lower energy content. However, the two forms have never been differentiated by x-ray diffraction. The hemiof decomposition, is transformed to -y.CaSO, a t much higher temhydrate of commercial plaster is of the beta form. prraturee. By boiling gypsum in water for several hours Davis ( 6 )obtained MECHAYISM OF HYDRATION a material which analysis showed to be 2CaS01.Hz0 (6.2% The oldest and most widely accepted theory of the mechanism water) but which lac,ked satisfactory setting properties. IIoppeof the hydration or setting of plaster of Paris is generally credited Seyler ( 1 1 ) , Le Chatelier ( I S ) , Van’t Hoff (26), and others earlier to Lavoisier, who proposed it in 1765 ( 7 ) . It assumes the soluhad prepared mat,erials corresponding in water content with of the lesstion of the hemihydrate and subsequent Drecipitation hemihydrate by heating gypsum in \vat,er and in solutions of ~soluble needlelike various acids and dihydratc crystals. salts Some of thew B C A These later develop materials had hyinradiating growths draulic propertics; from many adjacent others apparently centers, and the inhad not or were unterlocking of the tested, Excellently spines or needles setting, highaccounts a t least in strength plaPtrre part for the strength can be produced by of the set mass ( I S ) . dehydration of gypLe Chatelier’s sum in an atmostheory was considphere oi saturated ered to be satissteam (19) or in factory for well over high concenlrationk a quarter of a cenof magnesium sultury, but in more fate ( 2 1 ) The recent years some plaster produced by investigators have the latter process regarded it as inadeconsists of definite quate to explain all crystals of hemithe phenomena athydrate. tending the setting Of the lower hyprocess The alterdrates of calcium Each part shows same field at successive stage8 native theory most sulfate which have A. Initial stage B. After 5 minutes profrequently C. After 10 minutes 1 Present address, posed has postuD. After 15 minutes Stanolind Oil and Gas E. After 30 minutes lated that, in the F. After 45 minutes, final stage Co., Ulysses, Kan. 2402
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ammonium acetate increases the solubility of plaster t o a greater extent than does ammonium sulfate, the former retards and the latter accelerates setting. Weiser and Moreland (28) explained the accelerating and retarding effects of electrolytes through the application of von Weimarn’s theory of precipitation from supersaturated solution t o estimate the velocity of initial nucleation and of the Nernst-Noyes equation to predict the velocity of crystal growth. The fact that some substances, such as glue, gelatin, borax, citric acid, and ammonium acetate, retard setting was explained on the assumption t h a t these materials are absorbed upon the initially formed gypsum nuclei and thus inhibit crystal growth. The exact reasons for the structural strength of plaster are unknown. Because of the softness (number 2 on the Moh hardness scale) of gypsum and set plaster, preparation of thin sections for microscopic examinations of the internal structure is extremely difficult. Visual examination is futher complicated by lack of contrast between the different crystal forms. Hence it is not surprising that Tavasci ( 8 4 )observed no typical dihydrate needles in the mass and compared the Eurface of set plaster with the compact crystals formed upon solidification of a molten metal. On the other hand, Stratta ( 2 3 ) maintained t h a t the crystals in plaster-sand mortar are always needle C D shaped when the set is complete. Assuming that Figure - 2. Crystal Forms Occurring in Normal Hydration the crystals were needle shaped, Haddon ( I O ) A, B. Less t h a n 5 m i n u t e s after mixing plaster a n d water C. 10 m i n u t e s after mixing postulated an intercrystalline film similar to that D. 1 hour after mixing phenomenon in metals which causes adhesion between adjacent crystals and binds them into a coherent mass. Direct proof of his theory is lacking, and Le hydration of plaster, an intermediate gel stage occurs at some time Chatelier’s supposition that crystal interlocking is the basis of between the solution of the hemihydrate and the precipitation of plaster strength is largely accepted. thedihydrate. The colloidal theory is based principally on: the formation under special conditions of a gel-like material supposed to be a gypsum gel; the effects on setting of the addition of various electrolytes and nonelectrolytes; measurement of the change of viscosity with time during the course of the setting process; and measurement of temperature changes and expansion-contraction phenomena during setting. Chief advocates of the colloid theory have been Cavazzi ( 3 ) ,Baykov ( 2 ) ,Traube ( 2 5 ) , Ostwald and Wolski ( I 7 ) ,and Neville (15, 16). Both Cavazzi and Baykov prepared and described materials supposed t o be gypsum gels. Cavazzi produced a “gel” by the addition of ethyl alcohol t o a saturated solution of plaster of Paris and on this evidence inferred that the formation of an unstable gel was the first step in plaster hydration. Baykov formed a A B gelatinous mass, which was said to resemble silica gel, by shaking Figure 3. Electron Micrographs of Uncalcined Gypsum a suspension of plaster in ammonium sulfate solution. H e beand Unhydrated Plaster of Paris lieved that a similar gel was an intermediate step in the setting A. Uncalcined gypsum B. Unhydrated plaster of Paris of plaster. Chassevant ( 4 ) investigated thermodynamic effects in plaster EXPERIMENTAL PROCEDURE hydration and found no evidence of gel formation. Weiser and The investigations reported herein invoived the observation Moreland (28)agreed with Chassevant that the initial period of inhibition of hydration was caused by lack of gypsum nuclei of specially prepared specimens of gypsum plaster during hydraupon which gypsum crystals could form and not by any intermedition under controlled conditions and of set masses of the plaster ate gel stage. after hydration was complete. The time required for the setting of plaster can be controlled The plaster of Paris used was made by calcining a t 105” to Vy the use of additives which increase or retard the rate of hydra110’ C., for 36 to 48 hours C.P. quality calcium sulfate dihydrate, which had the following analysis: tion. Rohland ( 2 0 ) observed the effect of adding various soluble salts t o setting plaster and concluded that those salts which inChloride % 0.005 Mg and hkalies, % ’ 0.20 crease the solubility of calcium sulfate accelerate the hydration 0.001 Iron (Fe) % Other hedvy metals, % 0.000 and t h a t those salts which decrease the solubility have a retarding action. Through generally true, there are some notable After calcination, the plaster was allowed to stand exposed to atmospheric moisture for several hours t o ensure the conversion exceptions. For example, Haddon ( I O ) pointed out that although
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B
A
C
Each part shows the s a m e field at successive stages A. B. C.
Initial stage After 5 m i n u t e s After 10 m i n u t e s
D. After 15 m i n u t e s E. After 20 m i n u t e s F. After 25 m i n u t e s
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the petrographic microscope immediately after mixing and subsequently after 5 , 10, 15, 30, and 45 minutes. Photomicrographs are shown in Figure 1. The plaster went into solution rather slowly, and the first needlelike crystals of gypsum were visible T or 8 minutes after mixing (Figure l a through c). Blthough the hydration was essentially complete at the end of 45 minutes, some tiny plaster particles were still undissolved (Figure If). Figure 2 shows electron micrographs of normal hydrated plaster, samples of which were prepared by shaking 1 gram of plaster in 15 ml. of distilled water. Specimens were transferred t o the electron microscope screen by the wire-loop method previously described. Characteristic forms of crystals were present, needles and platelet,. predominating with occasional mallowtail twins occurring. The swallowtail twins were more common in specimens taken from freshlv mixed sam-
which, in this case, were obtained by evaporation of the thin layer of water on the screen.
of any soluble anhydrite to hemihydrate. The plaster was then screened through a No. 200 NBS (0.074 mm.) sieve and placed in tightly closed bottles for future use. For photomicrographic study with a light microscope, equipment of conventional design was used. It consisted of an Xgfa viewback camera, mounted on a n adjustable support above a Leitz petrographic microscope, with its optic lens serving also as the camera lens. The light source was a ribbon-filament tungsten lamp with a condenser lens. Plane-polarized light was used in making the photographs. The specimens photographed by aid of the light microscope were prepared by placing the prepared plaster with distilled water in t h e concave depression of a special microscope slide. The cover glass was then placed over the depression in such a way as t o exclude air bubbles and wa8 then sealed t o the slide with paraffin wax t o prevent evaporation. The specimen was then ready for immediate examination. The electron microscope used was an R.C.A. Universal Model EMU-1. I n use, this instrument requires t h a t the specimen be observed in a vacuum of 0.1 micron or better; hence, it could not be used t o study the progress of plaster hydration directly. A cell has been described ( 1 ) which might conceivably have been used, but difficulties of construction and doubtful value of the results discouraged any attempt to use it. Therefore the electron microscope studies were limited to those products which could be observed in the absence of volatile material. It was possible t o use the instrument only because of the prolonged induction of gypsum dehydration in a vacuum. The normal techniques used in sample preparation were relatively simple. A Parlodion film supported on a 200-mesh stainless steel screen was prepared by the usual methods. The material t o be observed was placed on the film in such a way as to give a suitable dispersion; for material in liquid suspension a technique similar t o that described by Sliepcevich, Gildart, and Katz ( 6 2 ) was empldyed. The suspended material was dipped from its container with a */B-inch loop of platinum wire and the adhering droplet placed on the prepared Parlodion film; dry material, such as unhydrated hemihydrate, was dispersed on the film by electrostatic forces produced by a Tesla coil Special techniques needed for some of the specimens are described later. GENERAL OBSERVATIONS
NORMAL HYDRATION. The progress of the hydration of pure calcined plaster in distilled water waq photographed by use of
Figure 5. Forms Found in Hydrated I'Iaster Accelerated with Sodium Chloride (15,200 X)
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F
Figure 6. Plaster Hydration with Bristle Retarder (160 X ) A. B. C.
