Properties of portland cement paste reinforced with mica flakes

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Cement and Concrete Research, 13, March 2, pp. 153-160, 1983-03-01

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Properties of portland cement paste reinforced with mica flakes

Beaudoin, J. J.

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no.

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'

C *

_{lational Research Council of Canada}

'onseil national de recherches du Canada

l - - - - i

PROPERTIES OF PORTLAND

CEMENT PASTE REINFORCED WITH

MICA FLAKES

By J.J.

Beaudoin

Reprinted from

Cement and Concrete Research

Vol. 13, No. 2, 1983

p.

153-160

DBR Paper No. 1082

Division of Building Research

CEMENT and CONCRETE RESEARCH. Vol. 1 3 , pp. 153-160, 1983. P r i n t e d i n t h e USA. 0 0 0 8 - 8 8 4 6 / 8 3 / 0 2 0 1 5 3 - 0 8 $ 0 3 . 0 0 / 0 Copyright (c) 1983 Pergamon P r e s s , L t d .

PROPERTIES OF PORTLAND CEMENT PASTE REINFORCED WITH MICA FLAKES

J. J. Beaudoin

D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research C o u n c i l Canada Ottawa, O n t a r i o K1A 0R6, Canada

(Communicated by G.G. L i t v a n ) (Received J u n e 1 5 , 1982)

ABSTRACT

R e s u l t s of a n i n v e s t i g a t i o n of t h e u s e of h i g h - a s p e c t - r a t i o mica f l a k e s i n hardened p o r t l a n d cement m a t r i c e s a r e r e p o r t e d . 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 by a f a c t o r of 2 and i n f r a c t u r e toughness ,by a f a c t o r of 4 , depending on m a t r i x p o r o s i t y , a r e o b t a i n e d by t h e a d d i t i o n of s m a l l amounts of mica. The dependence of t h e s e p r o p e r t i e s on m a t r i x p o r o s i t y i s determined and t h e e f f e c t of mica f l a k e a d d i t i o n on m a t r i x c h a r a c t e r i s t i c s d i s c u s s e d , e.g., p o r o s i t y , s u r f a c e a r e a , non-evaporable w a t e r c o n t e n t .

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

I n r e c e n t y e a r s t h e technology of f i b r e - r e i n f o r c e d cement h a s become a n i m p o r t a n t segment of r e s e a r c h and development a c t i v i t y i n t h e c o n c r e t e i n d u s t r y ( 1 ) . I n c o r p o r a t i o n of i n o r g a n i c and o r g a n i c f i b r e s i n cement m a t r i c e s h a s r e s u l t e d i n i n c r e a s e d f l e x u r a l s t r e n g t h and t o u g h n e s s ( 2 ) . I n t e n s i v e r e s e a r c h e f f o r t h a s been d i r e c t e d towards t h e development and u s e of g l a s s - f i b r e - r e i n f o r c e d cement p r o d u c t s (GRC) ( 3 ) . A l k a l i - r e s i s t a n t (AR) g l a s s f i b r e s have a t t r a c t i v e p r o p e r t i e s t h a t f a v o u r t h e i r s e l e c t i o n a s r e i n f o r c e m e n t b e c a u s e of h i g h v a l u e s of modulus of e l a s t i c i t y and t e n s i l e s t r e n g t h . There remains, however, concern about long-term d u r a b i l i t y . A f t e r a few y e a r s of n a t u r a l w e a t h e r i n g , o r i g i n a l v a l u e s of f l e x u r a l s t r e n g t h and toughness of GRC m a t e r i a l s a r e s i g n i f i c a n t l y reduced (41, and t h e r e i s

concern r e g a r d i n g i t s u s e i n s p i t e of a t t e m p t s t o modify and improve AR g l a s s f i b r e s ( 5 ) .

Mica f l a k e r e i n f o r c e m e n t h a s r e c e i v e d some a t t e n t i o n from t h e p r e c a s t c o n c r e t e i n d u s t r y because of i t s ready a v a i l a b i l i t y and s t a b i l i t y t o a l k a l i a t t a c k and f o r economic c o n s i d e r a t i o n s a s w e l l ( 6 ) . Recent s t u d i e s have a l s o demonstrated t h a t h i g h a l u n i n a cement m a t r i c e s r e i n f o r c e d w i t h high-aspect- r a t i o mica f l a k e s show 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 f r a c t u r e toughness ( 7 ) .

