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Cement and Concrete Research, 1, November 6, pp. 607-620, 1971-11-01
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Differentiation of interlayer and adsorbed water in hydrated portland
cement by thermal analysis
Feldman, R. F.; Ramachandran, V. S.
https://publications-cnrc.canada.ca/fra/droits
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https://nrc-publications.canada.ca/eng/view/object/?id=b4ac3998-e414-488a-b168-c01216200b3c https://publications-cnrc.canada.ca/fra/voir/objet/?id=b4ac3998-e414-488a-b168-c01216200b3cCEMENT and CONCRETE RESEARCH. V o l . 1 , pp. 607-620, 1971. Pergamon Press, I n c . P r i n t e d i n t h e U n i t e d S t a t e s .
~ N A
L
yZED
DIFFERENTIATION OF INTERLAYER AND ADSORBED WATER IN HYDRATED PORTLAND CEMENT BY
THERMAL ANALYSIS
R. F. F e l d m a n and V. S. R a m a c h a n d r a n
Division of Building R e s e a r c h , National R e s e a r c h Council of Canada Ottawa 7 , O n t a r i o , Canada
(Communicated by G. L. ~ a l o u s e k )
ABSTRACT
Work leading t o the new m o d e l of h y d r a t e d portland c e m e n t h a s shown that m u c h of t h e w a t e r p r e v i o u s l y c o n s i d e r e d t o r e s i d e on f r e e s u r f a c e s e x i s t s a s i n t e r l a y e r w a t e r . Ln the p r e s e n t w o r k
sample s we r e conditioned a t v a r i o u s r e l a t i v e humidities f o r t i m e s varying f r o m l f t o 14 m o n t h s and w e r e d r i e d t o two d i f f e r - e n t l e v e l s before conditioning, one being d - d r y . Conditions w e r e chosen, b a s e d on the m o d e l , s o that the different s t a t e s of w a t e r could be c l e a r l y i l l u s t r a t e d . I t w a s concluded that t h e r m a l a n a l y s i s allows differentiation of i n t e r l a y e r and physically
a d s o r b e d w a t e r and t h a t the different d e g r e e s of drying and v e r y long t i m e s allowed f o r equilibration support a l l sorption r e s u l t s on which the new m o d e l of hydrated portland c e m e n t w a s based. SOMMAIRE
L e s t r a v a u x m e n a n t
B
l a nouvelle f o r m u l e de c i m e n t portland h y d r a t e ont dCmontrC qulune g r a n d e p a r t i e de l l e a u qulon c r o y a i t qui d e m e u r a i t a u p a r a v a n t s u r l e s s u r f a c e s l i b r e s , e x i s t e s o u s f o r m e d'entrecouche d l e a u . Dans l a prCsente Ctude, l e s Cchanti- llons ont CtC conditionnCsB
d i v e r s e s humidites r e l a t i v e s pour d e s pCriodes de t e m p s v a r i a n t e n t r e 1f m o i s & 14 m o i s , e t f u r e n ts o u m i s au sCchage & deux niveaux diffCrents avant le conditionne
-
m e n t , un d e s niveaux Ctant "d-dry." L e s conditions f u r e n t c h o i s i e s , basCes s u r l a nouvelle f o r m u l e , afin que l e s d i v e r s Ctats de l ' e a u p u i s s e n t Etre c l a i r e m e n t illustrCs. On en a conclus que l'analyse t h e r m i q u e p e r m e t l a diffgrenciation d e s e n t r e - couches e t de l l e a u adsorbCe physiquement, e t que l e s d i f f e r e n t s degrCs de sCchage e t l e s t r h s longues pCriodes de t e m p s p e r m i s e s pour llCquilibration supportent t o u s l e s rCsultats de d6sorption s u r l e s q u e l s l a nouvelle f o r m u l e de ciment portland hydrat6 Ctait basCe.
P r e s e n t e d at t h e Second Cement Symposium held a s p a r t of t h e 73rd Annual Meeting of t h e A m e r i c a n C e r a m i c Society, Chicago, A p r i l 1971.
