High-strength cement pastes : a critical appraisal

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High-strength cement pastes : a critical appraisal

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

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HIGH-STRENGTH CEMENT PASTES

-

A CRITICAL APPRAISAL

by

J.J.

Beaudoin and R.F. Feldman

A N A L Y Z E D

Reprinted from

Cement and Concrete Research

Vol. 15, No.

1,

1985

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Les d c a n i s m e s d e r e n f o r c e m e n t q u i a g i s s e n t dans l a p l t e de c i m e n t 2 h a u t e r e s i s t a n c e s o n t e v a l u e s d a n s c e t e s s a i v i s a n t _B d e f i n i r l e s c a r a c t e r i s t i q u e s communes B c e r t a i n e s n o u v e l l e s t e c h n o l o g i e s comme l e s p a t e s d e ciment p r e s s e e s 3 chaud, impregnees e t s a n s d 6 f a u t s m a j e u r s . Les f a c t e u r s i m p o r t a n t s m o d i f i a n t l a r e s i s t a n c e d e s m a t e r i a u x poreux s o n t , e n t r e a u t r e s , l a p o r o s i t e , l a dimension d e s p o r e s , l a c o m p o s i t i o n C-S-H, l e comportement d e s c o m p o s i t e s e t l e s c r i t e r e s d e r u p t u r e . C e t t e communication t r a i t e egalement d e s e f f e t s d e f a i b l e s r a p p o r t s eau-ciment, d e s i n c l u s i o n s d e n s e s , d e s m o d i f i c a t i o n s d e s c o n t r a i n t e s dans l e s zones c r i t i a u e s e t d e l e u r r 8 1 e d a n s 1 1 a m 6 1 i o r a t i o n e t l e c o n t r a l e d e l a - f a b r i c a t i o n d e l a p l t e d e ciment B h a u t e r e s i s t a n c e .

!

CEMENT and CONCRETE RESEARCH. Vol. 1 5 , pp. 105-116, 1985. P r i n t e d i n t h e USA.

I

0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon P r e s s , Ltd.

HIGH-STRENGTH CEMENT PASTES

- A

CRITICAL APPRAISAL

J.J. Beaudoin and R.F. Feldman

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 of B u i l d i n g Research, National Research Council Canada, Ottawa, Canada, KIA 0R6.

(Communicated by M. Daimon) (Received June 5 , 1984)

The mechanisms r e s p o n s i b l e f o r h i g h s t r e n g t h i n cement p a s t e systems a r e a s s e s s e d i n an a t t e m p t t o d e f i n e f e a t u r e s common t o s e l e c t e d new t e c h n o l o g i e s , e.g., hot-pressed, impregnated, and macrodefect-free p a s t e s . Important f a c t o r s i n f l u e n c i n g s t r e n g t h of porous systems i n c l u d e p o r o s i t y , p o r e geometry, C-S-H composition, composite behaviour and f r a c t u r e c r i t e r i a . The e f f e c t s of low w a t e r c e m e n t r a t i o , d e n s e i n c l u s i o n s , s t r e s s m o d i f i c a t i o n i n c r i t i c a l a r e a s and t h e i r r o l e i n o p t i m i z i n g and c o n t r o l l i n g p r o d u c t i o n o f h i g h - s t r e n g t h cement p a s t e a r e d i s c u s s e d .

Energy consciousness i n t h e 1980's h a s i n c r e a s e d t h e need f o r i n n o v a t i o n i n t h e cement and c o n c r e t e i n d u s t r y , w i t h t h e r e s u l t t h a t h i g h - s t r e n g t h

l i g h t w e i g h t m a t e r i a l s have been developed f o r v a r i o u s a p p l i c a t i o n s such a s c l a d d i n g p a n e l s and composites f o r r e t r o f i t purposes. The a r e a of h i g h s t r e n g t h concrete* h a s a l s o been t h e s u b j e c t of r e c e n t symposia c a l l e d t o o b t a i n consensus o n p r i o r i t i e s t o be s e t among c o n c r e t e s c i e n t i s t s , s t r u c t u r a l e n g i n e e r s , and mechanics t h e o r i s t s ( 1 ) .

Development .of new m a t e r i a l s h a s g e n e r a l l y been c a r r i e d o u t by m a t e r i a l s c i e n t i s t s , i n c l u d i n g r e s e a r c h on t h e s u p e r p l a s t i c i z e r s t h a t have made p o s s i b l e t h e u s e of low w/c r a t i o c o n c r e t e . Although t h e r e a r e many

,

c o n v e n t i o n a l ways of producing h i g h - s t r e n g t h c o n c r e t e ( i n c l u d i n g j u d i c i o u s s e l e c t i o n of a g g r e g a t e , admixture and c u r i n g c o n d i t i o n s ) work h a s g e n e r a l l y been r e l a t e d t o t h e cement p a s t e b i n d e r phase. New t e c h n o l o g i e s r e s u l t i n g i n I

h i g h - s t r e n g t h c o n c r e t e p r o d u c t s have evolved from r e s e a r c h on p o r t l a n d cement p s t e . T h i s paper examines some of t h e s t r e n g t h e n i n g mechanisms i n o p e r a t i o n

i

2

h i g h - s t r e n g t h cement p a s t e and c o n s i d e r s t h e m e c h a n i s t i c f e a t u r e s common t o s e l e c t e d new t e c h n o l o g i e s concerned w i t h t h e p a s t e .

*

For normal weight a g g r e g a t e c o n c r e t e , h i g h s t r e n g t h i s g e n e r a l l y d e f i n e d a s c o n c r e t e having compressive s t r e n g t h exceeding t h a t used i n c u r r e n t d e s i g n p r a c t i c e and i s nominally 240 MPa.

