Influence of silica fume on the microstructural development in cement mortars

(1)

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Cement and Concrete Research, 15, March 2, pp. 285-294, 1985-03-01

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=c69d76a8-64b1-4611-a59b-b804015bd166

https://publications-cnrc.canada.ca/fra/voir/objet/?id=c69d76a8-64b1-4611-a59b-b804015bd166

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.1016/0008-8846(85)90040-7

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Influence of silica fume on the microstructural development in cement

mortars

(2)

- -

-

Ser

National Research

Conseil national

13 3 . d

7 T H ~

Council Canada

de recherches Canada

N21

d

no,

1288

c .

2 BLDG

INFLUENCE OF SILICA FUME ON THE MICROSTRUCTURAL

DEVELOPMENT I N CEMENT MORTARS

by Huang Cheng-y i and R.F. Feldman

ANALYZED

Reprinted from

Cement and Concrete Research

Vol. 15, No. 2, 1985

p. 285

-

294 DBR Paper No. 1288

Division of Building Research

(3)

Gsm;

Des mortiers de ciment contenant OX, 10% et 30% de silice fine ont 6tk pr6par6s avec des rapports eaulciment + silice fine de 0,45 et 0,60. La resistance 3 la compression, les teneurs en

Ca(OH)2 et en eau non evaporable et la repartition dimensionnelle des pores ont kt6 soumises 3 un contr6le continu

pendant 180 jours. La silice fine reagit avec la plus grande partie du Ca(OH)2 form6 lors de l'hydratation en moins de 28 jours et augmente la rksistance 3 la compression du mortier.

De plus, elle modifie la repartition dimensionnelle des pores des mortiers en reagissant avec le Ca(OHI2 form6 autour des grains de sable et celui disperse dans la pSte de ciment.

IIII~I[\IIII!

809

I~~~~~IIIIIIIIIIII~

02

0

500

IIIIII

- -

- - - -- +

(4)

INFLUENCE OF SILICA FUME ON THE MICROSTRUCTURAL DEVELOPMENT I N CEMENT MORTARS

CEMENT and CONCRETE RESEARCH. Vol. 1 5 , pp. 285-294, 1985. P r i n t e d i n t h e USA. 0008-8846185 $3.00+00. Copyright ( c ) 1985 Pergamon P r e s s , L t d .

Huang Cheng-yi* and R.F. Feldman

*Engineer, Research I n s t i t u t e of B u i l d i n g M a t e r i a l s B e i j i n g , China

D i v i s i o n of B u i l d i n g Research National Research Council Canada

Ottawa, Canada K1A OR6 (Communicated by M. Daimon)

(Received J u l y 24, 1984)

ABSTRACT

Cement m o r t a r s c o n t a i n i n g 0 , 10 and 30 p e r c e n t s i l i c a fume were p r e p a r e d a t waterlcement

+

s i l i c a fume r a t i o s of 0.45 and 0.60. Compressive s t r e n g t h , Ca(0H) and non-evaporable w a t e r c o n t e n t s and p o r e - s i z e d i s t r i b u t i o n were monitored up t o 180 days. S i l i c a fume r e a c t s w i t h most of t h e Ca(OH)2 formed d u r i n g h y d r a t i o n w i t h i n 28 d a y s and improves t h e compressive s t r e n g t h of t h e mortar. I n a d d i t i o n i t a f f e c t s t h e p o r e s i z e d i s t r i b u t i o n of m o r t a r s by r e a c t i n g w i t h Ca(0H) formed around t h e sand g r a i n s and a l s o w i t h t h a t d i s p e r s e d throughout t h e cement p a s t e .

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

Waste m a t e r i a l s s u c h a s f l y a s h and b l a s t f u r n a c e s l a g , when mixed w i t h p o r t l a n d cement and h y d r a t e d , produce a p o r e s t r u c t u r e more d i s c o n t i n u o u s and impermeable t h a n t h a t of h y d r a t e d p o r t l a n d cement p a s t e ( 1 ) . Recent work s u g g e s t s t h a t "condensed s i l i c a fume", a byproduct of t h e f e r r o - s i l i c o n i n d u s t r y , produces a s i m i l a r e f f e c t ( 2 , 3).

