Study of the Setting Expansion of Gypsum

From: The Department of Technology, the Royal Dental College, Copenhagen, Denmark.

STUDY OF THE SETTING EXPANSION OF GYPSUM bU

KNUD DREYER JBRGESSEN

The present investigation is a continuation of the author’s previous studies of the setting expansion of gypsum (Jbrgensen & Posner, 1959; Jbrgensen, 1960, 1961). The purpose was to find a basic explanation of the empirical fact that the setting expansion of gypsum can be influenced, to a material degree, by a number of factors, such as addition of chemicals to the slurry of plaster and water, prolongation of mixing time, or introduction of insoluble, chemically inactive powders.

Earlier studies by other authors have shown that the morphology of gypsum crystals is changed when they form and grow in different solutions (Muthis, 1919; Weiser & Morelund, 1932; Tschepelewetzki & Jewslina, 1938; Andrews, 1951 ; McCartney & Alexunder, 1958, and others). It has been suggested that there may be a regular relationship between crystal shape and setting expansion (Andrews, 1951; Jbrgensen & Posner, 1959) ; the validity of this hypothesis will be considered subsequently. The experimental work is divided into two main sections, viz. measurement of the setting expansion, and a metric examination of

This investigation was supported, in part, by a research grant, D-842, to the Royal Dental College, Copenhagen, from the National Institute of Dental Research, U. S. Public Health Service.

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220 KNUD DItEPI;IL J$5IiGENSEN

the inorphology of gypsutii crystals formed and grown under conditions to be specified.

The setting expansion was iiieasurcd with a Mtihr d id gauge, which perinits readings to an accuracy of f one iiiicron; the pressure of the dial gauge was approx. 50 g. This pressure was distri- buted over the expanding gypsum by nieans of a circular brass plate, 20 niiii in diaineter and 2 inin thick.

The gypsuin used in the tests was Merck's precipitated calcium sulfate pro analysis (CaS04 - 2H20; molecular weight, 172.18). This gypsuiii was partially dehydrated in shallow glass dishes by 24 hours' heating in a therinostat oven at a temperature of 123 f 3' C. .4fter the dehydration the dishes with the plaster were kept :it rooin temperature for 24 hours to eliminate any soluble anhydrite which inight be present, and the final preparation was stored in sealed bottles. Analyses of the hemihydrate so obtained revealed a content of crystal water which was equi- valent, with a ~ i i ; i s i i i iu~~i inaccuracy of 0.1 %, to the forinula

The cylindrical test s:imples were prepared by pouring the spatulated plaster into cylinders of thin paper. Measureinents were started as soon as the plaster mix was able to resist the dial gauge pressure, and were continued for one hour after start of mixing, i.e. until the gradient of the expansion curve had be- conie insignificant.

In the nornial tests 2.5 g of plaster were mixed with 25 in1 of water by :I standardized hand spatulation for 30 seconds. The test specimen was covered with water before it became hygroscopic to avoid the influence of possibly varying capillary forces on the expinsion (cf. Jbrgensen, 1960). A number of ten tests gave a mean value of 0.44 Ch with variations not exceeding f 0.02 % . For nieasureinenfs carried out by this method such variations are in good agreement with previous experience.

To determine the significance of possible s y s t e 111 a t i L: e r- r o I' s the effect of the following factors on the expansion was examined.

CaSOd - % HrO.

I. .Wiring lime

These tests were conducted without hygroscopic expinsion. Spatulation varied between 30 and 300 seconds. There was not

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SETTINQ EXPANSION OF GYPSUM 229

enough time to place the long-stirred mixes (4 and 5 minutes) in paper cylinders, so in such cases the measurements were taken on free test specimens. The experiments were carried out with plaster of the same batch and burning, but of different age reckoned from the burning, uiz. approximately one week and three months, respectively. The results appear in Table 1 and Figs. 1 and 2.

Table 1

Setting Expansion of the Gypsum as affected by Spatulation Time and Age.

