Hydration reactions in Portland cement-silica fume blends

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Hydration reactions in Portland cement-silica fume blends

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Hydration Reactions in

Portland Cemen CSilica

Fume Blends

by Huang Cheng-yi and R.F. Feldman

Reprinted from

Cement and Concrete Research

Volume 15, No. 4, 1985

p. 585

-

592 (DBR Paper No. 1305)

Price $1.00

NRCC 24755

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La calorimetric par conduction et l'estimation de la teneur en Ca(OH)2 ont servi

a

suivre

a

leur debut le cours des reactions d'hydratation de melanges de ciment Portland et de poudre de silice; par la suite, le contr8le s'est fait par determination

de la teneur en et de la teneur en eau non haporable.

Au cours des premieres heures, la poudre de silice a acc€l€r€ l'hydratation du C3S et du C3A. Sur des periodes prolongbes, les pates de m6langes s'hydratent davantage que les mortiers, peut-Stre parce que le sable pdsent capte le Ca(OH)2 produit.

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CEMENT and CONCRETE RESEARCH. Vol. 1 5 , pp. 585-592, 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 .

HYDRATION REACTIONS I N PORTLAND CEMENT-SILICA FUME BLENDS

Huang Cheng-yi and R.F. Feldman D i v i s i o n of B u i l d i n g Research

B u i l d i n g M a t e r i a l s S e c t i o n N a t i o n a l R e s e a r c h C o u n c i l Canada

Ottawa, K1A 0R6, Canada.

(Communicated by F.H. Wittmann) (Received Oct. 4, 1984)

ABSTRACT

E a r l y h y d r a t i o n r e a c t i o n s of p o r t l a n d c e m e n t - s i l i c a fume b l e n d s were followed by conduction c a l o r i m e t r y , Ca(OH)2 e s t i m a t i o n , and l a t e r r e a c t i o n s by Ca(OH)2 and non-evaporable w a t e r c o n t e n t s . S i l i c a fume a c c e l e r a t e d b o t h C3S and C3A h y d r a t i o n i n t h e f i r s t few h o u r s . A t l o n g e r p e r i o d s p a s t e s of b l e n d s h y d r a t e d t o a g r e a t e r e x t e n t t h a n m o r t a r s , p o s s i b l y because sand a c t e d a s a Ca(OHl2 s i n k i n t h e m o r t a r s .

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

When s i l i c e o u s by-products s u c h a s condensed s i l i c a fumes, f l y a s h and b l a s t f u r n a c e s l a g a r e mixed w i t h p o r t l a n d cement and h y d r a t e d , t h e y 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 cement p a s t e (1-4).

M i c r o s t r u c t u r a l s t u d i e s show t h a t 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 a r t of t h e change i n p o r e d i s t r i b u t i o n r e s u l t s from s i l i c a fume r e a c t i n g w i t h

t h e Ca(OH)2 formed around t h e s a n d g r a i n s . S i l i c a fume i n t h e m o r t a r a l s o improves t h e bond between t h e h y d r a t e d cement m a t r i x and t h e s a n d , and enhances freeze-thaw r e s i s t a n c e ( 5 ) .

These i n v e s t i g a t i o n s r e v e a l e d t h e importance of t h e sand-matrix

i n t e r f a c e a f f e c t i n g t h e p r o p e r t i e s of c o n c r e t e s and m o r t a r s . An u n d e r s t a n d i n g of t h e r o l e of t h e sand-cement i n t e r f a c e r e q u i r e s a s t u d y of t h e h y d r a t i o n o f silica-fume-cement blend. The o b j e c t of t h i s p a p e r i s t o f o l l o w t h e h y d r a t i o n c h a r a c t e r i s t i c s of cement c o n t a i n i n g 0-30% s i l i c a fume and formed a t a

w a t e r l s o l i d r a t i o of 0.25 o r 0.45, and compare some of t h e s e r e s u l t s w i t h m o r t a r s .