L i t t l e work h a s been done on m i c r o s t r u c t u r a l changes i n m o r t a r s

c o n t a i n i n g s i l i c a fume. P r e f e r e n t i a l d e p o s i t i o n of Ca(OH)2 i n t h e i n t e r f a c i a l zone around a g g r e g a t e s h a s been observed ( 4 ) . Also, s i n c e t h e d i f f e r e n c e between t h e m i c r o s t r u c t u r e of h y d r a t e d p a s t e s and b l e n d s h a s been r e l a t e d t o t h e Ca(0H) component ( I ) , s i g n i f i c a n t d i f f e r e n c e s i n t h e p r o p e r t i e s of p o r t l a n d cement m o r t a r s w i t h and w i t h o u t s i l i c a fume can be a n t i c i p a t e d . The o b j e c t i v e of t h i s work was t o f o l l o w changes, s u c h a s compressive s t r e n g t h , pore s t r u c t u r e , Ca(OH)2 and non-evaporable w a t e r c o n t e n t s of m o r t a r s

c o n t a i n i n g 0 t o 30 p e r c e n t s i l i c a f ume.

(5)

Vol. 1 5 , No. 2 286

Huang Cheng-yi and R.F. Feldman

Experimental M a t e r i a l s

Type I p o r t l a n d cement w i t h a C3A c o n t e n t of 11.82% and s i l i c a fume c o n t a i n i n g 95.2% SiOp, 1.56% carbon, 0.27% K20, 0.10% Na20, and h a v i n g a

s u r f a c e a r e a 21,000 m2/kg were used. Ottawa s i l i c a sand p a s s i n g ASTM-Clog was

'

used f o r making mortar a t a sand-binder r a t i o of 2.25. Binder i n t h e m o r t a r

c o n t a i n e d cement w i t h 0 , 10 o r 30 p e r c e n t s i l i c a fume. Mixes were p r e p a r e d a t w/(c+s.f .) (water/(cement

+

s i l i c a fume)) of 0.45 and 0.60. Melment a d m i x t u r e was used a t a dosage of 0.3 and 2% o n l y i n m o r t a r s made w i t h w/(c+s.f.) o f 0.45 and c o n t a i n i n g 10 and 30 p e r c e n t of s i l i c a fume, r e s p e c t i v e l y .

Mixing

Cement was mixed w i t h a p o r t i o n of t h e w a t e r i n a Hobart Model N-50

mixer (ASTM C-305). The remaining w a t e r was t h e n added w h i l e mixing a t a slow speed f o r 2 min; s i l i c a fume was t h e n mixed a t a low s p e e d f o r 2 min and a t a medium speed f o r a n o t h e r 2 min. The sand was t h e n added a t a slow s p e e d f o r 2 min and a t a medium s p e e d f o r 3 min.

P r o p e r t i e s determined

Compressive s t r e n g t h ( 5 1 mm cubes) ; Ca(0H) c o n t e n t ( t h e r m a l a n a l y s i s ) ; non-evaporable w a t e r ( t hermogravimetry-weight-loss between 1 00-1000°C) ; and p o r e s i z e d i s t r i b u t i o n (Hg p o r o s i m e t r y t o 408 MPa) were determined. The d e t a i l s of t h e s e a r e d e s c r i b e d i n r e f e r e n c e 1.

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

Compared t o t h e r e f e r e n c e m o r t a r , t h a t w i t h s i l i c a fume showed g r e a t e r s t r e n g t h s a t a w/ (c+s .f .) of 0.45. The i n c r e a s e d s t r e n g t h was o b s e r v e d a t c u r i n g p e r i o d s a s e a r l y a s one day. A f t e r 14 d a y s c u r i n g t h e s t r e n g t h s were

I

57, 44 and 40 MPa f o r 30, 10 and 0 p e r c e n t s i l i c a fume c o n t e n t , r e s p e c t i v e l y . Corresponding s t r . e n g t h s a f t e r 90 days c u r i n g were 77, 54 and 48 MPa. A t a

