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Surface area of cement paste conditioned at various relative humidities
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SURFACE AREA OF CEMENT PASTE
CONDITIONED AT
VARIOUS RELATIVE HUMIDITIES
by
G. G.
Litvan and R.
E.
Myers
Reprinted from
ANALYZED
Cement and Concrete Research
Vol. 13, No. 1. 1983
p. 49-60
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DBR Paper No. 1065
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SURFACE AREA OF CEMENT PASTE CONDITIONED AT VARIOUS RELATIVE HUMIDITIES
G . G . L i t v a n and R.E. Myers
B u i l d i n g M a t e r i a l s S e c t i o n , D i v i s i o n o f B u i l d i n g Research, N a t i o n a l Research Council o f Canada, Ottawa, Canada K1A OR6
CEMENT and CONCRETE RESEARCH. Vol. 1 3 , pp. 49-60, 1983. P r i n t e d i n t h e USA.
0008-8846/83/010049-12$03.00/0 Copyright ( c ) 1982 Pergamon P r e s s , Ltd.
I
(Communicated by F.H. Wittmann) (Received J a n u a r y 25, 1982)
ABSTRACT
The N 2 BET s u r f a c e a r e a (SA) o f b o t t l e h y d r a t e d p o r t l a n d cement i s
g r e a t l y reduced i f exposed t o changing r e l a t i v e h u m i d i t i e s . The maximum SA l o s s , from 8 3 m2/g, o c c u r r e d on exposure t o between 40 and 60% RH. I n a d d i t i o n , t h e number and r a t e o f humidity changes appear t o a f f e c t t h e v a l u e o f t h e SA, which i n c r e a s e s o n l y when exposed t o r e l a t i v e h u m i d i t i e s i n e x c e s s o f 75%. The phenomenon c a n n o t be e x p l a i n e d by t h e u s u a l c a u s e s o f a g i n g o f h i g h s p e c i f i c SA s o l i d s . I t i s proposed t h a t t h e s o l i d , deformed by non-uniform m o i s t u r e d i s t r i b u t i o n d u r i n g changing humidity c o n d i t i o n s , b r e a k s and makes i n t e r p a r t i c l e bonds. A unique f e a t u r e o f cement p a s t e i s t h a t t h e s e bonds can e a s i l y form and remain i n t a c t even a f t e r t h e m o i s t u r e g r a d i e n t v a n i s h e s .
I n t r o d u c t i o n
Hunt, Tomes and B l a i n e (1) r e p o r t e d i n 1960 t h a t t h e BET n i t r o g e n s u r f a c e a r e a (SA) o f h y d r a t e d cement p a s t e depends t o a g r e a t e x t e n t on t h e r e l a t i v e humidity (RH) a t which t h e specimens a r e s t o r e d p r i o r t o t e s t i n g . G e n e r a l l y , t h e r e i s a d e c r e a s e i n SA, w i t h t h e g r e a t e s t l o s s o c c u r r i n g a t an i n t e r m e d i a t e r e l a t i v e humidity o f 40%.
The phenomenon, which h a s become known a s aging, was s t u d i e d by C o l l e p a r d i ( 2 ) , Bye and Chigbo ( 3 ) , and P a r r o t t , Hansen and Berger (4), b u t no agreement c o n c e r n i n g i t s c a u s e was r e a c h e d . Because s u r f a c e a r e a i s one o f t h e most c h a r a c t e r i s t i c p h y s i c a l p r o p e r t i e s o f d i s p e r s e systems, an a l t e r a t i o n of i t s
v a l u e i s i n d i c a t i v e o f s i g n i f i c a n t changes i n t h e m i c r o s t r u c t u r e , w i t h a p o t e n t i a l b e a r i n g on s u c h i m p o r t a n t macroscopic phenomena a s c r e e p and d r y i n g s h r i n k a g e . For t h e s e r e a s o n s an e x t e n s i o n o f t h e i n v e s t i g a t i o n of SA l o s s seems u s e f u l .