Each part shows the same field at successive stages Initial stage D. After 40 minutes After 15 minutes E. After 50 minutes After 30 minutes F. After 60 minutes G . After 90 minutes, final stage
The relatively large amount of irregularly shaped material in Figure lA, B, and C is probably undissolved plaster, since the solid hemihydrate was always added in excess of its solubility. The initial stages of the growth of gypsum crystals are clearly evident in Figure 1B. For general comparison, electron micrographs of the uncalcined gypsum and of the unhydrated plaster of Paris are shown in Figure 3A and 3B, respectively. Plaster hydration is, in general, ACCELERATED HYDRATION. accelerated by the addition of an electrolyte. Since sodium chloride is known t o exert a pronounced accelerating effect and since its cubic crystals would be readily distinguishable from those of either plaster or gypsum, it was selected for use in this study. Samples of dry plaster containing 0.1 weight yo of finely ground C.P. sodium chloride were prepared for observation in the manner previously described. Light micrographs of accelerated hydration were taken at 5-minute intervals and are presented in Figure 4. Five minutes after mixing, a relatively large amount of the plaster was dissolved and gypsum crystals had already
begun t o form. After 20 minutes (Figure 4E) the plaster was nearly all dissolved and gypsum crystals were still growing, whereas after 30 minutes of normal hydration a comparatively large amount of plaster was still visible (Figure 1E). Electron micrographs presented in Figure 5A and B show typical crystal formations observed in the accelerated specimens. All of the forms recognized in normal hydration were present with the addition of burrlike clusters, apparently composed of many tiny gypsum platelets. Only traces of material which might have been undissolved plaster were present
B
A
Figure 7 . Plaster Hydration Retarded with Bristle Retarder (7000 X) A.
5 minutes after mixing
B.
1 hour after mixing
Figure 8. Plaster Retarded with 0.1% Citric Acid (15,200X)
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amount than is the solubility of gypsum, L. Thus the per cent supersaturation (Q -L ) / L is increased, thereby causing an increase in gypsum nucleus formation. The Nernst-Noyes equation states that the rate of nucleus growth (of nuclei already formed) is proportional t o the absolute saturat,ion (Q-L). Since L is small and is increased less than & in a sodium chloride solution, giving (& - L ) a greater value, crystal growth will proceed more rapidly than in pure water and a larger number of small, imperfectly formed crystals will result. A study of Figures 1, 4, and 5 shows, a t least qualitatively, that the solubility of hemihydrate is visibly larger in water containing sodium chloride, gypsum crystals are somewhat smaller and of imperfect confirmation, and initial crystal growth occurs earlier. Taken collectively, the photomicrographic evidence directly establishes the validity of the explanation proposed by Weiser and Moreland for the accelerating action of electrolytes. RETARDED HYDRATION.A retarded plaster was prepared by adding 0.1 weight % of a commercial set retarder made from hog bristles t o the dry normal plaster, and its hydration characteristics were observed in the usual manner. Light micrographs showing a typical sequence of retarded hydration are presented in Figure 6. The inhibition period was increased to about 15 minutes, a t which time the first gypsum crystalliteE appeared; in marked contrast t o both nornial and accelerated hydration, undissolved plaster remaining after 60 minutes appeared to be from 50 to 75% of that initially present. Hydration was no6 quite complete 90 minutes after mixing. Electron micrographs of specimens taken 5 minutes after mixing show large quant,ities of nonuniform material which is probably undissol~edplaster (Figure 7A); much of this same material persists even nn hour after mixing (Figure 7B). Comparable dispersions of plaster containing 0.1 weight yo of citric acid, another retarder, showed appearances quite similar t o those containing the bristle retarder (Figure 8). As is indicated by the slow solution rates of the hemihydrate, it appears that large protein molecules of the bristle retarder probably were adsorbed onto the hemihydrate particles, reducing its solubility and lowering accordingly t'he relative supersaturation. This would have the effect' of reducing the velocity of nucleation and would give rise to slow formation of regularly ehaped crystals of gypsum. STRUCTURE O F SET PLASTER
Figure 9.