I w/(c+s.f.) of 0.60, however, n o s i g n i f i c a n t i n c r e a s e i n s t r e n g t h o c c u r r e d d u e

t o s i l i c a fume a d d i t i o n . A comparison of t h e compressive s t r e n g t h s of m o r t a r s and p a s t e s c o n t a i n i n g 0 t o 30 p e r c e n t s i l i c a fume a t a w/(c+s.f.) of 0.45 i s p r e s e n t e d i n F i g s . 1 and 2. The r a t e of s t r e n g t h development f o r cement o r m o r t a r c o n t a i n i n g 0 o r 30 p e r c e n t s i l i c a fume i s shown i n Fig. 1. Each e x p e r i m e n t a l p o i n t i s t h e mean v a l u e f o r t h r e e specimens; t h e b a r s g i v e t h e maximum and minimum v a l u e s . With n o s i l i c a fume, t h e p a s t e i s s t r o n g e r t h a n

mortar t h r o u g h o u t , having a s t r e n g t h of 68 MPa, compared t o 5 5 MF'a f o r m o r t a r , C a t 180 days. The mixes c o n t a i n i n g 30 p e r c e n t s i l i c a fume show t h e r e v e r s e

t r e n d . A f t e r seven days c u r i n g , t h e c u r v e s r e p r e s e n t i n g m o r t a r and p a s t e b e g i n t o d i v e r g e ; t h e mean s t r e n g t h of m o r t a r a f t e r 180 d a y s c u r i n g i s 8 2 MPa compared t o 74 MPa f o r p a s t e .

(6)

Vol. 1 5 , No. 2 28 7 SILICA FUME, CEMENT, MORTARS, STRENGTH, PORE STRUCTURE

1 1 . 1 I I 80 m &

-

8 0 - L + rn - 0 rn 4 0 - S I L I C A

-

~u FUME PASTE MORTAR.

0% -a-

--.--

rn rn 20

-

3%. -

.

- -

-.--

- 0. I 0 " 0 1 1 1 I I I 0 1 3 1 1 4 2 1 PO 180 A G E , d a y s FIG. 1 Compressive s t r e n g t h of cement p a s t e s and m o r t a r s c o n t a i n i n g 0 and 30 p e r c e n t s i l i c a f ume (w/(c+s.f .) = 0.45). The r a t i o of compressive s t r e n g t h s of m o r t a r t o p a s t e c o n t a i n i n g 0, 10 o r 30 p e r c e n t s i l i c a fume i s p l o t t e d a s a f u n c t i o n of c u r i n g t i m e i n Fig. 2. A f t e r a n i n i t i a l d e c r e a s e , t h i s r a t i o f o r specimens c o n t a i n i n g 30 p e r c e n t s i l i c a fume i n c r e a s e s t o a v a l u e of 1.11 a f t e r 180 d a y s c u r i n g ; f o r specimens c o n t a i n i n g n o s i l i c a fume t h e r a t i o b e g i n s t o d e c l i n e a f t e r 7 d a y s c u r i n g , t o a v a l u e of 0.80 a f t e r 180 days. Specimens c o n t a i n i n g 10 p e r c e n t s i l i c a fume d o n o t show a s r a p i d a d e c l i n e i n r a t i o . The r a t i o a f t e r 180 d a y s c u r i n g i s 0.90.

Calcium hydroxide and non-evaporable water c o n t e n t s

The change i n Ca(OH)2 c o n t e n t w i t h c u r i n g a g e f o r m o r t a r s w i t h t h r e e s i l i c a fume c o n t e n t s (0,10,30%) a t two w/(c+s.f.) r a t i o s (0.45 and 0.60) i s p r e s e n t e d i n Fig. 3 . The nomenclature f o r t h e s i x mixes i s shown i n t h i s f i g u r e . Specimens

cH

and C' (0% s i l i c a fume) a t t a i n v a l u e s of 20.3 and 17.3% Ca(0H) 2, r e s p e c t i v e l y , w h i l e t h e 10 p e r c e n t b l e n d s (B:~ and B:~) show

-

maximum Ca(0H) c o n t e n t s a t around seven days; t h e y d e c r e a s e t o 7.5 and 4.6%

a f t e r 180 d a y s Z c u r i n g . Blends B e 0 and B f o show d e c r e a s i n g v a l u e s a f t e r o n e

R H

d a y ' s c u r i n g . B 3 0 r e g i s t e r s z e r o Ca(0H) a f t e r 3 days' c u r i n g w h i l e B30 does s o a f t e r 14 days.

