CALCIUM SULFATE PLASTERS GEOFFREY BROUGHTONI Massachusetts Institute of Technology, Cambridge, Mass.
Four modifications of calcium sulfate-gypsum (CaS04,2H20), plaster of Paris (CaS04. '/2HzO), soluble and insoluble anhydrites (CaSO4)-exist. Soluble anhydrite is not normally transformed into insoluble anhydrite, the stable anhydrous phase, until a temperature between 300' and 400' C. is reached; there is a lag dependent upon conditions. Insoluble anhydrite does not ordinarily set with water, but grinding and the presence of free lime by thermal decomposition accelerate set, a maximum tensile strength being obtained at about 600' C.
A
LTHOUGH calcium sulfate is one of the more important inorganic salts, widely used in plaster of Paris and flooring plasters or as a retarder in portland cement, the interrelations between its polymorphic forms and hydrates have been but little understobd. This situation, exemplified by a voluminous literature full of conflicting statements, may be traced ( a ) to the fact that much of the work was done in the form of technical investigations on plaster production, (b) to insufficient allowance for attainment of equilibrium in many of the phase transformations, and (c) to frequent neglect of the effect of vapor pressure and impurities on the transitions. It is highly probable that the occasional failure of gypsum products may be attributed to lack of knowledge of the phase equilibria. When calcium sulfate dihydrate (gypsum) is heated in the open air, the heating curve shows an arrest a t about 130" C., and plaster of Paris (CaSO*.l/%HZO)is formed. Further heating gives the anhydrous salt, with an arrest a t approximately 165' C. Thus prepared, it has a considerable affinity for water, by which it is immediately converted to the hemihydrate. For this reason, and in contrast to the unreactive insoluble anhydrite formed by its ignition a t higher temperatures, it is called "soluble anhydrite." Since it reacts with water to give plaster of Paris, soluble anhydrite sets; but insoluble anhydrite, according to most workers, does not set unless it is ground or ignited a t high temperatures, variously stated as from 500" to 1200" C. The plasters obtained by such ignition find considerable use in Europe for flooring plasters and other purposes; they set very slowly to give hard, dense casts. I n the past considerable controversy has arisen as to the number of polymorphs of the anhydrous salt, their relative stability with regard to the hydrates, their zones of stability, and their setting powers. These questions have additional importance because the method of preparation of plaster of Paris and of incorporation of calcium sulfate as a retarder in portland cement are such that the products may contain several of the different modifications. Since the system calcium sulfate-water was imperfectly understood until recently and is essential for interpretation of the hightemperature results, it is first briefly considered.
sence of a liquid phase, investigation of this system has offered considerable difficulty; it appears better to base the study on solubility rather than vapor pressure measurements. Figure 1 shows the solubility curves of the various modifications, from theresults of a number of investigators. Insoluble anhydrite is the stable phase a t all temperatures above 40" C.; soluble anhydrite is absent because with water it immediately yields the hemihydrate which is itself unstable throughout the temperature range investigated. Although several workers (13, 18, 22) found no change when gypsum was heated with water at 100" C., this was undoubtedly 0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0 .I
The System Calcium Sulfate-Water
0.0
Because of the extreme slowness with which equilibrium is attained, months or even years being required in the ab1
Preeent address, Vassar Apartments, Rochester, N. Y.
