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Journal of Applied Chemistry, 13, 4, pp. 158-167, 1963-06-01
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Use of compacts to study the sorption characteristics of powdered
plaster of Paris
Feldman, R. F.; Sereda, P. J.
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N21r2
no. 188
c.
2
BLDG
NATIONAL
RESEARCH
COUNCIL
C A N A D A
DIVISION O F BUILDING R E S E A R C H
USE O F COMPACTS TO STUDY THE
SORPTION CHARACTERISTICS O F POWDERED PLASTER OF PARIS
BY
R.
F. FELDMAN AND P. J. SEREDA
R E P R I N T E D FROM
J O U R N A L O F APPLIED CHEMISTRY, VOL. 13, NO. 4, APRIL 1963
P. 158
-
1671
JUL 15
1953
1
Ii
i
R E S E A R C H P A P E R N O . 188 O F T H EDIVISION O F BUILDING R E S E A R C H
O T T A W A
J U N E 1963
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Reprinted froin the Jorrrnal of Applied Clleilzistry, 1963, Vol. 13, p p . 158-167
USE OF COMPACTS TO STUDY THE SORPTION
CHARACTERISTICS OF POWDERED PLASTER OF PARIS
By R. F. FELDMAN and P. J. SEREDA
Compacts made froin powdered material are used in an eftort to explain anomalous results obtained from tlie isothermal uptalte of water vapour by various types of dehydrated gypsum. In the sorption isotherms, a sliarp discontinuity occurred a t R.H. between 0.25 and 0.40 and a secondary hysteresis was observed. Sorption of illethanol on these materials was also measured, and results were compared with those obtained with water.
The determination of the expansion isotherm enabled an application of the Gibbs adsorption equation to be made, whicli led t o a method by which i t was possible to differentiate bet\veen hemihydrate ancl adsorbed water and thus allow a calculation of a B.E.T. surface area from the water isotherm for the material.
From this calculation and others obtained by use of a modified Kelvin equation, i t was concluded that the anomalies noted above were due to sites on the surface of the plaster that rcquired an activation energy for 'sorption' to occur. The generation of these sites was considered dependent on the method of preparation of the anhydrous calcium sulphate. The water attached to these sites would thus be termed 'chemisorbed' water.
Introduction
The uptalte of water by dehydrated gypsum has already been investigated, but inconsis- tencies in the results and in the derived hypotheses rennin ~ n e x p l a i n e d . l - ~
A
previous paper" describes the preparation of rigid porous bodies, 'compacts', from powders. The present work describes the use of several types of dehydrated gypsum in the observation of the dimensional changes associated with sorption of water vapour. This has made possible the study of the effect of the decrease of surface free energy and the development of spreading pressure as the adsorbate associates itself with the surface.159 FELLIMAN G SEREDA-SORPTION 01; PLASTER OF P A R I S
Experimental
The apparatus used for this work was designed to esposc 12 samples si~nultaneously to the same vapour pressure conclitions. Six were mounted in individual tubes on quartz spirals of the McEain-Baltr type, giving the results ol' the weight changes t o a sensitivity of 2.5
x
10-5 g. lor sorption isotherms. The reinaining six samples were mounted on modified Tuckerman optical estensometers (see9
and placed in individual cells cquipped with optical windows. This procedure yielded diinensional changes to a sensitivity of 2x
in./in. with changes in vapour pressure.A three-stage oil diffusion pump backed by a rotary vacuum pump was employed to reduce the pressure of the system to mm. Hg or less before adsorbate was introduced from a bulb immersed in a bath that could be controlled a t temperatures between 0" and 70°1;, within 0 . 1 " ~ . The samples were maintained a t 70'1; by immersing the lower ends of the tubes and cells containing them to a depth of 12 in. in a controlled temperature bath. Room temperature was controlled a t 73 lor..
Water-free methanol was introduced into the apparatus, when required, by placing high- purity methanol over calcium oxide in a vacuum-tight vessel which could be attached to the apparatus. The air was then evacuated from the apparatus and the methanol was allowed to remain in contact with the calcium oxide for 48 11. Occasional agitation ensured intimate contact. The methanol was then slowly distilled over into the cold bulb, and lcept there as a source of this adsorbate.