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HIGH-STRENGTH CEMENT PASTES, APPRAISAL, MDF PASTES, FRACTURE

Some a u t h o r s (12) have c i t e d t h e e x i s t e n c e of macropores a s a s t r e n g t h c o n t r o l l i n g f a c t o r i n cement systems. Evidence f o r t h e e x i s t e n c e of

I

macropores based on d i f f e r e n c e s i n p o r o s i t y v a l u e s o b t a i n e d by w a t e r

I

pycnometry o r pycnometry u s i n g o t h e r f l u i d s i s n o t ' v a l i d i n h y d r a t e d p o r t l a n d cement systems. Macropores i n v e r y low w/c r a t i o cement p a s t e p r e p a r a t i o n s

I _(w/c

G 0.20) may o c c u r a s a r e s u l t of poor compaction i f r h e o l o g i c a l a i d s , I

t

s p e c i a l a d m i x t u r e s , o r s p e c i a l compaction p r o c e d u r e s a r e n o t used. Such v o i d s

would undoubtedly be a s o u r c e of weakness. 1.2 Composition of C-S-H

It i s known t h a t f o r t h e h y d r a t e d p o r t l a n d cement s y s t e m t h e d e g r e e of p o l y m e r i z a t i o n and p o l y s i l i c a t e c o n t e n t i n c r e a s e s w i t h t i m e . It h a s n o t y e t been demonstrated, however, t h a t t h e dependence of s t r e n g t h on p o l y s i l i c a t e c o n t e n t i s independent of t h e t y p e of C-S-H.

The dependence of s t r e n g t k p o r o s i t y r e l a t i o n s o n chemical composition of t h e h y d r o s i l i c a t e s (and, hence, d e n s i t y and c r y s t a l l i n i t y ) h a s been s t u d i e d by t h e a u t h o r s , who o b t a i n e d a f a m i l y of l o g s t r e n g t k p o r o s i t y c u r v e s from t h e r e s u l t s of measurements on a v a r i e t y of a u t o c l a v e d c e m e n t - s i l i c a systems (5)

(Fig. 2a). I n g e n e r a l , h i g h e s t s t r e n g t h s a p p e a r t o b e o b t a i n e d i f t h e r e a c t i o n product i s a blend of p o o r l y - c r y s t a l l i n e and s e m i - c r y s t a l l i n e t y p e s of C-S-H ( c u r v e s f o r 20 and 30% S i 0 2 , Fig. 2a). The p r o p o r t i o n of c r y s t a l l i n e t y p e s g i v i n g h i g h e s t s t r e n g t h is dependent on p o r o s i t y and s t a r t i n g m a t e r i a l . The f l y ash/cement b l e n d ( c u r v e EF, Fig. 2a) h a s a n i n c r e a s i n g p r o p o r t i o n of c o a r s e , dense, c r y s t a l l i n e m a t e r i a l and h a s h i g h e s t s t r e n g t h s o v e r t h e p o r o s i t y r a n g e s t u d i e d . Very h i g h s t r e n g t h o b t a i n e d a t low p o r o s i t y (below t h e normal working r a n g e ) w i t h a h i g h d e n s i t y m a t r i x m a t e r i a l w i l l b e d i s c u s s e d a s a s p e c i a l c a s e i n s e c t i o n 2.2.

T a y l o r ( 6 ) and Crennan e t a l . (13) p r e s e n t e d t h e a u t h o r s ' d a t a i n a n o t h e r way by p r e p a r i n g a p o r o s i t y p a r t i c l e - t y p e s t r e n g t h diagram (Fig. 2b) f o r cement p a s t e s and s i m i l a r m a t e r i a l s . P o r o s i t y i s p l o t t e d a g a i n s t amount of c o a r s e , dense, c r y s t a l l i n e m a t e r i a l f o r a s e r i e s of c o n s t a n t s t r e n g t h c o n t o u r s . The l i n e s AB, EF, and CD c r o s s i n g t h e s t r e n g t h c o n t o u r s a r e f o r normally cured cement p a s t e , a u t o c l a v e d c e m e n t - s i l i c a p r o d u c t s and v e r y " h i g h d e n s i t y " cement m a t e r i a l s . The s i g n i f i c a n c e of t h e l i n e s w i l l b e d i s c u s s e d i n s e c t i o n s 2.1 and 2.2.

1.3 Cement P a s t e M i c r o s t r u c t u r e

Cement p a s t e c a n be c o n s i d e r e d a s a composite m a t e r i a l c o n s i s t i n g l a r g e l y of C-S-H, CH, unhydrated cement and p o r e s p a c e , t h e amount of e a c h component depending on t h e d e g r e e of h y d r a t i o n and w/c r a t i o . I n a s s e s s i n g t h e r e l a t i v e c o n t r i b u t i o n of e a c h component t o s t r e n g t h t h e a p p l i c a t i o n o f composite t h e o r y i s u s e f u l ; when a p p l y i n g b i n a r y m i x t u r e r u l e t h e o r y C-S-H, CH, and p o r e s p a c e c a n b e c o n s i d e r e d a s one p h a s e and unhydrated cement a s t h e o t h e r . The f o l l o w i n g form of mixing r u l e e q u a t i o n r e l a t i n g microhardness* of t h e composite (Hc) t o volume f r a c t i o n of t h e second phase p r o v i d e s some i n s i g h t i n t o e x p e c t e d composite behaviour (14) :

Hc = H I ( 1 + { a v 2 / ( 1

+

Y V ~ ) } ) where a = ( G /G - l ) / ( G / G ~ ) } B

Y =

1

a-fE,fE,)+l)f ( E , / E , - ~ )

V2 = volume f r a c t i o n of second phase E i s modulus of e l a s t i c i t y

*

Microhardness c o r r e l a t e s w i t h b o t h compressive and f l e x u r a l s t r e n g t h of cement systems (5).