Experimental M a t e r i a l s

Normal Type I p o r t l a n d cement and s i l i c a fume from e a s t e r n Canada were used. T h e i r chemical composition and some p h y s i c a l p r o p e r t i e s a r e g i v e n i n

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586 Vol. 1 5 , No. 4

Huang Cheng-yi and R.F. Feldman

Table I. Ottawa s i l i c a sand ground t o p a s s 100 mesh s i e v e and a f l y a s h were a l s o used i n some of t h e e x p e r i m e n t s a s r e p l a c e m e n t s f o r cement. P a s t e s were p r e p a r e d a t w/(c+sf) ( w a t e r / c e m e n t + s i l i c a fume) r a t i o s of 0.25 a n d 0.45, c o n t a i n i n g 0 , 10 and 30% s i l i c a fume a t e a c h water-to-binder r a t i o , w i t h Melment i n amounts of 0 , 0.3 and 2.0% by w e i g h t of b i n d e r . Heat e v o l u t i o n f o r p e r i o d s l e s s t h a n t h r e e days was measured on b l e n d s a t a w / ( c + s f ) of 0.60, u s i n g 0, 10, 20 and 30% s i l i c a fume.

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 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 then added and mixed a t a low speed f o r 2 min. and a t a medium s p e e d f o r a n o t h e r 2 min.

P r o p e r t i e s determined Ca(OH), c o n t e n t

-

A D i f f e r e n t i a l Scanning C a l o r i m e t e r (DSC c e l l ) , a module of t h e Du P o n t 1090 Thermal A n a l y s i s System, was u s e d t o o b t a i n thermograms from which t h e r e l a t i v e a r e a s of t h e peaks r e s u l t i n g from Ca(OHI2 decomposition were

determined. In e a c h experiment a 20-mg sample was h e a t e d i n N 2 atmosphere a t

a r a t e of 20°C/min. Measurements were made a t a w / ( c + s f ) of 0.25 and 0.45 f o r

h y d r a t i o n t i m e s of 1 , 3, 7, 14, 28, 90 and 180 d a y s , and a t a w / ( c + s f ) of 0.6 f o r h y d r a t i o n t i m e s of 1.5, 4, 6, 8 , 10, 12, 16, 24 and 72 h.

TABLE I

Chemical Composition and some P h y s i c a l P r o p e r t i e s of Cement, S i l i c a Fume and F l y Ash

P o r t l a n d Cement S i l i c a Fume F l y Ash

% % % Si02 21.16 95.17 55.6 A 2 20 5.87 0.21 22.7 Fe O3 2.21 0.13 4.3 C a8 63.07 0.23 13.3 MgO 1.56 0.15 2.4 I .L. 1.36 2.30 0.6 3 3.29 0.12 0.16 Na2 0 0.09 0.10 K2° 1.08 0.27 C 1.56 C3S 43.96 C2 27.47 C3A 11.82 C4AF 6.72 S p e c i f i c s u r f a c e a r e a , B l a i n e (m2 /kg) 332 S p e c i f i c s u r f a c e a r e a , N 2 a d s o r p t i o n (m2Ig) 2 1 P o z z o l a n i c a c t i v i t y i n d e x w i t h p o r t l a n d cement 110% w i t h lime 5.8 MPa

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Vol. 1 5 , No. 4 587

CEMENT, S i 0 2 FUME, BLENDS, HYDRATION, CALORIMETRY

Non-evaporable w a t e r con t e n t

T h i s was measured a t h y d r a t i o n t i m e s of 1, 3, 7, 14, 28, 90 and 180 days by d e t e r m i n i n g t h e l o s s i n weight between 100 and 1000°C u s i n g a Thermogravimetric Module of t h e Du Pont 1090 system.

Rate of h e a t e v o l u t i o n

I

T h i s was d e t e r m i n e d by a conduction c a l o r i m e t e r ( s u p p l i e d by T e c h n i c a l

1

P h y s i c s Department, TNO and TH, D e l f t , Holland) of s e n s i t i v i t y 16.5 mV/W.

i _{Measurements were made a t a w/(c+sf) of 0.60 u s i n g 0 , 1 0 , 20 and 30% s i l i c a}

'

fume c o n t e n t p a s t e s , a 30% ground s i l i c a sand p a s t e , and a 30% f l y a s h p a s t e .