G.G. L i t v a n , R.E. Myers
Vol. 1 3 , No. 1
Method
The SA of t h e specimens was determined by t h e volumetric method i n a NUMINCO Model MIC 103 apparatus. P r i o r t o measurement a specimen a t room temperature was pumped through a l i q u i d n i t r o g e n t r a p u n t i l t h e r e s i d u a l p r e s s u r e was l e s s t h a n 1 x 10-I Pa. In some c a s e s , where so i n d i c a t e d and a f t e r determination of SA, specimens were heated t o 80 and t o llO°C,
r e s p e c t i v e l y , during evacuation p r i o r t o r e p e a t i n g t h e measurement t o a s s e s s t h e e f f e c t of t h e p r e p a r a t o r y t r e a t m e n t on SA.
The l e v e l of t h e l i q u i d n i t r o g e n surrounding t h e specimen bulb was maintained ( a u t o m a t i c a l l y ) a t a c o n s t a n t h e i g h t (+1 mm).
The SA was c a l c u l a t e d from f i v e a d s o r p t i o n p o i n t s with a c o r r e l a t i o n c o e f f i c i e n t >0.9999.
S a t u r a t e d s o l u t i o n s of K B r , NaC1, NaN02, NaBr, K2C03, CaC12 and L i C l a s well a s anhydrous Mg(C104)2 provided c o n s t a n t humidity i n g l a s s d e s i c c a t o r s i n which t h e specimens were s t o r e d . The s o l u t i o n s were s t i r r e d with magnetic s t i r r e r s f o r 1 min i n every 6-min period i n o r d e r t o minimize h e a t i n g e f f e c t s . M a t e r i a l s
Type 10 normal p o r t l a n d cement was used t h a t had been b o t t l e hydrated (150 g cement i n 750 m l water) i n continuous r o t a t i o n f o r 288 days. Specimens f o r s t o r a g e were obtained by withdrawing a dose of c o n s t a n t volume from t h e well-mixed suspension with a s p e c i a l p i p e t t e . I t was c e n t r i f u g e d f o r 8 min and t h e c l e a r l i q u i d decanted, y i e l d i n g a s l u r r y weighing approximately 11 g.
Procedure
F i r s t S e r i e s : The sequence of t h e experimental procedure i s shown i n Fig. 1. Point AO: A specimen of t h e thoroughly mixed, c e n t r i f u g e d s l u r r y was t r a n s f e r r e d i n a gloved box t o a vacuum apparatus f o r drying. The atmosphere i n t h e gloved box was maintained moisture- and C02-free with anhydrous
Mg (C104) 2 and I n d i c a r b .
The sample was pumped a t room temperature through a l i q u i d N 2 t r a p f o r 28 days, during which time weight l o s s was l e s s than 3 x gg-l day-'. F i n a l degassing t o 1 x 10-I Pa was accomplished by pumping, s t i l l a t room temperature, i n t h e s u r f a c e a r e a a p p a r a t u s . (In some i n s t a n c e s , where i n d i c a t e d , t h e specimen was heated t o 80 o r 100°c during t h e l a s t drying s t e p . ) The described drying procedure was followed r o u t i n e l y with every specimen i n v e s t i g a t e d i n t h e p r o j e c t . The s u r f a c e a r e a of t h e o r i g i n a l p a s t e i s l a b e l l e d AO.
Vol. 13, No. 1 51
SURFACE AREA, CEMENT PASTE, RELATIVE HUMIDITY
Point Al: A t t h e beginning of t h e experiment, on 0 day, a thoroughly mixed, c e n t r i f u g e d s l u r r y was placed i n each of t h e e i g h t c o n d i t i o n i n g
d e s i c c a t o r s which were then pumped down t o 30 KPa p r e s s u r e . Humidities i n t h e d e s i c c a t o r s were 0, 11, 32, 42, 57, 66, 75 and 85%. Conditioning l a s t e d f o r a minimum of 133 days, during which period t h e d e s i c c a t o r s were l e f t
undisturbed i n an a i r - c o n d i t i o n e d room.