It i s t h e o b j e c t of t h i s s t u d y t o d e t e r m i n e t h e e f f e c t of mica f l a k e a d d i t i o n on s t r e n g t h , toughness and o t h e r p r o p e r t i e s of p o r t l a n d cement p a s t e i n o r d e r t o a s s e s s i t s p o t e n t i a l a s a replacement f o r g l a s s f i b r e s i n cement m a t r i c e s .

154 Vol. 1 3 , No. 2

J . J . Beaudoin

Experimental M a t e r i a l s

Cement: P o r t l a n d cement composition was C4AF = 6.7%; C3A = 12.7%;

C S = 51.4%; C2S = 20.3%; and CaS04 = 5.4% a s c a l c u l a t e d by t h e Bogue method. ~ ? a i n e f i n e n e s s was 300 m2/kg.

Mica: It i s p h l o g o p i t e t y p e and h i g h - a s p e c t - r a t i o r e i n f o r c i n g grade, marketed a s " S u z o r i t e Mica" ( M a r i e t t a Resources I n t e r n a t i o n a l Ltd.,

5083 S t . Denis S t r e e t , Montreal, Quebec). The a v e r a g e a s p e c t r a t i o of t h e f l a k e s i s a p p r o x i m a t e l y 80. They a r e g e n e r a l l y of i r r e g u l a r s h a p e , w i t h a mean w i d t h v a r y i n g between 250 and 1400 um. P h y s i c a l and c h e m i c a l p r o p e r t i e s of t h e f l a k e s have been p u b l i s h e d ( 7 ) .

Mixes: E i g h t s e r i e s of cement p a s t e mixes were made u s i n g a c o n v e n t i o n a l Hobart mixer, each s e r i e s w i t h a d i f f e r e n t volume c o n c e n t r a t i o n of mica f l a k e s v a r y i n g from 0 t o 7% i n i n c r e m e n t s of 1%. For a g i v e n volume c o n c e n t r a t i o n of mica f l a k e s , mixes w i t h t h e f o l l o w i n g waterlcement r a t i o s were prepared: 0.25, 0.30, 0.35, 0.40, 0.45 and 0.50. It was n o t p o s s i b l e t o produce low w/c mixes when volume f r a c t i o n of mica, V f , e q u a l l e d 7%. They were c a s t i n two sample g e o m e t r i e s : 5.1 cm cubes f o r c o m p r e s s i o n t e s t s and 2.5 x 2.5 x 25.4 cm l o n g prisms f o r f l e x u r a l t e s t s . Three cubes and t h r e e prisms were made w i t h each mix. Samples were demolded a f t e r one day and m o i s t c u r e d f o r approximately 32 d a y s p r i o r t o t e s t i n g .

Techniques: F l e x u r a l t e s t s (mid-span l o a d i n g ) were c a r r i e d o u t on a

T i n i u s Olsen t e s t i n g machine u s i n g a cross-head speed of 0.127 cm/min. Load d e f l e c t i o n t r a c e s were o b t a i n e d from machine r e c o r d s and used t o d e t e r m i n e f l e x u r a l s t r e n g t h s and r e l a t i v e f r a c t u r e toughness. Areas under l o a d d e f l e c t i o n c u r v e s were determined and used a s i n d i c e s of f r a c t u r e toughness.

Compression t e s t s were a l s o c a r r i e d o u t on a T i n i u s O l s e n t e s t i n g machine u s i n g a l o a d i n g r a t e of 0.2 MPa s-l

.

P o r o s i t y measurements were made on s m a l l chunks from e a c h specimen.

Samples were c o n d i t i o n e d t o 11% RH and t h e n pumped i n a vacuum d e s i c c a t o r f o r a b o u t 45 min p r i o r t o t e s t i n g . A methanol d i s p l a c e m e n t method employing Archimedes P r i n c i p l e w a s used f o r a l l p o r o s i t y d e t e r m i n a t i o n s ( 8 ) . P o r e volumes were e x p r e s s e d a s p e r c e n t a g e s of m a t r i x volume, i . e . , volume occupied by mica f l a k e s was excluded from t o t a l a p p a r e n t volume t o p r o v i d e a b a s i s f o r comparing composites w i t h d i f f e r e n t amounts of mica.