I

E x p e r i m e n t s w e r e c o n t i n u e d f o r 7 2 h . R e s u l t s Ca(OH), c o n t e n t

The amount of Ca(OH)2 l i b e r a t e d d u r i n g t h e e a r l y s t a g e s of t h e cement p a s t e h y d r a t i o n (0-72 h ) was determined a t a w / s of 0.60 f o r b i n d e r s

c o n t a i n i n g 0 and 30% s i l i c a fume o r 30% ground s i l i c a sand. These r e s u l t s a r e p r e s e n t e d i n Fig. 1 a s t h e p e r c e n t of Ca(OH)2 p e r u n i t w e i g h t o f i g n i t e d sample. The t h r e e c u r v e s a r e v e r y s i m i l a r up t o 8 h , b u t a f t e r t h i s p e r i o d t h e Ca(OH)2 c o n t e n t i s g r e a t e s t f o r t h e cement p a s t e a l o n e , f o l l o w e d by t h e sand blend and t h e s i l i c a fume blend. This i n d i c a t e s t h a t b o t h ground sand and s i l i c a fume a c c e l e r a t e t h e p r o d u c t i o n of Ca(0H) from t h e cement i n t h e f i r s t e i g h t hours. Although t h e s e b l e n d s a r e o n l y f 0 % cement, t h e Ca(OH)2 I _{c o n t e n t s a r e a s h i g h a s f o r p u r e cement.} _{The h e a t e v o l u t i o n d a t a c o n f i r m}

t h e s e r e s u l t s . Beyond e i g h t h o u r s , Ca(OH)2 i n b o t h sand and s i l i c a fume b l e n d s d e c r e a s e s g r e a t l y r e l a t i v e t o t h e cement. However, based on cement c o n t e n t , t h e sand blend and t h e cement have t h e same Ca(OH)2 c o n t e n t , w h i l e

i w i t h t h e s i l i c a fume i t i s lower. I I I I I I I

-

1 4

-

I +- I 0 10 -

I

3 I 2 8 - -

i

Z

_---

---O FIG. 1

-

6 -

-

Change i n Ca(0H) c o n t e n t a t e a r l y i I

0 t i m e s f o r cement and cement b l e n d s .

"

0-0 CEMENT

0 - - - 0 CEMENT + 30% Si02 FUME

0-- CEMENT + 30% Si02

0

.'

D 10 20 30 4 0 5 0 60 70 I I M E . h

The Ca(0H) c o n t e n t f o r t h e 0 , 10 and 30% s i l i c a fume b l e n d s mixed a t a w / ( c + s f ) of 0.2% and 0.45 and h y d r a t e d up t o 180 d a y s a r e p r e s e n t e d i n Fig. 2, a l o n g w i t h t h e nomenclature f o r t h e s e mixes. A t a w / ( c + s f ) of 0.45, w i t h 0%

H

s i l i c a fume ( S ), t h e r e i s l i t t l e i n c r e a s e i n Ca(OH)2 beyond 14 days; i t

a t t a i n s a v a l u e of a b o u t 16.2% a f t e r 180 days h y d r a t i o n . The e q u i v a l e n t v a l u e I

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Huang Cheng-yi and R.F. Feldman I i I I 1 S I L I C A FUME. W l l C 4 Sf) S Og 0.45. 0 o 5 / m 5 . H - 10 a?,$ * v y 0

-

30

i

TIME, d a y s FIG. 2 Change i n Ca(OH)2 c o n t e n t t o 180 d a y s f o r v a r i o u s s i l i c a fume-cemen t m i x t u r e s f o r m o r t a r , p r e s e n t e d p r e v i o u s l y ( 5 ) , i s 17.3%. T h i s v a l u e was o b t a i n e d by d e t e r m i n i n g t h e Ca(OH)2 c o n t e n t t h e r m o g r a v i m e t r i c a l l y , u s i n g a 1000 mg specimen f o r g r e a t e r a c c u r a c y ; t h e c a l c u l a t i o n was based on b i n d e r c o n t e n t , o m i t t i n g t h e l a r g e j u a n t i t y of s a n d i n t h e mortar. The p a s t e p r e p a r e d a t a w/(c+sf) of 0.25 (S ) shows s i m i l a r r e s u l t s i n t h a t l i t t l e change o c c u r r e d a f t e r 14 d a y s , y i e l d i n g a v a l u e of 10.4% a f t e r 180 days.

