Properties of Portland cement-silica fume pastes: II. Mechanical properties

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Properties of Portland cement-silica fume pastes: II. Mechanical

properties

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Properties of Portland

Cement-Silica Fume Pastes

11.

Mechanical Properties

by R.F. Feldman and Huang Cheng-yi

ANALYZED

Reprinted from

Cement and Concrete Research

Vol. 15, No. 6, 1985

p. 943

-

952

(IRC Paper No. 1350)

Price $2.75

NRCC 25342

Nwc

-

ClsTI

BLDG.

RES.

On a mesure la rgsistance en compression, le module de Young et la microdurete en fonction de la porosit6 de pates ne contenant que du ciment ou contenant du ciment et 10 ou 30 pour cent de silice fine dans des rapports e/(c+sf) de 0,25 et 0,45. Les porosites sont plus faibles pour les pates sans silice fine, mais les r6sistances sont superieures pour un rapport e/(c+sf) de 0,25 apres 180 jours de cure. Toutefois, pour un rapport e/(c+sf) de 0,45, l'incorporation d'une grande quantit6 de silice n'augmente que legerement la resistance. Le module d861asticit6 est plus 61ev6 sans silice fine. On montre que pour un rapport e/(c+sf) de 0,45, l'addition de silice fine donne un Eo relativement faible mais un So 6lev6. Ce ph6nomZne est dO B la combinaison d'un hydrate disperse de faible masse

volumique qui contient peu de CaO/Si02, et d'une faible teneur en Ca(OH)2, ce qui donne un composite relativement homogene. Les correlations entre les caracteristiques mecaniques ( B

l16chelle logarithmique) et la teneur en eau captive sont bonnes pour tous les 6chantillons, avec ou sans silice fine.

CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 943-952, 1985. Printed in the

USA,

0008-8846185 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.

PROPERTIES OF PORTLAND CEMENT-SILICA FUME PASTES

11. MECHANICAL PROPERTIES

R.F. Feldman and Huang Cheng-yi Division of Building Research National Research Council of Canada

Ottawa, Canada, KIA OR6

(Communicated by F.H. ~ittmann) (Received Jan. 11, 1985)

ABSTRACT

Compressive strength, Young's modulus, and microhardness are measured as a function of porosity for pastes of silica fume-cement blends made with silica fume contents of 0, 10 and 30 per cent at w/(c+sf) ratios

of 0.25 and 0.45. Porosities are lower for pastes without silica fume

addition, but strengths are greater at w/(c+sf) of 0.25 at 180 days curing. At a w/(c+sf) of 0.45, however, strengths are marginally greater with large additions of silica fume. Modulus of elasticity is greater without addition. It is shown that silica fume addition at a w/(c+sf) of 0.45 results in a relatively low Eo but high So. This is due to a combination of low-density dispersed hydrate of low CaO/Si02

and low Ca(OH), content, leading to a relatively homogeneous

composite. ~o;relations between log mechanical property and non-evaporable water content are good for all specimens, with and without silica fume.

Iqtroduction

The first paper of this series (1) reported an investigation of the

development of pore-size distribution, measurement of surface area by

different techniques, and measurement of the drying shrinkage of various mixes of portland cement-silica fume blended pastes. It demonstrated that the properties of blended pastes are quite different from those of pure pastes or blended mortars. Calcium hydroxide, which is deposited in mortars at the interface of inclusions such as sand, subsequently reacts with silica fume

(2). In blended pastes, silica fume reacts with calcium hydroxide only in the

matrix phase (3).

r : In order to understand the factors that affect mechanical properties,

the total porosity of the blends has been measured. Mechanical properties such as microhardness, Young's modulus, and compressive strength were

determined for the paste blends at 0, 10 and 30 per cent silica fume content prepared at w/(c+sf) of 0.25 and 0.45 and cured for up to 180 days.

Correlations were also investigated for mechanical properties, non-evaporable water, and porosity.

R.F. Feldman and H. Cheng-yi

Vol. 1 5 , No. 6

I

1

Experimental

D e t a i l s of m a t e r i a l s , mixes, and c u r i n g p r o c e d u r e have been g i v e n i n

p a r t one of t h i s s e r i e s ( 1 ) .