1002
AUGUST, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
apparatus was immersed in a boiling water FICCRE bath, or the solntion in the flask itself gently boiled with the aid of a small flame. The following table shows the observed conductivities at. the end of 3 hours; they indicate that the hemihydrate has t,he higher solubility and hence is the unstable form, at least below 100" C.: Temp., 'C. 98.7 100.4
Conduclivity, Mhos.--
7
Gypsum 4100 x 10-8 4072
Hernihydrale
4180 4103
x
10-6
The most remarkable features of the system are the instability of the hemihydrate and soluble anhydrite throughout the temperature range and of gypsum above 40" C. (104" F.). It is significant that the hemihydrate and soluble anhydrite are never found free in nature. The resistance of gypsum to change into the more stable insoluble anhydrite is of great importance, because above 40" C . all plast.er casts or set gypsum plasters should, according to Figure 1, become thermally unstable anti yield insoluble anhydrite and water. If such a change actually occurred, all plaster materials of construction would become mechanically unsound and tend to lose their strength in very hot weather. The following talrle indicates that such deterioration probably occurs, although perhaps not to the extent which might be anticipated. Briquets were made up of two parts of plaster of Paris and one part of water according to A. S.1'.>I. specifications (2). They were kept for 7 days at 20", 50°, or 85' C. in atmospheres of 100 per cent relative humidity before determination of their tensile strengths. Simultaneously duplicates containing 10 per cent insoluble anhydrite (prepared by ignition of the plaster at red heat for 8 hours) were made up and tested in the same manner. In every experiment at least 10 briquets were broken. -6tren.th
TemD. 0
c:
Unseeded Lb./ac. in
20 60
227
85
180
176
of piaster Cast----. Seeded 10%.
insoi. anhvdrrte Lb./an. in. 208 121 122
These data show that the presence of insoluble anhydrite, as might he expected, accelerates disintegration; at 20' C. the
1003
RADI~GEA FEOX M ~ S*.\rrLes IGNITED AT 225" C. (top), 300' C. (cenier), ASD 7SQ" C. (bullom) reduction in strength is about 10 per cent, bot a t 50' and 85' C. it is nearly 30 per cent. Even so the resisiance to change into insoluble anhydrite, the stable form at tliesc temperatures, is remarkable. This is further excmplified by the fact that whenever caleium sulfate is formed in solution hy metatliesis, i t is precipitated as gypsum or hemihydrate, neyer as insoluble anhydrite below 100" C. (22). This hehavior may well be bound up Nith the close similarity in crystal structure between gypsum, plaster of Paris, and soluble anhydrite, whereas insoluble anhydrite has a completely different crystal lattice.
Anhydrous Modiiications of Calcium Sulfate When soluble anhydrite is heated above about 250' C., it slowly changes into a modification which, d t h water, either remains inert or slowly hydrates without hardening or expansion. At higher temperatures products are obtained x~hichagain set slowly with water to hard masses but without expansion. Ahout 800" C. or slightly ahove, decomposition of calcium sulfate commences and liherates free lime (6, 26). Until recently there was much doubt as to the number of pulyinorphic forms of anhydrous calcium sulfate produced at these high temperatures, hut during the course of tlre present work Bussern, Cosmann, and Schuster (8) and Piewrnan anti Wells (27) shoTr-ed that only two existsoluble and insoluble anhydrite. X-ray investigation of calcium sulfate and its hydrates revealed only four distinct patterns, corresponding to gypsum, hemihydrate, soluble and insoluble anhydrites, respectively. Nevertheless, the setting properties of the plasters formed at high temperatures remained obscure, since it was uncertain to what extent setting depended on grinding and presence of free lime. In the first series of experiments, the existence of only two modifications of the anhydrite was confirmed (Table I). Figure 2 shows typical radiograms obtained from samples at 225O, 300*, and 790" C . (6). Breakdown into lime was
INDUSTRIAL AND ENGINEERING CHEMISTRY
1004
TABLEI. IGNITION OF CALCIUM SULFATE (c. P.) AT HIGH TEMPERATURES Initial Temp. X-Ray Rise in Ignition Diffraotion Water, Temp., O C. Pattern O C . CaSOc.2Hz0 I i:3 CaSOc. '/rHnO I1 100 120 185 225 300
I11 I11
420
IV IV IV
510 630
790 950
1070
1250
CaO Natural anhydrite
I11
I11
IV
IV
IV IV IV, v
V V
%
Free Lime 0 0
12.2 12.2 12.4 10.5 4.9
0 0 0 0 0
1.5
0
0.4 0.0
2.0 2.2
.. ..
0 0
0.17 0:93 37.8
a .
100
..
..
