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Freezing of water in hydrated cement pastE/Le gel de l'eau contenue
dans la pate de ciment hydrate
Litvan, G. G.
https://publications-cnrc.canada.ca/fra/droits
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S e r
' I / , . TH1N211-2
no.
446
c .2
L
yzED
BLDG
INATIONAL
RESEARCH
COUNCIL
OFCANADA
CONSEIL
NATIONAL
DERECHERCHES
DUCANADA
FREEZING O F WATER IN HYDRATED CEMENT PASTE
BY G. G. L I N A N
REPRINTED FROM
PRELIMINARY REPORT. PART I
RlLEM INTERNATIONAL SYMPOSIUM O N DURABILITY O F CONCRETE
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1 9 6 9 PRAGUE, 1969P. 6 1 5 3
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8160RESEARCH PAPER NO. 4 4 6
O F THE
44732
DIVISION O F BUILDING RESEARCH
O I T A W A
Litvan, G. G. Ph.D.
Division of Building R e s e a r c h
National Re s e a r c h Council of Canada, Ottawa, Canada
F r e e z i n g of Water in Hydrated Cement P a s t e
L e Gel de 1'Eau Contenue dans l a P d t e de Ciment Hydratk
Summary;
P r e s e n t theory of f r o s t action in'cement i s re-examined in the light of the r e s u l t s of r e c e n t experiments (dimensional changes, a d s o r p - tion c h a r a c t e r i s t i c s , c a l o r i m e t r y , DTA). It i s found that hydrostatic p r e s s u r e in cement i s not generated by the 9 p e r cent difference between the specific volume of w a t e r and i c e but by the changed interaction be- tween the cement and the adsorbed w a t e r on i t s solidification. The proposed m e c h a n i s m r a i s e s the possibility that new methods f o r f r o s t ~ r o t e c t i o n m a y be developed. Similarly, f r o s t susceptibility m a y be t e s t e d in a m o r e r e l i a b l e way.
Lbauteur examine l a th6orie accept6e de lfaction du g e l dans l e ciment
B
l a l u m i e r e d e s r k s u l t a t s dfexpkriences rkcentes, p a rdetermination d e s changements dimensionnelles e t d e s c a r s c t k r i s t i q u e s dsadsorption, p a r c a l o r i m k t r i e et p a r analyse thermique diffkrentielle.
Il
a trouve que l a p r e s s i o n hydrostatique c r k k e5
lfintCrieur du ciment n'est p a s dueh
l a diffkrence de 9 pour cent e n t r e l e s volumes spkci- fiques de l'eau e t de l a glace, m a i sB
l a modification d e s interactions e n t r e l e ciment e t l'eau adsorbCe en c o u r s de solidification. Ce m e c a n i s m e entrevu ouvre d e s possibilit6s d'klaboration de nouvelles mgthodes de protection du ciment contre l e gel. On p o u r r a i t de m6me2
-
LitvanT h e r e s t i l l e x i s t s a need t o improve f r o s t r e s i s t a n c e of con- c r e t e in spite of the advancement achieved by the a i r - e n t r a i n m e n t technique. It would be of g r e a t benefit to:
a ) improve the reliability of, and shorten the t i m e r e q u i r e d t o complete, the a c c e l e r a t e d t e s t f o r f r o s t susceptibility;
b ) extend the capabilities of p r e s e n t engineering p r a c t i c e by developing methods of manufacturing concrete with even b e t t e r f r o s t r e s i s t a n c e in a s i m p l e r and m o r e economical way.
Because of the climatic conditions prevailing i n Canada the Division of Building R e s e a r c h of the National R e s e a r c h Council of Canada h a s a p a r t i c u l a r i n t e r e s t in the study of the freezing of water in porous building m a t e r i a l s such a s cement, stone, brick, m o r t a r , e t c . F r e e z i n g of water in concrete i s a special c a s e of phase t r a n s i - tion of a d s o r b a t e s in porous solids. Because t h i s phenomenon i s
s t i l l insufficiently understood, experiments t o elucidate the g e n e r a l problem w e r e included i n the work.