V o l . 1, N o . 6 WATER, INTERLAYER, ADSORBED, THERMAL A N A L Y S I S
Introduction
A new m o d e l f o r hydrated portland c e m e n t h a s been r e c e n t l y d e s c r i b e d ( 1 ) . P o r t i o n s of the m o d e l w e r e based on r e s u l t s f r o m w a t e r sorption e x p e r i m e n t s ( 2 ) and conclusions w e r e m a d e with r e g a r d t o the s t a t e of the w a t e r and i t s r o l e in the s y s t e m . The m o d e l r e c o g n i z e s that the hydrated calcium s i l i c a t e s in hydrated portland cement a r e l a y e r - s t r u c t u r e d but poorly oriented, and with a spacing between l a y e r s that m a y v a r y f r o m region t o region. Much of the w a t e r which had previously been c o n s i d e r e d t o r e s i d e on f r e e s u r f a c e s i s now considered t o e x i s t a s i n t e r l a y e r w a t e r , and m a n y of the physical and m e c h a n i c a l p r o p e r t i e s of hydrated portland cement a r e a s c r i b e d t o this.
The m o d e l h a s a r o u s e d considerable i n t e r e s t but doubts have been e x p r e s s e d ( 3 ) . In a continuing r e s e a r c h p r o g r a m a t the National R e s e a r c h Council on the new m o d e l , significant new evidence w a s developed on the m a n n e r in which w a t e r o c c u r s in C-S-H. In work on c e m e n t s s o f a r r e p o r t e d no s y s t e m a t i c work s e e m s t o have been done on the t h e r m a l a n a l y s i s of
s a m p l e s a t controlled conditions of humidity (4, 5). However, t h e r m a l a n a l y s i s of s a m p l e s equilibrated a t different r e l a t i v e humidities, based on the m o d e l , c l e a r l y differentiates between a d s o r b e d and i n t e r l a y e r water. The r e s u l t s a r e p r e s e n t e d i n t h i s r e p o r t .
E x p e r i m e n t a l Mate r i a l s
A Type I portland cement was bottle-hydrated f o r 1; y e a r s a t a w a t e r - t o - s o l i d r a t i o of 5. I t was a l m o s t completely hydrated; carbon dioxide content was 1 % and the s u r f a c e a r e a by nitrogen adsorption was
2
22 m / g m . The bottle-hydrated powder w a s d r i e d t o 1170 R. H. and s c r e e n - ed over a 100-mesh sieve. The residue of l a r g e C a ( 0 H ) c r y s t a l s was
2 ground and r e m i x e d with the powder.
Sample Conditioning
The bottle-hydrated powder was r e s t o r e d a t 1170 R. H. ( o v e r s a t - u r a t e d solution of LiC1) i n a d e s i c c a t o r f o r
6
m o n t h s and designated p r e p -V o l . 1, N o . 6 609 WATER, I N T E R L A Y E R , ADSORBED, THERMAL A N A L Y S I S
a r a t i o n A. One portion of A w a s s p r e a d 2 t o 3 m m thick in d i s h e s a n d p l a c e d i n a l a r g e d e s i c c a t o r with a l a r g e - d i a m e t e r tube fixed t o the lid. The tube led t o a pumping s y s t e m which consisted of a t r a p cooled with solid CO and
2 a two-stage r o t a r y pump. T h i s c o r r e s p o n d s t o the well-known d-drying p r o c e d u r e . A f t e r 5 weeks of pumping the m a t e r i a l was placed in a vacuum v e s s e l and heated f o r 3 h r a t 100°C. T h i s sample i s designated p r e p a r a t i o n B. Another portion of A designated p r e p a r a t i o n C was placed in a vacuum v e s s e l and heated t o 70°C f o r approximately 2 h r . It was found t o contain approximately 370 m o r e w a t e r than the d - d r i e d s a m p l e . S m a l l amounts of preparations A , B and C w e r e placed in d e s i c c a t o r s o v e r s a t u r a t e d s a l t
solutions ( i n t e r m i t t e n t l y s t i r r e d by magnetic s t i r r e r s ) t o control a t 11, 32, 44, 58, 66, 76, 84 and 10070 RR. H., respectively. After a period of t i m e s a m p l e s of p r e p a r a t i o n s B and C exposed at different humidities a s d e s c r i b e d above w e r e placed in another vacuum d e s i c c a t o r conditioned a t 11% R.H. T h e s e reconditioned s a m p l e s w e r e designated B(R) and C(R). T h e s c h e m a below i l l u s t r a t e s how the s a m p l e s w e r e designated and t h e i r history.