Remarks Starting material Prepared by heating selenite at 90' C . until water content became 6.05% Set with water Same Same ~. ~. Same Set only to give friable crumbly cast Did not set with water Same Same Same Same Same Same
....
....
VOL. 31, NO. 8
Figure 1 and the above results seem to show that soluble anhydrite is always the unstable phase; its conversion temperature into the insoluble modification is dependent entirely upon the experimental conditions. Thus, with rapid heating, as in the determination of the heating curve, there is a time lag, and conversion takes place at a temperature considerably higher than that at which conversion takes place on prolonged heating. This theory is also supported by the
8
26
6
25
25
24
E
F
z
23 22.1
22
observed at above 1000" C. in an open crucible. The hemiFIGURE 3 hydrate was also found to give a pattern distinct from that of soluble anhydrite, confirming the results of Weiser, Milliconversion of soluble into insoluble anhydrite at much gan, and Ekholm (32). lower temperatures in the presence of water which acts as a Precipitated calcium sulfate (Baker's c. P.) was ignited in 15catalyst ( 1 1 , 27). Hence, in industrial work, irrespective of gram portions at temperatures ranging from 100" to 1250" C.; the temperature, the possibility of this change must always the temperature of the electric furnace was controllable to be borne in mind. 10" C. Preliminary experiments showed that samples heated for 48 and 96 hours behaved identically in subsequent test, while a sample heated for only 5 hours did not check. All samples, Properties of Ignited Calcium Sulfate therefore, were heated for 170 hours to ensure complete change. After cooling, B. E. Warren made x-ray diffraction patterns on a h'one of the samples described showed any signs of setting portion of the sample, using primary K a radiation with a time of with water when ignited above 300" C.; but in order to inexposure of 3 t o 5 hours. Another portion (5 grams) of the sample vestigate the effect of grinding, determine tensile strengths, was mixed with 5 ml. of water in a test tube, mounted inside a etc., it was imperative to have available larger quantities of Dewar flask. After being stirred vigorously with the thermometer for 30 seconds, the initial temperature rise was noted. Free lime material. Consequently, commercial plaster of Paris, conwas determined in several of the samples by direct titration. taining 5.56 per cent water, was used for the experiments to be described ( 1 ) : The existence of but one phase change up to 650' C. was confirmed by differential heating curves (S),using aluminum Times of set, densities, solution rates, heat of hydration curves, oxide as the inert mass in a small Monel metal cylinder. A free lime, tensile strengths, and porosities (of the set plasters) heating rate of 12" C. per minute was used. The temperawere determined on samples ignited from 150' to 1040' C. Throughout the work, unless otherwise stated, 5:2 plaster: water ture was obtained with a calibrated Chromel-Alumel thermoratios were used. Times of set were measured in the usual mancouple, and the phase changes were indicated by a spot ner ( 2 ) with a modified Vicat peedle wzighing 150 grams. Dengalvanometer connected to the differential thermocouple. sities were determined at 25 * 0.1 C. by displacement in A typical curve for gypsum, ground to pass a 100-mesh toluene with a specific gravity bottle. Complete removal of air from the powder, sieve, is shown in preliminary to the acFimre 3. After the tual volume adjustbreaks at 110" and ment, was secured by 190" C., due to deevacuating the air space surrounding hydration of the dit h e bottle. Soluhydrate and hemibility-time curves at h y d r a t e , respec25" 1 C. were obtively, only one tained by adding 10 grams of the Sam le further break octo 1 liter of distilEd curred, at 335" * water; samples were 5" C. This was in withdrawn from time the opposite direct o time for analysis by precipitation as tion to the previous calcium oxalate and breaks a n d indititration with 0.04 N cated an exothermic potassium permanreaction, the magniganate. Agitation was provided by bubtude of which could bling compressed air, be roughly estiwashed through pomated by comparit a s sium hydroxide son with the dihyand water, through the suspension. Heat drate and hemihyof hydration curves drate peaks as 200 were secured by' mixTIME IN MINUTES calories per gram ing 3 grams of plaster mole. FIGURE 4 with 3 ml. of water in O