Four different types of dehydrated gypsuin were studied:
(i) Pottery plaster-produced by the 'aridised process' consisting of heating gypsum with traces of calcium chloride.
(ii) Reagent grade precipitated gypsum (Fisher Scientific Co.)
(iii) 'B-Base' hydrocal-pure gypsuin rock dehydrated in the presence of steam (U.S. Gypsum Co.) (iv) 'Alba-Floc' dihydrate ( U S . Gypsum Co.), prepared by a precipitatioil technique.
The samples were made illto compacts compressed a t 46,400 p.s.i. following the technique outlined by Sereda & Feldman." Dehydration was achieved by heating the samples at 150°c or 200°c under vacuum of
<
mm. Hg for 4 11.For equilibrium to be attained at any one conditioil along the isotherm, a period of 1-2 days was generally allowed; a 15 11. period of negligible weight change mas always allowed to elapse before the system was considcrecl to be in equilibrium.
Results and discussion
Water sorptiogz on calciunt sttl~lznte (anhydrous) (1) The sorption isotherllts
The sorption isotherms for the four types of calcium sulphate inaterials were plotted as shown in Figs. 1-G(a).
All
samples were dehydrated at 150 or 200°c before esposure to the water vapour.I n general, the isotherin inay be divided into four regions:
(i) That represented by OA sho~vs a large increase in weight at a low relative humidity, mainly due to the formation of CaS0,2,0.5H,0 and the presence of some 'sorbed' water. I t may be observed that this point exceeds the theoretical amount of water (-6.6%) required for hemillydrate formation for samples dehydrated at 150°, escept that of the 'B-Base', but for samples dehydrated a t 200" this point is below the theoretical.
(ii) The region of the isotherms designated AB represents a small amount of water compared with that initially talcen up; i t is relatively independent of the temperature of dehydration. The amount of water sorbed in this region increases consecutively through 'B-Base', pottery plaster, 'Alba-Floc' and 'precipitated gypsum'.
(iii) At point B a discontinuity in the slope of the isotherin occurs in most cases, followed by a steady rise in the curve from points B to C. This discontinuity occurs for the different samples bet~veenp/povalues of 0.25 and 0.40, while with the 'B-Base' sample, there are two discontinuities. The effect of the temperature of dehydration on the occurrence of these discontinuities appears to be only a modifying one. I n pottery plaster samples the higher temperature of dehydration causes two discontinuities to occur, as compared with one for the lower temperature of clehy- dration (Figs. 3a and 4 4 . For precipitated gypsum there appears to be little discontinuity.
(iv) On desorption a hysteresis loop occurs and extends for the whole range of pressures
FELDMAN G SEREDA-SORPTION OF P L A S T E R OF P A R I S
Fig. 1. Res~rlts from colnpact of 'B-Base' Ircr~iihj~drate del~j~drated a! 150"
(a) sorption isotherms (b) expansion isotherms (c) dinlensional change as related to water sorbed 0 adsorption desorption
Fig. 2. Sorptioi~ isotl~evi>~s for water or1 con~pact of 'B-Base' Iremzl~ydrate del~ydrated at IZiffere?zt temper-
atures
(a) dehydrated a t 200" (b) dehydrated at > 150 < 200" 0 adsorption dcsorption
without rejoining the adsorption cycle. Although it never meets the adsorption part of the loop, i t levels off and continues almost parallel to the adsorption part of the cycle AB. The extent of the separation between the desorption and sorption curves in the region AB is ternled secondary hysteresis and is present to a larger clegree in the 'B-Base' and pottery plaster samples than in the other two. ?'his was also observed for the extent of the discontinuities.
(2) Di?tzensional clznnges resz~lti~zg from water sorption on plaster (expansio~z isotlzerms)
Expansion isotherms for plaster materials can be divided into regions corresponding to those used above for describing the sorption isotherms (Figs. 1, 3-5 and 6b) and have similar characteristics involving the discontinuity and double hysteresis. To represent inore clearly the expansion produced by water sorbed at any particular pressure, a plot of sorbed water @W% vs expansion @l/l%, is shown on Figs. 1, 3-5 and 6(c). 0 , A, B, C represent the same points as in the previous plots.