P o r e s i z e d i s t r i b u t i o n s were d e t e r m i n e d on s e l e c t e d samples by mercury i n t r u s i o n a t 408 MPa, u s i n g an American I n s t r u m e n t Co. p o r o s i m e t e r . S u r f a c e a r e a measurements were made on s e l e c t e d samples u s i n g a Numinco-Or s u r f a c e a r e a a n a l y s e r w i t h N2 a s a d s o r b a t e .

Non-evaporable w a t e r d e t e r m i n a t i o n s were made by oven d r y i n g samples a t 110°C f o r 3 h and d e t e r m i n i n g t h e l o s s on i g n i t i o n a t 1000°C. The w e i g h t of mica p r e s e n t i n t h e specimens was s u b t r a c t e d f o r d e t e r m i n i n g non-evaporable w a t e r c o n t e n t .

R e s u l t s and D i s c u s s i o n

Logarithm of f l e x u r a l s t r e n g t h v e r s u s p o r o s i t y c u r v e s f o r cement m a t r i c e s c o n t a i n i n g 0

-

6% mica f l a k e s a r e p r e s e n t e d i n Fig. 1 . Data f o r 7% mica c o n t e n t were i n c o m p l e t e owing t o d i f f i c u l t i e s i n sample f a b r i c a t i o n and a r e n o t p r e s e n t e d . A t m a t r i x p o r o s i t i e s g r e a t e r t h a n about 21% a l l c u r v e s ( w i t h t h e e x c e p t i o n of t h e c u r v e f o r mica volume f r a c t i o n , Vf = 6 % ) l i e above t h e

Vol. 1 3 , No. 2 155

CEMENT PASTE, M I C A REINFORCEMENT

c u r v e f o r Vf = 0%. A t p o r o s i t i e s <21% a l l c u r v e s l i e above t h e Vf = 0% c u r v e e x c e p t t h o s e f o r Vf = 2 and 4%. The c u r v e f o r Vf = 4% c r o s s e s t h e Vf = 0% c u r v e a t about 18% p o r o s i t y . Logarithm of compressive s t r e n g t h v e r s u s p o r o s i t y c u r v e s a r e p l o t t e d i n F i g . 2. A l l t h e c u r v e s l i e below t h e Vf = 0% curve. I n g e n e r a l , compressive s t r e n g t h d e c r e a s e s w i t h mica f l a k e c o n t e n t over most of t h e p o r o s i t y range. S i m i l a r r e s u l t s have been observed f o r g l a s s - f i b r e - r e i n f o r c e d cements (9). R e g r e s s i o n a n a l y s i s d a t a f o r c u r v e s i n F i g s . 1 and 2 a r e g i v e n i n T a b l e s 1 and 2. The f o l l o w i n g e q u a t i o n , based on composite t h e o r y , h a s 2 0 25 3 0 3 5 M A T R I X P O R O S I T Y . % FIG. 1 o f t e n been used t o e s t i m a t e F l e x u r a l s t r e n g t h v e r s u s m a t r i x p o r o s i t y

composite compressive o r f o r mica-flake-reinforced p o r t l a n d cement

f l e x u r a l s t r e n g t h v a l u e s a t p a s t e

d i f f e r e n t p o r o s i t i e s f o r f i b r e - r e i n f o r c e d cement composites ( 1 0 ) :

where acf = composite s t r e n g t h

a m = m a t r i x s t r e n g t h

o f = maximum f i b r e o r f l a k e s t r e s s when composite peak s t r e s s i s o b t a i n e d

Vf = volume f r a c t i o n of f i b r e o r f l a k e r e i n f o r c e m e n t

4

= e f f i c i e n c y f a c t o r used t o account f o r f i b r e l e n g t h and o r i e n t a t i o n .