I
P r e p a r a t i o n B P r e p a r a t i o n CJ/
P r e p a r a t i o n B(R) d - d r i e d 370 m o r e H z 0 thanJ,
P r e p a r a t i o n C ( R )'4
11,Table I shows the length of t i m e s a m p l e s of p r e p a r a t i o n s A, B, C, B(R) and C ( R ) w e r e held a t e a c h condition before they w e r e analyzed. A s can be s e e n , the t i m e s w e r e usually about s e v e r a l months. E n t r i e s in the columns under TGA and DTA show when s a m p l e s w e r e t e s t e d by e a c h technique.
p r e p a r a t i o n B
61
0
Vol. 1 , No.
6
W A T E R , I N T E R L A Y E R , A D S O R B E D , T H E R M A L A N A L Y S I S
T A B L E I P e r i o d s ( i n M o n t h s ) D u r i n g W h i c h S a m p l e s W e r e C o n d i t i o n e d a t R e s p e c t i v e R e l a t i v e H u m i d i t i e s a c k t o 11% R.H. a c k t o 11% R. H. a c k t o 1 1 % R . H . a c k t o 11% R . H . a c k t o 11% R . H . a c k t o 11% R . H . a c k t o 1 1 % R . H . B I D T A 3 - 6 - 3 - 6 - 3 - 6 - 3 - 6 - 3 - 6 - --
3 - 6 - 3 - 6-
T h e r m a l A n a l y s i sIn t h i s w o r k DTA and TGA w e r e c a r r i e d out u n d e r controlled con- ditions of humidity, the d e t a i l s of which have been published e l s e w h e r e ( 6 ) .
D T A TGA 1 . 2 5 - 1 . 2 5 - 1 . 2 5 - 1 . 2 5 - 1 . 2 5 - -
-
1 . 2 5 - 1 . 2 5-
D T A (1) T h e r m o g r a v i m e t r i c A n a l y s i sA s p e c i a l l y c o n s t r u c t e d gloved e n v i r o n m e n t a l box w a s u s e d with a Cahn R. H. e l e c t r o b a l a n c e (6). S a m p l e s could be weighed and loaded onto the b a l a n c e and r u n s p e r f o r m e d without t h e s a m p l e s being exposed t o outside conditions. N o r m a l TGA r u n s w e r e p e r f o r m e d a t a heating r a t e of 1 0 " ~ / m i n under vacuum. S t a t i c r u n s w e r e p e r f o r m e d by heating a t 100°C f o r 3 h r . S a m p l e s w e r e compacted a t 5000 p s i t o p r e v e n t blowing and l o s s of powder d u r i n g evacuation and heating.
( 2 ) D i f f e r e n t i a l T h e r m a l A n a l y s i s
T h e 900 Du P o n t t h e r m a l A n a l y s e r w a s u s e d f o r d i f f e r e n t i a l t h e r m a l m e a s u r e m e n t s . T h i s unit h a s a r e m o t e plug-in module called the c a l o r i - m e t r i c c e l l . The c e l l i s placed i n a gloved box controlled a t the r e q u i r e d humidity and connected to t h e r e c o r d e r t h r o u g h a length of shielded w i r e . S a m p l e s of r e q u i r e d quantity (16 m i l l i g r a m s ) w e r e weighed in a balance i n
TGA TGA D T A TGA
Vol. 1, N o . 6 61
1
WATER, I N T E R L A Y E R , ADSORBED, THERMAL A N A L Y S I S
the gloved box. T h e r m a l c u r v e s w e r e obtained a t heating r a t e s of both 10 and 20°C/min. Results a r e r e p o r t e d h e r e f o r 10"C/min only.
Re sult s ( 1 ) T h e r m o g r a v i m e t r i c Analysis
The dynamic TGA r u n s did not show a s c l e a r a delineation of differ
-
ent types of w a t e r a s the DTA c u r v e s . Hence, the TGA equipment was a l s o used under static conditions, where the s a m p l e s equilibrated a t e v e r y con- dition w e r e heated in vacuum f o r 3 h r a t 100°C. It w a s found previously ( 2 ) that t h i s condition w a s equivalent t o the d - d r y condition, and thus the r e s u l t s would yield quantitative information with r e g a r d t o the amount of w a t e r ine a c h s a m p l e above the d - d r y condition. A s a r e s u l t the intensity of e a c h DTA curve could be well defined f o r the region of "evaporable" w a t e r ( 7 ) on the b a s i s of the above.