(7)

Huang Cheng-yi and R.F. Feldman Vol. 1 5 , No. 2 A G E , d a y s FIG. 2 Dependence of r a t i o of compressive s t r e n g t h s of mortar t o p a s t e on a g e and s i l i c a fume c o n t e n t (w/(c+s.f .) = 0.45). 4 10%

locf

0 R,

lmc:

B.,

1

A G E , d a y s FIG. 3 Ca(0H) c o n t e n t v e r s u s h y d r a t i o n a g e f o r m o r t a r s c o n t a i n i n g s i l i c a fume.

(8)

SILICA FUME, CEMENT, MORTARS, STRENGTH, PORE STXVCTURE A G E , d a y s FIG. 4 Nonevaporable w a t e r v e r s u s h y d r a t i o n time f o r m o r t a r s w i t h and w i t h o u t s i l i c a fume P O R E D I A M E T E R , n m FIG. 5 P o r e s i z e d i s t r i b u t i o n c u r v e s of cement m o r t a r 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 (w/ (c+s .f .) = 0.45) ( a ) 0 % s i l i c a fume ( b ) 10% s i l i c a fume ( c ) 30% s i l i c a f m e .

(9)

Huang Cheng-yi and R.F. Feldman

Vol. 1 5 , No. 2 290

The change i n non-evaporable w a t e r c o n t e n t w i t h t i m e i s shown i n

Fig. 4. The r e s u l t s a r e p l o t t e d a s non-evaporable w a t e r p e r i g n i t e d weight of cement p l u s s i l i c a fume.

cH

and C' a t t a i n e d v a l u e s of approximately 20.2 and 18.9%, r e s p e c t i v e l y a f t e r 180 days' curing.

_H

B y O and B: have v a l u e s i n e x c e s s of t h a t of C a t one day and a b o u t t h e same a t 3 J a y s , b u t a t 7 d a y s

cH

h a s t h e l a r g e s t v a l u e and a t approximately 180 d a y s t h e v a l u e s a r e 20.2, 17.3 and 15.2% f o r

cH,

B y g and B:~, r e s p e c t i v e l y . This i s p r o b a b l y due t o lower combined w a t e r a s s o c i a t e d w i t h calcium s i l i c a t e h y d r a t e (CSH) produced i n t h e b l e n d s t h a n w i t h t h e t o t a l combined w a t e r a s s o c i a t e d w i t h Ca(OHl2 o r t h e CSH produced i n cH. The CSH produced i n t h e b l e n d s h a s a lower CaO/Si02 r a t i o t h a n t h a t produced i n normal mortar ( 3 ) . I n a d d i t i o n , t h e CSH produced i n t h e b l e n d s may g i v e o f f more combined w a t e r below 100°C ( r e f e r e n c e t e m p e r a t u r e from which i t i s determined). S i m i l a r r e s u l t s were o b t a i n e d f o r c', B t 0 , BPIO; t h e i r v a l u e s a t 3 days were 13.1, 11.8 and 10.9, r e s p e c t i v e l y , and a t 180 days were 18.9, 12.4 and 11.8, r e s p e c t i v e l y . The r e a s o n f o r s u c h a d e c r e a s e i n t h e v a l u e f o r 8 t 0 i s n o t known b u t a n amount of u n r e a c t e d s i l i c a fume i n B : ~ i s p a r t l y r e s p o n s i b l e f o r i t s low value. I n a d d i t i o n t h e v a l u e s of non-

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 B t 0 and B: do n o t change much from 3 t o 180 d a y s , b u t l a r g e changes i n Ca(0H) ? f o r B t O ) , compressive s t r e n g t h , and p o r e s i z e d i s t r i b u t i o n and p o r o s i t y , i n d i c a t e t h a t a s i g n i f i c a n t r e a c t i o n i s o c c u r r i n g i n t h i s p e r i o d .