Stirring

I

30

I 6o I 90

120 150 1 180

I 200 ; 230 I

- Expansion %

-. - _. One week old I Three months old

0.264.28-0.24-0.24-0.25

0.25--0.26-0.25

0.40-0.44-0.43 0.69-0.66-0.67 0.$54.98-0.%3

0.36-0.36-0.37 0.41-0.40-0.40

0.60--0.57-0.57

1.20-1.17-1.24

1.51-1.63-1.56 240 1.60-1.65-1.57

! ____ . 1 .---'

The results of these tests show that the spatulation time does not affect the amount of expansion for plaster which is about one week old after the burning, if it is not stirred beyond 90 seconds. Fig. 2 is a semi-logarithmic representation of the same experimental data as those shown in Fig. 1, lower curve, and shows that after an initial period of just under two minutes the logarithm of the expansion is proportional to the mixing time. It is highly probable that the findings are related to the amount, size, and shape of the dihydrate nuclei present in the plaster. Since these factors may vary much with the brands, it must be stressed that the findings apply only to the material used in these tests.

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230

0 see. Fig. 1. The influence of the mising time upon the sctting expansion. Lower curve: the plaster one \wek old after the burning. L‘pper curve: the plaster

three months old after the t)urning.

2. I’lnrter/ri~nler ratin

These experiiiients were made without hygroscopic expansion and the mixing proportion was varied hetween 22% and 35 g of powder per 2,5 id of witer. The results are given in Table 2 and Fig, 3.

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SETTING EXPANSION OF GYPSUM

log [ %Exp. x lo]

-.

Grams plaster/25 ml water

40 80 120 160 200 240 280 :

Setting expansion %

23 1

10 sec. Fig. 2. The same curve as the lower curve in Figure 1, but in semi-

logarithmic presentation.

I

35 I 0.42-0.42-0.41 0.39-0.39 0.34-0.36 0.28-0.31

30

25 0.25-0.25 0.18-4.18-0.17 aa%

i 32%

2734 I

I _ - ~- .-

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232 K S U D DRETEll JgRCEh'SEX

It is :iyparent from these ineasureiiients that proportioning has a considerable influence on the expansion. On the other hand, this need not produce systematic errors if weighing and measuring are carried out with accuracy.

Exp %

2fml YO Fig. 3. The influelice of the proportion hemihydrute/waler upmi the

setting expansion.

3. Temprrdarc

Apparatus and materials were placed in cold chamber or hot chaiiiber at constant temperatures at which the experiments were carried out, see Table 3.

Table 3 Inllrtence nf the Tempercititre on the Seffing Expnnsion.

__. ___ -. __ __ ._ - ~- I

I Temperature C" Setting expansion %

3 0.53-0.55 16 0.47-0.49 22 0.44-0.41 37 0.30-0.31

-. - ..-__--____ ____

The temperature need not be 8 source of error, provided the 1nbor:itory temperature can be kept reasonably const :in t .

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SE'Pl'ING EXPANSION OF GYPSUM 233

4. Paper matrix

The tests were made on specimens of about the same size, partly with and partly without paper matrix, partly in air and partly under hygroscopic conditions. Table 4 shows the results.

Table 4

Effect of the Matrix on the Setting Expansion in %.

Setting in air i With matrix Without matrix With matrix Without matrix I Setting nnder water

I

I 0.25-0.25-0.25 0.32--0.31--0.30 O . W . 4 4 - 0 . 4 4 0.46-Q.44-0.45 I In the experimental investigation of the influence exerted upon

the setting expansion by the factors mentioned in the introduction, the following precautions were taken against systematic errors, (1Y the same batch of hemihydrate was used in all the tests, (2) the experiments ran for about two weeks, and were carried out with hemihydrate burned approximately one week before the tests started, (3) stirring was as nearly standardized as possible and lasted for 30 seconds, (4) to avoid appreciable errors due to proportioning the hemihydrate was weighed with an accuracy of k 5 mg, while the water phase was measured with an accuracy of f 0.1 ml, ( 5 ) the temperature was 22 f 2" C in all the experiments, except in some cases where the tests were planned at higher or lower temperatures; in the low-temperature tests the temperature was 3 f 1" C, while it was 37 f 0.2" C in those at the higher temperature, (6) the matrix was standardized in all the tests; a few experimental series with accelerators, in which the setting proceeded very rapidly, were run without matrix; the resulting inaccuracy is considered to be without influence on the conclusions.