A t t h e end of t h e s t o r a g e period 1 . 5 g samples were removed and prepared f o r s u r f a c e a r e a determination.
P o i n t s A2 and A3: The remainder of t h e p a s t e i n t h e e i g h t d e s i c c a t o r s was a t t h i s s t a g e (205 days a f t e r 0 day) divided i n t o two p o r t i o n s ; one was t r a n s f e r r e d t o one l e v e l higher humidity and t h e o t h e r p o r t i o n t o one l e v e l lower humidity. A f t e r 124 days of s t o r a g e the specimens were d r i e d and SA v a l u e s obtained. The SA v a l u e s of t h e specimens conditioned a t t h e s e
h u m i d i t i e s were A2 and A3, r e s p e c t i v e l y . P o i n t s A4 and AS: On day 426 t h e specimens l e f t i n t h e d e s i c c a t o r s were
1 > r e t u r n e d f o r 224 days t o t h e h u m i d i t i e s
0 1 3 3 205 3 2 9 4 2 6 6 5 0 d a y s a t which t h e y had been o r i g i i ~ a l l y conditioned. A4 and A5 a r e t h e SA v a l u e s of t h o s e samples exposed t o FIG. 1 higher and lower h u m i d i t i e s ,
r e s p e c t i v e l y . Thus A l , A4 and A5 were Sequence of experimental obtained a t t h e same r e l a t i v e humidity. procedures followed i n I t should be noted t h a t specimens, once F i r s t S e r i e s . Numbers i n vacuum d r i e d f o r SA determination, were boxes i n d i c a t e RH n o t replaced i n s t o r a g e u n l e s s
s p e c i f i c a l l y s t a t e d . The e n t i r e c y c l e r e q u i r e d 650 days.
Gravimetric S e r i e s : Companion specimens t o t h o s e i n t h e F i r s t S e r i e s were placed, on 0 day, i n a second s e t of e i g h t d e s i c c a t o r s maintained a t t h e same humidity l e v e l s a s t h o s e chosen f o r t h e o t h e r s e r i e s , These specimens were weighed p e r i o d i c a l l y t o monitor t h e p r o g r e s s of e q u i l i b r a t i o n of t h e samples i n t h e F i r s t S e r i e s . By following t h i s procedure i t was not necessary t o open t h e d e s i c c a t o r s of t h e F i r s t S e r i e s .
The r a t e of weight change a f t e r 28 days of s t o r a g e was between
70 and 5 x gg-l d a y - l ; t h i s decreased t o between 4 and 0.7 x gg-l daye1 a t t h e end of t h e c o n d i t i o n i n g p e r i o d .
Second S e r i e s : In a few c a s e s t h e specimens conditioned a t 0, 11, 32 and 42% RH were d - d r i e d ( 7 x Pa water vapor p r e s s u r e ) and following SA
determination (Al) were divided i n t o two p a r t s ; one p o r t i o n was re-exposed t o 100% RH and t h e o t h e r t o t h e humidity a t which t h e companion A 1 was obtained. Carbonation: The C02 content of t h e o r i g i n a l s t o c k , bottle-hydrated p a s t e , was 0.616, a v a l u e t h a t i n c r e a s e d t o only 0.96% a f t e r it was handled s i x
Vol. 13, No. 1
G.G. Litvan, R.E. Myers
times, indicating 100
.
I 1 1 I 1 1 I I I the effectiveness of the precautions 9 0-
taken to avoid carbonation. 80- -
D R I E D A T R O O M TEMP Results---
D R I E D A T 80°C ... D R I E D A T llO°C Point A0 70-
or The SA value ofthe original paste
-
on 0 day was 83.0 m2 g-l. In
-
the course of thestudy the
-
determination wasrepeated several times. The good
-
reproducibility indicated that no 20-
-
further change (such as hydration) 10-
-
affecting SA took place on storingI I I I I I I I I the paste in water.