7,

P o r o s i t y

Damp-dry specimens (1.3 mm t h i c k ) were p l a c e d i n d e s i c c a t o r s

-

c o n d i t i o n e d w i t h s a l t s o l u t i o n t o 11% RH. The weight change from t h e damp-dry c o n d i t i o n was recorded a s a p e r c e n t a g e of t h e volume of t h e specimen i n t h e d-dried s t a t e . (D-drying was a c h i e v e d by h e a t i n g t h e samples i n a vacuum a t 100°C f o r 8

-

14 h . ) Apparent volumes were determined by measuring t h e t h i c k n e s s and d i a m e t e r of t h e d i s c samples.

Compressive S t r e n g t h , Young's Modulus and Microhardness

S t r e n g t h was measured i n compression on 51 x 51mm c u b e s , t h r e e cubes f o r each p r e p a r a t i o n , a t each of t h e c u r i n g t i m e s ( 1 , 3 , 7 , 1 4 , 18, 90 and 180 days).

Young's modulus was measured on 32-m d i a m e t e r p a s t e d i s c s 1.3 mm t h i c k , u s i n g seven d i s c s t o o b t a i n an a v e r a g e v a l u e ( 4 ) . D e f l e c t i o n was measured by l o a d i n g a s a t u r a t e d p a s t e specimen a t i t s c e n t r e w h i l e s u p p o r t e d a t t h r e e p o i n t s on t h e c i r c u m f e r e n c e of a c i r c l e 25 mm i n d i a m e t e r .

Microhardness was measured by means of a L e i t z microhardness machine w i t h a V i c k e r s pyramid i n d e n t e r on t h e same d i s c s a s were used f o r d e t e r m i n i n g t h e modulus of e l a s t i c i t y . Measurements were c a r r i e d o u t i n a c o n d i t i o n e d box f r e e of Co2. Each v a l u e was t h e a v e r a g e of 40 r e a d i n g s t a k e n on two

d i s c s ( 4 ) .

R e s u l t s Compressive S t r e n g t h

The r e s u l t s of t h e development of compressive s t r e n g t h w i t h time f o r t h e s i x mixes, t o g e t h e r w i t h t h e nomenclature f o r e a c h mix, a r e p r e s e n t e d i n Fig. 1 and T a b l e I. A t a w / ( c + s f ) of 0.25 t h e s t r e n g t h f o r P' is g r e a t e s t (of t h e t h r e e mixes) t o between 28 and 90 days of c u r i n g , w h i i g S' and P' a r e s i m i l a r up t o 14 days. By 90 d a y s , S' h a s t h e h i g h e s t s t r e n g t h , i . e . , 138 MPa a f t e r 180 days of c u r i n g ; P ; ~ i s l o w e s t , a t 106 MPa. The r a t e of s t r e n g t h development a f t e r 28 days i s much g r e a t e r f o r S' t h a n f o r t h e o t h e r two mixes. A t a w / ( c + s f ) r a t i o of 0.45, t h e s t r e n g t h of

~7~

i s h i g h e s t between one and 180 days; but t h e r a t e of s t r e n g t h development i s g r e a t e s t f o r

sH

a f t e r 28 d a y s , e x c e e d i n g t h e s t r e n g t h of p y 0 between 90 and 180 days.

Microhardness

The development of microhardness w i t h t i m e i s i l l u s t r a t e d i n F i g . 2.

The s p r e a d of r e s u l t s f o r t h e t h r e e TABLE I

s i l i c a fume c o n t e n t s a t b o t h v a l u e s of Nomenclature f o r Mixes w / ( c + s f ) i s much l e s s t h a n t h a t f o r

compressive s t r e n g t h . A t a w/(c+sf) of w/(c+sf )*

0.25, P' d e v e l o p s maximum h a r d n e s s , Per c e n t s i l i c a fume 0.25 0.45

i.e., 688 MPa a f t e r 180 d a y s , w h i l e 'S

0

s'

sH

1

and f f 0 have similar v a l u e s o f h a r d n e s s I

o f a b o u t 650 MPa a f t e r t h e same p e r i o d . 10 _{pP;o pllI0}

A t a w / ( c + s f ) of 0.45, p t O shows t h e 3 0 P:O

~5

.I

g r e a t e s t microhardness up t o 14 days.

