Stress and strains in the hardened cement paste - water system

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Stress and strains in the hardened cement paste - water system

Beaudoin, J. J.; Feldman, R. F.

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i

TKZ.

I

National Research

Conseil national

I

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+

Council Canada

de recherche Canada

no.

1187

I

IRC

PUB

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~ I K E S S E S

AND STRAINS I N THE HARDENED CEMENT PASTE

-

i

_{WATER SYSTEM}

by

J.J. Beaudoin and R.F. Feldman

Reprinted from

Cement and Concrete Research

Vol. 14, 1984

P.

231 -

237 DBR Paper No. 1187

Division of Building Research

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&SU&

La t h g o r i e d e s v a r i a t i o n s de longueur propos'e p a r Flood e t Heyding pour l e s s o l i d e s microporeux e s t appliqu'e

2

l a pBte de c i m e n t , e n c o n s i d ' e r a n t l ' e f f e t d e p o r o s i t ' e e t d e c o m p r e s s i b i l i t ' e s u r l a phase s o l i d e . la t h h r i e s e r e 6galement

2

Bvaluer l a dependance du module d ' e l a s t i c i t ' e v i s - a - v i s de l'humidit'e, e t e s t e n g e n e r a l e n accord avec l'exp'erience. On a r r i v e 3 mieux e x p l i q u e r l e changement m i c r o s t r u c t u r a l e t s o n e f f e t s u r l a v a r i a t i o n d e longueur a u moyen d ' e s t i m a t i o n s d'un f a c t e u r de s t r u c t u r e K . k s v a l e u r s de r s o o t

2

pen p r b c o n s t a n t e s pour des taux d ' h m i d i t ' e a l l a n t jusqu'8 56%.

T o u t e f o i s d e s changements de s t r u c t u r e importants s e p r o d u i s e n t

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CEMENT and CONCRETE RESEARCH. Vol.

1 4 ,

pp. 231-237, 1984. P r i n t e d i n t h e USA. 0006-8846184 $3.00+00. Copyright ( c ) 1984 Pergamon P r e s s , L t d .

STRESSES AND STRAINS I N THE HARDENED CEMENT PASTE

-

WATER SYSTEM

J.J.

Beaudoin and R.F. Peldman Research O f f i c e r s D i v i s i o n of Building Research N a t i o n a l Research Council Canada

Ottawa, O n t a r i o KIA OR6

(Communicated by M. Daimon) (Received June 28, 1983)

ABSTRACT

The l e n g t h change theory proposed by Flood and Heyding f o r microporous s o l i d s i s a p p l i e d t o cement p a s t e i n c o n s i d e r i n g t h e e f f e c t of p o r o s i t y and c o m p r e s s i b i l i t y on t h e s o l i d phase. The t h e o r y i s a l s o used t o e s t i m a t e t h e dependence of modulus of e l a s t i c i t y on humidity and is i n g e n e r a l agreement w i t h experiment. Some i n s i g h t i n t o m i c r o s t r u c t u r a l change and i t s e f f e c t on l e n g t h change i s provided by e s t i m a t e s of a s t r u c t u r e f a c t o r , K . For h u m i d i t i e s up t o

56%,

v a l u e s of K a r e approximately c o n s t a n t , but major changes i n s t r u c t u r e occur a t h i g h e r h u m i d i t i e s .

I n t r o d u c t i o n

Hardened p o r t l a n d cement p a s t e ( H E )

-

a term used t o d e s c r i b e t h e r i g i d porous body, e x c l u d i n g any f r e e o r adsorbed w a t e r , formed when cement h y d r a t e s

-

i s a multicomponent, microporous, m o i s t u r e - s e n s i t i v e m a t e r i a l . Numerous s t u d i e s have been conducted on t h e mechanical p r o p e r t i e s of HCP and t h e model proposed by Feldmsn and h i s co-workers (1,2) h a s been u s e f u l i n e x p l a i n i n g r e s u l t s , e.g., t h e dependence of modulus of e l a s t i c i t y (E) on humidity where i n c r e a s e s i n E w i t h humidity were a t t r i b u t e d t o t h e s t i f f e n i n g e f f e c t of i n t e r l a y e r water.