P o r o u s 96 p e r cent silica g l a s s adsorbent h a s been extensively studied. Like cement, i t i s porous and siliceous, but h a s the advan- tage of being uniform and stable i n chemical composition and geom- e t r y . Owing t o the complexity of the phenomenon it was n e c e s s a r y t o employ a number of experimental methods. The m e a s u r e m e n t of dimensional changes, i s o t h e r m s w d i s o s t e r e s in an adsorption a p p a r a t u s was complemented by c a l o r i m e t r y , differential t h e r m a l analysis, and i n f r a r e d spectroscopy.
The m a i n findings of e a r l y work ( 1 , 2) m a y be s u m m a r i z e d a s follows:
a ) No freezing will take place if the amount of water p r e s e n t in a porous body i s l e s s than t h a t r e q u i r e d t o f o r m two complete l a y e r s on the surface.
b )
If
freezing does take place, i t will occur a t t e m p e r a t u r e s below the n o r m a l freezing point. As i n s o many other a s p e c t s , water behaves differently f r o m a l l the other liquids investigated. The f r e e z i n g point of n o r m a l liquids in the adsorbed state i s a function of concentration,approaching the n o r m a l f r e e z i n g point
st
saturation; w a t e rm a v have s e v e r a l freezing r a n g e s , some concentration- dependent, o t h e r s not.
c ) Melting, and probably f r e e z i n g too, c a n be classified a s an
anomalous f i r s t o r d e r phase change, i. e . , i t extends o v e r a wide range of t e m p e r a t u r e .
3
-
Litvand ) The values of t h e latent heat of fusion p e r g r a m of adsorbed water in porous g l a s s varied between 49.7 and 55.5 ~ a l / ~ , depending on the concentration, o r approximately two-thirds of the n o r m a l value of 79 ~ a l / ~ .
T h e s e r e s u l t s indicate that w a t e r adsorbed in a porous solid h a s c h a r a c t e r i s t i c s v e r y d i f f e r e n t f r o m those of bulk w a t e r , a fact brought about by the interaction of the w a t e r molecules with the s u r - face of the solid. The solid itself i s a l s o p e r t u r b e d a s i s d e m o n s t r a - ted by the anomalous changes of dimensions during adsorption o r freezing.
Following t h i s understanding of the f r e e z i n g phenomenon in the model s y s t e m , the cement-water s y s t e m was examined. T h i s paper gives s o m e of the r e s u l t s ; a detailed r e p o r t will be given e l s e - where.
Experimental
Apparatus
The specific heat and dimensional changes w e r e determined i n an adiabatic c a l o r i m e t e r incorporating an extensometer (3).
M e a s u r e m e n t s w e r e m a d e of dimensional changes and the amorlnt of w a t e r adsorbed in cement s p e c i m e n s a s the function of t e m p e r a t u r e . Companion sample s w e r e examined simultaneously i n a h e r m e t i c a l l y sealed apparatus. T h i s included an optical extensom- e t e r (4) and a q u a r t z s p i r a l f o r m e a s u r i n g weight changes. The m a - jor and e s s e n t i a l p a r t of the apparatus was enclosed in a well cooled by a c o m p r e s s o r . With the aid of a controller-governed h e a t e r , t h e t e m p e r a t u r e could be maintained constant to fO.05OC i n the range between +20°C and -45°C.