161 FELDMAN 6 SEREDA-SORPTION OF PLASTER OF PARIS
(a) sorption isotherms (b) expansion isothcr~ns (c) dimensional changes as related to water sorbed
0 adsorption desorption
(a) sorption isotherms (b) expansion isothcr~ns (c) climensiollal changes as related t o \vater sorbecl
0 adsorption desorption
Four regions are again apparent:
(i) OA, where hemihyclrate formation occurs and whcrc thc average slope of the curve of
All1 vs AW is the lowest for the whole plot.
(ii) AB, where a linear plot may be observed ancl ivhere the slope is a t its greatest value. (iii) BC, where the slope decreases gradually iron1 B to C in Figs. 3, 4, 6(c), and in Figs. 1 and 5(c) ivhere the slope actually increases over a portion of this region. Nevertheless a transition from AB to BC can always be observed.
FELDMAN & SEREDA-SORPTION OF PLASTER OF PARIS
Fig, 3. I ? E s I [ I ~ A J Y O I I I C O I I I ~ X L C ~ OJ 'illbn-FIoc' ~ L I L J ~ ~ Y C L I E de1~)ld~nted nt 150"
(a) sorption isotherms (b) expansion isotherms (c) dirnensional changcs as rclatcd t o water sorbed 0 adsorption desorption
sorption
Fig. 6. X e s ~ t l t s J y o n ~ cori~pccct ~Jpvccipitrcted g y p s u m clel~yyd~ated nt 200"
isotherms ( b ) cspansion isotherm (c) dinlensional changcs a s related to 0 adsorption desorption
water sorbed
(iv) The desorption loop (in the region BC) lor Fig. G(c) falls below the adsorption loop but rejoins the linear portion AB. Sorption is carried up to a ;h/;ho value of 0.83. I11 Fig. 5(c) the desorption curve does not drop below the adsorption curve, but sorption is continued only to a
$/Po
value of 0.70.Sor$tion of metl~anol on anlrydvo?is calciunz su1;hhate
Having observed unusual characteristics of the isotherm for water it was of interest to learn whether this was specific for water, and an isotherin for methanol on plaster was therefore determined.
A smooth type I1 isotherin4a was obtained with carefully dried methanol as adsorbate and anhydrous calcium sulphate as adsorbent (Fig. 7b). After early sorption any increase with relative
163 FELDllfAiV & SEKEDA-SORPTION OF PLASTER 01; PAIIIS
vapour pressure was very low ancl a significant increase was observed only at a relative pressure of 0.75 for the sample obtained from clehydrated pottery plaster. For that obtained Iroin clehydrated precipitated gypsum the sorption curve was similar, although greater sorption occurrecl (Fig. 7a). On the desorption curve, secondary hysteresis was present for both samples, but to a larger extent for the dehydrated precipitated gypsum. I t may bc seen that very little clesorption occurred after a relative pressure of 0.85 for this latter sample, although there was a pronounced hysteresis loop at a relative pressure of 0.56 for the other sample.
The discoiltinuity observed ~ v i t h water sorption is not present with methanol sorption, although secondary hysteresis is present, to a larger extent for dehydrated precipitated gypsum. Assuming that the methanol molecule occupies an area of 15.2A2, an area of 11.4 nl."g. Ivas obtained by the B.E.T. method of calculation for the dehydrated pottery plaster sample, and one of 16.7 m.Z/g. for the dehydrated precipitated gypsum. I t must be stressed, however, that t h e inethanol isotherms were determined in both cases on the anhydrous calcium sulphate.