Thermodynamic t r e a t m e n t of HCe length-change d a t a i s complicated because of i r r e v e r s i b l e p r o c e s s e s t h a t occur when t h e system is w e t t e d . I n a d d i t i o n t o s u r f a c e a d s o r p t i o n t h e s e p r o c e s s e s i n c l u d e i n t e r c a l a t i o n of w a t e r between s h e e t s , s w e l l i n g , agglomeration e f f e c t s and p o s s i b l y t h e e f f e c t s of s h e a r s t r e s s e s i n t h e s o l i d m a t e r i a l . Flood and Heydiog used a thermodynamic approach ( s u b s e q u e n t l y r e f e r r e d t o a s t h e F-H approach) t o determine

t h e o r e t i c a l l y t h e l e n g t h change i s o t h e r m s f o r w a t e r vapor on carbon and f o r water vapor on porous g l a s s ( 3 ) . They d e r i v e d a s i m p l e e x p r e s s i o n f o r l e n g t h change a s a f u n c t i o n of p o r o s i t y , c o m p r e s s i b i l i t y of t h e s o l i d , m i c r o s t r u c t u r e of t h e a d s o r b e n t , and amount of H O adsorbed. T h e i r t h e o r y p r o v i d e s a p o s s i b l e means of a s s e s s i n g b o t h i r r e v e r s i t l e e f f e c t s on l e n g t h changes of XCP and t h e assumption t h a t t h e s e e f f e c t s a r e due t o e x i t and e n t r y of i n t e r l a y e r water.

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232

Vol. 14, No. 2

3 .

Beaudoin and R.F. Feldman

This paper describes the application of the F-H thermodynamic approach to the

cement paste-water system and assesses the results based on the model for

HCP

developed by Feldman and his co-workers.

Length Change Isotherms

F-H Approach

Briefly, the F-H theory assumes that assemblies of volumes of pure

adsorbable gas and assemblies of volumes of pure adsorbent can exist

separately, in equilibrium with externally applied forces, in states

thermodynamically identical to those in the adsorbent-adsorbate system. The

conditions of reversibility and equilibrium lead to the following expression

for the pressure of the pure adsorbate, pa, in the pore volume Va

where

pa

is the mean density of the substance in Va, and pl and

p l

are the gas

pressure and density of the gas surrounding the sample. The term,

a,

is the

mean value of pa/pl averaged over the pressure interval dpl.

If

$ =

Va/Vs, where Vs is the non-porous solid volume, it may be shown that

ps

=

(1

+

-

a+)p

,

which is the pressure on the solid adsorbent in

equilibrium with tbe surrounding gas at pressure pl.

If the pressure on the solid adsorbent is uniform, the length change

isotherm can be obtained from the equation

where

B

is the compressibility of the porous body.

Where solid pressures are not constant,

K ,

a small numerical factor

dependent upon the structure of the porous solid, is introduced into eq.

(I),

i.e., the term in brackets becomes (1 +

+ K

-

$ ~ a ) . K ,

the ratio of the linear

average pressures to volumetric average pressures, is generally independent of

the nature of the adsorbate, but is a characteristic of the structure of the

adsorbent. Flood and Heyding determined values of

K

for various ideal models

of pore structure, e.g.,

K =

1.0 for a system of continuous non-intersecting

straight capillaries, and

K =

5.6 for a system of continuous intersecting

straight capillaries

( 3 ) .

If

the shape of the average pore is not

statistically independent of the surface free energy of the solid enclosing the

average micropore, then

K

will become a function of pl. For large adsorptions

a >>

1 and

Porous Glass

The weight change isotherm for porous glass in the adsorption region is

reversible and glass is considered to be relatively stable. Using the length

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Vol. 1 4 , No. 2 233 STRESS, STRAIN, HARDENED CEMENT PASTE, WATER, STRUCTURE FACTOR

change d a t a of Amberg and McIntosh ( A ) , Flood and Heyding ( 3 ) were a b l e t o o b t a i n good agreement between t h e c a l c u l a t e d isotherm and e x p e r i m e n t a l d a t a , s u g g e s t i n g t h a t eq. ( 2 ) i s a p p l i c a b l e t o systems i n which w e t t i n g and d r y i n g i n v o l v e mainly r e v e r s i b l e p r o c e s s e s .