An advantage gained by performing the s t a t i c t h e r m a l g r a v i m e t r i c a n a l y s i s i s that sorption i s o t h e r m s can be constructed. These a r e shown in Fig. 1 f o r . t h e p r e p a r a t i o n s A, B and C. Another advantage i s that t h e s e i s o t h e r m s a r e constructed f r o m s a m p l e s that have been conditioned s i m u l t a - neously a t v a r i o u s h u m i d i t i e s f o r 1: t o 11 months ( s e e Table I). This m e a n s that e a c h point on the i s o t h e r m h a s been a t i t s respective condition f o r that period. To accomplish this by a n o r m a l vacuum balance, the period involved would have been i n c r e a s e d several-fold. As i t t u r n s out, t h e s e lengths of t i m e a r e u n n e c e s s a r y in the regions inve,stigated.
The points enabled outer boundary sorption c u r v e s t o be drawn f o r p r e p a r a t i o n s B and C, i. e . B ( a d s ) and C ( a d s ) respectively. F r o m the points obtained f r o m s a m p l e s that had been exposed t o various r e l a t i v e humidities and returned t o 11% R. H. and the adsorption curve f o r p r e p a r a t i o n A, approximate scanning c u r v e s could be outlined. F r o m the sample that had been returned f r o m 100 t o 11%
RR.
H. the outer boundary desorption c u r v e could be estimated. ( T h i s e s t i m a t e i s based on previous work with anidentical sample ( 2 ) . ) The adsorption curve f o r p r e p a r a t i o n A w a s drawn on the i s o t h e r m loop of both p r e p a r a t i o n s B and C, I t s slope i s very low, m u c h lower than the outer boundary sorption c u r v e s f o r p r e p a r a t i o n s B and C. As
61 2 V o l . 1, No. 6 WATER, I N T E R L A Y E R , ADSORBED, THERMAL A N A L Y S I S
0
20
40
60
80
100
PER CENT, RH
FIG.
1V o l . 1 ,
No.
6 613
WATER, INTERLAYER, ADSORBED, THERMAL ANALYSIS
mentioned above, the scanning c u r v e s w e r e p a r t i a l l y located by observing the slope of this c u r v e , P r e v i o u s work ( 2 ) has a l r e a d y noted the difference in slope of the outer boundary c u r v e s and the scanning c u r v e s . Also shown in Fig. 1 i s the f a c t that a l l the w a t e r (including hydrate w a t e r ) removed when the s a m p l e s a r e d - d r i e d ( B ) o r p a r t i a l l y d r i e d ( C ) f r o m 1170 R. H. i s returned a f t e r the sample h a s been rewetted. The scanning c u r v e s show that t h i s r e t u r n takes place over the range of humidities 11 t o 10070 R. H, DTA will show that p a r t of the w a t e r i s r e t u r n e d below 1170 R. H.
Ln comparing the two i s o t h e r m s f o r p r e p a r a t i o n s B and C i t can be observed that the sorption boundary curve f o r B ( d - d r i e d ) has a g r e a t e r slope. To i l l u s t r a t e t h i s , the l a t t e r i s drawn on the s a m e axis a s the C preparation.
P r e p a r a t i o n C contained approximately 370 m o r e m o i s t u r e than p r e p - a r a t i o n B a t the s t a r t of the i s o t h e r m , but t h i s difference w a s regained completely by p r e p a r a t i o n B a t higher humidities. This w a s apparent b e - cause both p r e p a r a t i o n s had completely regained a l l the w a t e r lost a f t e r being exposed to 10070 R 0 H . T h i s lack of r e c o v e r y at the low humidities of a l l the e x t r a w a t e r removed explains the m u c h l a r g e r h y s t e r e s i s loop of preparation B (d-dried).
S e v e r a l points m a y be taken f r o m t h e s e results!