Pore s i z e d i s t r i b u t i o n

The p o r e s i z e d i s t r i b u t i o n f o r t h e w/ (c+s.f .) of 0.45 and 0.60 i s shown i n F i g s . 5 and 6, . r e s p e c t i v e l y . R e s u l t s f o r m o r t a r w i t h o u t s i l i c a fume a r e shown i n Fig. 5a; t h e t o t a l p o r o s i t y down t o 100 nm p o r e d i a m e t e r f o r t h e

180 day specimen i s a b o u t 5.5% of volume, a s compared w i t h l e s s t h a n 2.0% f o r t h e e q u i v a l e n t cement p a s t e ( 2 ) . The volume of mercury i n t r u d e d i n t o t h e specimen d e c r e a s e s w i t h c u r i n g time o v e r t h e f u l l r a n g e of p r e s s u r e i n a p r o g r e s s i v e manner, e x c e p t f o r t h e t h r e e - d a y specimen, which h a s a lower t h a n e x p e c t e d p o r o s i t y above 300 nm diameter. The c u r v e s f o r a l l samples a t h i g h p r e s s u r e s a r e concave t o t h e p r e s s u r e a x i s .

R

The p o r e s i z e d i s t r i b u t i o n c u r v e s f o r specimens B 1 0 a r e p r e s e n t e d i n Fig. 5b. A t v a l u e s below 60 nm, t h e t o t a l i n t r u d e d volume g e n e r a l l y d e c r e a s e s a s t h e c u r i n g age i n c r e a s e s . A f t e r t h r e e days t h e c u r v e s a t t h e h i g h p r e s s u r e end a r e convex t o t h e p r e s s u r e a x i s . T h i s t r e n d i s s i m i l a r t o t h a t observed p r e v i o u s l y f o r f l y a s h and s l a g b l e n d s ( I ) , where i t was r e l a t e d t o t h e d i s r u p t i o n of d i s c o n t i n u o u s p o r e s by t h e h i g h p r e s s u r e i n t r u s i o n and t o t h e e x t e n t of t h e r e a c t i o n of t h e C a ( 0 ~ ) ~ . A p l o t of t h e s l o p e of t h e c u r v e s i n F i g s . 5 and 6 a t t h e maximum i n t r u s i o n p r e s s u r e dV/dlo@ (%/nm)lO v e r s u s a g e of c u r i n g i s p r e s e n t e d i n Fig. 7. There i s a l a r g e i n c r e a s e i n s l o p e w i t h time f o r t h o s e specimens where C ~ ( O H ) ~ c o n t e n t i s low o r d e c r e a s i n g . A t

100 nm t h e c u r v e s i n Fig. 5b f o r a l l t h e c u r i n g a g e s , e x c e p t one d a y , f o r m o r t a r s w i t h 10 p e r c e n t s i l i c a fume, merge w i t h t h e same p o r e volume of a b o u t 7.5%, w h i l e from 200 nm t o 3000 nm, t h e p o r e volumes f o r 9 0 a n d 180-day c u r e d specimens a r e g r e a t e r t h a n a l l b u t t h e o n e d a y c u r e d specimen.

2

The d i s t r i b u t i o n c u r v e s f o r specimens B 3 0 a r e p r e s e n t e d i n Fig. 5c. The t r e n d i s t h e same a s i n Fig. 5b i n t h a t i n t h e 100 t o 3000 nm r a n g e t h e t o t a l p o r e volume i s g r e a t e r i n many c a s e s f o r t h e specimens c u r e d f o r l o n g e r times. The t o t a l p o r e volume down t o a p o r e s i z e of 300 nm f o r t h e 90- and 28-day c u r e d specimens i s g r e a t e r t h a n t h a t f o r t h e o n e d a y specimen; t h e t h r e e - d a y

(10)

Vol. 15, No. 2 291 SILICA FUME, CEMENT, MORTARS, STRENGTH, PORE STRUCTURE

PORE D I A M E T E R . n m FIG. 6 P o r e s i z e d i s t r i b u t i o n c u r v e s of cement mortar 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 (w/(c+s.f .) = 0.60) ( a ) 0% s i l i c a fume ( b ) 10% s i l i c a fume ( c ) 30% s i l i c a fume. specimen h a s t h e l o w e s t p o r o s i t y . A t t h e p o r e s i z e of 30 nm, t h e t o t a l i n t r u d e d volume d e c r e a s e s w i t h age of c u r i n g and w i t h a g e s g r e a t e r t h a n t h r e e

d a y s , t h e c u r v e s a r e s h a r p l y convex t o t h e p r e s s u r e a x i s . The change i n s l o p e

R

w i t h time of t h e s e c u r v e s a t maximum p r e s s u r e (Fig. 7 ) shows t h a t f o r B 3 0 and

B f 0 , l a r g e changes o c c u r up t o 28 d a y s , w h i l e f o r C' t h e change i s s l i g h t .