A t 180 d a y s , sHand P:~ b o t h exceed

pya,

*Water-to-cement p l u s s i l i c a fume

Vol. 15, No. 6

R . F . Feldman and H. Cheng-yi

A t a w / ( c + s f ) of 0.45, t h e p a s t e s w i t h o u t s i l i c a f u m e ,

sH and

pya,

a r e

H

s i m i l a r f o r t h e f i r s t 14 d a y s , b u t S a-gain shows t h e l a r g e s t d e c r e a s e i n

p o r o s i t y a f t e r 28 d a y s ; v a l u e s a t 180 days a r e 28.8, 31 and 32.2 f o r

sH,

P ! ~ t

and p H O , r e s p e c t i v e l y . P r i o r t o 28 d a y s ,

pya,

sH

and py0 have l o w e s t p o r o s i t i e s i n t h a t o r d e r . These a r e t r e n d s v e r y s i m t l a r t o t h o s e of t h e

Young's modulus.

P o r o s i t y / M e c h a n i c a l P r o p e r t y R e l a t i o n

S e v e r a l e x p r e s s i o n s have b e e n 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 , f r a c t u r e e n e r g y , and Young's modulus. The f o l l o w i n g e q u a t i o n h a s o f t e n been used f o r m e c h a n i c a l p r o p e r t i e s : -b P M = M e M 0 where M = m e c h a n i c a l p r o p e r t y , Mo = m e c h a n i c a l p r o p e r t y a t z e r o p o r o s i t y , P = p o r o s i t y , and bM i s a p o r e s h a p e f a c t o r . P l o t s of l o g m e c h a n i c a l p r o p e r t y a g a i n s t p o r o s i t y were made f o r specimens c u r e d from 3 t o 180 d a y s ; t h e p l o t s d e v e l o p e d s i x s t r a i g h t l i n e s f o r each m e c h a n i c a l p r o p e r t y . The r e s u l t s f o r l i n e a r r e g r e s s i o n a n a l y s i s , r e c o r d e d i n T a b l e 11, show t h e i n t e r c e p t , t h e m e c h a n i c a l p r o p e r t y a t z e r o p o r o s i t y ( S o , Ho, Eo, i . e . , s t r e n g t h , m i c r o h a r d n e s s , and Young's modulus, r e s p e c t i v e l y ) , and t h e s h a p e f a c t o r ( b S ,

qI

and bE) f o r e a c h specimen. The c o r r e l a t i o n c o e f f i c i e n t f o r e a c h l i n e i s a l s o g i v e n .

Compressive S t r e n g t h

The v a l u e s of So d e r i v e d f r o m c o m p r e s s i v e s t r e n g t h a n a l y s i s a t a w / ( c + s f ) of 0.25 d e c r e a s e w i t h i n c r e a s e i n s i l i c a fume c o n t e n t . T h i s s y s t e m i s v e r y complex, e s p e c i a l l y s i n c e a l a r g e amount of u n h y d r a t e d cement r e m a i n s f o r S II even a t 180 d a y s , and bS v a l u e s d e c r e a s e w i t h S o v a l u e s . At a w / ( c + s f ) of 0.45, So v a l u e s f o r py0 and pF310 a r e much h i g h e r t h a n t h a t f o r

sH,

i . e . , 746.5,

669.9 and 448.7 MPa, r e s p e c t i v e l y . T h i s i s c o n s i s t e n t w i t h t h e f a c t t h a t i n py0 and p H O , Ca(0H) c o n t e n t i s g r e a t l y reduced, and d e s p i t e t h e h i g h e r

H

PorosAtg f o r

PY*

an2

,yo

a t 180 days t h e s t r e n g t h f o r P,, was h i g h e r t h a n t h a t f o r S

.