108

J . J . Beaudoin and R.F. Feldman

10 20 30 40 5 0 60 P O R O S I T Y . % I I I ( I I I , , / P O R T L A N D C E M E N T P A S T E

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I I I , I , I , 0 0 2 4 6 8 Vol. 15, No. 1 10 FIG. 2 ( a ) Compressive s t r e n g t h v e r s u s p o r o s i t y f o r v a r i o u s a u t o c l a v e d and room t e m p e r a t u r e h y d r a t e d cement and c e m e n e s i l i c a p r e p a r a t i o n s F L E X U R A L S T R E N G T H . M P a 0 I N C R E A S I N G P R O P O R T I O N O F C O A R S E , D E N S E . C R Y S T A L L I N E M A T E R I A L

-

FIG. 2 ( b ) P o r o s i t y - p a r t i c l e t y p e s t r e n g t h p l o t f o r cement p a s t e s and s i m i l a r m a t e r i a l s . Curves of c o n s t a n t compressive s t r e n g t h a r e l a b e l l e d i n MPa. The broken l i n e XC j o i n s p o i n t s of maximum s t r e n g t h a t a g i v e n p o r o s i t y and f r a c t i o n of n o n - c r y s t a l l i n e m a t e r i a l . For s i g n i f i c a n c e of l i n e s AB, CD and

EF

s e e t e x t

k

FIG. 2 ( c ) R e l a t i o n between compressive s t r e n g t h and f l e x u r a l s t r e n g t h f o r normally h y d r a t e d p o r t l a n d cement p a s t e

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HIGH-STRENGTH CEMENT PASTES, APPRAISAL, MDF PASTES, FRACTURE

G i s s h e a r modulus

f? i s a s t r e s s c o n c e n t r a t i o n f a c t o r and i s a f u n c t i o n of E2/E and

geometry of t h e second phase ( i . e . major t o minor a x i s r a t i o of a n e l i p t i c a l i n c l u s i o n )

( S u b s c r i p t s r e f e r t o f i r s t o r second phase and H 1 i s t h e microhardness of t h e f i r s t phase, u s u a l l y t h e c o n t i n u o u s m a t r i x . )

An example where eq. ( 1 ) might be a p p l i e d i s a cement s y s t e m c o n t a i n i n g l a r g e amounts of unhydrated cement s e p a r a t e d by t h i n l a y e r s of C-S-H.

Consider phase 1 t o be t h e d e n s e , unhydrated cement p a r t i c l e s p r e s e n t i n s u f f i c i e n t numbers t o comprise t h e c o n t i n u o u s phase, and t h e second,

d i s c o n t i n u o u s phase t o be t h e h y d r a t e d m a t e r i a l i n c l u d i n g C-S-H, CH and p o r e space. Equation ( 1 ) i n d i c a t e s t h a t t h e c o n t r i b u t i o n of t h e second phase t o composite s t r e n g t h depends on t h e s h a p e of t h e second-phase p a r t i c l e s , modulus r a t i o , and volume f r a c t i o n . The i n n e r b r a c k e t e d term ( n e g a t i v e ) e x p r e s s e s t h e d e g r e e t o which t h e s t r o n g unhydrated cement p a r t i c l e s c a n c o n t r i b u t e t o composite s t r e n g t h . Other examples i n c l u d e polymer-impregnated o r modified- cement systems. I n g e n e r a l , t h e composite t h e o r y w i l l b e used t o supplement o t h e r arguments i n s u p p o r t of p a r t i c u l a r s t r e n g t h e n i n g mechanisms.

1.4 Crack Formation

Some workers have employed t h e G r i f f i t h e q u a t i o n t o p r e d i c t s t r e n g t h ( o ) of ceramic and cement systems ( 5 ) .

where E i s Young's modulus, T i s s u r f a c e e n e r g y , and c i s h a l f t h e f l a w l e n g t h .

One d i f f i c u l t y w i t h t h e d i r e c t a p p l i c a t i o n of eq. ( 2 ) i s t h a t work t e r m i n a d d i t i o n t o s p e c i f i c s u t f a c e energy a f f e c t f r a c t u r e . Secondly, n e i t h e r f r a c t u r e energy nor Young's modulus i s c o n s t a n t ; b o t h a r e dependent on p o r o s i t y , w/c r a t i o , environment, and o t h e r m a t r i x p r o p e r t i e s (2,15). F o r example, some workers have i n c o r r e c t l y compared polymer-odified cement

systems w i t h o r d i n a r y p o r t l a n d cement p a s t e (9), u s i n g t h e same v a l u e of T f o r b o t h systems i n e q , ( 2 ) .

E q u a t i o n ( 2 ) i s a p a r t i c u l a r c a s e of t h e I n g l i s s o l u t i o n f o r t h e s t r e s s f i e l d round a n e l l i p t i c a l h o l e i n a n i n f i n i t e p l a t e w i t h i t s major a x i s normal t o a uniform t e n s i l e s t r e s s (16). A fundamental assumption i n t h e G r i f f i t h e q u a t i o n i s t h a t t h e f l a w s a r e a t o m i c a l l y s h a r p and t h e major/minor a x i s r a t i o l a r g e , i . e . , t h a t t h e s h a p e of t h e f l a w i s f i x e d . Pores ( h a v i n g d i f f e r e n t pore s h a p e s ) , a s such, do n o t f i t t h e G r i f f i t h d e f i n i t i o n of a flaw. Some ceramic s c i e n t i s t s have approached t h i s problem by c o n s i d e r i n g p o r e s themselves a s an i n t e g r a l p a r t of f l a w s , i.e., t h e l e n g t h of t h e f l a w i s c o n s i d e r e d t o be t h e p o r e d i a m e t e r p l u s one g r a i n d i a m e t e r on e i t h e r s i d e of t h e pore (17). T h i s i s based on t h e concept t h a t c r a c k s w i l l p r o p a g a t e a l o n g g r a i n boundaries emanating from t h e p o r e w a l l i n t o t h e body u n t i l t h e n e x t l a y e r of g r a i n s i s encountered. E f f e c t s of v a r i a t i o n i n p o r e shape t h a t a l t e r t h e s t r e s s f i e l d i n t h e v i c i n i t y of t h e p o r e a r e n o t considered. T h i s concept was a p p l i e d t o cement p a s t e by Wittman and Z a i t s e v , who modelled t h e system a s

a p l a t e c o n t a i n i n g a c y l i n d r i c a l h o l e w i t h two edge c r a c k s emanating from p o i n t s of maximum t e n s i l e s t r e s s (18). V e r i f i c a t i o n of i t s v a l i d i t y f o r normally h y d r a t e d cement p a s t e by d i r e c t measurement i s d i f f i c u l t s i n c e g r a i n s i z e i s extremely f i n e . I n systems t h a t c o n t a i n l a r g e numbers of unhydrated cement g r a i n s ( 5

-

50 um) i t may be p o s s i b l e . It i s , however, u n c e r t a i n whether macropores i n p o o r l y compacted low waterlcement r a t i o p a s t e s , which c o n t a i n l a r g e amounts of unhydrated cement, c a n b e c o n s i d e r e d a s f l a w s i n t h e G r i f f i t h s e n s e ( 9 ) .