M e a s u r e m e n t s w e r e taken under both equilibrium and non- equilibrium conditions. In the f o r m e r c a s e the t e m p e r a t u r e was changed by 2 t o 3'C and then kept constant until conqtant weight (f 0.01 p e r cent) and dimensions (f 0.001 p e r cent) had been achieved. The t i m e r e q u i r e d v a r i e d f r o m 3 to 8 days and the completion of a r u n took 5 months. The r e s u l t s , the final equilibrium values, a r e shown in F i g u r e s 1 and 2. Under non-equilibrium conditions the t e m p e r a t u r e was changed continuously a t a r a t e of approximately 0 . 2 5 " ~ / minute. The t e m p e r a t u r e was m e a s u r e d with a t h e r m o - couple attached to the sample. The values s o obtained a r e shown in F i g u r e 1 (curve c).
The differential t h e r m a l c u r v e shown in F i g u r e 3 was obtained i n a non-commercial apparatus, which consisted of a h e r m e t i c a l l y s e a l e d c e l l containing cement and g l a s s r e f e r e n c e s a m p l e s , a 1 - m v f u l l ~ s p a n r e c o r d e r , and a t e m p e r a t u r e p r o g r a m m e r . The r a t e of the t e m p e r a t u r e change was 5"c/minute.
4
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LitvanF i g u r e 1 Dimensional changes a s a function of t e m p e r a t u r e
F i g u r e 2 Weight change a s a func- tion of t e m p e r a t u r e .
F i g u r e 3 Differential t h e r m a l curve.
5
-
LitvanM a t e r i a l s
P o r s u s 96 p e r cent silica g l a s s was obtained f r o m Corning Glass Works in the f o r m of plates with various thicknesses. Cement s a m p l e s with uniform porosity and exclusion of C 0 2 and a i r were p r e p a r e d by the method of S e r e d a and Swenson (5). After a minimum hydration period of one y e a r 1 . 3 - m m thick specimens were cut with a c i r c u l a r saw having a diamond blade. The samples were kept in 100 p e r centrRH a t a l l t i m e s .
Discussion
The Existence of Hydraulic P r e s s u r e
The c u r v e s i n F i g u r e 1 r e p r e s e n t the length changes of the s a m p l e s on slow equilibrium cooling ( c u r v e s a and b) and on rapid non-equilibrium cooling (curve c). It m a y be seen that whereas curve c i s s i m i l a r in shape to those reported in the l i t e r a t u r e (6) c u r v e s a and b indicate contraction which h a s previously been ob- s e r v e d only f o r a i r - e n t r a i n e d cement. It m a y be noted again that c u r v e s a and b r e p r e s e n t the final values which the samples attained a t a given t e m p e r a t u r e . Initially, the samples expanded a f t e r the t e m p e r a t u r e was altered, but during the constant t e m p e r a t u r e period they continued t o change and finally contracted.
T h e s e observations a r e consistent with the assumption of the p r e s e n t theory (6) that hydraulic p r e s s u r e was generated on cooling. The p r e s s u r e was subsequently relieved gradually because uncompen- sated l a r g e internal p r e s s u r e cannot be maintained in equilibrium.
Confirmation of the existence of the hydraulic p r e s s u r e s e e m s t o be valuable because i t was proved only in an indirect way. It was
considered a s evidence that samples cooled a t the r a t e of 0 . 2 5 " ~ / m i n - Ute continued t o expand when kept a t constant t e m p e r a t u r e . I t was argued that t h e r e i s no mechanism other than the hydrostatic p r e s
-
s u r e hypothesis t o explain the phenomenon and that t h e r e f o r e it m u s t be t r u e . This reasoning i s not quite justified, however, because the experimental r e s u l t s indicate only that the t e m p e r a t u r e change i n - itiated a p r o c e s s that i s slow t o r e a c h completion. The nature of the p r o c e s s i s undefined and m a y be, besides a generation of p r e s s u r e , a r e a r r a n g e m e n t of the molecules in the adsorbed l a y e r , changes i n the a r e a accessible t o the water molecules, and s o on.Origin of the Hydraulic P r e s s u r e
According t o the theory the well known 9 p e r cent volume i n c r e a s e of water on freezing g e n e r a t e s hydrostatic p r e s s u r e in cement; Since the cement i s completely saturated, i. e., a l l the p o r e s a r e completely filled, the internal volume cannot accommod- a t e the . l a r g e r volume of i c e and the p r e s s u r e i n c r e a s e s . As a r e s u l t the cavity m u s t dilate o r the e x c e s s ' w a t e r be expelled f r o m it.