M i c r o h a r d n e s s

.E

The v a l u e s of Ho and bH a r e a l s o p r e s e n t e d i n T a b l e 11. P10 h a s t h e l a r g e s t Ho v a l u e a t a w / ( c + s f ) of 0.25, b u t i t i s n o t much d i f f e r e n t from t h a t of

sP.

q l H c h a n g e s i n t h e same manner a s Ho. A t a w / ( c + s f ) of 0.45, Ho v a r i e s a s py0 > P 3 0 > >

sH,

i n t h e same o r d e r a s f o r bH, So and bS. T h i s i s

c o n s i s t e n t w i t h t h e c o m p r e s s i v e s t r e n g t h r e s u l t s and shows t h a t , d e s p i t e t h e H H H h i g h e r p o r o s i t y of P10 and P 3 0 i n r e l a t i o n t o S

,

t h e m i c r o h a r d n e s s v a l u e s a f t e r 180 d a y s of c u r i n g do n o t d i f f e r f r o m e a c h o t h e r t o any l a r g e e x t e n t . Young ' s modulus

The v a l u e s f o r Eo p r e s e n t e d i n T a b l e I1 show a d i f f e r e n t t r e n d from t h o s e

06

c o m p r e s s i v e s t r e n g t h and m i c r o h a r d n e s s . A t a w/ c + s f ) of 0.45,

H

S > > pq0 > pH in t h i s c a s e modulus v a l u e s f o r

Sb

a r e a l s o g r e a t e r t h a n P l o and

<

a t lt3d0iays. A t a w/ =+sf)

_k

o f 0.25, no p a r t i c u l a r t r e n d i s e v i d e n t , t h e vayues b e i n g l'fo > S' > P l o _{The r e s u l t s a p p e a r t o be dominated by t h e} t

r e s i d u a l u n h y d r a t e a cement.

Mechanical P r o p e r t y

-

Non-evaporable Water R e l a t i o n

P l o t s of t h e l o g a r i t h m of t h e t h r e e m e c h a n i c a l p r o p e r t i e s v e r s u s

non-evaporable w a t e r c o n t e n t a t a w / ( c + s f ) of 0.45 a r e p r e s e n t e d i n F i g s . 5-7. A l l a r e f o r specimens c u r e d from 3 t o 180 d a y s . C o r r e l a t i o n c o e f f i c i e n t s from

Vol. 1 5 , No. 6 9 4 7 SILICA FUME, CEMENT PASTES, STRENGTH, MODULUS, POROSITY, MICROHARDNESS

FIG. 3

-. ,

0 1 3 7 1 4 28 9 0

T I M E , d a y s (&%LEI

Young's modulus o f cement p a s t e s w i t h d i f f e r e n t s i l i c a fume c o n t e n t s v e r s u s h y d r a t i o n t i m e 0 0 1 3 7 14 28 9 0 1 8 0 TIME, d a y s

LEI

FIG. 4 P o r o s i t y d e t e r m i n e d by a d j u s t e d w a t e r t e c h n i q u e f o r cement p a s t e s w i t h d i f f e r e n t s i l i c a fume c o n t e n t s v e r s u s h y d r a t i o n t i m e

Vol. 15, No. 6 R.F. Feldman and H. Cheng-yi

TABLE I1

Linear Regression Analysis of Compressive Strength, Microhardness, and

Modulus of Elasticity Versus Porosity Results I

Correlation coefficient

Sample So(MPa) _{b~ r}

s

a

610.94 0.0960 0.98

linear regression analysis are all over 0.93, and for compressive strength and microhardness are above 0.95. The larger amount of non-evaporable water associated with specimen

sH for a similar gain of compressive strength or

microhardness is evident from increased slope for Py0 and pro; pH has the greatest slope in the plot of strength and microhardness (F'lgs. loand 6,

1

respectively). Similar results may be observed for Young's modulus, except

that the slopes for py0 and py0 are similar. At a w/(c+sf) of 0.25 there is ,

relatively less difference between the slopes for

say

on the one hand, and pt0 and Pt0, on the other, for the plot of the three mechanical properties. This is due to the increased influence of the unhydrated phase and the influence of the initial low w/(c+sf) on degree of hydration.