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Vol. 1 5 , No. 1

1.5 Applied S t r e s s Mode

The d i s c u s s i o n of f r a c t u r e c r i t e r i a h a s been based o n specimens s u b j e c t e d t o t e n s i l e s t r e s s . A l i n e a r c o r r e l a t i o n between compressive and f l e x u r a l s t r e n g t h s was observed (19) f o r normally h y d r a t e d cement p a s t e (Fig. 2c); t h e a v e r a g e compressive/f l e x u r a l s t r e n g t h r a t i o was approximately 10. Roy and Gouda ( 7 ) found a s i m i l a r c o r r e l a t i o n between compressive and t e n s i l e s t r e n g t h s f o r b o t h h o e p r e s s e d and c o l d - p r e s s e d cement p a s t e

specimens and argued t h a t most m i c r o s t r u c t u r a l p r o p e r t i e s (such a s p o r o s i t y ) a f f e c t i n g h i g h s t r e n g t h i n compression c o n t r i b u t e i n a p a r a l l e l manner t o t h e h i g h s t r e n g t h i n t e n s i o n .

F a i l u r e t h a t o c c u r s i n most compressive s t r e n g t h t e s t s i n v o l v e s t h e g e n e r a t i o n of t e n s i l e s t r e s s e s because of end e f f e c t s . Crack growth i n v o l v i n g a p o r e l i n k a g e p r o c e s s h a s a l s o been d e s c r i b e d a s o c c u r r i n g d u r i n g compression of p o r t l a n d cement p a s t e (17). It a p p e a r s t h a t t h e r e a r e common f a c t o r s a f f e c t i n g c r a c k growth i n compression and f l e x u r a l t e s t i n g . The r o l e p o r e s p l a y i n f r a c t u r e under compression o r i n t e n s i o n may, however, be d i f f e r e n t . R i c e h a s s u g g e s t e d t h a t t e n s i l e ( o r f l e x u r a l ) s t r e n g t h may b e c o n t r o l l e d p r i m a r i l y by p o r e and g r a i n s i z e and by t o t a l p o r o s i t y , i n c l u d i n g s h a p e e f f e c t s , a s d i s c u s s e d p r e v i o u s l y ; h e f u r t h e r a r g u e s t h a t , i n compression, p o r e s a l o n e a c t a s s t r e s s c o n c e n t r a t o r s : t h a t , i n compression, c r a c k development from p o r e s i s i n h i b i t e d u n t i l much h i g h e r s t r e s s e s have been r e a c h e d ( 1 7).

1.6 Pore Shape

S e v e r a l e x p r e s s i o n s have been u s e d t o d e s c r i b e t h e p o r o s i t y dependence o f s t r e n g t h and f r a c t u r e energy f o r c e r a m i c and cement systems (6,20). F o r example, i t h a s been found t h a t t h e r o l e of pore shape i n f r a c t u r e energy can be accounted f o r by t h e f o l l o w i n g r e l a t i o n ( 2 1) :

Y = Y , ~ -bp ( 3 )

where Y = f r a c t u r e e n e r g y , p = p o r o s i t y , yo = f r a c t u r e energy a t z e r o p o r o s i t y , and b i s a p o r e s h a p e f a c t o r (Fig. 3). S i m i l a r e x p r e s s i o n s have been used f o r t h e p o r o s i t y dependence of e l a s t i c modulus and s t r e n g t h . A model was developed by Knudsen (22) t o a c c o u n t f o r t h e e f f e c t o f p o r e s h a p e by assuming t h a t f r a c t u r e energy should be p r o p o r t i o n a l t o t h e f r a c t i o n of s o l i d a r e a f r a c t u r e d , and t h a t f r a c t u r e w i l l g e n e r a l l y f o l l o w t h e p a t h of m a x i m pore and minimum s o l i d c r o s s - s e c t i o n a l a r e a . The a n a l y s i s , which assumes a uniform d i s t r i b u t i o n of p o r e s , r e s u l t e d i n t h e non-linear model c u r v e s i n Fig. 3. The "b" v a l u e s used i n eq. ( 3 ) , which r e s u l t i n l i n e a r c u r v e s i n c l o s e agreement w i t h t h e model c u r v e s , a r e g i v e n i n Fig. 3. Values of "b" d e r i v e d from s t r e n g t h d a t a f o r a u t o c l a v e d c e m e n t s i l i c a p r e p a r a t i o n s v a r y from

4.4 t o 7.7. Assuming t h a t t h e r e i s some c o r r e l a t i o n between s t r e n g t h r a t i o s (S/So) and f r a c t u r e energy r a t i o s (-flyo), t h e s e systems seem t o have a v a r i e t y of pore shapes. It might be argued t h a t p o r e shape and composition of C-S-H a r e n o t two independent v a r i a b l e s . It a p p e a r s , however, t h a t p o r e s h a p e m o d i f i c a t i o n f o r a system w i t h a g i v e n composition p r o v i d e s a means of

i n c r e a s i n g s t r e n g t h .