0 o 1 0 20 30 40 50 60 70 30 90 100 Point A1 R E L A T I V E H U M I D I T Y . % The SA values of specimens FIG. 2 conditioned at various humidities
N2 BET surface area of cement paste stored at various are shown in
humidities (Al) and dried prior to SA determination of Fig. 2. Drastic
20, 80 or llO°C decrease in SA is
particularly noticeable at intermediate humidities. After conditioning at 42% RH SA was less than 25% ofthe original value of samples initially conditioned at 100% RH.
These results are in general agreement with those of Hunt et al. (I), although there are significant differences: in the study by Hunt et al. conditioning was not done above 80% RH and the shape of the curve in the 80 to 100% RH region (showing only small changes in SA) was not recorded. Because of the larger number of RH levels in the present study, the curve is generally better defined. At very low humidities Hunt et al. obtained SA values greater than those of the original wet paste, probably the result of
high drying rates, the effects of which have been well documented (5). In a
recent study Parrott et al. (4) observed low SA values not much larger than
those obtained at 40% RH following storage at below 40% RH.
The SA of the A1 points in Fig. 2 for specimens conditioned at various humidities and dried at temperatures of 20, 80 and 110°C prior to SA
determination are similar in shape, indicating that the observed features are not affected by the rigidly held water. Elevation of the temperature from 20 to llO°C resulted in 20% higher A1 values. Whether or not chemically bound water was also removed in this process cannot be ascertained. The results once again underline the need for standardizing every aspect of the SA determination if quantitative comparisoll is contemplated.
Vol. 1 3 , No. 1 53
SURFACE AREA, CEMXNT PASTE, RELATIVE HUMIDITY P o i n t s A2 t o A5
The SA v a l u e s of t h e specimens a t t h e previous s t a g e s of t h e c o n d i t i o n i n g c y c l e , A2 t o AS, a r e shown i n Figs. 3 and 4. The following o b s e r v a t i o n s were made :
1) A s A2 and A3 a r e i n every c a s e l e s s than t h e r e s p e c t i v e A1 v a l u e s , it i s c l e a r t h a t SA l o s s occurred with changes i n humidity t o e i t h e r h i g h e r o r lower l e v e l s .
2) For samples o r i g i n a l l y conditioned a t 32, 42, 75 and 85% RH, A2 = A3; f o r t h o s e conditioned a t 57 and 66% RH, A2 > A3; and f o r t h o s e conditioned a t
11% RH, A2 < A3.
3) In t h e range from 11 t o 66% RH completion of t h e humidity c y c l e by r e - e s t a b l i s h i n g t h e RH of p o i n t A 1 r e s u l t e d i n a f u r t h e r d e c r e a s e i n SA: A2 > A4 and A3 > AS.
4) A t 11% RH A4 < AS; between 32 and 57% RH A4 = AS; and a t 66% RH A4 > AS. 5) In t h e c y c l e s o r i g i n a t i n g a t 75 and 85% RH A4 > > A? and A5 >> A3;
A4 >> A5 even A4 > A l .
6) The c y c l e s commencing a t 32 and 42% RH appear t o be uniquely symmetrical inasmuch a s changes i n r e l a t i v e humidity r e s u l t i n s i m i l a r SA l o s s e s , i r r e s p e c t i v e of d i r e c t i o n .
7) P l o t s of SA v a l u e s , A 1 t o AS, a s a f u n c t i o n of r e l a t i v e humidity (Fig. S),
show t h a t changing h u m i d i t i e s below 66% RH r e s u l t i n a converging t o a common low v a l u e t h a t occurs between 42 and 57% RH.
Second S e r i e s
The r e s u l t s shown i n Fig. 6 i n d i c a t e t h e following:
1 ) SA l o s t on exposure t o low h u m i d i t i e s can be recovered only by exposure t o very high h u m i d i t i e s .
2) A s expected, t h e e x t e n t of recovery i s i n v e r s e l y p r o p o r t i o n a l t o t h e magnitude of t h e SA l o s s ( s e e Fig. 6, p l o t a t lower r i g h t ) .