( a ) The C p r e p a r a t i o n which was not d - d r i e d did not allow complete regain of water when exposed a t low o r i n t e r m e d i a t e humidities, even a f t e r extended periods. This i s explained by i n t e r l a y e r h y s t e r e s i s d e s c r i b e d in the new model,
( b ) If the BET calculations a r e applied t o the boundary sorption curve of p r e p a r a t i o n s B o r C, r e s u l t s would be completely fictitious since i n t e r l a y e r w a t e r i s r e -entering in the 0-3370 R. H. range. Values would be controlled by how the i n t e r l a y e r w a t e r r e - e n t e r e d the sample. Conclusive evidence that w a t e r e n t e r s in the BET region i s derived f r o m the scanning c u r v e s and the f a c t that p r e p a r a t i o n B ( d - d r i e d ) h a s a s t e e p e r curve in that region.
If
the e x t r a w a t e r removed f r o m p r e p a r a t i o n B was solely phys- ically adsorbed, i t would have r e a d s o r b e d below 1070 R. H . , where i t w a sWATER, I N T E R L A Y E R , ADSORBED, THERMAL A N A L Y S I S
V o l . 1, N o . 6
removed. The i n c r e a s e d s t e e p n e s s i n the c u r v e in the BET region i s due to i n c r e a s e d i n t e r l a y e r r e - e n t r y in this region.
A calculation f o r s u r f a c e a r e a could be m a d e f r o m the c u r v e f o r the A p r e p a r a t i o n , since no i n t e r l a y e r w a t e r a p p e a r s t o be entering on t h i s c u r v e in the BET region. However, the position f o r z e r o adsorbed w a t e r i s not known. Special p r o c e d u r e s involving simultaneous m e a s u r e m e n t of sorption and length change can be used t o d e t e r m i n e t h i s . They have been used by Helmuth ( 8 ) and r e f e r r e d t o a s the Bangharn, F r e u n d l i c h , F e l d m a n and S e r e d a method. The s u r f a c e a r e a obtained by Helmuth f o r the curve equivalent t o c u r v e C was s i m i l a r to the a r e a obtained by N 2 adsorption.
( 2 ) Differential T h e r m a l Analysis
The s a m p l e s used in this study w e r e in some c a s e s conditioned f o r longer p e r i o d s at the v a r i o u s r e l a t i v e humidities. The t i m e s a r e a l s o t a b - ulated on Table I.
F o r p r e p a r a t i o n B and B(R) a t h e r m o g r a m i s p r e s e n t e d on F i g . 2 f o r e v e r y condition on the sorption boundary c u r v e s e r i e s (A) a s well a s f o r a l l the points r e t u r n e d t o 11% R. H . , i. e.
,
the points on the scanning curves e r i e s ( B ) a t a heating r a t e of 10"C/min. F o r s e r i e s A it m a y be observed that two endothermic peaks with peak t e m p e r a t u r e s varying in the range 65-80 and 90-1 05°C f o r m and grow simultaneously with i n c r e a s i n g humidity. It should be m a d e c l e a r h e r e that t h e s e t e m p e r a t u r e s a r e not exactly definable and a r e known t o v a r y t o some extent depending on the e x p e r i m e n t a l con- ditions. The peaks f o r t h e heating r a t e of 10"C/min a r e v e r y c l e a r l y recognizable although the baseline shifts g r e a t l y at this r a t e . Of the two peaks the 90-105°C peak i s l a r g e r up t o 84% R.H. ; a t 11 and 32% R . H . , the 65-80°C peak i s s m a l l , showing that in the region up to 11% R. H. m o s t of the w a t e r sorbed i s a s s o c i a t e d with the higher t e m p e r a t u r e peak.