I n a d d i t i o n t o s h a p e , t h e "b" v a l u e i n eq. ( 3 ) i s a l s o s e n s i t i v e t o pore l o c a t i o n and s i z e . F r a c t u r e h a s g e n e r a l l y been s e e n t o o r i g i n a t e i n l a r g e , n o t s m a l l , p o r e s i n a body h a v i n g b o t h t y p e s (17). Bodies h a v i n g o n l y f i n e p o r e s a r e g e n e r a l l y a t l e a s t a s s t r o n g a s comparable b o d i e s w i t h

p r i m a r i l y l a r g e p o r e s . Samples h a v i n g more uniformly shaped and spaced p o r e s tend t o have lower "b" v a l u e s . Local c o n c e n t r a t i o n s of p o r e s c a n a l t e r "b" v a l u e s whether t h e p o r e s a r e i n t e r g r a n u l a r o r i n t r a g r a n u l a r , o r mixed. In

Vol. 15, No. 1 111 HIGH-STRENGTH CEMENT PASTES, APPRAISAL, MDF PASTES, FRACTURE

FIG. 3

F r a c t u r e energy r a t i o , y/yo vs

volume f r a c t i o n p o r o s i t y f o r v a r i o u s s i m p l e p o r e shapes. The arrows from each p o r e shape i n d i c a t e t h e d i r e c t i o n of s t r e s s i n g r e l a t i v e t o t h e p o r e geometry

-. -

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 V O L U M E F R A C T I O N P O R O S I T Y , P

p o r e s a t g r a i n boundaries. The "b" v a l u e i n compression tests i s

s i g n i f i c a n t l y h i g h e r . The r e a s o n f o r t h i s a p p a r e n t h i g h e r p o r o s i t y dependence of compressive s t r e n g t h i s n o t known.

It f s a p p a r e n t t h a t c o n p a r i s o n of s t r e n g t h d a t a on t h e b a s i s o f a n e x p o n e n t i a l e q u a t i o n s i m i l a r t o eq. (3)

i s

i n a d e q u a t e t o e x p l a i n a l l t h e r e a s o n s f o r a t r e n g t h d i f f e r e n c e s among v a r i o u s cement systems

.

The

a p p l i c a t i o n of eq. (3) t o s t r e n g t h d a t a i s , however, a u s e f u l s t a r t i n g p o i n t f o r comparing d i f f e r e n t systems.

These comments a d d r e s s what t h e a u t h o r s c o n s i d e r t o b e some o f t h e i m p o r t a n t f a c t o r s i n t h e development of v e r y h i g h s t r e n g t h cement p a s t e s ( i n t h i s review compressive s t r e n g t h i n e x c e s s of 6 0 MPa). C o n s i d e r a t i o n w i l l be g i v e n t o t h e s e f a c t o r s i n m e c h a n i s t i c arguments p e r t a i n i n g t o h i g h s t r e n g t h . 2.0 O p t i m i z a t i o n of F a c t o r s C o n t r o l l i n g P r o d u c t i o n of H i g h t h Cement P a s t e 2.1 Low Water-Cement R a t i o A s d i s c u s s e d , h i g h e r s t r e n g t h s c a n b e a c h i e v e d w i t h normally h y d r a t e d low-porosity cement paste.* T h i s i s u s u a l l y accomplished by k e e p i n g t h e w/c r a t i o a s low a s p o s s i b l e , but i t should be recognized t h a t low w/c r a t i o cement p a s t e ( u s u a l l y w/c t0.25) h a s a c h a r a c t e r i s t i c m i c r o s t r u c t u r e d i f f e r e n t

.

from t h a t of p a s t e w i t h h i g h e r w/c r a t i o . The low w/c r a t i o p a s t e i s a more heterogeneous composite m a t e r i a l t h a n normal cement p a s t e because i t c o n t a i n s s i g n i f i c a n t amounts of anhydrous cement p a r t i c l e s embedded i n a C-S-H m a t r i x . T h i s , i n t u r n , h a s c h a r a c t e r i s t i c s d i f f e r e n t from t h o s e produced a t h i g h e r w/c r a t i o (23). The C-S-H m a t r i x and t h e unhydrated cement p a r t i c l e s b o t h

*

An i m p o r t a n t assumption i s t h a t t h e low p o r o s i t y p a s t e s a r e w e l l compacted and do n o t c o n t a i n l a r g e v o i d s t h a t may o c c u r due t o poor c o n s o l i d a t i o n .

Vol. 1 5 , No. 1

J . J . Beaudoin and R . F . Feldman

c o n t r i b u t e t o s t r e n g t h . T h i s C-S-H d i f f e r s from t h a t p r e p a r e d a t h i g h e r w/c r a t i o i n t h a t i t i s l e s s dense, c o n t a i n s t r a p p e d microspace between s h e e t s , h a s a d i f f e r e n t C/S r a t i o , a s i g n i f i c a n t l y lower N 2 s u r f a c e a r e a , a p o r e s i z e d i s t r i b u t i o n skewed towards s m a l l e r p o r e s i z e s , and c o n t a i n s p o r e s p a c e t h a t

cannot be p e n e t r a t e d by mercury. I n t h e s e p a s t e s , e f f e c t s d e s c r i b e d a s a g i n g

\

a r e more prominent, i . e . , rearrangement and c o n s o l i d a t i o n of C-S-H l a y e r s ,

g e n e r a l l y r e s u l t i n g i n s t r e n g t h i n c r e a s e s . The r e l a t i v e c o n t r i b u t i o n s of t h e J

C-S-H m a t r i x and cement i n c l u s i o n s i s important i n d e v e l o p i n g h i g h s t r e n g t h , and t h e s e s p e c i a l c h a r a c t e r i s t i c s of low w/c r a t i o p a s t e s p l a y a r o l e i n d e t e r m i n i n g t h e e x t e n t of t h e i r c o n t r i b u t i o n .