The additives used in investigating the effect of different soluble substances on the setting expansion were chemical re- agents; the gelatin used was Difco's Bacto-Gelatin for microbio- logical procedures. Of these substances solutions of certain concentrations were prepared and used for stirring with hemihydrate. Three expansion tests were made for each solution; the variations in the experimental results were very small in all the

17 - Acta odont. scand. Vol. 21.

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234 KNUD DREYER JfiRGENSEN

cases, and did not exceed 5 0.02 %. The findings are listed in Table 5.

Expansion tests were also made with hemihydrate which had been further coiiiiiiinuted by five minutes’ grinding in a large porcelain mortar after the burning.

The investigation of the effect of cheniically inactive powder on the setting expansion was carried out with pulverized pumice, which was mixed with the hemihydrate powder. The test specimens were prepared with such a quantity of water that the amount of hemihydrate per unit voluine of the mixture was the same as in the experiments with pure hemihydrate. In the pumice powder tests 10 g of this powder was added to the usual 25 g sample of hemihydrate. As the former has a specific gravity of 2.27 g/cc, and thus occupies 4.4 cc, the whole mixture required 25 less 4.4 = 20.6 cc of water. The pumice powder was divided into three grain-size fractions, viz. one fraction below 20 p, one between 20 and 35 ,ti, and one above 35 p. The expansion found for the three fractions was 2.05, 1.48, and 0.90 %, respectively, i.e. a considerable increase on the 0.44 % for pure hemihydrate.

INVESTIGATION OF THE MORPHOLOGY OF THE DIHFDRATE CRYSTALS

During the preliminary investigation of the morphology of microscopic gypsum crystals formed and grown in different solutions, it was observed that as a rule they retained their typical inonoclinic foriii with constant angles between the various faces and with development of the saiiie faces (110, 111, and 010). Further it could be noted that the proportion between the a and the b diiiiensions (breadth and thickness of the crystals) was without great variations, while the c dimension (length) and the proportion between the a and the c dimension varied to a marked degree.

So it became clear that the investigation had to be directed towards dimensions and proportions of the crystals, and that it was therefore essential to secure so many single crystals in each inicropreparation that the data obtained by the measurements could be subjected to statistical treatment.

A number of attempts to obtain satisfactory preparations made it clear that the main difficulties would be to eliminate various

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SETTING EXPANSION OF GYPSUM 235

germ-producing factors which were not characteristic of the preparation concerned, and to keep the concentration of the dissolved crystal modifier reasonably constant. The final preparations were made as follows. Ordinary microscope slides and cover glasses were thoroughly cleaned and polished. Then they were boiled in demineralized water for about an hour, care being taken that the glasses did not touch each other or the boiling vessel. After boiling they were washed in alcohol and ether, and stored dust proof until use. They were handled in such a way that those areas of the glasses which would get in contact with the gypsum preparations were not touched. When the solutions of crystal growth modifiers had been prepared, a drop of a standardized size (approx. 35 mg) was placed on the slide together with approx. 1 mg of hemihydrate powder. By means of a corner of the cover glass, solution and powder were mixed, the cover glass was put in place, and the preparation sealed up with Canada bal- sam dissolved in xylene.

Measurements and photographs of any one preparation were taken within a few days of its completion. The single crystals were measured in Reichert’s Visopan microscope (Fig. 4) with a linear magnification of 500. The crystal images were projected on the ground glass of the microscope and measured directly there with a millimeter rule. All measurements below 5 mm (10 p ) were correct to within f 0.25 mm (0.5 p ) ; measurements from 5-10 mm (10-20 p ) , to within k 0.5 mm (1 p ) ; and measurements beyond 10 mm (20 p ) , to within f 1 mm (2 p). In order to ensure a random selection of crystals, measurements were taken of all the regular single crystals which the movements of the preparation had placed, wholly or partly, inside the circle drawn on the ground glass.