S e r i e s B shows c l e a r l y how 11% R. H. on the sorption i s o t h e r m i s not a uniquely defined point a s a l s o can be s e e n in F i g . 1 , where the amount of w a t e r contained a t 1170 R . H . v a r i e s with the conditions of exposure. The higher t e m p e r a t u r e peak g r o w s bigger a s the r e l a t i v e humidity to which the
V o l . 1, N o . 6
WATER, I N T E R L A Y E R , ADSORBED, THERMAL A N A L Y S I S 1 I I ) I I I PREPARATION B 0 100 200 300 4 0 0 TEMPERATURE ,'C PREPARATION B (R) TEMPERATURE, 'C FIG. 2 DTA t h e r m o g r a m s of p r e p a r a t i o n B and B ( R ) S e r i e s A - P r e p a r a t i o n B e q u i l i b r a t e d a t R . H. of 0 to 10070; S e r i e s B
-
P r e p a r a t i o n B ( R ) f r o m R , H . of 0 to 1000/0, e q u i l i b r a t e d a t 1170
R . H .s a m p l e w a s exposed i n c r e a s e s . The 65-80°C peak, on the o t h e r hand, i s always r e d u c e d t o a v e r y s m a l l s i z e on r e t u r n t o 11% R. H. T h u s it i s evident that the h y s t e r e s i s effect m a n i f e s t e d by the scanning c u r v e s o b s e r v - e d on the s o r p t i o n i s o t h e r m i s a s s o c i a t e d with the high t e m p e r a t u r e p e a k . T h i s h y s t e r e s i s effect h a s been p r e v i o u s l y a t t r i b u t e d t o i n t e r l a y e r w a t e r
6 1 6 V o l . 1, N o . 6
WATER, I N T E R L A Y E R , ADSORBED, THERMAL A N A L Y S I S
r e - e n t r y ( Z ) , and i t i s apparent that the two peaks distinguish between i n t e r - layer w a t e r and w a t e r a d s o r b e d on f r e e s u r f a c e s .
The t h e r m o g r a m s f o r p r e p a r a t i o n s A, C(R) and C a r e p r e s e n t e d on F i g . 3. S e r i e s A for p r e p a r a t i o n C(R) shows the c u r v e s f o r the s a m p l e s that had been a l l r e t u r n e d to 1170 RR. H. a f t e r exposure t o various humidities.
It
i s c l e a r that t h i s p r e p a r a t i o n contained m o r e w a t e r a f t e r drying than p r e p a r a t i o n B since an extended endothermal effect i s visible below 100°C f o r the 0% R.H. c u r v e . Subsequent e x p o s u r e t o higher humidities and back to 1170 R . H . c a u s e s the 90-105°C peak t o grow. The 65-80°C peak i s s m a l l o r not visible. This s e r i e s i s thus s i m i l a r t o that f o r p r e p a r a t i o n B. S e r i e s B , the s a m p l e s exposed a t different humidities shows, a s f o r p r e p a r a t i o n B, a simultaneous growth of both peaks, the higher t e m p e r a t u r e peak not b e - ing s u r p a s s e d in intensity by the lower t e m p e r a t u r e peak until 7670 R. H.S e r i e s C r e p r e s e n t s p r e p a r a t i o n A exposed to various r e l a t i v e humidities. T h i s sample shows a well-developed peak a t 90-105°C and a s m a l l peak a t 65-80°C. A s the relative humidity i s r a i s e d the 65-80°C peak i n c r e a s e s , s u r p a s s i n g the 90-105°C peak a t 7670 R. H. The l a t t e r a p p e a r s t o r e m a i n relatively constant, and the 65-80°C peak now a p p e a r s t o grow independently of the h i g h e r t e m p e r a t u r e peak. This explains the sorption curve f o r p r e p a r a t i o n A which h a s a v e r y low slope in c o n t r a s t t o that of p r e p a r a t i o n B o r C ( F i g . 1 ) . F o r p r e p a r a t i o n A a l l the w a t e r a s s o c i - ated with the 90-105°C peak i s a l r e a d y p r e s e n t , while f o r B and C it i s s o r b - ing a s the humidity i s increasing. The sorption h y s t e r e s i s i s explained a l s o by this peak, and since p r e p a r a t i o n C h a s s o m e of this w a t e r a l r e a d y p r e s e n t
i t s h y s t e r e s i s i s not a s l a r g e .
Comparing s e r i e s B and C on Fig. 3, it a p p e a r s that the lower t e m - p e r a t u r e peak f o r C i s l a r g e r f o r the s a m e humidity. This m a y be due t o a l a r g e r s u r f a c e a r e a of the undried s a m p l e . Some o t h e r e x p e r i m e n t a l r e s u l t s confirming t h i s have been obtained and will be published l a t e r .
D i s c u s s i o n
Despite the e x t r a o r d i n a r y lengths of t i m e allowed f o r equilibrium a t the v a r i o u s conditions d e s c r i b e d above t h e s e r e s u l t s a r e essentially the s a m e
V o l
.
1 , No. 6 WATER, TEYPERATURE, *C INTERLAY ER , ADSORBED TEYPERATURE, 'CFIG.