Hardened Cement P a s t e (HCP)

HCP-water isotherm. A b r i e f comment on t h e HCP-water i s o t h e r m and on t h e assumptions made i n u s i n g i s o t h e r m d a t a t o apply t h e F-H procedure f o l l a u s . Along t h e a d s o r p t i o n branch of t h e HCP-water i s o t h e r m s e v e r a l r e v e r s i b l e and i r r e v e r s i b l e e f f e c t s occur. I f r e l a r i v e humidity i s reduced from a p a r t i c u l a r v a l u e on t h e a d s o r p t i o n curve, a seaming curve i s o b t a i n e d , f o r example, c u r v e s 1 , 2 , 3, Fig. 1. It has been argued t h a t ( t o a f i r s t approximation) mainly r e v e r s i b l e p r o c e s s e s occur over a

l a r g e p a r t of t h e s c a n n i n g curve. A " r e v e r s i b l e " i s o t h e r m can be

c o n s t r u c t e d by a p p r o p r i a t e summation of r e v e r s i b l e p o r t i o n s of t h e scanning curves. D e t a i l s of t h i s procedure have been published ( 5 ) . It i s assumed t h a t i r r e v e r s i b l e e f f e c t s can t h u s be

s e p a r a t e d from r e v e r s i b l e ones. I n I a d d i t i o n , i t is recognized t h a t t h e + r e v e r s i b l e a d s o r p t i o n p r o c e s s e s o p e r a t i v e

5

a l o n g each scanning curve a r e a c t i n g on a ;

3 m a t e r i a l t h a t has changed s i n c e t h e p r e v i o u s scanning c u r v e , i . e . , t h e m a t e r i a l i s d i f f e r e n t f o r each of curves 1, 2 , 3, Fig. 1. Thus, i n t r a n s f e r r i n g from p r o c e s s e s c u r v e s a r e I t o o p e r a t i v e . 2 t o 3, i r r e v e r s i b l e A p p l i c a t i o n of

E?l

R E L A T I V E H U M I O I T Y , % t h e F-H procedure t o t h e r e v e r s i b l e i s o t h e r m assumes, however, t h a t t h e F i g u r e 1 i r r e v e r s i b l e changes i n t h e s o l i d m a t e r i a l ( i n t e r l a y e r p e n e t r a t i o n , e t c . ) Schematic of primary a d s o r p t i o n do not a f f e c t t h e n a t u r e of t h e curve w i t h scanning curves r e v e r s i b l e a d s o r p t i o n p r o c e s s e s o c c u r r i n g

on t h e s u r f a c e of t h e l a y e r s , and t h a t s u r f a c e energy changes a r e a c t i n g on a

d i f f e r e n t m a t e r i a l w i t h d i f f e r e n t p r o p e r t i e s when p o s i t i o n s a r e changed on t h e isotherm. I t i s assumed, t h e n , t h a t a step-by-step a p p l i c a t i o n of t h e P-H procedure a l o n g t h e r e v e r s i b l e a d s o r p t i o n p a t h is v a l i d , even where

i n t e r c a l a t i o n of t h e l a y e r e d s i l i c a t e h y d r a t e occurs; i . e . , s i n c e t h e scanning i s o t h e r m i s r e v e r s i b l e , t h e F-H procedure i s a p p l i c a b l e . I f s c a n n i n g isotherms a r e i r r e v e r s i b l e , t h e n changes have occurred i n t h e porous system and t h e F-H procedure cannot be a p p l i e d . Thus, t h e procedure may n o t be a p p l i c a b l e when major changes i n pore s t r u c t u r e occur.

A p p l i c a t i o n of F-H procedure t o HCP. To apply eq. ( 2 ) t o t h e HCP system i t

is n e c e s s a r y t o e v a l u a t e t h e i n t e g r a l

1''

%

:d

.

Consider t h e f o l l o w i n g : 0

For a cement p a s t e w i t h w/c

-

0.50, p o r o s i t y i s approximately 26%. T h e r e f o r e , $ = Va /Vs = 0.351. Assuming d e n s i t y of t h e non-porous s o l i d = 2.2 g / c c , t h e mass/cc of t h e porous sample i s 2.20 x 0.74 = 1.63 g / c c o r 1 g of porous sample

-

111.63 = 0.610 cc.