Discussion
The good agreement between t h e r e s u l t s of previous s t u d i e s (1,4) and t h e p r e s e n t work l e a v e s l i t t l e doubt t h a t N 2 BET s u r f a c e a r e a l o s s on s t o r a g e a t varying h u m i d i t i e s i s a r e a l i t y . Surface a r e a r e d u c t i o n was a l s o observed using water a s a b s o r b a t e ( 6 ) , and t h e phenomenon cannot be considered a s an a r t e f a c t of n i t r o g e n a d s o r p t i o n . Despite t h e l a r g e d i s c r e p a n c i e s between N 2
and H20 BET a r e a i n numerical v a l u e s they appear t o be q u a l i t a t i v e l y s i m i l a r . Furthermore, t h e a r e a a t 52% RH was only h a l f of t h a t a t s a t u r a t i o n when examined by t h e small-angle X-ray s c a t t e r i n g method ( 7 ) , r e q u i r i n g no a d s o r b a t e . I t s u g g e s t s t h a t t h e r e s u l t s r e f l e c t r e a l p h y s i c a l changes. Rate and Frequency Dependence
The r e s u l t s i n d i c a t e t h a t SA i s a f f e c t e d n o t only by t h e a b s o l u t e l e v e l of humidity during s t o r a g e but a l s o by t h e number and r a t e of changes i n
r e l a t i v e humidity. Hunt e t a l . (1) observed t h a t crushed specimens undergo s m a l l e r SA d e c r e a s e than do 12 mm diameter c y l i n d e r s because t h e former specimens d r y f a s t e r . An important element of t h e s o l v e n t replacement technique (5) was t h e very high drying r a t e t h a t r e s u l t e d i n N 2 SA v a l u e s comparable t o t h o s e obtained with water a s adsorbent.
I t seems q u i t e p o s s i b l e t h a t t h e SA v a l u e s below 42% RH i n t h e F i r s t S e r i e s would, a f t e r a number of RH changes, d e c r e a s e t o l e s s t h a n 10 m 2 g-l.
5 4 Vol. 13, No. 1
G.G. Litvan, R.E. Myers
100 90 80 70 m
-
N 6 0 - a Y E -x 50 W U u I I I '1 A-
100% AO.-
-
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-
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I 1 1 4 RELATIVE H U M I D I T Y , 90 F I G . 3SA at various stages of conditioning cycle, A 0 to A 5
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I 1 1 1 40 50 60 70 5 0Vol. 1 3 , No. 1 55
SURFACE AREA, CEMENT PASTE, RELATIVE HUMIDITY
In t h i s r e g i o n t h e specimens experience t h e h i g h e s t r a t e of drying i f t h e moist samples a r e placed i n t h e r e s p e c t i v e d e s i c c a t o r s , minimizing t h e e f f e c t s of SA l o s s . Several subsequent c o n d i t i o n i n g s t e p s a r e probably r e q u i r e d t o reduce SA t o i t s f i n a l low v a l u e , a t which s t a g e t h e A 1 v e r s u s RH curve would c l o s e l y resemble t h a t of P a r r o t t e t a l . ( 4 ) . The r a t e of drying above 47% RH I I I I I I I I I a l s o a f f e c t s t h e 0 o 10 2 0 30 40 50 60 7 0 8 0 90 100 u l t i m a t e v a l u e of drying shrinkage R E L A T I V E H U M I D I T Y , % (8) and c r e e p FIG. 5 A 1
.
. .