Before being measured all the preparations were inspected in a Leitz Panphot microscope in polarized light. This examination showed that for a few preparations there was a tendency in the peripheral areas with relatively low concentration of calcium sulfate to develop atypical crystal proportions (the proportion c/a, unusually small) during crystal growth; such zones were avoided when the measurements were taken. In the denser areas of the preparations, on the other hand, the concentration could not be seen to affect the crystal shape. A few preparations had

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236 KXUD DRBTER JBROESSES

Pig. 4. lleichcrt’s Visopan microscope which was used in measuring the crystals. (Courtesy of C. Reichert, Vienna).

zones of minute, closely packed crystals, probably formed at places where the corner of the cover glass had scraped against the object glass when mixing was performed.

The majority of single crystals in nearly all the preparations were orientated with the b axis parallel to the optical axis of the microscope. It was therefore natural to measure the c axis dimension (crystal length! as the length of one side of the paral- lelograinshaped profile of the crystal ; further the crystal breadth was measured as the dimension in the a-c plane at right angZes to the c axis (i.e. not parallel to the a axis).

As already pointed out the greater number of the preparations did not display visible differences in the proportion between a and b axis dimensions. It is therefore possible metrically to ex- press the end surface area, the end surface periphery, and the volume of a crystal by means of symbols, which can be calculated

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from the two measurements, and the magnitude of which is proportional to the true values for the abovementioned three properties. If the end surface area of a crystal is understood to mean the area of the projection of the crystal on a plane at right angles to the c axis, this area may be expressed by the square of the breadth since this product is proportional to the true end surface area. In the same way, the end surface periphery may be defined as the periphery of the projection of the crystal on a plane at right angles to the c axis, this periphery becomes proportional to the breadth and may therefore be expressed by it. Further, the true volume of the crystal becomes proportional to the product of the breadth to the second power and the length, and hence it may be expressed by means of this product.

After the deliberations following measurement of the dimensions of the many single crystals it was decided that only relative measurements were of interest, and therefore the metric charac- teristics of the various crystals were expressed as specified.

To examine the accuracy with which measurements of in- dividual preparations could be reproduced, a total number of 35 crystals were measured for each of three preparations made with distilled water by the same procedure. For both length and breadth the mean values (m) and the standard deviations (s) were calculated with the following results:

S I m 41.3--40.1-39.1 1 1.33-1.30-1.27

15.4-1 6.1-1 6.0 1 0.75-0.73-0.74

A similar control carried out in a few other cases (0.5 n K2S04; 4 n NaCl; 0.5 n CuSO4; 0.2 $6 gelatine) gave corresponding results with relatively small variations of the mean values and the standard deviations for each modifier.

Accordingly the method described was considered dependable and well suited for obtaining a metric expression for the effect of growth modifiers on gypsum crystals. This impression was further strengthened during measurement of the various prepara-

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Num

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0.56

0.

33

0.20

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KN

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:::I;: 2 3

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1 6

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8

18 I 19

1 0

9 IQ

20 21 22 23 24 25

Fig. 5. ”Average” crystals from the microscopic preparations; each preparation is represented by the mean length and the mean breadth of the crystals.

The numbers indicate the sequence of volume.

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tions, since it was found that the majority of them could be readily distinguished from each other by microscopic inspection (cf. Fig. 10).

RESULTS AND DISCUSSION

The results of the measurements and of some of the calculations are collected in Table 5, while Fig. 5 shows the ”average crystals” in profile seen from the b face.

An examination of the measurements with the view of statistical treatment showed in most cases no normal distribution, so it was with some hesitation that the statistical calculations listed in Table 5 were carried through (it can be noted, f. ex., that in many cases the standard deviation amounts to more than one third of the corresponding mean value). A mathematical analysis of co-variations had to be abandoned owing to the abovementioned type of distributions.

It must be warranted, however, to make a graphical analysis of co-variations, though it must still be borne in mind that the co-variants fail to form normal distributions.

The analysis shows that there is no correlation between on the one hand the setting expansion, and on the other (1) the mean length of the crystals, Fig. 6, (2) the end face area of the cry-

EXD. %

Fig. 6. The relation between the mean length (M) of the crystals and the setting expansion.

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212

E m %

KNUD DREYER JBRGENSEN

)O A

Fig. 5. The relation between the end face area of the crystals per unit volume of dihydrate ( A ) and the setting expansion.

stals per unit volume of dihydrate, Fig. 7, or (3) the sum of breadth + breadth + length per unit rolunie of dihydrate.