TABLE 1

R e g r e s s i o n A n a l y s i s of F l e x u r a l S t r e n g t h - P o r o s i t y Curves f o r P o r t l a n d Cement-Mica F l a k e Composites

Volume F l e x u r a l S t r e n g t h ( a c f ) C o r r e l a t i o n Mica F l a k e s , % MPa C o e f f i c i e n t , % 0 a c f = 21.04 exp (-0.064)p* 74.6 1 a c f = 15.90 exp ( - 0 . 0 4 6 ) ~ 86.6 2 acf = 14.39 exp ( - 0 . 0 4 7 ) ~ 90.1 3 a c f = 14.26 exp ( - 0 . 0 3 7 ) ~ 96.7 4 a c f = 9.60 exp ( - 0 . 0 2 2 ) ~ 76.1 5 a c f = 20.55 exp ( - 0 . 0 5 4 ) ~ 91.8 6 a c f = 123.02 exp ( - 0 . 1 4 8 ) ~ 94.1

I

*

p = m a t r i x p o r o s i t y

Vol. 1 3 , No. 2 J . J . Beaudoin

Matrix s t r e n g t h i s expressed i n terms of p o r o s i t y by t h e f o l l o w i n g e x p r e s s i o n :

where p' = p o r o s i t y of u n r e i n f o r c e d cement p a s t e ( p ' i s dependent on p, t h e m a t r i x p o r o s i t y of t h e r e i n f o r c e d cement p a s t e and t o a f i r s t approximation p = p ' )

b = c o n s t a n t

a, = m a t r i x s t r e n g t h a t z e r o p o r o s i t y .

The v a l u e of a, i s o b t a i n e d by e x t r a p o l a t i n g t o z e r o p o r o s i t y t h e Vf = 0% curve i n Fig.

2.

For p o r t l a n d cement p a s t e t h e r e i s some e v i d e n c e

TABLE

2

R e g r e s s i o n A n a l y s i s of Compressive S t r e n g t h - P o r o s i t y Curves f o r P o r t l a n d Cement-Mica Flake Composites

Volume Compressive S t r e n g t h ( a c c ) C o r r e l a t i o n Mica F l a k e s , % MF'a C o e f f i c i e n t , %

a,,

= 400.10 exp (-0.090)p* 89.4 a,, = 120.23 exp ( - 0 . 0 5 9 ) ~ 89.2 a,, = 131.83 exp ( - 0 . 0 6 8 ) ~ 96.7 a,, = 79.43 exp ( - 0 . 0 4 8 ) ~ 80.3

a,,

-

87.50 exp ( - 0 . 0 5 8 ) ~ 83.4 ucc = 86.49 exp ( - 0 . 0 5 6 ) ~ 91.5 a,, = 86.70 exp (-0.063)~ 99.1

*

= m a t r i x p o r o s i t y of t h e v a l i d i t y of t h e e x t r a p o l a t i o n t o z e r o p o r o s i t y ( 1 1 ) . E s t i m a t e s of microhardness ( t h e r e

i s

a good c o r r e l a t i o n between microhardness and s t r e n g t h of cement p a s t e ) of t h e non-porous cement p a s t e samples made by employing composite t h e o r y on cement p a s t e samples completely impregnated w i t h a n o t h e r phase were s i m i l a r t o v a l u e s determined by e x t r a p o l a t i o n .

The s t r e n g t h r a t i o u C f / a m (which e x p r e s s e s t h e change i n s t r e n g t h of t h e composite r e l a t i v e t o t h e u n r e i n f o r c e d m a t r i x ) i s o b t a i n e d by d i v i d i n g e q u a t i o n (1) by e q u a t i o n ( 2 ) i n which p' = p

2E.r The s t r e s s i n t h e f i b r e o f =

-

,

t

where T = i n t e r f a c i a l s h e a r s t r e s s between r e i n f o r c e m e n t and m a t r i x ,

Illt = a s p e c t r a t i o , o b t a i n e d by d i v i d i n g a v e r a g e l e n g t h of f l a k e by i t s t h i c k n e s s ( R l t = 80 f o r t h e mica f l a k e s i n t h i s s t u d y ;

Orno = 21.04 MPa and

b = 0.064, both of which have been o b t a i n e d from curve f o r Vf = 0 % ) .