This i s t h e p e r i o d i n which t h e major p o r t i o n of Ca(OH)2 i s b e i n g formed by

h y d r a t i o n r e a c t i o n s o r b e i n g consumed by t h e r e a c t i o n w i t h s i l i c a fume.

Previous work w i t h f l y a s h and s l a g ( 1 ) r e v e a l e d t h a t t h e convex c u r v e s were a r e s u l t of t h e f o r m a t i o n of a d i s c o n t i n u o u s s t r u c t u r e when Ca(0H) r e a c t e d w i t h

t h e pozzolan. It was concluded t h a t d u r i n g t h e mercury i n t r u s i o n experiment,

r u p t u r e of t h e s t r u c t u r e occurred. In t h i s c a s e w i t h s i l i c a fume, t h e

p o z z o l a n i c r e a c t i o n and t h e f o r m a t i o n of t h e d i s c o n t i n u o u s s t r u c t u r e commences only a f t e r a b o u t t h r e e days' curing.

The r e s u l t s f o r samples made a t t h e w/(c+s.f .) of 0.60 (Fig. 6 ) show a

s i m i l a r t r e n d a s f o r t h e w/(c+s.f .) of 0.45. The t o t a l volumes down t o 100 nm

p o r e diameter f o r specimens

cH,

BY

and B~ in Figs. 6a,b,c a r e approximately

3, 7 and 8 p e r c e n t , r e s p e c t i v e l y , !or the3#oday c u r e d specimens. T o t a l

p o r o s i t y f o r t h e whole p o r e s i z e r a n g e f o r

cK

(Fig. 6 a ) d e c r e a s e s generally

w i t h c u r i n g time i n a p r o g r e s s i v e manner. Specimen BH (Fig. 6b) d i s p l a y s

some merging i n t h e d i s t r i b u t i o n c u r v e s and a t a b o u t

380

nm, samples cured

f o r

14 and 28 days have t o t a l p o r o s i t i e s t h a t a r e lower t h a n samples c u r e d f o r 90

and 180 days. These e f f e c t s a r e even more a p p a r e n t f o r specimen B:~, where

(11)

Huang Cheng-yi and R.F. Feldman Vol. 1 5 , No. 2 AGE, days FIG. 7 Dependence of s l o p e of p o r e s i z e d i s t r i b u t i o n c u r v e ( a t maximum i n t r u s i o n p r e s s u r e ) on a g e and s i l i c a fume c o n t e n t .

cured f o r l o n g e r p e r i o d s ; t h e 180-day c u r e d specimen h a s a s h i g h a p o r o s i t y a s t h e one-day specimen a t 2000 nm. These e f f e c t s a r e u s u a l l y accompanied by a b r u p t jumps i n t h e d i s t r i b u t i o n curves. Both specimens B t O and B H O have a n i n c r e a s i n g s l o p e a t t h e maximum i n t r u s i o n a f t e r a b o u t t h r e e d a y s

02

c u r i n g . T h i s e f f e c t , p l o t t e d i n Fig. 7, i s s i m i l a r t o t h a t f o r B t o o r B:~.