-4 lznlysis of reszilts
I t has been observed for a long time that the water content of plaster of Paris varies and even exceeds the theoretical. Dunn" suggested a series of subhydrates ranging from 0 to 213 H,O, as did B u n i ~ . ~ Another area of disagreement is provided by the questioil of whether CaS04,0.5H,0 is a hydrate or a zeolite. Jury LO Light3 observed a discontinuity in the isotherm, but this has not received much further attention. The explanation was that further hydration had taken place, and that 12CaS04,SH,0 had been formed froin 12CaS0,,6H20. The two extra water molecules were attached to two 'forbidden sites' that required a further activation energy for their hydration.=
This explanation has been discounted here. The All1 vs W A plots (Figs. 1, 3-5 and G(c)) show three distinct regions, the slope being very
low for the region where henlihydrate is formed,
,
and large for the region where the discontinuity
is observed. As the two forbidden sites are part 2 I 2 of the same structural cell as the others in the
12CaSO4,SH,O unit, a similar expansion for unit weight gain might be expected. This is not the case, however, and it may be concluded that tlie discontinuity is not due to the forination of a further hydrate.
Fig. 7. S o r p t ~ o ? i zsothertns for ~ i ~ e t l ~ n ? ~ o l o ~ n ~ ~ l r y d ~ o z i s calczz~?i~ s u l ~ l ~ n l e
(a) froin precipitated gypsum (b) from pottery plaster 0 adsorption
0
desorption0 0 1 2 0 2 6 0 3 6 0 6 8 0 6 3 0 7 2 0 8 4 0 9 6
?/Po
I t is possible that this discontinuity is clue to multiillolecular adsorption ancl subsequent capillary condensation in small pores, although 011 this basis a sharp rise a t a some~vhat similar
R.H. should have been observed with inethanol as aclsorbate. Based on the assumption of capillary condensation, ho~vever, the pore size distribution was calculated from the desorption curve for pottery plaster (Fig. 3b) down to the point where it flattened out at a R.H. of 32%. This calculation was made by the use of a modified form of the I<elvin equation,
FELDMAN 6 SEREDA-SORPTION OF PLASTER OF PARIS 164
where it is assumed that the adsorbate completely wets the solid, and that at any point on the desorption isotherm all pores have an adsorbecl film of thicliness
t ,
wliich may vary with pressure of adsorbate $. A11 pores sinaller than radius Ri arc completely filled by aclsorbed water, and for a filled pore having a radius 01 Ri the meniscus is assumed to have a radius of (R, - t ) . Thevalue of the tllicltncss of the aclsorbecl layer
t
as a function of pressure was talien from an estimate by Powers7 for cement gel in which it was assunled that the thicltness of the strongly adsorbed layer does not exceed two nlolecular diameters.The surface area of the aclsorbent may be calculated from tlle pore size distribution by a numerical integration of the area of all the pores. Assuming cylindrical pores,
where
A
Ai and AV i
are the area and volume pcr g. of sample for each group of capillaries talten to have X i as its corrected mean pore radius. Pierce's methods was used in this calculation; a correction is inade for desorption that occurs from the film of larger pores that have already lost their menisci, as ~vell as for the fact that the area obtained is that of the film and not the pore \valls.Thc cu~nulative area calculatecl from the pore size distribution was 34.6 m.?/g. This calcula- tion was lnacle on the sainple of pottery plaster, ancl, since the clesorption loop of the dehyclrated precipitated gypsum was not very clifferent from that of the pottery plaster, the calculation mias not repeated for the other samples, although it was assuinecl that the result would be similar.
\Vhen this result is coinpared wit11 those obtained from the methanol isotherms by the B.E.T. area calculation, it is found that there is a thi-ee-fold clifferencc for pottery plaster and a two-fold
difference for dehyclrated precipitated gypsum. For a true con~parison to be made, however, the
area should be calculated from the water isothcrm by the B.E.T. procedure. As can be seen froin the first point obtained on the water isotllerins of Figs. 1-6(a), the weight gained varied froin 7.0% for the 150"-deliydrated sample of 'Alba-Floc' dihydrate to 4.20% for the 200"-deliydrated sample of B-Basc l~einihydrate. This malies it impossible to calculate the surface area of the samples by the B.E.T. method from the water-vapour isothenns; clearly, one cannot determine what portion of the water first talien up is used for the formation of the hemihydrate and what portion is physically adsorbed.
X method based on the Gibbs aclsorption equatioil has been worked out here to estimate the quantity of tlle sorbed \\rater and thus allon a calculation of the area by the B.E.T. method. This equation relates the lowering of the surface free energy, A F , as sorption occurs on a surface, as follows
Al; = RT
$:
s/+.d+...