S e v e r a l p a p e r s d e s c r i b i n g t h e p r e p a r a t i o n and p r o p e r t i e s of a well- c o n s o l i d a t e d , low p o r o s i t y m a t r i x w i t h a r e l a t i v e l y h i g h d e g r e e of h y d r a t i o n have been p u b l i s h e d by Yudenfreund e t a l . (24-30). A t y p i c a l composition o f a cement p a s t e p r e p a r e d by them i s a s f o l l o w s : w/c = 0.20, t y p e I cement w i t h f i n e n e s s 600

-

900 m2/kg (much f i n e r t h a n normal), a g r i n d i n g a i d ,

e.g., d i e t h y l c a r b o n a t e a t 0.5% by weight of cement, 1% by weight of d r y d x e d calcium l i g n o s u l p h o n a t e and 0.5% potassium c a r b o n a t e ( a s e t - c o n t r o l l i n g

a d d i t i v e ) . This t y p i c a l cement p a s t e ( d e g r e e of h y d r a t i o n (10%) h a s a compressive s t r e n g t h of a b o u t 204 MPa a t 28 days. P o r o s i t y determined by water pycnometry was r e c a l c u l a t e d by t h e p r e s e n t a u t h o r s t o g i v e a n e q u i v a l e n t v a l u e of approximately 5% c o r r e s p o n d i n g t o t h a t which would have been o b t a i n e d by helium pycnometry. The d a t a p o i n t f o r t h i s p r e p a r a t i o n l i e s on t h e

s t r e n g t h p o r o s i t y c u r v e f o r normally h y d r a t e d cement p a s t e ( l i n e AB, Fig. 2a). I n t h i s c a s e , t h e a b i l i t y t o a c h i e v e low p o r o s i t y by means of g r i n d i n g and r h e o l o g i c a l a i d s ( w i t h o u t which e f f e c t i v e c o n s o l i d a t i o n would be d i f f i c u l t t o a c h i e v e ) a p p e a r s t o be r e s p o n s i b l e f o r h i g h s t r e n g t h .

2.2 E f f e c t of Dense I n c l u s i o n s

S t r e n g t h s s i g n i f i c a n t l y h i g h e r t h a n 204 MPa c a n b e o b t a i n e d a t low p o r o s i t i e s by i n c r e a s i n g t h e c o n t r i b u t i o n of d e n s e phases s u c h a s unhydrated cement. Roy e t a l . (31) demonstrated t h a t w i t h " h o t - p r e s s i n g " t e c h n i q u e s s t r e n g t h s a s h i g h a s 644 and 6 3 MPa (compressive and t e n s i l e s t r e n g t h s ,

r e s p e c t i v e l y ) can be achieved. These low p o r o s i t y m a t e r i a l s , p r e s s e d a t up t o 1020 MPa, and t e m p e r a t u r e s g r e a t e r t h a n 150°C c o n t a i n e d l a r g e amounts of unhydrated cement ( d e g r e e of h y d r a t i o n ranged from 29

-

37%). There a r e s e v e r a l o t h e r examples of v e r y s t r o n g cement p a s t e systems a t low p o r o s i t i e s . These i n c l u d e s e v e r a l of t h e h y d r a t e d a l u m i n a t e cement m i n e r a l s , e.g., C4AF, C3A, CA (32-34).

Low s t r e n g t h of a l u m i n a t e cements h y d r a t e d a t e l e v a t e d t e m p e r a t u r e s i s

o f t e n a t t r i b u t e d t o t h e f o r m a t i o n of t h e C3AH6 phase. Contrary t o g e n e r a l o p i n i o n , however, t h e c u r i n g of a l u m i n a t e systems a t h i g h e r t e m p e r a t u r e

a c t u a l l y r e s u l t s i n improvement i n s t r e n g t h , provided t h e w a t e r s o l i d r a t i o i s low. Higher s t r e n g t h s a t v e r y low w / s r a t i o s may b e a t t r i b u t e d t o d i r e c t f o r m a t i o n of t h e c u b i c C3AH6 phase on t h e o r i g i n a l s i t e s of t h e cement

m i n e r a l s . T h i s r e s u l t s i n i n t i m a t e c o n t a c t between d e n s e p a r t i c l e s t h a t form * a c l o s e l y welded, c o n t i n u o u s network w i t h enhanced mechanical s t r e n g t h . An

a n a l a g o u s e x p l a n a t i o n i s o f f e r e d f o r enhanced s t r e n g t h of low p o r o s i t y , a u t o c l a v e d , c e m e n t - s i l i c a m i x t u r e s a t low s i l i c a c o n t e n t . These p a s t e s c o n t a i n s i g n i f i c a n t amounts of d e n s e aC2SH and a r e weak a t h i g h p o r o s i t i e s where t h e i n t e r p a r t i c l e c o n t a c t s a r e fewer, weaker, and p o s s i b l y have s h a r p boundaries t h a t c a n c o n t r i b u t e t o h i g h e r l o c a l s t r e s s e s . A t low p o r o s i t y , t h e c o n t r i b u t i o n t o s t r e n g t h of t h e aC SH p a r t i c l e s

-

formed i n c l o s e p r o x i m i t y t o one a n o t h e r

-

i s r e f l e c t e d i n t h e $ a r g e i n c r e a s e i n s t r e n g t h of t h e

m a t e r i a l .

amounts of dense c r y s t a l l i n e m a t e r i a l demonstrate t h e p r i n c i p l e e x p r e s s e d by t h e m i x t u r e r u l e , i . e . , eq. ( 1 ) . The c o n d i t i o n s of f o r m a t i o n of t h e d e n s e i n c l u s i o n s , however, determine t h e r e l e v a n c e of what i s p r e d i c t e d by