Surface of Set Plaster Mass (540X)
Lower photo taken after shadow-casting with a l u m i n u m
Other than the effects of mechanical interference, there is no reason t o believe that the gypsum crystals present in a solid mass of set plaster are any different from those obtained in a very dilute suspension of the hemihydrate in water such as described. Because of the softness of and lack of contrast, in set plaster, conventional techniques for the preparation of thin seetioils are unsatisfactory. If it can be assumed that the surface of a mass of set plaster exhibit's the same characteristic cryst,al forms and arrangement as does t'he internal portion of the mass, it shoud be possible to gain some information of value through studies of
It has been mentioned earlier that Weiser and Moreland regarded the acceleration of hydration by an electrolyte as a function of the velocity of nucleation, their conclusions being based upon von Weimarn's theory of precipitation
where TV is the velocity of initial precipitation, Q is the quantity of substance that is to precipitate, L is the solubility of the precipitate, and P is the absolute supersaturation. Rest,ated, this simply says that the initial velocity of nucleus formation is proportional to the relative supersat,uration. In the case of an electrolyte such as sodium chloride, it is known that the solubility of the hemihydrate, Q, is increased proportionately by a greatpr
B
A
Figure 10.
Gypsum Keedles and Swallow-tail Twins A.
15,OOOX
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the surface itself. Direct microscopic examination is of little help, b u t through the use of an adaptation and combination of polystyrene and collodion replica methods widely used in electron microscope surface studies, an accurate reproduction of a plaster surface was obtained. The procedure was as follows: inch in diameter and inch thick was cast A plaster pellet with one face on a clean glass microscope slide. After the plaster hardened it was carefully removed from the slide by soaking in water (to retain the smoothness of the surface) and was allowed t o dry. A film of a transparent, colorless insulation compound composed of polystyrene in benzene was formed upon one side of a glass microscope slide and allowed t o dry t o such hardness t h a t i t could not easily be indented by a fingernail. The smooth surface of the plaster pellet was placed flat against the polystyrene film and both were placed, glass down, on a flat electric hotplate at a temperature of about 90" C. Sufficient weight was placed on the pellet t o impress it into the polystyrene film. After about half an hour the impression was deep enough; the pellet was soaked off the film in water and residual plaster was dissolved from the film with dilute hydrochloric acid. The completed impression was then examined in the light microscope by transmitted light. To improve the depth of focus, a thin circular disk with a pin-hole aperture was inserted in the microscope above the objective lens. The appearance of a plaster surface is shown in the upper part of Figure 9. The surface is composed largely of acicular crystals of gypsum with occasional swallowtail twins appearing. Information about t h e crystal forms present can be gained but the picture lacks sufficient depth t o show much more than two dimensional structure. An illusion of depth was given by shadow-casting a polystyrene impression with aluminum. The aluminum metal was evaporated onto the polystyrene surface a t an oblique angle in an RCA Model EMV-1 high vacuum apparatus and the shadowed replica was photographed as before. The result is shown in the lower part of Figure 9. Crystals are arranged in random fashion with no regularity of orientation. There is no doubt but that the intertwining and interlocking of crystals held t o one another at points of contact by intercrystalline attractive forces are responsible for at least part of the strength of set plaster. GYPSUM GELS
Since no evidence of an intermediate colloid gel stage was found in the foregoing studies, gels were prepared in accordance with the methods described by previous investigators. Cavazzi prepared a gelatinous-appearing precipitate as follows (3): A saturated solution of plaster of Paris was prepared by adding 0.5 gram plaster t o 25 ml. of cold distilled water, mixing thoroughly, and filtering. An equal volume of ethyl alcohol was rapidly added to the filtrate after both filtrate and ethyl alcohol had been cooled in a n ice bath. This procedure was followed and a specimen was immediately removed for examination in the electron microscope (Figure 10). The material contained, in general, two types of crystals, tiny needlelike monoclinic p r i s m and feathery swallowtail twins. That the latter were extremely thin is clearly shown by the evident overlapping of some of the crystals. The thickness of the swallowtail twins is estimated t o be between 100 and 500 A. Acicular crystals less than 10 mp in width were present. There was no indication of the presence of gel or of any noncrystalline material. Electron diffraction patterns of the material were made to confirm identification (Figure 11). Spacings calculated from measurement of the diffraction rings checked very closely with those calculated from x-ray diffraction patterns of gypsum, indicating that the crystals observed were CaSOa.2H20 and not an alcoholate. The heavier arcs in Figure 12B show the presence of preferred orientation and probably are given by the crystal twins. None of the crystals observed resulted from evaporation of the subpension medium on the specimen film. This fact was
Figure 11. Upper.