Vol. 1 3 , No. 2

CEMENT PASTE, M I C A REINFORCEMENT

P O R T L A N D C E M E N T M A T R I X A G E 32 d 70 6 0 _\ M A T R I X P O R O S I T Y . % FIG. 2 Compressive s t r e n g t h v e r s u s m a t r i x p o r o s i t y f o r m i c a - f l a k e - r e i n f o r c e d p o r t l a n d cement p a s t e . 2 % M I C A F L A K E V O L U M E

n

PORTLAND CEMENT M A T R I X AGE 32 d 14 16 18 20 22 24 24 28 30 M A T R I X P O R O S I T Y , % FIG. 3 C a l c u l a t e d v a l u e s of i n t e r f a c i a l s h e a r s t r e n g t h v e r s u s m a t r i x p o r o s i t y f o r m i c a - f l a k e - r e i n f o r c e d p o r t l a n d cement p a s t e .

The f o l l o w i n g e x p r e s s i o n f o r +T was used t o f i t e q u a t i o n ( 3 ) t o

[s

,

p] d a t a o b t a i n e d e x p e r i m e n t a l l y (p = p o r o s i t y of t h e m a t r i x i n r e i n f o r c e d sample).

+T = K exp {-a(p-a)} [l-expi-a(p-a)}] ~ x P ( - ~ P ) _{( 4 )}

where K, a and a a r e c o n s t a n t s and o t h e r terms a r e a s p r e v i o u s l y defined.

(5

cf

The

-

v e r s u s p o r o s i t y c u r v e s ( n o t p r e s e n t e d ) i n c r e a s e t o a maximum and

'm

t h e n d e c r e a s e a s p o r o s i t y i n c r e a s e s . It was noted t h a t mica f l a k e s modify c h a r a c t e r i s t i c s of t h e m a t r i x , i n c l u d i n g p o r e s t r u c t u r e ( t o be d i s c u s s e d l a t e r ) ; t h e pore systems i n r e i n f o r c e d and u n r e i n f o r c e d cement p a s t e s a r e t h e r e f o r e d i f f e r e n t . The d i f f e r e n c e i n t o t a l p o r o s i t y f o r t h e r e i n f o r c e d and u n r e i n f o r c e d systems i s n o t l a r g e , and a s a f i r s t approximation t h e y have been c o n s i d e r e d e q u a l . The term exp(-bp) c a n c e l s o u t when e q u a t i o n ( 4 ) i s s u b s t i t u t e d i n e q u a t i o n ( 3 ) and

-

Ocf _{becomes a f u n c t i o n of t h e m a t r i x}

O m

p o r o s i t y , p, a l l o t h e r terms b e i n g c o n s t a n t .

Using a v a l u e of = 116 f o r a random d i s t r i b u t i o n of f l a k e s i n t h r e e dimension (121, v a l u e s of T were c a l c u l a t e d and a r e p l o t t e d a g a i n s t m a t r i x p o r o s i t y i n Fig. 3. There i s a s e p a r a t e c u r v e f o r e a c h volume f r a c t i o n of mica f l a k e s . For each curve t h e s h e a r s t r e s s i n c r e a s e s t o a maximum and t h e n

Vol. 1 3 , No. 2 J . J . Beaudoin

d e c r e a s e s a s p o r o s i t y i n c r e a s e s . The l a r g e s t v a l u e of maximum s h e a r s t r e s s was o b t a i n e d f o r 2% volume f r a c t i o n of mica f l a k e s .

F r a c t u r e t o u g h n e s s / p o r o s i t y c u r v e s a r e p l o t t e d i n Fig. 4. Toughness v a l u e s f o r each curve d e c r e a s e t o a minimum v a l u e a s p o r o s i t y i n c r e a s e s , t h e n i n c r e a s e w i t h f u r t h e r i n c r e a s e s i n p o r o s i t y . Toughness v a l u e s a l s o i n c r e a s e a s mica f l a k e a d d i t i o n i s i n c r e a s e d ; t h e c u r v e s a r e c l u s t e r e d t o g e t h e r

between 2 and 62, s u g g e s t i n g t h a t i n some c a s e s 2% a d d i t i o n may be s u f f i c i e n t t o g i v e t h e d e s i r e d r e s u l t s . The s h a p e of t h e t o u g h n e s s / p o r o s i t y curve i s s i m i l a r f o r t h e c o n t r o l c o n t a i n i n g 0% mica f l a k e s . It i s suggested t h a t i n c l u s i o n s s u c h a s unhydrated cement p a r t i c l e s , Ca(OH)2 p a r t i c l e s , and p o r e s may play a r o l e i n t h e f r a c t u r e behaviour of hardened cement p a s t e and a f f e c t t h e s h a p e of t h e t o u g h n e s s / p o r o s i t y curve.