Vol. 1 5 , No. 6 949 SILICA FUME, CEMENT PASTES, STRENGTH, MODULUS, POROSITY, MICROHARDNESS

Mechanical P r o p e r t y I n t e r r e l a t i o n

I L i n e a r p l o t s of microhardness v e r s u s compressive s t r e n g t h , Young's

modulus v e r s u s compressive s t r e n g t h , and Young's modulus v e r s u s microhardness a r e p r e s e n t e d i n F i g s . 8-10, r e s p e c t i v e l y , f o r a w / ( c + s f ) of 0.45. F i g u r e 8 shows good l i n e a r c o r r e l a t i o n between microhardness and compressive s t r e n g t h , and t h e s l o p e of t h e l i n e i s independent of s i l i c a fume c o n t e n t . On t h e o t h e r hand, c o r r e l a t i o n s between Young's modulus and compressive s t r e n g t h o r

m i c r o h a r d n e s s , a l t h o u g h good, show s l o p e s dependent on s i l i c a fume c o n t e n t . It i s c l e a r t h a t t h e r e s p o n s e of t h e Young's modulus measurement t o changes brought on by s i l i c a fume i s d i f f e r e n t from t h a t of compressive s t r e n g t h and microhardness measurements.

D i s c u s s i o n

P o r o s i t y measurements ( F i g . 4) show t h a t p a s t e s w i t h o u t a d d i t i o n s of s i l i c a fume have lower p o r o s i t y v a l u e s because h y d r a t i o n r e a c t i o n s between 28 and 180 days c o n t i n u e a t a f a s t e r r a t e . T h i s d i f f e r e n c e was d i s c u s s e d i n a p r e v i o u s paper ( 3 ) i n which i t was s u g g e s t e d t h a t t h e d e c r e a s e i n p e r m e a b i l i t y of t h e h y d r a t i o n p r o d u c t s i n t h e p r e s e n c e of s i l i c a fume i s due p a r t l y t o low Ca(OH)2 c o n t e n t . The d i f f e r e n c e i n p o r o s i t y , however, i s n o t s o l a r g e a s would be e x p e c t e d from t h e d i f f e r e n c e i n t h e e x t e n t of t h e r e a c t i o n ( a s s e s s e d from t h e non-evaporable w a t e r c o n t e n t ) because of t h e o f f s e t t i n g e f f e c t of d e n s i t y of p r o d u c t s . The d e n s i t y of C-S-H formed i n t h e p r e s e n c e of s i l i c a fume i s 2.05 g/mL, whereas i t s v a l u e when formed i n t h e absence of s i l i c a fume i s 2.24 g/mL. The C-S-H formed i n t h e p r e s e n c e of s i l i c a fume would t h u s be more e f f e c t i v e , p e r u n i t weight of non-evaporable w a t e r , i n f i l l i n g p o r e s of t h e m a t r i x (1).

The v a l u e of t h e mechanical p r o p e r t y of porous composite b o d i e s s u c h a s c e m e n t - s i l i c a fume b l e n d s i s dependent on s e v e r a l f a c t o r s , i n c l u d i n g p o r o s i t y , d e n s i t y of p r o d u c t , bonding between r e a c t i o n p r o d u c t s , and homogeneity ( 6 ) . Although lower p o r o s i t y could i n d i c a t e a h i g h e r v a l u e f o r a mechanical propert-#, o t h e r f a c t o r s might have a c o u n t e r i n g i n f l u e n c e . A t a w / ( c + s f ) of 0.45, S does n o t have t h e h i g h e s t compressive s t r e n g t h o r microhardness a f t e r 180 days c u r i n g d e s p i t e t h e lower p o r o s i t y . It i s c l e a r t h a t t h e lower

Ca(OH)2 c o n t e n t of py0 and pH ( g i v i n g a more homogeneous body, p o s s i b l y w i t h 30

fewer a r e a s f o r c r a c k initiation) can i n c r e a s e t h e s t r e n g t h of t h e s e p a s t e b l e n d s . T h i s f a c t o r i s n o t s o dominant w i t h r e g a r d t o Young's modulus, a s may be s e e n i n t h e r e s u l t s a t a w / ( c + s f ) of 0.45 where t h e v a l u e f o r

sH

i s g r e a t e r t h a n e i t h e r pHO o r

pya;

i n t h i s c a s e t h e h i g h e r d e n s i t y product w i t h t h e lower p o r o s i t y r e s u l t s i n a h i g h e r Young's modulus.