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234

J . J . Beaudoin and R . F . Feldman

Vol. 1 4 , No. 2

Thus Va/g of sample = 0.260 x 0.610 = 0.160 c c

and pa =

bY/V=

6 - 2 6

-

AW

W ( 3

'a

where AW/W = weight change per u n i t weight of sample. The mass of t h e g a s i n t h e void volume, Va, i s n e g l i g i b l e i n r e l a t i o n t o t h e mass of t h e adsorbed phase. The d e n s i t y of t h e g a s ( w a t e r vapor) p i s 18.34 x g / c c a t 21°C

and 100% RH. 1

.-

Data from weight and l e n g t h change isotherms a r e t a k e n from Feldman ( 5 ) . Using s c a n n i n g l o o p s , t h e s e isotherms were s e p a r a t e d i n t o i r r e v e r s i b l e and

r e v e r s i b l e i s o t h e r m s . The r e v e r s i b l e i s o t h e r m i s used i n t h e f o l l o w i n g c a l c u l a t i o n s :

P

A p l o t of -2 v e r s u s p i s given i n Fig. 2; and t h e i n t e g r a l dpl is

P 1 1 0 1 e v a l u a t e d g r a p h i c a l l y by determining t h e a r e a under t h e curve. T h i s p r o v i d e s a measure of pa = upl = p r e s s u r e of a d s o r b a t e . A curve of up v e r s u s p l ( n o t p r e s e n t e d ) was c o n s t r u c t e d t o a s s i s t i n t h e c a l c u l a t i o n s . Thus, eq. ( 2 ) can be f i t t e d t o t h e e x p e r i m e n t a l l e n g t h change d a t a by assuming a n a p p r o p r i a t e v a l u e f o r BK. F i g u r e 3 i s a p l o t of l e n g t h change v e r s u s vapor p r e s s u r e , g i v i n g curves f o r eq. ( 2 ) and e x p e r i m e n t a l v a l u e s . 4 - A s i n g l e average v a l u e of BK = 1.76 x ~ ~ a - l was chosen t o g i v e a c l o s e f i t between t h e O A e x p e r i m e n t a l l e n g t h change d a t a and eq. ( 2 ) . Although t h i s v a l u e of BK g i v e s an approximation of t h e e x p e r i m e n t a l d a t a , BK v a r i e s w i t h p and a more e x a c t f i t can be o b t a i n e d by c a l c u l a t i n g BK a t

-

each d a t a p o i n t . Values of BK a t d i f f e r e n t h u m i d i t i e s a l o n g t h e

..

a d s o r p t i o n curve were c a l c u l a t e d from eq. ( 2 ) u s i n g e x p e r i m e n t a l v a l u e s of

6 & / ~ . These v a l u e s a r e given i n 1

-

Table 1. _{- A D S O R P T I O N}

Values of BK were a l s o c a l c u l a t e d f o r an HCP-water i s o t h e r m scanning curve

over t h e range 56 t o 11% RH. BK was o - ~ ~ ~ ~ ' I ~ I ~ I ' I I

0 4 8 1 2 16 20 2 4 28 approximately c o n s t a n t o v e r t h a t p o r t i o n

of t h e scanning curve (56

-

40% RH) used G A S P R E S S U R E , PI, MPa x 10 4 t o c o n s t r u c t t h e r e v e r s i b l e isotherm.

T h i s s u p p o r t s t h e assumption t h a t F i g u r e 2 m a t e r i a l p r o p e r t i e s a r e c o n s t a n t on t h e

segment of each scanning curve used t o R a t i o of a d s o r b a t e d e n s i t y t o c o n s t r u c t t h e r e v e r s i b l e isotherm. g a s d e n s i t y v e r s u s g a s p r e s s u r e

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Vol.

14,

No. 2

235 STRESS, STRAIN, HARDENED CEMENT PASTE, WATER, STRUCTURE FACTOR

TABLE 1 C a l c u l a t i o n of c o m p r e s s i b i l i t y and modulus of e l a s t i c i t y f o r HCP from a d s o r p t i o n d a t a RH

-

6 8

_xl00

_aP

_B

K 6

*

E

a

1 ₄ _-1

x 1 ~ 4

-5

% MPa

x10

MPa

x10

MPa

*

10.3

0.016

6.67

2.06

1.08

0.167

12.7

0.018

7.88

1.96

1.03

0.176

15.4

0.020

9.18

1.85

0.97

0.186

18.5

0.022

10.74

1.76

0.92

0.196

24.1

0.026

12.99

1.70

0.89

0.202

27.8

0.030

14.28

1.79

0.94

0.192

31.1

0.032

15.84

1.73

0.91

0.199

34 .O

0.033

16.93

1.68

0.88 0.206 '