The curves f o r i n t e r f a c i a l s h e a r s t r e s s

v e r s u s p o r o s i t y (Fig. 3 ) and f o r f r a c t u r e 500

toughness v e r s u s p o r o s i t y (Fig. 4 ) have i n v e r s e shapes. It i s known t h a 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 due t o

i n c r e a s e s i n i n t e r f a c i a l s h e a r s t r e s s a r e ,A

o f t e n achieved a t t h e expense of f r a c t u r e

-

-

Z 400

3

toughness. A b a l a n c e between t h e two

p r o p e r t i e s can a p p a r e n t l y be achieved by > LZ

c o n t r o l l i n g m a t r i x p o r o s i t y , and a LZ C - i n t e n t i o n a l m o d i f i c a t i o n of t h e m 300

matrix-mica f l a k e i n t e r f a c e would have t o LZ

t a k e t h i s i n t o c o n s i d e r a t i o n . V) M a t r i x p r o p e r t i e s may be a f f e c t e d by V) u Z mica f l a k e s . N i t r o g e n s u r f a c e a r e a I 0 measurements were made on samples I 200

0 c o n t a i n i n g 0

-

7 % volume f r a c t i o n of mica + f l a k e s . C o r r e c t i o n s were made t o t h e C Z W i g n i t e d w e i g h t s t o account f o r t h e mica LZ < p r e s e n t . S u r f a c e a r e a v e r s u s volume a f r a c t i o n of mica f l a k e s i s p l o t t e d i n 100 Fig. 5 f o r p r e p a r a t i o n s with w/c = 0.35 and 0.50; i t shows a d e c r e a s e i n s u r f a c e a r e a a s mica f l a k e a d d i t i o n i n c r e a s e s . The d e c r e a s e i n s u r f a c e a r e a o i s accompanied by a d e c r e a s e i n t h e 14 1 8 22 26 30 34 38 non-evaporable w a t e r c o n t e n t of t h e cement _{M A T R I X P O R O S I T Y , %} p a s t e , i n d i c a t i n g t h a t t h e degree of h y d r a t i o n i s lower because of f l a k e a d d i t i o n . S i m i l a r o b s e r v a t i o n s were FIG. 4 r e p o r t e d by M i k h a i l i n h i s work w i t h

g l a s s - f i b r e - r e i n f orced cement ( 9 ) . F r a c t u r e toughness v a l u e s T y p i c a l f i g u r e s f o r d e c r e a s e i n v e r s u s m a t r i x p o r o s i t y f o r non-evaporable w a t e r c o n t e n t a r e , f o r m i ca-f l a k e - r e i n f orced example, 0.181 g / g f o r a sample made w i t h p o r t l a n d cement p a s t e w/c = 0.50 and OX mica, d e c r e a s i n g t o

0.155 g/g f o r a s i m i l a r sample c o n t a i n i n g

6 % mica. The h y d r a t i o n p e r i o d was 32 d a y s

f o r both. The r e a s o n s f o r t h e s e r e s u l t s a r e n o t c l e a r , b u t Ca(OHl2 r i c h zones were observed i n t h e i n t e r f a c i a l r e g i o n s between f l a k e s and cement m a t r i x and i t i s s u s p e c t e d t h a t t h i s i n f l u e n c e s t h e h y d r a t i o n p r o c e s s .

Mica f l a k e a d d i t i o n a l s o h a s a n e f f e c t on t h e p o r e s y s t e m of hardened cement p a s t e . To i l l u s t r a t e t h i s , p o r e s i z e d i s t r i b u t i o n d a t a f o r t h e

Vol. 1 3 , No. 2 159

CEMENT PASTE, M I C A REINFORCEMENT

Ocf

,

f o r a s t r e n g t h r a t i o ,

-

O m w/c r a t i o of 0.35 o c c u r s a t about 3% volume f r a c t i o n of mica, and a t t h i s w/c r a t i o t h i s p a s t e h a s t h e l o w e s t p o r o s i t y . A t mica f l a k e volume f r a c t i o n s of 3% o r g r e a t e r t h e volume f r a c t i o n of f i n e pores (0.01

-

0.003 pm) i n c r e a s e s s i g n i f i c a n t l y

.