(3) where $ is the pressure 01 the adsorbate in dynes/cm." and s is the surface coilcentration ol adsorbate on adsorbent in g. mols./cinY \When A F is calculated by graphical integration, a plot of All1 vsAF
shoulcl, according to the Bangham relation,"yield a straight line tllrough the origin. Al/l is the expansion clue to sorption and ?, is a constant related to the elastic coefficient of the material. All the values required for these plots are known except the absolute values for the weight gain and dimensional change clue to sorption, and the B.E.T. surface area calculated from the water sorption isotherm.
The samples at approsimately 10% humidity may be considered as CaSO,,O.SH,O as adsorbent with some water present as adsorbate. If it were possible to remove this adsorbed water without decomposition of the hemihydrate, the climensional change Al/l(A) resulting from the adsorption of water AW(A) and the dimensional change Al/l(H) from hemihydratc formation AW(H) would be known. Considering these expansions and weight gains as indepen- dent of one another we have the equ a t ' ions
where Al/l(T) and AW(T) are the actual weight gain and dimensional change observed. Since no further helllillydrate forination will talie place beyond approximately 10% humidity, line AB on the ,5111 vs AT* plot (Figs. 1, 3, 4, 5, 6c), which is approximately linear,
165 FELDMAN & SEREDA-SORPTION OF PLASTER OF P A R I S
is a relation between Al/l(A) and AW(A). This same plot for other adsorbents, e.g. Vycor glasslo, calcium carbonate and Cab-0-Sil silicaqor low coverages from no adsorbed water to one or two ~nolecular layers, shows a linear portion through the origin.
I t nlay thus be concluded that if the linear portion AB were extrapolated back, this line would pass through the point where both AW(A) and Al/l(A) are zero. The residual AW(T) and Al/l(T) would be equal to AW(H) and Al/l(H) as seen frorn the above equations. A trial and error procedure is used by taking any point on this extrapolated line as the zero for AW(A) and Al/l(A), calculating the B.E.T. area and determining A F froin the Gibbs equation for several $/$, values; a plot of Al/l vs A F did yield a straight line; this procedure was repeated until a straight line through the origin was obtained (Fig. 8). The values obtained from this are shown in Table I.
A value for Young's modulus for the plaster may be calculated from another relation from Banghain & i l l a g g ~ , ~
E = pA/h
...
(5)where p is the density (g./c.c.), A is the internal surface area ( ~ m . ~ / g . ) and h is the constant as in
equation (4).
Fig. 8. Eupni2siorz oJ conzpacts oJ plflsler as a Jz~~zcti07z OJ the calcz~lated spreading Joyce
dehydrated precipitatcd gypsum
A
pottery plaster dehydrated a t 150' 0 pottery plaster dehydratccl a t 200' dehydrated 'Alba-Floc' dihydrateclehydratcd 'B-Base' hemihydratc (numerals on curves are PIPo values)
Table I
Expansion \Vcight gain R.E.T. I<elvin B.E.T, due to due to area frorn equation arca frolu Dehydration henlihydrate hemihydrate ~iiethanol area, water temperature, formation, forruation,
01 E , isotherm, isothcr~u,
Oc % /o dynes/cm.= m.'/g. r ~ i . ~ / g . m.?/g. Pottery plastrr A 1.50 0.060 6.60 I.OG x lot0 1 1 .4 34.6 12.3
(wm* at,PlP. of 0.22) Pottery plaster R Dehydrated precipitated 200 0.10 5.68 2.15 x 10" 1G.7 34.6 34.1 g y p ~ ~ i m (IVm* a t PIP. of 0.31) 'B-Base' 1.50 0.090 5.53 1.34 x IO'O - - 5.0 hemihydrate (Jf"m* a t PIP. of 0.21) 'Alba-Floc' (lil~ydrate 150 0.16 G.G2 5.67 x loio - 25.10
Of'm* a t f l P o of 0.231 'Weight of water required for one complete ~ilonolnycr on the surface of the adsorbent.