37.4

0.035

18.09

1.65

0.86

0.209

42.7

0.037

20.13

1.56

0.82

0.221

45.3

0.039

20.67

1.60

0.84

0.216

48.5

0.040

21.90

1.56

0.82

0.221

51.5

0.041

22.71

1.52

0.80

0.226

53.6

0.042

23.46

1.52

0.80

0.226

56.6

0.043

24.48

1.49

0.78

0.230

62.2

0.047

26.04

1.57

0.82

0.219

66.9

0.052

27.54

1.60

0.84

0.216

72.5

0.057

29.85

1.63

0.85

0.245

88.0

0.080

36.72

1.85

0.97

0.265 *

C a l c u l a t i o n made u s i n g K =

1.91

0.10 1 1 , , 4 1 1

, ,

, , I , Values of 6 a r e c a l c u l a t e d from

B K

v a l u e s , assuming K =

1.91.

_0.09

_-

Choice of t h i s K v a l u e g i v e s a n i n i t i a l v a l u e of

B

comparable t o

B

0.08

-

c a l c u l a t e d from t h e r e l a t i o n ₀ I 0.07-

!

B

=

2(1

_E

-

2u),

where u i s P o i s s o n s r a t i o of

0.20,

a s w e l l a s

B

determined by o t h e r methods

(6).

LU ( C a l c u l a t i o n s of

BK

were a l s o made a z 0.05

-

< u s i n g l e n g t h change d a t a f o r t h e x

porous glass-water system, b u t

-

t h e s e a r e n o t t a b u l a t e d . ) o Z w 0.03

-

2 0 4 8 12 16 20 24 28 G A S P R E S S U R E . P1, M P a x lo4 F i g u r e

3

"Reversible" l e n g t h change isotherm f o r p o r t l a n d cement p a s t e , w/c =

0.50,

e x p e r i m e n t a l and t h e o r e t i c a l

(9)

Vol. 1 4 , No. 2 J . J . Beaudoin and R.F. Feldman

C E M E N T P A S T E R E L A T I V E H U M I D I T Y . %

F i g u r e 4

Dependence of

BK

on r e l a t i v e humidity f o r cement p a s t e and

porous g l a s s 0. 26

-

0 . 2 4

-

0 . 2 2 - THIS WORK, K = 1.91

I

... .. . . .. . THIS WORK. K = 1.91 - 2. R E L A T I V E H U M I D I T Y . % F i g u r e 5 Modulus of e l a s t i c i t y v e r s u s r e l a t i v e humidity f o r cement p a s t e , w/c = 0.50 F i g u r e 4 i s a p l o t of

BK

f o r b o t h HCP and porous g l a s s .

B K

f o r HCP i s dependent on RH, whereas f o r porous g l a s s it i s independent of RH. It would be expected t h a t porous g l a s s would be s t a b l e , i . e . , n o t undergo a m a t e r i a l change a s RH i n c r e a s e s . Thus, t h e r e l a t i v e l y c o n s t a n t v a l u e of

BK

determined by t h e F-H procedure

-

i s a p p r o p r i a t e . The dependence of

BK

on RH i s expected f o r HCP s i n c e t h e h y d r a t e d calcium s i l i c a t e s a r e u n s t a b l e , i . e . , C-S-H s o l i d s change a s RH i n c r e a s e s . F i g u r e 5 i s a p l o t of E v e r s u s RH f o r HCP. One of t h e c u r v e s i s a p l o t of t h e c a l c u l a t e d E (determined by F-H procedure u s i n g K = 1.91). P O R O U S G L A S S

:

,

, j

0 . 3 0 20 40 60 80 100 R E L A T I V E H U M I D I T Y . % C E M E N T P A S T E

-1

R E L A T I V E H U M I D I T Y , % F i g u r e 6 ( a ) S t r u c t u r e f a c t o r K v e r s u s r e l a t i v e humidity (b) D e n s i t y of HCP exposed t o v a r i o u s r e l a t i v e h u m i d i t i e s and r e t u r n e d t o 11% RH

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Vol. 1 4 , No. 2 237 STRESS, STRAIN, HARDENED CEMENT PASTE, WATER, STRUCTURE FACTOR

Another c u r v e i s a p l o t of e x p e r i m e n t a l v a l u e s of E

( 7 ) .