3 THERMAL A N A L Y S I S TEMPERATURE. OC DTA t h e r m o g r a m s of p r e p a r a t i o n sA,
C ( R ) and C S e r i e sA
-
P r e p a r a t i o n C ( R ) f r o m R. H. of 0 t o 100y0, e q u i l i b r a t e d at 1170
R. H. S e r i e sB
-
P r e p a r a t i o nC
e q u i l i b r a t e d a t R . H . of 0 t o 100%. S e r i e s C-
p r e p a r a t i o nA
e q u i l i b r a t e d a t R . H. of 0 t o 1000/oo.WATER, INTERLAYER, ADSORBED, THERMAL A N A L Y S I S
V o l . 1, No. 6
a s t h o s e obtained before when the equilibration p e r i o d s w e r e m u c h l e s s ( 2 ) . The r e s u l t s c l e a r l y show the following:
( a ) adsorption and desorption c u r v e s a r e well defined by the e x p e r i m e n t a l points and a l a r g e h y s t e r e s i s loop i s confirmed;
( b ) scanning c u r v e s e x i s t and a r e the r e s u l t of r e - e n t r a n c e of i n t e r l a y e r w a t e r ;
( c ) p r a c t i c a l l y a l l w a t e r r e m o v e d on f i r s t drying f r o m llyo R. H. i s regained by rewetting;
( d ) i n t e r l a y e r w a t e r r e - e n t e r s throughout the BET range; thus conventional s u r f a c e a r e a calculations f r o m w a t e r sorption a r e not valid.
In t h i s p a p e r i t w a s possible t o c o r r e l a t e the 90-105°C peak with i n t e r l a y e r w a t e r and thus i t was possible t o o b s e r v e d i r e c t l y how t h e amount of i n t e r l a y e r w a t e r changes with humidity. P r o g r e s s i v e r e - e n t r y of i n t e r - l a y e r w a t e r into the d - d r i e d m a t e r i a l t a k e s place in s t a g e s beginning with e x p o s u r e a t v e r y low humidities and i s not complete until i t e x c e e d s about 75yo R. H. T h e r e s e e m s t o be a f o r m of equilibrium a t e a c h humidity; this was d i s c u s s e d in an e a r l i e r p a p e r (2). F r o m the c o r r e l a t i o n of the 90-105°C peak with i n t e r l a y e r w a t e r and f r o m p r e p a r a t i o n C, the p a r t i a l l y d r i e d p r e p - a r a t i o n , i t was possible t o o b s e r v e that i n t e r l a y e r w a t e r e n t e r e d when the s a m p l e s w e r e exposed f r o m the d r y to 1170 R.H. by a growth in the 90-105°C peak; this growth a l s o o c c u r r e d in the BET region.
The significance of the 1170 humidity position i s obvious in t h e
t h e r m o g r a m s . F o r a n ideal adsorption s y s t e m the amount a d s o r b e d a t 11% R. H. should be uniquely defined. At about 11
yo
RR. H. a monolayer usually e x i s t s without the complicating f e a t u r e s of c a p i l l a r y condensation. Also in t h i s region adsorption i s expected to be r e v e r s i b l e along the adsorptionc u r v e . However, in the cement s y s t e m any number of positions e x i s t b e - tween the upper and lower boundaries of the h y s t e r e s i s loop. This i s a l s o evident f r o m t h e peak i n t e n s i t i e s a t 90- 105°C in the t h e r m o g r a m s . The 90-105°C peak in p r e p a r a t i o n A a t 1170 R - H . i s s i m i l a r t o that in p r e p a r a -
V o l . 1, No. 6
WATER, INTERLAYER, ADSORBED, THERMAL A N A L Y S I S
tions B and C exposed t o 84 o r 10070 R . H . and r e t u r n e d t o 11%
R.
H. E x - p o s u r e of p r e p a r a t i o n A t o d i f f e r e n t h u m i d i t i e s c a u s e s only peak 65-80°C t o g r o w . An explanation based on the r e a s o n i n g t h a t the 90-105°C p e a k i s due t o t h e d e s o r p t i o n of a m o n o l a y e r of a d s o r b e d w a t e r c a n be r e ~ e c t e d f o r t h e following r e a s o n s .( 1 ) When t h e peak i s r e m o v e d by d r y i n g i t c a n only be r e s t o r e d by e x p o s u r e t o high h u m i d i t i e s . ( 2 ) S a m p l e s c a n be p r e p a r e d by e x p o s u r e t o d i f f e r e n t
h u m i d i t i e s and m a i n t a i n e d a t 11
70
R. H. e v e n though they contain d i f f e r e n t a m o u n t s of w a t e r . T h i s i s a l s o m a n i f e s t e d by the 90-105°C peak being of d i f f e r e n t s i z e s . The 1170RR.