Electron Diffraction Patterns
Cavazzi's gel.
Lower. Pattern of s a m e specimen as Figure 12A, showing preferred orientation
demonstrated by the fact t h a t specimens of the supernatant liquid taken from samples a week old and then evaporated on the specimen film showed no solid material visible in the electron microscope at magnification of 10,000 diameters. Furthermore, specimens of the precipitate examined immediately after preparation of the gel sample were identical with those taken from the same sample after it had been allowed t o stand for a week. It was apparent t h a t the material observed was the dihydrate and t h a t it wab formed as such at the time of preparation and remained unchanged for at least a week. A colloidal system is characterized by very extensive phase boundaries. The particles found in Cavazzi's gel, particularly
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t o the striations the crystal would appear as a row of elongated hexagons joined a t opposite vertices. From the ease with which they are split along these lines, it is possible that the attractive forces between sections of crystals are provided by rows of water molecules which give relatively weak bond?. B y forming as twin crystals, gypsum acquires a third symmetry and thus becomes a more stable conformation. Consequently, the twinned variety of gypsum has a lower free energy content than the untwinned varieties For that reason the presence of greater or lesser quantities of these crystal twins of gypsum may explain the different apparent calcining temperatures of some types of gypsum.
B
A
Figure 12.
Swallowtail Twin Crystals of Gypsum (15,200X) A.
B.
Before burning in electron beam After burning in eleetron beam
the twinned crystals, would impart to the system the qualifying characteristics for designating it as being of colloidal nature, but no evidence was found which would warrant classifying the niaterial as a gel in the sense of the classical definition. Baykov's gel ( 2 ) was prepared by adding plaster of Paris t o a solution of ammonium sulfate in distilled water. Since such a mixture probably involves the formation of ammonium syngenite ( 5 ) , which is not a true plaster hydration product, it will not be discussed. DEHYDRATION OF GYPSUM B Y THE ELECTRON BEAM
An interesting observation made during the cousee of these studies was the fact that high electron beam currents sometimes caused a major change in the hydrated plaster specimens. This change was particularly noticeable in the case of the swallowtail twins which appeared t o be subject to splitting along the fine striations which characterized their structure (Figure 12) It is assumed that enough energy from the high speed beam of electrons was absorbed by the gypsum crystals t o heat them t o calcination temperature in the high vacuum existing within the specimen chamber. The resulting particles have not been positively identified, but may be any of the less-hydrated forms of calcium sulfate. An attempt was made to identify the r e d u e material by electron diffraction, but the burning proress seemingly cannot be carried out a t will and it was never possible t o burn an area on one specimen large enough for satisfactory diifraction study. It is not likely that the effects produced hy the beam result from mechanical impacts of electrons.