On a d s o r p t i o n up t o 56% RH, t h e two c u r v e s have a maximum d i f f e r e n c e i n E ( a t any RH) of o n l y 0.01 x

lo5

m a , and E i n c r e a s e s w i t h RH. A t h i g h e r h u m i d i t i e s u s e of t h e s t r u c t u r e f a c t o r K = 1.91 g i v e s d e c r e a s i n g v a l u e s of E t o 68% RH, followed by a n o t h e r i n c r e a s e i n E a s RH i n c r e a s e s f u r t h e r . By a d j u s t i n g K a t each humidity

above 56% RH, however, (and d e t e r m i n i n g a new v a l u e of 6 from B K ) , t h e

c a l c u l a t e d v a l u e of E i n c r e a s e s monotonically a s t h e dashed c u r v e (Fig. 5) i s

extended from 56 t o 88%

RH.

The extended curve a l s o g i v e s v a l u e s of E c l o s e t o

.*

t h o s e determined by experiment.

F i g u r e 6a i s a p l o t of K v e r s u s RH f o r HCP and porous g l a s s . K i s c o n s t a n t

f o r HCP up t o 56% RH and i s humidity-dependent t h e r e a f t e r . K f o r porous g l a s s

i s c o n s t a n t o v e r t h e whole humidity range. S e v e r a l i r r e v e r s i b l e changes t a k e p l a c e i n HCP a t h i g h e r h u m i d i t i e s . It i s known, f o r example, t h a t r e w e t t i n g a d r i e d sample above 50% RH s i g n i f i c a n t l y i n c r e a s e s c r e e p and d r y i n g s h r i n k a g e .

It i s i n t h e h i g h humidity range t h a t t h e m i c r o s t r u c t u r e , and hence p o r e shape a n d / o r c o n t i g u i t y , may be a l t e r e d ; f o r example, i n F i g . 6b i t i s shown t h a t a b s o l u t e d e n s i t y (measured on samples of HCP exposed t o v a r i o u s RH's and d r i e d t o 11% RH) i s c o n s t a n t t o a b o u t 42% RH and d e c r e a s e s t h e r e a f t e r . Experiments have shown t h a t a f t e r w e t t i n g HCP t o 42% RH, He g a s

is

a b l e t o p e n e t r a t e f u l l y s p a c e s p r e v i o u s l y v a c a t e d by w a t e r ( 8 ) . It i s noteworthy t h a t e a c h p o i n t on t h e curve i n F i g . 6b r e p r e s e n t s t h e d e n s i t y of HCP t h a t h a s been c o n d i t i o n e d a l o n g e a c h d i f f e r e n t s c a n n i n g c u r v e of t h e i s o t h e r m , s i n c e t h e m a t e r i a l on e a c h i s d i f f e r e n t . The change i n d e n s i t y a t h i g h e r h u m i d i t i e s s u g g e s t s t h a t m i c r o s t r u c t u r a l changes, i n a d d i t i o n t o t h o s e o c c u r r i n g a t lower h u m i d i t i e s , occur (changes a t lower h u m i d i t i e s a r e a t t r i b u t e d mainly t o i n t e r l a y e r

p e n e t r a t i o n ) . These changes a p p e a r t o be r e f l e c t e d i n t h e changing v a l u e of K

a t h i g h e r h u m i d i t i e s . Concluding Remarks

The Flood-Heyding l e n g t h change t h e o r y f o r microporous a d s o r b e n t s c a n be u s e d t o e s t i m a t e t h e r e v e r s i b l e l e n g t h change i s o t h e r m f o r hardened cement p a s t e and g i v e s credence t o t h e model of cement p a s t e developed by Feldman and h i s co-workers. The t h e o r y i s a l s o i n agreement w i t h t h e observed humidity dependence of modulus of e l a s t i c i t y f o r t h e cement p a s t e system. Although many of t h e i r r e v e r s i b l e m i c r o s t r u c t u r a l changes r e s u l t i n g from w e t t i n g and d r y i n g of HCP a r e i n d e t e r m i n a t e , an a p p r e c i a t i o n of t h e s e e f f e c t s can be o b t a i n e d from e s t i m a t e s of t h e s t r u c t u r e f a c t o r K . S i g n i f i c a n t changes i n t h e v a l u e of K a t h u m i d i t i e s g r e a t e r t h a n 56% RH a r e c o n c u r r e n t w i t h changes i n t h e s o l i d d e n s i t y of HCP.