A5 v a l u e s a s a f u n c t i o n of RH (9-11). ~ r y i n ~ a t r a p i d r a t e s induces s t r e s s e s of magnitude p r o p o r t i o n a l t o r a t e of d r y i n g . Cady e t a l . (12) observed t r a n s i e n t d e c r e a s e of t h e t e n s i l e s t r e n g t h of cement mortar b r i q u e t s on drying, and Litvan (13) has r e p o r t e d s i m i l a r t r a n s i e n t r e d u c t i o n of Young's modulus of hardened cement p a s t e . Repeated wetting and drying c y c l e s appear t o a f f e c t c r e e p of cement p a s t e (9,lO) and t h e volume changes of Ca(011)2 compacts (14).40 t o 60% RH Range
The g r e a t e s t d e c r e a s e i n SA occurs a t o r i n t h e v i c i n i t y of 40% RH. A r e l a t i v e p r e s s u r e of 0.4 i s of g r e a t s i g n i f i c a n c e i n s e v e r a l a r e a s of s u r f a c e chemistry. In hundreds of a d s o r p t i o n isotherms, including hardened cement p a s t e , t h i s i s t h e inception p o i n t of t h e h y s t e r e s i s loop (15,16); below t h i s p r e s s u r e micropore f i l l i n g and above it c a p i l l a r y f i l l i n g occur. For t h i s reason 0.4 r e l a t i v e p r e s s u r e was chosen a s t h e r e f e r e n c e s t a t e f o r s u r f a c e a r e a determination by t h e a-method (17).
I t i s a l s o known t h a t when d r y cement p a s t e i s wetted, 65 t o 70% of t h e expansion t a k e s p l a c e below 50% RH; and t h a t when d - d r i e d and exposed t o higher h u m i d i t i e s a p o t e n t i a l f o r i r r e v e r s i b l e shrinkage develops (18).
Creep r a t e i n c r e a s e s r a p i d l y a t h u m i d i t i e s above 42% RH (19); t h e
compressive s t r e n g t h of cement mortar and t h e e l a s t i c modulus of cement p a s t e (20), i f conditioned a t 40 t o 50% RH, a r e a t t h e i r lowest l e v e l . The
c o e f f i c i e n t of thermal expansion of cement p a s t e i s maximum a t 40% RH (14). In f i r s t - d r y i n g shrinkage 47% RH was found t o be a most s i g n i f i c a n t
56 Vol
.
G.G. Litvan, R.E. Myers
RELATIVE HUMIDITY. %
13, No.
RELATIVE HUMIDITY. % RH % OF
CONDITIONING STEP
FIG. 6
SA regeneration after conditioning.
100% RH ( ~ 0 ~ )
Sequence of procedure 100%
RH
(AO) + X% RH (Al) + 0%RH ( ~ 0 ~ ) Top graphs: SA at various stages of conditioning.
Bottom, left: A1 and ~ l l vs
RH.
Right : AO' as a function of RH of A1 conditioning step.
condition; and Helmuth and Turk drew attention to the link between shrinkage and SA loss (8). Feldman showed (21) that, on rewetting, tile layers of the cement paste open up significantly only after 42%
RH.
Shrinkage of lime compacts following carbonation (22) and conditioning at various humidities(14) is maximum at 50% RH. Reversibilitv
Whereas changing humidities during storage, either increasing or decreasing, result in lower surface area values, the trend is reversed on exposure to very high RH, at least to 75% (Fig. 4). Recovery of the SA loss on resaturationin water has been reported previously (4,7). The reversible portion of drying shrinkage manifested itself only on rewetting (8).
Vol. 13, No. 1 57
SURFACE AREA, CEMENT PASTE, RELATIVE HUMIDITY
P o s s i b l e Causes of Aging
A time-dependent a l t e r a t i o n of high s p e c i f i c s u r f a c e a r e a s o l i d s i s a w e l l known phenomenon (23) a t t r i b u t e d t o s e v e r a l causes. I t w i l l be examined f o r r e l e v a n c e t o t h e p r e s e n t c a s e of cement p a s t e . F i r s t , however, t h e p o s s i b i - l i t y t h a t t h e o b s e r v a t i o n s can be accounted f o r by assuming continued
h y d r a t i o n has t o be considered. Obviously, t h e formation of new compounds could a l t e r s u r f a c e a r e a . T h i s hypothesis must be r e j e c t e d , however, because t h e b o t t l e hydrated cement used was well hydrated on 0 day. I t may be
mentioned t h a t h y d r a t i o n , except a t high temperatures and f o r an i n i t i a l period (24), normally l e a d s t o an i n c r e a s e and n o t a d e c r e a s e of SA (25).