r' /
\ INDUSTRIAL AND ENGINEERINqCHEMISTRY
AUGUST, 1939
1005
of the sa\ple ignited a t 365" C. as due to unchanged soluble anhydrite. (KOfigures are available for the tensile strength of casts ma e from the unground material ignited a t 420' C., but the s e t w r was undoubtedly very weak and could be crumbled between the fingers.) The absence of a sharp change from soluble to insoluble anhydrite, in accordance with the view that the latter is always the intrinsically stable form, is shown by Figure 4, where the temperature rise on mixing with water is recorded against time. Here curves 1 to 5 show a sharp initial rise, due to immediate conversion of soluble anhydrite into the hemihydrate, and a second rise due to hydration of the latter; curves 7 to 9 show no signs of a second rise. Thus, the samples ignited a t 365" and 420" C. are probably mixtures of the two anhydrites. It seems safe to conclude from Tables I and 11, therefore, that insoluble anhydrite, like natural anhydrite, does not set under ordinary conditions unless ground. After being ground, all the samples set on mixing with water, although their setting times were long (Table 111). Presumably natural or insoluble anhydrite does not set when mixed with water and left open to the air, because the water evaporates and leaves the mixture dry before any considerable degree of supersaturation can be obtained (IO). This is supported by sealed tube experiments of Farnsworth (IS)
5
0
40
20
60
100
80
120
140
180
IbO
HOURS
FIGURE 5
a test tube set in a Dewar flask. Tensile strengths were determined by the methods prescribed by the American Society of Testing Materials (2). In determining the porosity, the plaster casts were allowed t o set one week, weighed, and covered with water in a vessel which could be evacuated. A pressure of 10 inches of mercury was maintained for 1.5 hours, when the casts were dried on their surface with a towel, placed in weighing bottles, and weighed. Samples ignited at 420' C. and above were ground in approximately one-pound lots in a gallon-jar rotatine oebbIe mill. Each mind lasted 24 hours at 78 ryi. m.; beyond this $me particle size did not appear to diminish further under the given conditions. The surface areas of the unground 6 samples ignited below 420" C., and the ground samples were compared by means of the Klein turbidimeter (80). Because this instrument is 5 designed for use with portland cement, only relative values could be obtained. Unlike pure calcium sulfate, samples of ignited plasters of Paris showed some setting power with water up to 420" C.; no samples set without grinding above this temperature (Table 11). The sudden rise in density between 310" and 365' C. suggests that the samples ignited a t 365" and 420" C. consist predominantly of insoluble anhydrite, although they will set to give comparatively weak casts. In the case of the former, the ratio of its tensile strength (Table 111) to that of the cast from plaster ignited at 200' C. is 0.557; this is very close to the ratio of the initial and final temperature rises of the samples taken from Table 11, which are 0.555 and 0.540, respectively. This points strongly to the setting
6 4
.
V
B
- 3
s
s 2
I 0
0
20
Ignition Temp.
c.
c.
Cas04 .2Hn0 C ~ S Ol ~/ z. ~ 150 200 255 3 10 365 420 480 540 600 660 715 775 830 890 945 1005 1040
-
Final Temp. Rise a
c.
i:o
ii:9
11.0 7.6 8.5 7.4 4.0 3.0 2.9 2.9 2.9 2.8 2.8 2.1 1.3 1.0 0.4 1.5 1.9
13.9 10.9 11.4 9.8 5.9 4.0
z ~
.... ..
Density Q./ml. 2.37 2.72 2.71 2.72 2.71 2.68 2.81 2.81 2.86 2.86 2.85 2.84 2.91 2.91 2.91 2.92 2.93 2.96 2.94
60
80 100 MINUTES
140
120
IbO
I8
FIGURE 6
TABLE11. PROPERTIES OF IGNITED PLASTER OF PARIS Initial Temp. Rise
40
Free Lime
Betting5
70
...