1

eq. ( 1 ) . 2.3 S t r e s s M o d i f i c a t i o n i n C r i t i c a l Areas a _{F a i l u r e i n b r i t t l e m a t e r i a l s , i n c l u d i n g cement p a s t e s , i s of t e n t h e} r e s u l t of i n i t i a t i o n and p r o p a g a t i o n of c r a c k s o r i g i n a t i n g i n h i g h s t r e s s r e g i o n s . M o d i f i c a t i o n of t h e s t r e s s f i e l d i n t h e s e r e g i o n s c a n r e s u l t i n h i g h s t r e n g t h . F i b r e r e i n f o r c e m e n t of b r i t t l e m a t r i c e s c a n r e s u l t i n s i g n i f i c a n t i n c r e a s e s i n f l e x u r a l s t r e n g t h and i s a p r a c t i c a l example of c r a c k c o n t r o l through s t r e s s m o d i f i c a t i o n p r o c e s s e s . T h i s m o d i f i c a t i o n c a n a l s o be a c h i e v e d i n u n r e i n f o r c e d m a t r i c e s , f o r example, by changing t h e geometry o f a c r a c k t i p , p o r e , o r v o i d s p a c e between p a r t i c l e s . This c a n be done i n s e v e r a l ways: (1) d e p o s i t i o n of a s m a l l amount of a d d i t i o n a l C-S-H b i n d e r i n h i g h - s t r e s s r e g i o n s , ( 2 ) impregnation of a porous system w i t h a second phase,

( 3 ) molecular b r i d g i n g of microspace by a n impregnant, ( 4 ) i n t e g r a l mixing w i t h a low-modulus polymer emulsion, and ( 5 ) f i l l i n g macrodefects and modifying t h e i r geometry.

F i g u r e 4 i s a s c h e m a t i c i l l u s t r a t i o n of some of t h e s e concepts. The "hot-pressed" samples

(7),

which had a d d i t i o n a l m o i s t - c u r i n g f o r 28 d a y s , had s i g n i f i c a n t i n c r e a s e s i n s t r e n g t h a l t h o u g h t h e i n c r e a s e i n d e g r e e of h y d r a t i o n was m a r g i n a l , I t i s suggested t h a t t h e s m a l l amount of a d d i t i o n a l h y d r a t i o n

( a l t h o u g h t o t a l p o r o s i t y i s e f f e c t i v e l y unchanged) i s s u f f i c i e n t t o "glue" t h e dense c l i n k e r p a r t i c l e s t o g e t h e r and modify t h e s t r e s s f i e l d a t r e g i o n s of i n t e r p a r t i c l e c o n t a c t (Fig. 4a). The mechanism of s t r e s s m o d i f i c a t i o n

probably i n v o l v e s change of pore shape due t o t h e f i r s t concept and r e d u c t i o n of t h e "b" f a c t o r i n eq. ( 3 ) .

I n c r e a s e s i n s t r e n g t h due t o impregnation of p o r e s w i t h a s o l i d , which i t s e l f h a s good e n g i n e e r i n g p r o p e r t i e s , a p p e a r s a l s o t o be ( i n a d d i t i o n t o p o r o s i t y r e d u c t i o n ) t h e r e s u l t of a s t r e s s m o d i f i c a t i o n mechanism c o n t r o l l i n g behaviour i n r e g i o n s of h i g h s t r e s s . F i g u r e 4b i l l u s t r a t e s how p o r e shape i n r e g i o n s of h i g h s t r e s s may b e modified. Large i n c r e a s e s i n s t r e n g t h a s w e l l a s h i g h t o t a l s t r e n g t h a r e o b t a i n e d f o r compositions c o n t a i n i n g dense

p a r t i c l e s , e.g.

,

h i g h p o r o s i t y systems c o n t a i n i n g aC2SH p a r t i c l e s , when t h e s e compositions a r e impregnated w i t h a second phase. The mechanism r e s p o n s i b l e f o r s t r e n g t h i n c r e a s e ( a t l e a s t i n p a r t ) i s s i m i l a r t o t h a t s u g g e s t e d f o r t h e "hot-pressed" systems. Another s t r e n g t h e n i n g mechanism i s p o s s i b l e when t h e impregnant i s f i l l i n g microspace between s u r f a c e s s e p a r a t e d by d i s t a n c e s of molecular dimension; f o r example, i t i s a r g u a b l e t h a t a polymethylmethacrylate molecule c o u l d b r i d g e two s u r f a c e s of a micropore, t h u s u t i l i z i n g t h e s t r e n g t h of c o v a l e n t bonds r a t h e r t h a n t h e s t r e n g t h of t h e i n t e r c o n n e c t i o n s of s e v e r a l of t h e polymethylme t h a c r y l a t e molecules themselves (Fig. 4c).

Microscopic e v i d e n c e s u p p o r t s arguments t h a t polymer emulsions,

e.g., l a t e x systems, b r i d g e m i c r o c r a c k s w i t h a polymer f i l m (35), r e s i s t i n g c r a c k growth and i n c r e a s i n g t e n s i l e s t r e n g t h and f a i l u r e s t r a i n ( F i g . 4d). I n c r e a s e d d u c t i l i t y i s dependent on t h e t y p e and amount of polymer. When t h e polymer forms a f i l m i n a r e g i o n of h i g h s t r e s s , t h e arguments p r e s e n t e d f o r

I

s t r e s s m o d i f i c a t i o n a l s o seem t o a p p l y . With r e s p e c t t o w a t e r s o l u b l e

polymers, i f t h e c o n c e n t r a t i o n of polymer i n t h e p o r e s p a c e a v a i l a b l e i s h i g h , t h e n t h e polymer w i l l have a d i r e c t r e i n f o r c i n g e f f e c t on t h e cement system and behave t h e same a s l a t e x e s .