D i s c u s s i o n

The s t r e n g t h of a composite m a t e r i a l i s n o t o n l y dependent on t h e r e l a t i v e p r o p o r t i o n s of components ( f o r example i n m o r t a r , t h e p r o p o r t i o n of cement p a s t e and s a n d ) b u t a l s o on t h e s t r e n g t h of t h e bond between t h e components. Although s i l i c a sand i s d e n s e r and s t r o n g e r t h a n cement p a s t e , Lhe s t r e n g t h of m o r t a r i s lower t h a n t h a t of p a s t e ( F i g s . 1 and 2) owing ' p a r t l y t o a weak sand-cement bond. The a d d i t i o n of s i l i c a fume t o t h e m o r t a r

a p p e a r s t o improve t h a t bond. A p r e f e r e n t i a l d e p o s i t i o n of c ~ ( o H ) , in t h e i n t e r f a c i a l zone ( l e s s t h a n 50 microns) around a g g r e g a t e s i n concr&e and f i b r e s i n p a s t e h a s been observed (4-6). It h a s a l s o been observed i n t h i s work f o r t h e m o r t a r s , a s shown i n Fig. 8. The s t r e n g t h of m o r t a r w i t h o u t s i l i c a fume d e c r e a s e s s i g n i f i c a n t l y i n comparison t o t h e p a s t e a f t e r 14 d a y s , when l a r g e q u a n t i t i e s of Ca(OHl2 have been formed ( F i g s . 2 and 3). H A t t h i s

R H R

p e r i o d l a r g e q u a n t i t i e s of Ca(0H) i n specimens B 3 0 , B O, B and B h a v e r e a c t e d w i t h s i l i c a fume. A f t e r $8 d a y s t h e s t r e n g t h development c u r v e s of B : ~ and p a s t e w i t h 30 p e r c e n t s i l i c a fume d i v e r g e , B ! ~ becoming s t r o n g e r . T h i s i s p r o b a b l y due t o a b e t t e r bond b e i n g formed between t h e sand g r a i n s by t h e new CSH formed from t h e r e a c t i o n of Ca(0H) w i t h s i l i c a fume. Although n o Ca(OH)2 a p p e a r s a f t e r t h r e e d a y s i n B : ~ (Fig.

5,

t h e Ca(OH) t h a t i s

c o n t i n u o u s l y b e i n g formed i s completely r e a c t e d w i t h s i l i c a fume t o g i v e b e t t e r bonding c h a r a c t e r i s t i c s .

The development of p o r e s t r u c t u r e l e a d s t o s i m i l a r c o n c l u s i o n s ( F i g s . 5 and 6 ) . The p o r e s t r u c t u r e developed between 3 and 14 d a y s shows lower v a l u e s of p o r e volume t h a n t h o s e c u r e d f o r l o n g e r p e r i o d s , f o r s i z e r a n g e

100-1000 nm. T h i s may be t h e r e s u l t of t h e l i m e c o n c e n t r a t e d around t h e sand g r a i n forming a n i n i t i a l s t r u c t u r e ; a s u b s e q u e n t s t r u c t u r e r e s u l t i n g from new

(12)

Vol. 15, No. 2 29 3 SILICA FUME, CEMENT, MORTARS, STRENGTH, PORE STRUCTURE

FIG. 8

Scanning e l e c t r o n micrograph showing l a r g e Ca(OH)2 f o r m a t i o n n e a r sand p a r t i c l e i n mortar (w/ (c+s .f .) = 0.60 a f t e r 7 days' c u r i n g ) .

CSH formation by r e a c t i o n between t h e s i l i c a fume and t h i s l i m e may dominate t h e p o r e s t r u c t u r e i n t h e 100-1000 nm range. The a b r u p t i n c r e a s e s i n i n t r u d e d volume s u g g e s t t h a t i n t h i s r e g i o n of mercury p r e s s u r e , r u p t u r e of t h e pore s t r u c t u r e may occur. Such a r u p t u r e t a k e s p l a c e a t h i g h e r p r e s s u r e s f o r hydrated f l y a s h and s l a g b l e n d s , and t h i s probably o c c u r s w i t h t h e s i l i c a fume b l e n d s a t b o t h i n t e r m e d i a t e and h i g h p r e s s u r e s ( 1 , 7 ) . These a b r u p t i n c r e a s e s i n i n t r u d e d volume a t i n t e r m e d i a t e p r e s s u r e s have n o t been observed 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 fumes ( 2 ) , i n c o n t r a s t t o t h e

mortars.