0
N E
... ... ... ... ...
E E E E
0:ois 0.056 0:403 0.683 0.694 0.694 0.717 0.840 0.762 0.951
* E expanded on setting: B = did not expand on setting; N not set; * set only with oonsiderable di5culty.
Klein Time of Set 'Time Turbidim- Combined Gaged with of Set Tensile eter Water in Satd. Lime (Needle) Strength Reading Briquets Porosity Water Lb:/ Microc. Hours eq. an. amps. % % Hours CaSOr.l/zHzO" 0 . 3 7 466 35.4 *. 16.5 CaSO4.'/zHnO 0.13 536 32.3 .. 150" 0.28 547 32.0 200a 0.25 520 31.7 15:s 19:5 0:23 255" 0.28 525 31.9 310" 0.33 493 31.7 22:s .. 365' 0.29 290 31.1 2 420 193 17:2 31.6 26:s 0:33 480 230 3.5 11.1 31.2 540 215 30.9 3.8 10.0 26:3 o:i3 4 600 360 29.2 13.9 4 660 305 26.7 13.2 24:7 2:i, 25.1 715 248 5 .. 5 24:8 775 210 22.7 830 165 23.0 5.5 105 23.0 6 890 i:4 23:9 3:o 123 23.5 6 945 25.3 125 24.7 6 23.5 1005 7:2 3:a 133 23.7 1040 6 .. .. .. * Unground. Ignition Temp.
..
E B* N N
N N N
N N N N N N
-
TABLE111. PROPERTIES OF IGNITED CALCIUM SULFATE
.. ..
..
did
....
INDUSTRIAL AND ENGINEERING CHEMISTRY
1006
in which set was secured even with natural anhydrite. Figure 5 shows that the rate of solution is considerably accelerated by grinding, and the maximum solubility may be larger. Thus, a satisfactory degree of supersaturation can be reached before evaporation removes the necessary water. 600
I
I
I
I
TEMP. of DEHYDRATION,'C
FIGURE 7
Another factor of importance, emphasized by earlier workers ( 1 , $1, 24, 33) is the accelerating effect of free lime on set shown in Table 111. Some light is thrown upon the mechanism of this catalytic action by Figure 6. If faster set is due to a more rapid attainment of supewaturation, it might be expected that supersaturation with respect to the dihydrate would be attained more quickly in lime water. Figure 6 shows this not to be the case. The solubility of the dihydrate in the solution of calcium hydroxide used (0.0198 N ) was 1.73 grams per liter, agreeing well with the value 1.75 found by Cameron and Bell (9). This figure was exceeded in about 80 minutes by the insoluble anhydrite ignited a t 890" C. In pure water the same sample exceeded the normal solubility of 2.12 grams per liter in about 45 minutes. This lends support to the theory of Hansen (15), according to which the effect of the accelerator upon the precipitation rate of the gypsum from its supersaturated solution is the fundamental factor governing speed of set. 500
5: >n I
age) and their tensile strength (Figure 8). Both these quantities show dips not exactly identical in temperature range at intermediate temperature levels, and then decrease with the temperature of heating. (b) Samples burned a t 420" C. and above not only require grinding to be able to set a t all, but, as seen from the turbidimeter readings (Table III), samples burned in the lou7er temperature ranges resist grinding more than those ignited a t higher temperature levels. (c) Above 590" C. tensile strength and water absorption decrease markedly (Figure 7), while the sample ignited at 890" C. has a solution rate substantially lower than that heated a t 540" C. These facts demonstrate that, despite increased fineness due to grinding, increase of temperature causes a lowered tendency to set even though no phase change occurs. The dip in tensile strength and degree of hydration in the temperature range from about 400" to 500" C. may perhaps be due to resistance of the material ignited at low temperatures to grinding, in some way associated with its low density (i. e., the material may become brittle as density increases). The temperature (600" C.) above which the second drop in tensile strength occurs corresponds roughly to the point a t which a rise in density and an increase in particle size take place; this is apparent on the x-ray diffraction patterns for 300" and 790" C. (Figure 2). This suggests crystal growth as a possible cause of decreased hydration power, although it may be that a surface film or glaze of a resistant basic salt, only partially broken down by grinding, is formed. Whatever the explanation, the results of the determination of the degree of hydration and tensile strength of the material, in contradistinction to the x-ray data which would indicate no marked changes above 300" C., demonstrate that the changes are far more complicated. The practical application to the production of high-temperature or flooring plasters is obvious; temperatures of approximately 600 " C. should be used to secure ease of grinding, and this should be made as effective as possible.