The v a l u e s of S n , Hn and En a t a w / ( c + s f ) of 0.45 i l l u s t r a t e t h e r e s p o n s e of t h e p r o p e r t i e s ofutheuvariou: composites t o d i f f e r e n t mechanical s t r e s s e s . For So and Ho, py0 > P:~ > >

sH,

but f o r E

,

sH

> py0 and P:~. I n p r e v i o u s work on a u t o c l a v e d c e m e n t - s i l i c a systems ? 6 ) , v a l u e s of Eo, So, Ho and bE, bS and bH were o b t a i n e d ; t h e v a l u e s f o r t h e a u t o c l a v e d 20% s i l i c a - c e m e n t m i x t u r e were s i m i l a r t o t h o s e f o r

sH

o b t a i n e d h e r e , b u t t h e v a l u e s of t h e t h r e e p r o p e r t i e s , Eo, So and Ho, of P: and P : ~ were n o t s i m i l a r t o t h o s e of a n

8

O f

t h e a u t o c l a v e d m i x t u r e s . The vayues of t h e r a t i o Eo/So f o r

sH,

pY0 and P30 a r e 133.9, 65.6 and 72.6, r e s p e c t i v e l y ; but t h i s v a l u e v a r i e s from 110 t o 186 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 m i x t u r e s c o n t a i n i n g 10 t o 50% s i l i c a . S i l i c a fume a d d i t i o n r e s u l t s i n t h e combination of a r e l a t i v e l y low Eo, probably due t o t h e low d e n s i t y h y d r a t e p r o d u c t , and a h i g h So, due t o low Ca(OH)2 c o n t e n t

( 7 ) and p o s s i b l y t o p o r e s h a p e s l e a d i n g t o low s t r e s s i n t e n s i t i e s . H H

The h i g h v a l u e s of mechanical p r o p e r t i e s f o r P10 and P 3 0 , w i t h r e l a t i v e l y low non-evaporable w a t e r c o n t e n t ( a ) , a s shown i n F i g s . 5-7, a r e s u r p r i s i n g . They a r e probably due t o t h e combination of a low-density, d i s p e r s e d - h y d r a t e

1 + 0

z ;

2

in" VI u > 0

-

0 in 2 in

-

u m a Vol. 1 6 , No. 6 R . F . Feldman a n d H. Cheng-yi COEFFICIENT • 10 0.99 0 M 0.95 8

I

I I I 1 I I 1 I I 1 0 1 1 . 2 1 2 . 4 1 3 . 6 1 4 . 8 1 6 1 7 . 2 1 8 . 4 1 9 . 6 20.8 2 2 N O N - E V A P O R A B L E W A T E R . g i g - I G N I T E D W E I G H T FIG. 5 C o m p r e s s i v e s t r e n g t h v e r s u s n o n - e v a p o r a b l e w a t e r c o n t e n t f o r cement p a s t e s c o n t a i n i n g s i l i c a fume ( w / ( c + s f ) = 0 . 4 5 ) COEFFICIENT 0.995

.

10 1 0

L

1 I I I I I I I I

I

1 1 12. 1 1 3 . 2 1 4 . 3 1 5 . 4 1 6 . 5 1 7 . 6 18.7 1 9 . 8 2 0 . 9 2 2 N O N - E V A P O R A B L E W A T E R . g i g - I G N I T E D W E I G H T FIG. 6 M i c r o h a r d n e s s v e r s u s n o n - e v a p o r a b l e w a t e r c o n t e n t f o r cement p a s t e s c o n t a i n i n g s i l i c a fume ( w / ( c + s f ) = 0 . 4 5 )

SILICA FUME. I CoRREmTloN

4

COEFF l C l ENT

+

i

-

<? 1 . 9

-

0 _M

1

0.94

-

L a

"=:

-

1 . 7 - C

zg

1 . 6

-

Y - 1.4 - 0 m 1.2

-

3

-

2 3 1 . 1 - c ?