The setting expansion was well correlated, however, both with the number of crystals per unit volume of dihydrate (Fig. 8 ) and with the end face periphery per unit volume of dihydrate (Fig. 9) .

In 1961 Jlrgensen described a hypothetical model to explain setting expansion and expansion pressure of gypsum. The model built on the assumption that an excess pressure would arise in the liquid in the narrow slit between two growing crystals, and that the magnitude of this pressure, and accordingly of the setting expansion, would be (1) inversely proportional to the sum of the slit peripheries present per unit volume of dihydrate, (2) directly proportional to the viscosity of the liquid, and (3) directly proportional to the setting rate (rate of crystal growth). The iiieasurements and calculations reported in the present work have demonstrated that setting expansion and end surface periphery*) are positively correlated, and there is nothing to indicate that the degree of setting expansion is dependent on the viscosity of the liquid or on the setting rate. So the abovementioned hypo- --

*) This periphery will to a large degree determine the magnitude of the slit periphery owing to the prevailing growth in length of the dihydratc crystals.

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Exp. Yo

0.5 E 20 4 1 60 80 100 120 140 160 N

Fig. 8. The relation between the number of crystals per unit volume of dihydrate (N) and the setting expansion. Experiments with the same ad- ditive in different concentrations are connected with lines. 1, gelatine. 2, water of different temperatures. 3, NaNOs. 4, Rochelle salt. 6, NaCI. 6, Na2S04.

7, KrSOd. 8, NaZBd07. 9, sodium citrate.

thesis must no doubt be abandoned. An alternative possibility is that crystal growth thrust and expansion may result from thermodynamic movements on the boundary faces between two united gypsum crystals. Undoubtedly such movements may cause the molecules and ions of the solution to diffuse into the less regular interfacial lattice and thereby expand it. A direct proof of such an assumption cannot be furnished by the data presented in this work, but for the present it seems the most plausible explanation.

Any factor increasing the number of crystals will also increase the number of boundary faces and thus the possibilities for thermodynamic, expanding particle movements. The following observations are therefore in good accordance with this hypo-

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244 KNUD DREYER J6RGENSEN

S Fig. 9. The relation between end face periphery per unit volume of dihydrate (S) and the setting expansion. The numbers indicate the same solutions

as in Figure 8.

thesis: (1) If the hemihydrate is crushed before spatulation the number of dihydrate crystals in the set gypsum will increase, and so will the expansion. (2) Spatulation prolonged beyond a certain time will increase the number of crystals and at the same time, the expansion. (3) Addition of foreign, chemically inactive powders (here pumice) will give a greater number of interfaces (here between gypsum and pumice) increasing with increased fineness of the pumice powder, and a higher expansion will result. The positive correlation between the setting expansion and the length of end face peripheries per unit volume of dihydrate also supports the thermodynamic explanation since the proba- bility for diffusion of particles into the interfacial lattice must increase with its extent.

The experiment with addition of dihydrate powder (approx. 30 % > to the hemihydrate powder in preparation of the slide specinien gave fewer crystals than pure hemihydra te, while meas-

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urements showed an increased setting expansion for such a mixture. This apparent departure from the rule that expansion and number of crystals are positively correlated may probably be explained in the way that during the spatulation incident to expansion tests, a comminution of the dihydrate particles occurs with resultant increase in the number of crystal nuclei.

Besides, the experiments seem to show (see Table 5) that different ions have a specific, different effect on expansion and crystal number. Thus the effect of the alkali metal ions appears to be correlated with their order in the periodic system (Li+ is least effective, K+ and Rb+, most), and of the common anions Sod-- has evidently a greater effect than NOS-, which again bas a greater effect than C1-.

Summarized the results of the investigation show that the various modifiers have a double effect since they influence both nucleation and crystal proportions. Formation of nuclei, and accordingly the number of crystals, obviously affects the degree of setting expansion; on the other hand, the influence of the crystal proportion on the properties of the gypsum is rather un- known.