R e f e r e n c e s

R.F. Feldman and P . J . Sereda, Eng. J .

53,

53 (1970).

R.F. Feldman and V.S. Ramachandran, Cem. Concr. Res.

k,

155 (1974).

E . A . Flood and R.H. Heyding, Can. J . Chem.

32,

660 (1954).

C.H. Amberg and R. McIntosh, Can. J. Chem.

30,

1012 (1952).

R.F. Feldman, Proc. Vth I n t . Symp. Chem. Cement, Tokyo, 111-23, 53 (1968).

R.A. Helmuth and D.H. Turk, Proc. HRB, Spec. Rep. 90, 135 (1966). V.S. Ramachandran, R.F. Feldman and J . J . Beaudoin, Concrete S c i e n c e , Heyden & Son, L t d . , pp. 398 (1981).

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CEMENT and CONCRETE RESEARCH. Vol

.

14, pp. 238-248, 1984. P r i n t e d i n t h e USA. 0008-8846/84 $3.00+00. C o p y r i g h t ( c ) 1984 Pergarnon Press, L t d .

MECHANISM

AND

KINETICS OF HYDRATION OF

C

3

A

AND C 4AF, EXTRACTED FROM CEMENT.

C . Plowman and

J.G.

C a b r e r a w

D e p a r t m e n t o f C i v i l E n g i n e e r i n g , The U n i v e r s i t y o f L e e d s , L e e d s LS2 9 J T , E n g l a n d .

(Cornrnun i c a t e d by F. H. W i ttrnann)

( ~ e c e i ved Ju 1 y 2 1 , 1983)

AESIRA(;T

The

e a r l y hydration of C3A

+

C,+AF extracted f r a n

carerit

and

mixes

with quartz, g y p m and pulverised f u e l ash (PFA) has been studied by x-ray d i f f r a c t i o n . The investigation has

shown

t h a t t h e hydration of both aluminates

is

e s s e n t i a l l y a mechanisn which obeys a modified diffusion equation. The values obtained f o r t h e reaction r a t e s show that the hydra- t i o n of C3A takes place a t seven

tires

t h e r a t e of hydration of C4A.F. PFA was s h a m t o be a very e f f e c t i v e

retarder.

A

mechanisn t o explain retardation

is

also proposed.

Introduction

Addition of gypsum during t h e grinding process f o r the production of cement

is

the i n d u s t r i a l accepted method f o r controlling f l a s h setting-which

arises

from the rapid hydration of cement.

I t

is

accepted

that

the main constituent of cemnt responsible f o r f l a s h s e t t i n g

is

C3A, and t h a t g y p m r e t a r d s the hydration of t h i s phase.

What

is still

a matter of controversy

is

the manner i n which g y p m r e t a r d s

hydration; a l s o there is very l i t t l e information regarding the ~ c h a n i s n and

'

k i n e t i c s of hydration of C3A and C4AF. The theories proposed t o explain

retardation of hydration can be broadly divided in two groups:

( a ) The protective layer theory: researchers supporting this theory disagree

'

mainly on t h e conposition of t h e protective layer. E a r l i e r work i n the

1950's (1) advanced the idea t h a t t h e protective layer consisted of a t h i n layer of e t t r i n g i t e f o n d around the aluminate p a r t i c l e s . Later m r k

(2,3,4)

including the investigations of Collepardi e t a 1 (5) have supported this theory. Other investigators, notably Gupta, C h a t t e r j i and J e f f e r y (6)

cam t o t h e conclusion

that

the impervious layer consists of

C4,AY,

which

is

formed on t h e aluminate grains i n t h e presence of QI and

c%,.

They indi- cated t h a t t h e t h i n C 4 4 layer

is

overlaid with e t t r i n g i t e ; when t h e

e t t r i n g i t e cover hems s u f f i c i e n t l y t h i c k , t h e m b i l i t y of t h e sulphate ion

is r e s t r i c t e d and thus the tetra-aluminate w i l l be converted t o mnosulphate instead of e t t r i n g i t e .