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SUMMARY
The following conclusions are drawn on the basis of the visual and photographic data obtained in this study The hydration of plaster 01 Paris appears t o be a solutionrecrystallization process in which the hemihydrate dissolves and then crystallizes as needlelike gypsum crystals. Though they may vary somewhat in size, the same tvpe of crystals are formed in the solid set mass of plaster of Pails that are formed in dilute suspensions. The addition of the set-accelerating and set-retarding mal erials used affectsthe rate of solution of hemihydrate. Contrary to expectations, no visual evidence was found of intermediate gel formation in the process of setting or hydration. The presence of the swallowtail twins of gypsuni may effect the calcining temperatures. However, because they apparently are not present in appreciable quantities, the effect on commercial calcination is doubtful. LITERhTURE CITED
RfcBain, J. W.,J . A p p l i e d Phys., 15, 607 (1944). (2) Baykov, M., Compt. rend., 182, 129 (1926). (3) Cavaaei, A., Kolloid-Z., 12, 196 (1913). Chassevant, L., Ann. chim., 6 , 264 (1926). D'Ans. J., Ber., 39, 3326 (1906); 40, 192 (1907). Davis, W. a., J . Soc. C h e m . I n d . ( L o n d o n ) , 26, 227 (1907). Desch, C. H., T r a n s . Eng. Ceram. Soc., 18, 15 (1918). Dunn, J. S., Chemistry a n d I n d u s t r y , 1938, 144-8. Faivre, R.,and Chaudron, G., C o m p t . r e n d . , 219, 29 (1944). Haddon, C. L., T r u n s . F a r u d a y Soc., 20, 337 (1924). Hoppe-Seyler, Pogo. A n a l . , 127, 161 (1886). Kelley, K. K., Southard, J. C., and ilnderson, C. T., U. d. Bur. LIines, Tech. P a p e r 625,3-9 (1941). 1.e Chatelier, H. L., C o m p t . rend., 96, 1668 (1893); Anta. ntines, (1) Abrams, I. lf,,and
1887, 346. GYPSCWI TWINS
The presence of the so-called swallowtail twins of gypsum in the hydration products has been mentioned frequently in this paper. So far as is known, these twins have not been described previously, and it seems likely t h a t their presence is largely responsible for one investigator's conclusion that plaster hydration involves a gel formation. Twins have been present in all types of hydration studied in t h e present work, including masses of set plaster, and the facts point toward their being formed in greatest quantity under conditions involving rapid precipitation of the dihydrate. At one time it was thought that the striations in the swallowtail twins were cleavages, since it is parallel to these lines that splitting occum in dehydration (Figure 12R). A crystallographic analysis of the twins showed, however, t h a t the principal cleavage is parallel t o the broad face of the twins. The possibility nom- considered most likely is that the striations are caused by extremely small, needlelike monoclinic forms lying parallel t o one another, joined together by a line of ionic bonds, so as t o form a continuous crystal. 'C-iewed from an edge in a line parallel
Linck, G., and Junp, E€., 2. anorg. ( 1924). Xeville,
u.
allgem. Chem., 137, 407
H.A , J . P h y s . C h e m . , 30, 1037 (1926).
Keville, arid Jones, Colloid S y m p o s i u m M o n o g r a p h , 6 , 30!1-18 (1928). Oetwald, W., and Wolski, P., Kolloid-Z., 27, 78 (1920). I'osnjak, E., Am. J . Sci., 35A, 247 (1938). Randel, W. B., and Dailey, M. C., U. S. Patent 1,901,051 (1933). Rohland. P.. 2. anoru. Chern.. 89. 352 (1914). Schoch, E. P., and C~iiniiiphani,'W..I.,T r a n s . Am. Inst. Chem. E n g r s . , 37, 1 (1941). Sliepcevich, Gildart, and Katz, IVD. ENG. CHEM..35, 1178 ( I 943). Stratta, R., Ann. c h h . applacuta, 36, 110 (1946). Tavasci, B., C h i m i c a e i n d u s t r i a ( M i l a n ) , 24, 309 (1942). Traube, J., Kolloid-Z., 25, 62 (1919). Van't Hoff et al., 2.P h y s . Chem., 45, 257 (1903). Weiser, H. B., Millipan, W. O., and Ekholm, W. C., J . Am. Chem. Soc., 58, 1261 (1938). Weiser, H. B., and Moreland, F. B., J . Phys. Charc., 36, 1 (1932). RECEIVED for review September 10, 1951.
ACCEPTEDApril 2 5 , 1952. Presented a t the 7th Southwest Regional Meeting of the AJIERICAX CHEMICAL SOCIETY, Austin, Te-?.,December 1951.