ADDENDUM

In order to study the morphology of gypsum crystals in set gypsum, test specimens were prepared from mixes made up of 25 g of hemihydrate and 25 ml of expansion modifier solution. The test samples were examined by incident light by means of a Leitz Ultropak with a magnification of 220. The purpose of the investigation was to establish to what degree the crystals in these preparations differed in appearance from the micropreparations previously described. The crystal proportions could not be measured because no single crystals occurred. The crystals in the various preparations were compared, by simple inspection, with the crystals in a normal preparation obtained by mixing hemihydrate with water. The same modifiers were used as those set out in Table 5.

The results showed that only a few modifiers will distinctly change the growth habits of the crystals (see Fig. 11). Thus, 4 n NaCl gave very large and coarse crystals; 0.1 n sodium citrate,

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246 K N U D DREYER JflRGENSEN

Fig. 10 a.

flat and very short crystals, much the saiiie as in the iiiicroscopic specimens; and prolonged syntulation resulted in very minute crystals.

Certain substances, such as KeSOl and gelatine, produce very considerable changes in the growth habits of crystals in the micropreparations, where the relative amount of modifier is much greater than in the spatulated preparations, but no visible modifications of the crystals in the latter preparations. This suggests that such substances disappear during the setting and probably are built into the gypsum crystals. This incorporation may take place in the way that molecules or ions of the modifier are adsorbed on the growing crystal faces and enclosed by ele- ments of the dihydrate lattice. The adsorption may occur to a more or less marked degree on the various faces of a crystal (selective adsorption), and will probably also be able to bring about changes in the growth rate of the faces, and hence in the proportions of the crystals.

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SETIING EXPANSION OF GYPSUY

Fig. 10 b.

247

Fig. 10 c.

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148 K S U D DREYER JBRCENSEN

Fig. 10 d.

Fig. 10 e.

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Fig. 10 f.

Fig. 10 g.

18 - Acta odont. scand. Vol. 21.

249

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250 R S U D DIIETEIi J0RGESSES

Fig. 10 h.

Fig. 10. The effect of growth modifiers on gypsum crystals. Tllc crystals are grown in distilled water (a), 1 11 Sac1 (b), 0.2 c /o gelatine (c), 0.2 n Rochelle s d t (d), 0.1 n sodium citrate (e), and 0.025 n borax Cf), respectively. (6) shows syngenite crystals grown from hemihydrate in 0.5 II KzSO4. ( h ) shows gypsum crystals grown in distilled \niter after mixing

a slurry of 25 ml water and 25 g hemihydrate for one minute. Jlagnificatinn 150 X.

SZ'MJIARY

The present study was designed to examine whether a relationship exists between on the one hand the aiiiount of the setting expansion of gypsum as varied, inter alia, by addition of different expansion modifiers, and on the other hand the morphology of the gypsuiii crystals.

After measurement of single crystals of gypsum in micropreparations (see Table 5 ) it could be demonstrated that the setting expansion is positively correlated with the number of crystals per unit volume of gypsuni, and with the sum of end face peripheries, likewise per unit volume of gypsum. The expansion is independent, on the other hand, of the mean length of the

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Fig. 11 a.

Fig. 11 b.

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‘135

Fig. 1 1 e.

Fig. 11. Surfaces of gypsum after setting in distilled water (a). 1 n NaCl solution (b), nncl 0.1 II sodium citrate solution (e), respectively.

Cltropnk, 650 X.

crystals, of the end face area of the crystals per unit volume of gypsum, and of the sun1 of breadth -t breadth + length per unit volume of gypsum.

It is possible that the expansion is due to thermodynaiiiic movements on the boundary faces between united gypsum crystals. Such rnoreiiients iiiay give rise to diffusion of molecules and ions into the interfacial lattice and thereby expand it.

Examination of the crystals in preparations of spatulated set gypsum showed that only exceptionally are their structure and shape affected by the expansion modifiers.

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HfiSLJMfi

ETUDE SUR L’EXPANSION DE PRISE DU PLATRE Le but de cette dtude est d’examiner s’il existe un rapport

entre les deux facteurs suivants: l’importance de l’expansion de prise du plltre pendant ses variations obtenues entre autre par addition de divers modificateurs d’expansion d’une part, et la morphologie des cristaux de plltre d‘autre part.