FELDMAN 6 SEREDA-SORPTIOA' OF PLASTER OF P A R I S 166
These values are included i n Table I and are o f t h e same order o f magnitude as those obtained b y Bangham
tb
Maggs for coal; n o value for plaster obtained b y mechanical means has beenloulld for comparison, b u t these values are included here t o show t h a t b y t h e use o f compacts i t is possible t o obtain a value for Y o u n g ' s nlodulus t h a t is o f t h e same order o f nlagnitude as for other materials.
B y comparisoll o f tlle surlace areas o f t h e samples obtained b y tlle tllree d i f f e r e n t methods, i t m a y b e observed t h a t , for pottery plaster, t h e results f r o m t h e methanol and water isotherms agree quite closely, although t h e y are only about one-third o f t h e values obtained from t h e Kelvin equation calculation. I t is concluded t h a t t h e discontinuity is not t h e result o f capillary condensatioll and t h u s a calculatioll based o n t h e I<elvin equation is in error. For dehydrated precipitated g y p s u m t h e values lrorn t h e water isothernl calculated b y t h e B.E.T. method and t h e Kelvin equation agree closely, but t h e B.E.T. area calculations lrolll t h e methanol and water isotherms differ greatly. A s a result o f these determinatiolls o f surface area b y t h e B.E.T. method f r o m t h e water sorption isotherm, i t was possible t o calculate t h e sorbed water at 5% hunlidity and t o determine t h e espansioll due t o sorption at this h u m i d i t y f r o m t h e extrapolated curve A B . T h n s , t h e curve for t h i s h u m i d i t y t o zero lor t h e sorption ancl expansion isotherms were drawn in as sl~o\vn o n Figs. 1 and 3-6, and i t m a y b e observed f r o m t h e original curve t h a t some hydration t o hemihydrate still occurred at 5% h u m i d i t y . I t m a y also b e observed t h a t expansion due t o hemihyclrate formation is low; this agrees wit11 observations o f Bunnl1 t h a t t h e dimensional change i n t h e lattice d u e t o hemillydrate formation is small.
T h e unusual form o f t h e water sorption isotherms on t h e various t y p e s o f plaster, in contrast \\lit11 t h e methanol sorption isotherm, can b e explained on t h e basis o f t h e experimental \vork presentecl here. T h e discontinuity is n o t considered t o b e t h e result o f capillary co~ldensatioll or formation o f subhydrate, b u t o f activated aclsorption.13
Tlle existence o f an intrinsic activation step i n chemi-sorption is now well recognised ancl t h e magnitude o f t h e energy barrier m a y v a r y considerably w i t h t h e system concerned. I t is a generally accepted h y p o t h e s i s 1 ~ 1 ~ a t t h e activation energy required t o surmount t h e barrier between t h e approaching molecule and t h e surface is supplied entirely b y t h e adsorbent. I n this case, as t h e adsorbate goes on t h e surface, changes t o t h e adsorbent occur. T h i s can b e well understood since t h e adsorbent is ionic in nature and b y n o means inert; there is considerable interaction between t h e adsorbent ancl adsorbate I n e f f e c t , an increase o f t h e area available for adsorption is generated during coverage o f t h e aclsorbent b y t h e adsorbate. W h e t h e r this is achieved b y modification o f t h e lattice spacing allowing g-reater penetration o f t h e adsorbent, or b y a change i n interaction energies nlaking more sites available lor adsorption, is a ~ n a t t e r o f speculation. T h u s , t h e cllaractel-istics ol t h e isotherms on t h e different plaster samples will depend o n their methods o f preparation and will v a r y considerably w i t h t h e adsorbate usecl. T h e fact t h a t n o colnparable discontinuity occurred for t h e methanol isotherm at l o w pressures is probably d u e t o t h e diflerence i n t h e clipole m o m e n t s per unit area o f t h e methanol and water molecules, as i t is t o b e expected t h a t t h e interaction between aclsorbate and adsorbent will influ- ence t h e generation o f further sites for adsorption.