,
Bibliography Abbott and Comley, Mass. Inst. Tech., B.S. thesis, 1937. Am. SOC.Testing Materials, Designation C26-33 (1933). Baron, S., Mass. Inst. Tech., B.S. thesis, 1938. Boyer-Guillon, -4., Ann. Conservs. Arts Metiers, [3] 2, 189 (1900). Broughton, Mass. Inst. Tech., Sc.D. thesis, 1938. Budnikoff, Chem.-Ztg., 47, 825 (1923). Budnikoff, Compt. rend., 183, 387 (1926). Bussem, Cosmann, and Schuster, Sprechsaal, 69, 405, 421, 433, 443 (1936).
5. 400 I t;
a 300 E
Y
d
p
VOL. 31, NO. 8
200 100
6
IO
80
PER CENT WATER
FIGURE 8
Nevertheless, particle size and presence of free lime, although amply sufficient to account for the fact that insoluble anhydrite does set, do not seem to be the only factors involved in the hydration of samples of the salt ignited a t different temperatures. Figures 5 and 6 are not strictly comparable as the rates of agitation employed were different; but comparison of Figures 6, 7, and 8 and Table 111, keeping in mind the fact that all samples burnt a t 420" C. and above were ground for equal lengths of time under identical conditions, leads to the following conclusions: (a) There is a striking parallelism between combined water content of the set briquets (determined by ignition of the briquets after break-
Cameron and Bell, J . Am. Chem. SOC., 28, 1220 (1906). Chassevent, Ann. chim. phgs., [ l o ] 6, 244, 313 (1926). ChassevenC, Compt. rend., 194, 786 (1932). Dunn, Chemistry & Industry, 57, 144 (1938). Farnsworth, IND. ENG.CHEX.,16, 967 (1925). Hall, Robb, and Coleman, J . Am. Chem. SOC.,48, 924 (1926). Hansen, IND.ENG.CHEM.,22, 611 (1930). Hill, J. Am. Chem. SOC.,59, 2242 (1937). Hill and Yanick, Ibid., 57, 645 (1935). Hoff, van't, Armstrong, Hinrichsen, Weigert, and Just, 2. physik. Chem., 45, 257 (1903). Hulett and Allen, J. Am. Chem. SOC.,24, 667 (1902). Klein, Proc. Am. SOC.Testing Materials, 33, Pt. 11, 303 (1934). Lafuma, Compt. rend., 194, 2309 (1932). Lambert and Schaffer, J. Chem. SOC.,1926,2648. Lashchenko and Kompansku, J . Russ. Phys. Chem. SOC., 60, 579 (1928).
Linck and Jung, 2. anorg. allgem. Chem., 137, 407 (1924). Marchal, J . chim. phys., 23, 38 (1926). Melcher, J . Am. Chem. SOC.,32, 5 0 (1908). Newman and Wells, J . Research Natl. Bur. Standards, 20, 825 (1938).
Partridge and White, J . Am. Chem. SOC.,51, 360 (1929). Ramsdell and Partridge, Am. Mineral., 14, 5 9 (1929). Straub, IND.ENG.CHEM.,24, 915 (1932). Toriumi and Hara, J . Chem. SOC.Japan, 55, 1051 (1934). Weiser, Milligan, and Ekholm, J. Am. C h m . Boo., 58, 1261 (1936).
Weissenberger, KoEZoid.-Z., 32, 181 (1923).