It a p p e a r s t h a t low p o r o s i t y and h i g h concen- t r a t i o n of f i n e p o r e s g i v e s maximum f l e x u r a l s t r e n g t h . A t 6% volume f r a c t i o n , however, t o t a l p o r o s i t y and f r a c t i o n of l a r g e pores (10

-

1 pm) . a r e b o t h h i g h e r , r e s u l t i n g i n lower s t r e n g t h s . The u n r e i n f o r c e d m a t r i x h a s a g r e a t e r volume c o n c e n t r a t i o n of p o r e s i n t h e medium p o r e s i z e range (0.1

-

0.01 pm and 1

-

0.1 pm) t h a n t h e m a t e r i a l w i t h 6% mica. T h i s may account f o r t h e small b u t r e a l i n c r e a s e d f l e x u r a l s t r e n g t h , r e l a t i v e t o u n r e i n f o r c e d m a t e r i a l , f o r some high mica c o n t e n t m a t e r i a l s . D i f f e r e n c e s i n t h e p o r e s i z e d i s t r i b u t i o n may account f o r some a s p e c t s of t h e s t r e n g t h of t h e s e composites, b u t o t h e r f a c t o r s such a s product d e n s i t y , s t r e n g t h of m a t r i x - f l a k e i n t e r f a c e and d e g r e e of c r y s t a l l i n i t y must a l s o be c o n s i d e r e d . T o t a l p o r o s i t y v a l u e s f o r t h e v a r i o u s p r e p a r a t i o n s a r e t a b u l a t e d i n -

able

3 . It i s a p p a r e n t t h a t f l a k e a d d i t i o n 2 5

:

I I I I I I

I

P O R T L A N D C I M L N I M A T R I X A C E 3 2 d 20

-

W I C

-

0. 50 10 V O L U M E F R A C T I O N M I C A F L A K E S . % FIG. 5 N i t r o g e n s u r f a c e a r e a of p o r t l a n d cement p a s t e r e i n f o r c e d w i t h mica f l a k e s TABLE 3 P o r o s i t y Values of Mica-Flake-Reinforced Cement P a s t e Samples Hydrated For 32 d a y s

WaterICement R a t i o changes t o t a l p o r o s i t y and t h a t t h e s e changes a r e dependent on t h e v a l u e of V f . High v a l u e s of V f , e.g., Vf " 5 2 , g e n e r a l l y l e a d s t o i n c r e a s e d p o r o s i t y f o r w/c 0.45. It was d i f f i c u l t t o p r e p a r e samples c o n t a i n i n g Vf = 7% a t w/c = 0.25. Aging i n hydrated p o r t l a n d cement systems can be d e s c r i b e d

I a s time-dependent p h y s i c a l and chemical p r o c e s s e s , o t h e r t h a n h y d r a t i o n , t h a t r e s u l t i n m i c r o s t r u c t u r a l change, e.g., s i l i c a p o l y m e r i z a t i o n , l a y e r i n g of s i l i c a t e s , e t c . I n t h e presence of admixtures t h a t i n f l u e n c e h y d r a t i o n r a t e s

Vol. 1 3 , No. 2 J

.

J

.

Beaudoin

s u r f a c e a r e a changes may be caused by s i l i c a p o l y m e r i z a t i o n (13). As mica f l a k e s a l s o i n f l u e n c e t h e r a t e of h y d r a t i o n , i t i s p o s s i b l e t h a t s i l i c a p o l y m e r i z a t i o n may be only one of t h e f a c t o r s c a u s i n g d e c r e a s e i n s u r f a c e a r e a .