F o r a g i v e n p o l y m e r c e m e n t r a t i o t h e c o n c e n t r a t i o n of polymer i n t h e pore s o l u t i o n 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 w/c r a t i o . T h i s can be a s i g n i f i c a n t f a c t o r i n v e r y low p o r o s i t y systems. F o r l a t e x - m o d i f i e d p o r t l a n d

J.J. Beaudoin and R.F. Feldman Vol. 15, No. 1 UNHYORATED CEMENT PARTICLE l a l IMPREONANT

2

I C S H ( b ) ( c J FILM ( d l 0 0 10 20 30 40 5 0 60 T O T A L P O R O S I T Y , % FIG. 4 FIG. 5 Schematic i l l u s t r a t i o n s of F l e x u r a l s t r e n g t h v s p o r o s i t y f o r o r d i n a r y v a r i o u s s t r e s s m o d i f y i n g p o r t l a n d cement p a s t e . MDF = cement p a s t e mechanisms cement p a s t e p u b l i s h e d d a t a i n d i c a t e t h a t ( f o r c o n s t a n t polymer c o n t e n t ) t e n s i l e s t r e n g t h i s p o r o s i t y dependent o v e r a wide r a n g e o f p o r o s i t y a n d s t r e n g t h i n c r e a s e s a s p o r o s i t y d e c r e a s e s (36). It a p p e a r s t h e n t h a t t o t a l p o r o s i t y i s one o f t h e c o n t r o l l i n g f a c t o r s t h a t d e t e r m i n e t h e t e n s i l e s t r e n g t h of t h e s e m a t e r i a l s .

An example o f a polymer-modified cement s y s t e m ( h a v i n g h i g h f l e x u r a l s t r e n g t h ) i n which mechanisms i l l u s t r a t e d i n F i g . 4(a, b and d ) a r e o p e r a t i v e i s a s y s t e m r e f e r r e d t o a s Macro-Defect-Free (MDF) cement p a s t e (37-42). T h i s p a s t e system i s p r e p a r e d a t v e r y low w/c r a t i o s (0.08

-

0.20). I n g e n e r a l , composition c o m p r i s e s t h e f o l l o w i n g : h y d r a u l i c cement, w a t e r , o n e p o l y m e r i c o r w a t e r - d j . s p e r s i b l e a d d i t i v e , p a r t i c u l a t e m a t e r i a l i n s o l u b l e i n t h e

c o m p o s i t i o n h a v i n g p a r t i c l e s i z e (0.1 um. A t y p i c a l composition i s a s a

f o l l o w s : 100 p a r t s p o r t l a n d cement, 0.7 p a r t s i l i c a powder (0.04 pm) and 16.5 p a r t s aqueous s o l u t i o n c o n t a i n i n g 3.5 p a r t s polyacrylamide.

Hydroxypropyl methyl c e l l u l o s e and hydrolyzed p o l y ( v i n y 1 a c e t a t e ) have been u s e d i n l i e u o f polyacrylamide.

F i g u r e 5 i s a f l e x u r a l s t r e n g t h v s p o r o s i t y p l o t f o r cement p a s t e , i n c l u d i n g a p l o t p o i n t f o r MDF cement p a s t e (41). The p o r o s i t y v a l u e s d e t e r m i n e d by w a t e r pycnometry a r e probably h i g h e r t h a n a c t u a l , b u t t h e g e n e r a l t r e n d of t h e d a t a would be s i m i l a r i f c o r r e c t p o r o s i t y v a l u e s were used. The ? l o t p o i n t f o r MDF cement p a s t e i s s i g n i f i c a n t l y h i g h e r t h a n t h e v a l u e on t h e c u r v e a t t h e same p o r o s i t y . The s t r e s s m o d i f y i n g c h a r a c t e r i s t i c s o f t h e polymer e n a b l e t h e d e n s e unhydrated cement p a r t i c l e s t o c o n t r i b u t e more

Vol. 1 5 , No. 1 115 HIGH-STRENGTH CEMENT PASTES, APPRAISAL, MDF PASTES, FRACTURE

f u l l y t o t h e s t r e n g t h of t h e composite i n accordance w i t h eq. (1). T h i s i s

t h e p r i n c i p a l r e a s o n why t h e MDF s y s t e m i s s t r o n g e r t h a n normal p a s t e a n d d o e s

n o t f i t t h e s t r e n g t h - p o r o s i t y c u r v e f o r t h e l a t t e r .

I

_{The f l e x u r a l s t r e n g t h v a l u e o f t h e MDF p a s t e (Fig. 5) i s a b o u t 6 0 MPa,}

a v a l u e a p p r o x i m a t e l y t h e same a s t h a t o b t a i n e d by Roy and Gouda7 f o r t h e

maximum t e n s i l e s t r e n g t h o f h o t - p r e s s e d cement p a s t e . I n b o t h c a s e s t h e

unhydrated cement a p p e a r s t o c o n t r i b u t e a n optimum amount t o composite

s t r e n g t h i n a c c o r d a n c e w i t h m i x t u r e r u l e t h e o r y and eq. ( 1 ) . It i s s u g g e s t e d

t h a t a s t r e s s m o d i f i c a t i o n mechanism i s p r i m a r i l y r e s p o n s i b l e . A

s i g n i f i c a n t l y lower compressive s t r e n g t h l f l e x u r a l s t r e n g t h r a t i o (approx. 3) f o r polymer-modified cement systems a p p e a r s t o s u p p o r t t h i s s u g g e s t i o n (36). The e f f e c t of macrovoids i n t h e MDF polymer cement s y s t e m h a s n o t b e e n determined. 3.0 Concluding Remarks The mechanisms d i s c u s s e d a r e t h o s e t h e a u t h o r s b e l i e v e t o b e r e l e v a n t and a p p a r e n t l y r e s p o n s i b l e f o r t h e h i g h s t r e n g t h of many p r o d u c t s d e s c r i b e d i n r e c e n t p a t e n t s . One of t h e p r i n c i p a l mechanisms a p p e a r s t o b e t h e m o d i f i c a t i o n of s t r e s s i n a r e a s of h i g h s t r e s s ( f o r example, c r a c k t i p s ) by c h e m i c a l a n d p h y s i c a l means. It i s hoped t h a t t h e o p i n i o n s e x p r e s s e d w i l l p r o v i d e i n s i g h t f o r f u r t h e r development. R e f e r e n c e s S.P. Shah. Concr. I n t . ,

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T h i s p a p e r i s a c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l Research Council of Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r o f t h e D i v i s i o n .

T h i s p a p e r , w h i l e b e i n g d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g R e s e a r c h , remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , O t t a w a , O n t a r i o , K1A 0R6.