Aprks avoir mesurk des cristaux de platre isoles sur des preparations microscopiques (voir tableau 5)’ il a 6th possible de mettre en evidence que l’expansion de prise du plltre est en correlation positive avec le nombre de cristaux par unite de volume de plltre et avec la somme des peripheries des faces terminales, calculee de mQme par unite de volume de plltre. L‘expansion est par contre independante de la longueur moyenne des cristaux, de la superficie des faces terminales des cristaux par unite de volume de platre et de la somme: largeur + largeur + longueur par unit6 de volume de plltre.

I1 est possible que l’expansion soit due B des mouvements ther- modynamiques sur les faces de jonction des cristaux soud6s entre eux; de tels mouvements peuvent faire diffuser des molecules et des ions dans le rkseau plus ou moins irregulier des faces de jonction, et provoquer ainsi son expansion.

L’Ctude de la morphologie des cristaux dans des prdparations de plltre glche et pris a montre qu’elle ne se trouve qu’excep- tionnellement influencde par les modificateurs de l’expansion.

ZUSAMMENFASSUNG

OBER DIE ABBINDEEXPANSION VON GIPS

Der Zweck dieser Untersuchung war zu untersuchen, inwieweit ein Zusammenhang besteht mischen einerseits der Griisse der Abbindeexpansion von Gips, so wie diese u. a. durch Zusatz von verschiedenen Expansionsreglern variiert, und andererseits der Morphologie der Gipskristalle.

Durch Messung von Einkristallen von Gips in Mikropriparaten (siehe Tabelle 5) wurde ermittelt, dass die Abbindeexpansion positiv korrelat ist mit der Anzahl von Kristallen pro Raumein-

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251 KSCD DRETEH JgIIt iESSES

heit Gips uiid iiiit der Suiiiiiie ron Endflachenperipherien, eben- falls pro Rauriieinheit Gips. Dagegen ist die Espansion unab- hiingig ron der ;\litlellQnge der Kristalle, von deiii Endflachen- mass der Kristalle pro Rauiiieinheit Gips und ron der Suiiiine Breite + Breite + LQnge pro Rauiiieinheit (;ips.

Es ist miiglich, dass die Expansion auf therinodgnainische Be- wegungen in den Grenzflachen zwischen zusaiiiinengewachsenen Cipskrist:illen zuruckzufiihren ist ; solche Bewegungen kiinnen eine Diffusion ron Molekeln und Ionen in das weniger regelinas- sige Grenzflachengitter hinein niit sich fuhren und so dasselbe expandieren.

Die Untersuchung der Morphologie der Kristalle in Praparaten von ausgeriihrtein, abgebundenem Gips ergab, dass sie nur aus- nahmsweise yon Espansionsreglern beeinflusst wird.

REFERENCES

dndrems, H., 1951 : The production, properties and uses of calcium sulpliate plasters. British Building Congress, division 2, part F, 135-144, London.

Jgrgenoen, K. D. h d. S. Porner, 1959: Study of the setting of plaster. .I. dent. Rcs. 38: 491-499.

JCrgensen, K. U., 1960: The hygroscopic setting expansion of gypsum. Bcta odont. wand. I f f : 461-475.

-*- 1961: Setting expansion of gypsum. In J. H. Ue Roer et al. (editors) Reuclivify of Solids. Proc. 4th inlern. symp. on the reactivity of solids. Amsterdam. Elsevier Publishing Co.

Mathis, E. H., 1919: A study of the behaviour of plaster of Paris as an in- vestment in the process of vulcanizing dental rubber. .J. nat. dent. Ass. 6: 1 3 2 4 1 5 .

AfcCurtney, E. R. & A. E. Alesander, 1958: The effect of additives upon the process of crystallisat ion. J. Coll. Sci. 23: 382-3W.

Tschepelewetzki, JI. L. & B. B. Jewslina, 1938: Dynamik des Krystalwachs- turns und die Veranderungen ilirer ausscren Form am Beispiel des Gipses und Calciumkarbonats. Reviewed in Chem. Zentralblatt fO9:

IVeIser, If. B. & F. B. .Iforeland, 1932: The setting of plaster of Paris. J. Phgs. 1307-1308.

Chem. 36: 1-30.

Address: Kpbenhauns TandltPqchdjskole 1G0, Jagtuej, Kbbenhctvn 8 Denmnrk

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Documents

Study of the Setting Expansion of Gypsum