It i s a p p a r e n t t h a t f a c t o r s a f f e c t i n g composite s t r e n g t h i n c l u d e mica c o n t e n t , e f f i c i e n c y of t h e r e i n f o r c e m e n t , t o t a l p o r o s i t y , p o r e s i z e

d i s t r i b u t i o n , a g i n g , and s h e a r s t r e s s a t t h e matrix-mica i n t e r f a c e . One o r more of t h e s e may be t h e predominant f a c t o r c o n t r o l l i n g s t r e n g t h . The amount of mica f o r h i g h e s t s t r e n g t h i s p o r o s i t y dependent, and a t a g i v e n p o r o s i t y i t i s probably i n f l u e n c e d by a l l t h e above f a c t o r s . Any predominating f a c t o r i s d i f f i c u l t t o d e t e r m i n e , e x c e p t t h a t a t h i g h mica c o n t e n t s a g r e a t e r

c o n t e n t of l a r g e p o r e s may be t h e main f a c t o r c o n t r i b u t i n g t o d e c r e a s e i n s t r e n g t h .

F u r t h e r work i s i n p r o g r e s s t o d e t e r m i n e whether mica h a s p o t e n t i a l f o r u s e i n v a r i o u s i n d u s t r i a l p r o d u c t s , i n c l u d i n g t h o s e w i t h b i n d e r s c o n t a i n i n g f i n e a g g r e g a t e s .

C o n c l u s i o n s

1. The i n c r e a s e i n f l e x u r a l s t r e n g t h of mica f l a k e - r e i n f o r c e d cement p a s t e i s dependent on t h e volume f r a c t i o n of f l a k e s and p o r o s i t y and i s s i g n i f i c a n t l y i n c r e a s e d by a few p e r c e n t of mica f l a k e s .

2. Compressive s t r e n g t h of p o r t l a n d cement p a s t e d e c r e a s e s w i t h a d d i t i o n of mica f l a k e s .

3 . F r a c t u r e t o u g h n e s s of p o r t l a n d cement p a s t e i s s i g n i f i c a n t l y i n c r e a s e d by t h e p r e s e n c e of mica f l a k e s and i s dependent on m a t r i x p o r o s i t y .

4. C h a r a c t e r i s t i c s of t h e m a t r i x s u c h a s s u r f a c e a r e a , p o r e s i z e d i s t r i b u t i o n , and non-evaporable w a t e r c o n t e n t a r e modified by t h e p r e s e n c e of mica f l a k e s . T h i s m o d i f i c a t i o n i s dependent on t h e volume f r a c t i o n of mica added t o t h e cement p a s t e .

5. A b a l a n c e between changes i n f l e x u r a l s t r e n g t h and f r a c t u r e t o u g h n e s s can

b e a c h i e v e d by c o n t r o l l i n g m a t r i x p o r o s i t y . Acknowledgement

The a u t h o r w i s h e s t o thank J. Wood f o r a s s i s t a n c e w i t h t h e e x p e r i m e n t a l work. 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 Research, 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 of t h e D i v i s i o n .

R e f e r e n c e s

1. Proc. RILEM Symp. on F i b r e R e i n f o r c e d Cement and Concrete (ed. by A.M. N e v i l l e )

1,

459 (1975).

2. V.S. Ramachandran, R.F. Feldman and J . J . Beaudoin, C o n c r e t e S c i e n c e , Heyden

& Son, L t d . , U.K. (1981).

3. A . J . Majumdar and R.W. Nurse, ? f a t s . S c i . and Eng.

15,

107 (1974). 4. B u i l d i n g Research E s t a b l i s h m e n t , U . K . , C u r r e n t Paper 38/76 (1976).

5. C . J . Cheetham and P. Maguire, Coating of G l a s s F i b r e s , U.S. P a t e n t 4,173,486 (1979).

6. P.E. Grattan-Bellew and J . J . Beaudoin, Cem. Concr. Res.

10,

789 (1980). 7. J . J . Beaudoin, Cem. Concr. Res.

12,

159 (1982).

8. R.F. Feldman, Cem. Tech. 3 , 5 (1972).

9. R. Sh. M i k h a i l , M. ~ b d - ~ l x h a l i k and A . Hassanein, Cem. Concr. Res.

5,

765 (1978).

10. R.N. Swamy and P.S. Mangat, Cem. Concr. Res. 4, 313 (1974). 11. R.F. Feldman and J . J . Beaudoin, Cem. Concr. R ~ S .

7,

1 4 3 (1977). 12. H.L. Cox, B r . J . Appl. Phys.

3,

72 (1952).