6
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LitvanAccording t o F i g u r e s 1 and 2 the s a m p l e s have c o n t r a c t e d and l o s t w a t e r with slow cooling, indicating t h a t not dilation but the e x - pulsion m e c h a n i s m was operative. If the t h e o r y i s c o r r e c t and the p r e s s u r e i s c a u s e d by t h e volume i n c r e a s e of w a t e r , t h e m a x i m u m amount of w a t e r t o be expelled f r o m t h e s y s t e m can not be m o r e t h a n 9 p e r cent of t h e t o t a l w a t e r p r e s e n t . The w a t e r actually expelled i s , however, s e v e r a l t i m e s g r e a t e r .
F i g u r e 2 shows t h a t t h e two s a m p l e s have l o s t 0.061 g w a t e r p e r 0.814 g c e m e n t and 0.102 g w a t e r p e r 0.7460 g c e m e n t , r e s p e c t - ively, r e p r e s e n t i n g 33 and 40 p e r c e n t of the t o t a l w a t e r p r e s e n t , a s d e t e r m i n e d by the D-drying method. T h e s e v a l u e s a r e i n c o n s i s t e n t with the t h e o r y and w a r r a n t re-examination.
I n t e r p r e t a t i o n of t h e P r e s e n t R e s u l t s
The findings of the p r e s e n t w o r k m a y be explained in t h e following way:
When w a t e r f r e e z e s , the m o l e c u l e s , which a r e i n a r e l a t i v e d i s a r r a y in t h e liquid s t a t e , have t o o r d e r t h e m s e l v e s a s r e q u i r e d by the c r y s t a l s t r u c t u r e of i c e . The mobility of the a d s o r b e d m o l e c u l e s . however, i s l i m i t e d owing t o the s u r f a c e f o r c e s .
In
t h e f i r s t twol a y e r s w h e r e the a t t r a c t i o n t o the s u r f a c e i s the s t r o n g e s t , no r e a r r a n g e m e n t can o c c u r even a t low t e m p e r a t u r e s , and in t h e sub- sequent l a y e r s i t m a y take place only below 0 ° C . On f o r m a t i o n of a n a d s o r b e d l a y e r with i c e - l i k e s t r u c t u r e , t h e i n t e r a c t i o n between the c e m e n t s u r f a c e and t h e w a t e r m o l e c u l e s d e c r e a s e s . A s a d i r e c t consequence, t h e m a x i m u m amount of w a t e r held in t h e s u r f a c e i s reduced. T h u s p a r t of the w a t e r r e q u i r e d f o r s a t u r a t i o n before f r e e z i n g b e c o m e s e x c e s s a f t e r i c e f o r m a t i o n . A s a l l t h e p o r e s w e r e completely filled a t s a t u r a t i o n , t h i s e x c e s s w a t e r cannot be accommo- dated and p r e s s u r e i s g e n e r a t e d . In a n open s y s t e m p r e s s u r e d i f f e r - e n c e s cannot be m a i n t a i n e d , s o t h a t the w a t e r h a s t o l e a v e t h e void s y s t e m by diffusion.