Surface a r e a l o s s of high s p e c i f i c SA s o l i d s i s o f t e n a r e s u l t of enhanced s o l u b i l i t y . Such s o l i d s a r e known t o d e c r e a s e t h e i r s u r f a c e f r e e energy through d i s s o l u t i o n and p r e c i p i t a t i o n . T h i s mechanism must a l s o be r u l e d out f o r cement p a s t e s f o r t h e following reasons:
1 ) SA d e c r e a s e occurs a t h u m i d i t i e s below 40% RH, y e t adsorbed water i n t h i s r e g i o n r e j e c t s s o l u t e s (26).
2) There i s l e a s t l o s s of SA a t high h u m i d i t i e s , when t h e c o n d i t i o n s f o r t h i s mechanism would be t h e most f a v o r a b l e .
3) On t h e b a s i s of t h e mechanism, SA d e c r e a s e s during prolonged exposure t o c o n s t a n t humidity, so t h a t i n s e n s i t i v i t y t o RH changes would be expected. The c o n t r a r y has been observed.
S i l i c e o u s s u r f a c e s a r e known t o undergo gradual dehydration of t h e SiOH groups l e a d i n g t o t h e formation of Si-0-Si b r i d g e s , a process accompanied by s u r f a c e a r e a r e d u c t i o n (27). Polymerization of s u r f a c e s i l a n o l groups a s one cause of aging was suggested by C o l l e p a r d i (28), who a l s o r e p o r t e d t h a t from t h e r a t e of SA d e c r e a s e a t d i f f e r e n t temperatures t h e a c t i v a t i o n energy i s 10 t o 12 kcal/mol. T h i s v a l u e i s i n t h e same range a s t h e a c t i v a t i o n energy of t h e s i l a n o l condensation r e a c t i o n determined by Kiselev (29).
In support of t h e s i l a n o l polymerization t h e o r y i s t h e w e l l - e s t a b l i s h e d f a c t t h a t r e h y d r a t i o n of dehydrated s i l i c a , a s f o r SA change, i s a f u n c t i o n of r e l a t i v e humidity and w i l l occur only i f l i q u i d water i s p r e s e n t i n t h e c a p i l l a r i e s (30). With t h i s mechanism t h e d i f f i c u l t y i s t h a t t h e e f f e c t of a l t e r n a t i n g h u m i d i t i e s cannot be explained. R e c r y s t a l l i z a t i o n of one o r more c o n s t i t u e n t s on a l a r g e s c a l e can be r u l e d o u t on t h e b a s i s of C o l l e p a r d i ' s work, which could not d e t e c t i n t h e X-ray d i f f r a c t o g r a m s of calcium s i l i c a hydrates any change due t o aging (28).
Collapse of t h e layered s t r u c t u r e of t h e s i l i c a t e h y d r a t e s on drying does not e x p l a i n SA r e d u c t i o n a d e q u a t e l y because t h e phenomenon i s observed a l s o a t r e l a t i v e l y high h u m i d i t i e s , and even on wetting ( s e e p o i n t s A 1 and A2 i n Fig. 4 ) . For t h e same reason t h e suggestion t h a t g e l p a r t i c l e s a r e brought c l o s e r t o g e t h e r during d e s o r p t i o n and t h a t c o n t a c t p o i n t s a r e c r e a t e d under p r e s s u r e (31) cannot be accepted a s t h e f u l l explanation.
The most important c h a r a c t e r i s t i c s o f t h e SA l o s s phenomenon t h a t must be accounted f o r a r e : 1 ) t h e a r e a r e d u c t i o n i t s e l f , 2) t h e s e n s i t i v i t y of t h e phenomenon t o t h e number and 3) t h e r a t e of humidity changes. There i s l i t t l e doubt t h a t f i r s t drying shrinkage (8) i s an important f a c t o r i n t h e i n i t i a l phases o f c o n d i t i o n i n g , but i t cannot account f o r a l l t h e f i n d i n g s .