-

z

1 I I 1 I I I I 1 1 1 2 . 1 1 3 . 2 1 4 . 3 1 5 . 4 16.5 1 7 . 6 18.7 19. 8 2 0 . 9 2 2 N O N - E V A P O R A B L E W A T E R . q l g - I G N I T E D W E I G H T

FIG. 7 Young's modulus v e r s u s n o n - e v a p o r a b l e w a t e r c o n t e n t f o r cement p a s t e s c o n t a i n i n g s i l i c a fume ( w / c + s f ) = 0 . 4 5 )

Vol. 16, No. 6 951 SILICA FUME, CEMENT PASTES, STRENGTH, MODULUS, POROSITY, MICROHARDNESS

SILICA FUME. CORRELATION

% COEFFICIENT

'5!

:2 3; 4; 4: i5 2: 6; i 6 8\ O!

C O M P R E S S I V E S T R E N G T H . M P a

FIG. 8 Microhardness versus compressive strength for cement pastes containing silica fume (w/(c+sf) = 0.45)

3.0 n I I I 1 1 I I I 1

-

9 ' , 2.4-

-

4 " 2.1- LA,

.

10

-

>- 1.8- 0 M 0.99 C

-

-

0 1.5-

-

C

w -

:

1.2-

-

2 w 0.9

-

L

-

0

,

0.6 - - = 2 3 0.3 - C1

-

0 2 0 2 2 8 4 5 6 6; 6 4 80 C O M P R E S S I V E S T R E N G T H . MPa

FIG. 9 Young's modulus versus compressive strength for cement pastes containing silica fume (w/(c+sf) = 0.45)

m 3.0 n I I I I I I I 1 I

-

-

w 2.1- • 10 0.91 - i 1.8 - C 0 M 0. % -

-

U_ 1 . 5 -

-

C V) < 1.2

-

2 - w 0 0 , 0.9 - 0

-

LO 0.6- 3 - 2 2 0.3- a - 0 I 0 I I I I I I I I I 10 14 18 22 26 30 34 38 4'2 46 50 M I C R O H A R D N E S S x MPa

FIG. 10 Young's modulus versus microhardness for cement pastes containing silica fume (w/(c+sf) = 0.45)

Vol. 15, No.

6

R.F. Feldman and H. Cheng-yi

product of low CaO/Si02 ratio (7,9) and to low Ca(OH)2 content, leading to a relatively more homogeneous composite.

Conclusions a

1. Porosity of pastes without silica fume is lower than that of pastes of

blends, especially at longer periods of curing. I

2. Young's modulus of elasticity for paste without silica fume is higher than that for pastes with silica fume, but compressive strength is lower. 3. Good correlations are found between log mechanical property and

porosity or non-evaporable water.

4. Silica fume addition results in the combination of a relatively low Eo, due to a low density product, and a high So, due to low Ca(OHI2 content. 5. Silica fume addition also results in the combination of a low-density,

dispersed-hydrate product of low CaO/Si02 and low Ca(OH)2 content, leading to a relatively homogeneous composite.

Acknowledgement

The authors wish to recognize the work of G.W. Chan who helped to perform the experiments. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is submitted with the approval of the Director of the Division.

References

1.

R.F. Feldman and Huang Cheng-yi, Properties of portland cement-silica fume pastes, I. Porosity and surface properties. To be published. 2. Huang Cheng-yi and R.F. Feldman, Influence of silica fume on the

microstructural development in cement mortars. To be published. 3. Huang Cheng-yi and R.F. Feldman, Hydration reactions in portland

cement-silica fume blends. To be published.

4. R.F. Feldman, Pore structure formation during hydration of fly-ash and slag cement blends. Proceedings, Annual Meeting, Materials Research Society, Boston, pp. 124-133 (1981).

5. R.F. Feldman and J.J. Beaudoin, Cem. Concr. Res.

6,

389 (1976).

6. J.J. Beaudoin and R.F. Feldman, Cem. Concr. Res.

5,

103 (1975).

7. M. Regourd and

B.

Mortureux, Proc. 4th Inter. Conf. on Cement Microscopy, Las Vegas, p. 249 (1982).

8. E.J. Sellevold, D.H. Bager,

E.

Klitgaard Jensen and T. Knudsen,

in

Condensed silica fume in concrete (Ed. O.E. Gjorn and K.E. Loland). Norwegian Inst. Technol., Univ. Trondheim. BML 82.610, p. 19 (1982). 9. H.F.W. Taylor, The chemistry of cement. London, Academic Press, Vol. 2,