H. position i s not unique. T h i s s i t u a t i o n i s w e l l explained by the new m o d e l of h y d r a t e d p o r t l a n d c e m e n t .( 3 ) T h e r m a l a n a l y s i s of p o r o u s g l a s s a t d i f f e r e n t
humidity conditions ( 6 ) shows that t h e m o n o l a y e r cannot be d i f f e r e n t i a t e d f r o m t h e m u l t i l a y e r s .
T h e h i g h e r t e m p e r a t u r e peak i s explained a s due t o t h e expulsion of i n t e r l a y e r w a t e r . T h i s m a y not n e c e s s a r i l y be due only t o a h i g h e r e n e r g y of binding of t h e s e H 0 m o l e c u l e s o v e r t h o s e p h y s i c a l l y a d s o r b e d but a l s o
2
due t o the m e c h a n i c a l b a r r i e r i m p o s e d on t h e i n t e r l a y e r w a t e r b e c a u s e of t h e i r i n t e r l a y e r positions. Although two p e a k s c a n be o b s e r v e d i t i s c l e a r t h a t t h e r e m u s t be s o m e o v e r l a p in the i n t e r a c t i o n e n e r g i e s .
Conclusions
T h e r m a l a n a l y s i s allows d i f f e r e n t i a t i o n of i n t e r l a y e r a n d p h y s i c a l l y a d s o r b e d w a t e r .
Different d e g r e e s of d r y i n g of s a m p l e s and long p e r i o d s allowed f o r e q u i l i b r a t i o n s u p p o r t a l l s o r p t i o n r e s u l t s on which t h e new m o d e l of h y d r a t e d p o r t l a n d c e m e n t i s b a s e d .
WATER, I N T E R L A Y E R , ADSORBED, THERMAL A N A L Y S I S
Acknowledgement
V o l . 1, N o . 6
The a u t h o r s acknowledge the valuable a s s i s t a n c e of M e s s r s . G . P o l o m a r k , M vl. LeGeyt and S. Dods in p e r f o r m i n g the e x p e r i m e n t s and
g a t h e r i n g d a t a . T h i s p a p e r i s a contribution f r o m the Division of Building R e s e a r c h , National R e s e a r c h Council of C a n a d a , and i s published w i t h the a p p r o v a l of the D i r e c t o r of the Division.
T h i s p a p e r w a s p r e s e n t e d a t the Second C e m e n t S y m p o s i u m held a s p a r t of the 73rd Annual M e e t i n g of the A m e r i c a n C e r a m i c Societ): Chicago, A p r i l 1971.
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2. R. F. F e l d m a n . P r o c e e d i n g s , FFfth I n t e r n a t i o n a l S y m p o s i u m on the C h e m i s t r y of C e m e n t held in Tokyo, 1968, P a r t III, Vol. III, 53 (1968). 3. S. B r u n a u e r , I. O d l e r and M . Yudenfreund. Highway R e s . B o a r d
R e c o r d No. 38, 89 (1970).
4. V. S. Rarnachandran. Applications of DTA in C e m e n t C h e m i s t r y . C h e m i c a l P u b l i s h i n g Co.
,
New Y o r k (1969).5, G. E n g l e r t and F. Wittmann. M a t 6 r . C o n s t r . ,
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1, 535 (1968).6. R . F. F e l d m a n and V. S. R a m a c h a n d r a n , Modified t h e r m a l a n a l y s i s equipment and technique f o r study u n d e r c o n t r o l l e d humidity conditions. T o be published in T h e r m o c h i m i c a A c t a .
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8. R. A. Helrnuth. D i m e n s i o n a l C h a n g e s and W a t e r A d s o r p t i o n of H y d r a t e d P o r t l a n d C e m e n t and T r i c a l c i u m S i l i c a t e . M , S c . T h e s i s , Illinois I n s t . of Technology, (1965).