The o b s e r v a t i o n s l i s t e d above i n d i c a t e , however, t h e o p e r a t i o n of a time- dependent process g r e a t l y a f f e c t i n g t h e behaviour of t h e p a s t e . Under changing humidity c o n d i t i o n s two such p r o c e s s e s can be i d e n t i f i e d : moisture t r a n s f e r , and volume change of t h e s o l i d . Time dependence of volume change i n response t o RH change has n o t , t o t h e a u t h o r s ' knowledge, been r e p o r t e d i n
5 8 Vol. 1 3 , No. 1
G.G. L i t v a n , R.E. Myers
t h e l i t e r a t u r e . I t seems, t h e r e f o r e , reasonable t o assume t h a t t h e r a t e - c o n t r o l l i n g process i n f l u e n c i n g t h e e v e n t s i s t h e t r a n s f e r of moisture.
This hypothesis i m p l i e s t h a t d e c r e a s e i n SA follows from t h e e x i s t e n c e of moisture g r a d i e n t , r e s u l t i n g i n t u r n i n non-uniform volume changes throughout t h e p a s t e . Such a c o n d i t i o n deforms t h e s o l i d network, breaking some bonds and c r e a t i n g new c o n t a c t p o i n t s , t h e formation of which was suggested by Powers (31). The mechanism i s a common occurrence with a l l t y p e s of porous s o l i d s . The unique f e a t u r e of hydrated cement p a s t e i s t h a t bonds betweenthe s u r f a c e s of s i l i c a t e p a r t i c l e s form r e a d i l y when t h e y a r e brought i n t o c l o s e proximity, a s evidenced by t h e g r e a t s t r e n g t h of c o l d (32) and hot (33) compacted cement.
Loss of SA can a r i s e from reduced a c c e s s i b i l i t y of r e g i o n s o r r e d u c t i o n of t h e a r e a of t h e p a r t i c l e s involved. Gregg and Langford found (34) s i l i c a deformable under compaction and suggested t h a t t h e p a r t i c l e s r e t a i n t h e i r a l t e r e d shape even a f t e r t h e newly formed j u n c t i o n s open up t o accommodate r e l a t i v e movement of t h e p a r t i c l e s when s t r e s s c o n d i t i o n s continue t o change.
I f t r u e , t h i s mechanism i m p l i e s t h e occurrence of a r e a l o s s whenever s t r e s s , i . e . , wetting o r drying, deforms t h e p a s t e .
Because t h e g r a d i e n t i s a f u n c t i o n of t h e r a t e of wetting and drying t h e l o s s of SA i s , understandably, a l s o a f f e c t e d . I n t e r p a r t i c l e bonds, which appear t o be involved i n t h e changes brought about by moisture change i n d i s p e r s e systems such a s c o n c r e t e , a r e most important. The mechanical
p r o p e r t i e s of such s o l i d s a r e determined more by t h e s t r e n g t h of t h e c o n t a c t s between t h e p a r t i c l e s t h a n by t h e s t r e n g t h of t h e p a r t i c l e s themselves (35). I t i s t h u s t o be expected t h a t a phenomenon r e s u l t i n g i n diminution of SAalso a f f e c t s c r e e p and drying shrinkage phenomena. Although t h e cause of SA l o s s i s not c l e a r , t h e r e i s l i t t l e doubt t h a t c a l l i n g t h e phenomenon "aging" i s i n c o r r e c t . The process i s n o t dependent on time but on change of moisture c o n t e n t and t h e r a t e of t h i s p r o c e s s .
T h i s paper i s a c o n t r i b u t i o n from t h e Division of Building Research, National Research Council of Canada, and i s published with t h e approval of t h e d i r e c t o r of t h e Division.
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