T h i s explanation i s c o n s i s t e n t with the e x p e r i m e n t a l r e s u l t s and with field e x p e r i e n c e . Expansion w a s o b s e r v e d only on r a p i d cooling. If sufficient t i m e w a s allowed f o r equilibration t o be r e a c h e d , the w a t e r did m i g r a t e out of the void s y s t e m and a n o v e r - a l l c o n t r a c t i o n took place. The amount of diffused w a t e r exceeded 9 p e r c e n t of t h e t o t a l w a t e r content p r i m a r i l y b e c a u s e of d e c r e a s e d a d s o r p t i o n c a p a c i t y and not volume i n c r e a s e , a s e x p e r i e n c e d with bulk w a t e r . I t h a s t o be e m p h a s i z e d t h a t t h e 9 p e r cent i n c r e a s e in volume i s specific t o a situation w h e r e t h e w a t e r changes f r o m the d i s o r d e r e d s t a t e , c h a r a c t e r i s t i c of bulk w a t e r , t o the o r d e r e d s t a t e of bulk i c e I. T h i s t r a n s i t i o n i s connected with a h e a t effect of 79 ~ a l / ~ . The l a t e n t h e a t of fusion of a d s o r b e d w a t e r w a s found t o be r e d u c e d by 30 p e r c e n t , indicating t h a t t r a n s i t i o n t a k e s place between s t a t e s , any o r both of which a r e d i f f e r e n t f r o m t h e r e s p e c t i v e s t a t e s
in the bulk.
Air entrainment, the provision of empty voids, i s expected to protect the concrete by accommodating the e x c e s s water and reducing the p r e s s u r e . The v e r y significant differences in behaviour of the two s a m p l e s , a s shown in F i g u r e s 1 and 2, can be explained by the l a r g e s u r f a c e a r e a s of the sample with w/c = 0.7.
Conclusion
The above explanation of the m e c h a n i s m of freezing in cement attributes the generation of hydraulic p r e s s u r e and migration t o c a u s e s different f r o m those postulated in the p r e s e n t theory. The c u r r e n t study indicates that a portion of the water in cement will always f r e e z e and r e s u l t in disruption u n l e s s empty r e s e r v o i r s a r e provided in suffi- cient numbers and spacing. Air voids weaken the cement and t h e r e i s a l i m i t beyond which their number cannot be i n c r e a s e d . If the explana- tion put forward in this paper i s c o r r e c t , then a i r entrainment i s not the.only method of protecting cement. A second possibility exists, a t l e a s t theoretically, namely, surface modification. As the change i n the interaction between water and cement on freezing i s held responsible for the damage, modification of the c h e m i s t r y o r geometry of the s u r - f a c e will affect the freezing c h a r a c t e r i s t i c s .
F u r t h e r m o r e , by measuring the adsorptive p r o p e r t i e s of cement samples i t m a y be possible t o a s s e s s t h e i r f r o s t susceptibility m o r e reliably and m o r e quickly than by the p r e s e n t accelerated t e s t s .
References
1. G. Litvan and R. McIntosh, P h a s e Transitiomof Water and Xenon in P o r o u s Glass, Can. J. Chem.,
41,
1963, 3095.2. G. Litvan, P h a s e Transitions of Adsorbates I. Specific Heat and Dimensional Changes of the P o r o u s Glass-Water System, Can. J.
Chem,
44,
1966, 2617.3. A. A. Antoniou and G. G. Litvan, C a l o r i m e t e r f o r Simultaneous Measurement of T h e r m a l P r o p e r t i e s and Dimensional Changes, Rev. Sci. I n s t r . ,
38,
1967, 1641.. 4. L. B. Tuckerman, Optical Strain Gages and Extensometers, P r o c . ASTM,
23,
11, 1923, 602.8
-
Litvan5. P. J. S e r e d a and
E.G.
Swenson, Apparatus f o r P r e p a r i n g P o r t l a n d Cement P a s t e of High Water-Cement Ratio, Mat. Res. and Stand.,L,
1967, 152.6. T. C. P o w e r s and R.A. Helrnuth, T h e o r y of Volume Change i n Hardened P o r t l a n d Cement P a s t e During F r e e z i n g , P r o c e e d i n g s , Highway Res. Board,
32,
1953, 285.T h i s paper 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 Canada, and i s published with the approval of the D i r e c t o r of the Division.