The p r o g r e s s i v e manner i n which t h e p o r e d i s t r i b u t i o n changes w i t h time f o r specimens C' and

cH

( F i g s . 5a and 6a) s u p p o r t s t h e i d e a t h a t t h e l i m e t h a t i s formed p r e f e r e n t i a l l y around t h e sand g r a i n i s converted by r e a c t i o n w i t h nearby s i l i c a fume t o CSH w i t h a r e l a t i v e l y d i s c o n t i n u o u s p o r e s t r u c t u r e .

I _Conclusions

1. S i l i c a fume r e a c t s w i t h most Ca(OH)2 produced i n h y d r a t e d c e m e n t - s i l i c a fume m o r t a r s w i t h i n 28 days.

2. The a d d i t i o n of s i l i c a fume t o m o r t a r s r e s u l t s i n a n improved bond between t h e hydrated cement m a t r i x and t h e sand.

3. Ca(OH)2 seems t o form p r e f e r e n t i a l l y around t h e s a n d g r a i n i n mortars.

4. Cement m o r t a r s have h i g h e r p o r e volumes i n t h e 100-4000 nm p o r e d i a m e t e r range t h a n e q u i v a l e n t cement p a s t e s cured under s i m i l a r c o n d i t i o n s .

5. S i l i c a fume a f f e c t s t h e p o r e d i s t r i b u t i o n of m o r t a r s by r e a c t i n g w i t h t h e Ca(OHI2 formed around t h e sand g r a i n s a s w e l l a s w i t h t h e Ca(0H) d i s p e r s e d throughout t h e cement p a s t e .

(13)

Vol. 1 5 , No. 2 Huang Cheng-yi and R.F. Feldman

Acknowledgement

r

The a u t h o r s w i s h t o acknowledge t h e work of Gordon Chan and Ed Quinn i n h e l p i n g t o c a r r y o u t t h e experiments and t a k e measurements. T h i s paper i s a c o n t r i b u t i o n of 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 .

References

( 1 ) R.F. Feldman, Proc., 1 s t I n t e r . Conf. o n Use of Fly-Ash, S i l i c a Fume, Slag and Other Mineral By-products i n Concrete, A C I SP.79

1,

p. 415 (1983).

( 2 ) R.F. Feldman and Huang Cheng-yi. RILEM Seminar on t h e D u r a b i l i t y o f Concrete S t r u c t u r e s under Normal Outdoor Exposure. 26-29 March 1984. Hanover, F e d e r a l R e p u b l i c of Germany.

( 3 ) M. Regourd and B. Mortureux, Proc., 4 t h I n t e r . Conf. on Cement Microscopy, Las Vegas, p. 249 (1982).

( 4 ) B.D. Barnes, S. Diamond and W.L. Dolch, J. Amer. Ceram. Soc. =(I-21, p. 21 (1979).

( 5 ) A. C a r l e s G i b e r q u e s , J. Grandet and J.P. O l l i v i e r , Proc. I n t e r . Conf. on Bond i n Concrete. P a i s l e y College of Technology, S c o t l a n d , p. 24, London (Appl. Sci.) (1 982).

( 6 ) R. B e r g e r , D.S. Cahn and J . D . McGregor, J. Amer. Ceram. Soc. p. 57 (1970).

(14)

T h i s p a p e r , w h i l e being 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 . I t 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 of C a n a d a , O t t a w a , O n t a r i o , K I A 0R6. Ce document e s t d i s t r i b u B s o u s forme de t i r B - % p a r t par l a D i v i s i o n d e s r e c h e r c h e s e n b a t i m e n t . Les d r o i t s d e r e p r o d u c t i o n s o n t t o u t e f o i s l a p r o p r i B t 6 d e 1 ' B d i t e u r o r i g i n a l . Ce document n e p e u t S t r e r e p r o d u i t en t o t a l i t 6 ou en p a r t i e s a n s l e consentement de l ' l d i t e u r . Une l i s t e d e s p u b l i c a t i o n s d e l a D i v i s i o n p e u t O t r e o b t e n u e en B c r i v a n t

a

l a S e c t i o n d e s p u b l i c a t i o n s . D i v i s i o n d e s r e c h e r c h e s e n b a t i m e n t , C o n s e i l n a t i o n a l d e r e c h e r c h e s Canada, Ottawa, O n t a r i o , KIA 0R6.