T h i s t y p e o f sorption cannot b e o f a reversible nature and t h e esplanatioll o f t h e discontilluity based o n t h e above consideratiolls would necessarily include a n irreversible isotherm; this will account for t h e secondary hysteresis in these isotherms. An explanation for t h i s seconclary hysteresis has been attempted b y Grcgg & Wil1ing;"solated pockets o f dihydrate are thought t o exist, having been formed at liigher humidities. X-ray diffraction work b y t h e authors and o t l ~ e r s , ~ i n which samples were esposed t o various h u m i d i t y conditiolls before final sealing i n sample tubes, gives n o evidence for this explanation.
I f t h e calculation o f t h e reduction o f surface free energy lowering or o f spreading pressure is carried o u t for relative pressures higher t h a n 25-10% for d i f f e r e n t samples, t h e plot o f
A1/1
v s
4
(Fig. 8) yields a surprising increase i n t h e slope ol this curve ilnmediately a f t e r t h e point equivalent t o t h e discontinuities. For t h e sample o f clehydrated precipitated g y p s u m t h e plot yields a straight line u p t o t h e point obtained at 60% h u m i d i t y , a f t e r which a clecreasc i n slope is observed. T h e s e plots can b e understood i f t h e discontinuity is considerecl t o b e t h e result o f activatecl-adsorbed water. I f new ssulace is m a d e available spontalleously t o a n adsorbate a t25% relative vapour pressure, t h e amount adsorbed \vould be for t h e relative vapour pressure
range f r o m 0 t o 25%. T h u s , this adsorbate would n o t b e clesorbed a t 25%, b u t would b e i n t h e range f r o m 25 t o 0%.
167 FELDMAN 6 SEREDA-SORPTION OF P L A S T E R OF P A R I S
The calculation of the spreading pressure, by the Gibbs equation using the pressure a t \vhich the discontinuity occurrcd, \vould be in error. Since the discontinuity is estrenlcly small for dehydrated precipitatecl gypsum, thc plot of All1 vs (G contiilues linearly up to a j,iP, value of
0.60, aftcr which presumably capillary condensation lnay commence. The history and method
of the preparation of the sample must affect chemi-sorption on tlic surface, since differences are observcd for the isotherlns of the difiei-ent types of plaster. I t is knolvn tliat, in the production of pottery plaster by the ai-idised process,14 large ordercd crystals of hemihydrate are formcd. The preparation of 'B-Case' hydrocal hemihydrate by the dehydration of gypsum in the presence of steam produces the largest and best ordered crystals of the four types of plaster.
This is well illustrated in the isotherm of the material and the very low surface area calcu- lated from it. The precipitated gypsum samples, both the reagent grade and the 'Alba-Floc' dihydrate, ha\-e small crystals to start with, and deliydration to anhydrous calcium sulpliate in vacuum would reduce further the size and order of the crystals. I t may be espectcd that there will be differences in the state of thesc samples in comparison with the other two, and more active sites in tlie case of the dehydrated gypsum samples nlay already exist before sorption takes place.
Conclusion
By the use of compacts of powdered plaster of Paris, it has been possible to determine the dimensional chailges associated with the sorption process on their surfaces. By application of the Gibbs adsorption equation to the sorption and dinlensional change data and an assumption that for low coverages of sorbed water the All1 vs AW plot is linear, a trial and error procedure was adopted from which quantities of sorbed and hemihydrate water were estimated. As a result, the surface areas of plaster samples, calculated by the B.H.T. method from the water sorption isotherm, were obtained. I t was concluded from a comparison of this surface x e a and those obtaiiied from other methods that the discoiitinuity observed at $I$, values of 0.25 to 0.40 was not froin capillary condensation.
Further results from the dimensional change-spreading pressure plot and a secondary hysteresis observed in all the isotherms provided evidence that the discontinuity resulted from cheini-sorption on activated sites, generated during the process of adsorption.
Acknowledgments
Thc authors gratefully ackno~vledge the valuable assistance of Messrs H. F. Slade and S. E.
Docls in setting up the apparatus and recorcling the information. This paper is published with the approval of the Director of the Division of Building Rcscarch, National Research Council, Canada.
Division of Building Research, National Research Council,
Ottawa 2,
Canada.
Received 24 August, 1962;
amended manuscript 19 October, 1962
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