H

The p a s t e P10 a t t a i n s a Ca(OH)2 c o n t e n t of 11.2% a f t e r a b o u t 1 0 d a y s and t h e n d e c r e a s e s t o 9.7 a t 28 d a y s and 8.1 and 8.3% a t 9 0 and 180 days. Thus c o n s i d e r a b l e change s t i l l o c c u r s between 28 and 90 days. The e q u i v a l e n t m o r t a r p r e p a r e d a t t h e same w / ( c + s f ) r a t i o of 0.45 and 10% s i l i c a fume a t t a i n s a maximum of 9.5% Ca(OH)2 a t about 7 d a y s , d e c l i n i n g g r a d u a l l y t o 4.6% a f t e r

180 d a y s , w i t h most of t h e d e c r e a s e t a k i n g p l a c e between 14 and 28 days. P a s t e P!,

_{- "}

d e c l i n e s from a maxirmm of 7.2% a t one day t o 2.2% a t 180 days.

H

a

H

P a s t e s P30 and P30 b o t h have Ca(OHl2 l e v e l s of z e r o a t 14 days; f o r P a t t h r e e and seven d a y s , however, t h e l e v e l s a r e 2.0 and 0.4% as compared t o 1.5 and 0% a t one and t h r e e d a y s f o r t h e e q u i v a l e n t mortar.

Non-evaporable w a t e r c o n t e n t

The change p e r u n i t w e i g h t of i g n i t e d sample o v e r t i m e i n

non-evaporable w a t e r c o n t e n t i s shown i n Fig. 3. The specimens a t a w / ( c + s f ) of 0.45, made a t a h i g h e r w a t e r c o n t e n t , n a t u r a l l y show t h e l a r g e s t g a i n ;

sH

a t t a i n s 19.2% a f t e r 28 days and 20.7% a f t e r 180 days. Specimen p y 0 a t t a i n s 15.1% a f t e r 28 days b u t o n l y 15.5 a f t e r 180 days, w h i l e pH h a s t h e

non-evaporable w a t e r c o n t e n t of 14.4% a f t e r 28 days and 1?!)3% a f t e r 180 days.

The non-evaporable w a t e r c o n t e n t l e v e l a t t a i n e d by pY0 i s l o w compared t o t h a t o f

sH,

even i f i t i s c a l c u l a t e d on t h e b a s i s of i g n i t e d w e i g h t of cement, assuming t h a t t h e s i l i c a fume e i t h e r does n o t r e a c t o r does n o t c o n t r i b u t e t o

H

t h e non-evaporable w a t e r ; on t h i s b a s i s , however, t h e v a l u e f o r P 3 0 i s c l o s e t o t h a t of

sH,

b u t f o r b o t h pHO and p t 0 ( e s p e c i a l l y

pya)

t h e y a r e a l m o s t c o n s t a n t a f t e r 28 days. ~ e s u i t s f o r comparable m o r t a r s , determined s i m i l a r l y t o t h o s e of Ca(OH)2 and based on b i n d e r c o n t e n t ( 5 ) , show s i m i l a r t r e n d s ; t h e 30% s i l i c a fume c o n t e n t specimen changed from 11.5% a t 14 days t o 11,9% a t 180 days; o v e r t h e same p e r i o d , measurements f o r t h e 10% s i l i c a fume m o r t a r

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Vol. 1 5 , No. 4

CEMENT, S i 0 2 F W , BLENDS, HYDRATION, CALORIMETRY

TIME, davs FIG. 3

Change i n non-evaporable w a t e r c o n t e n t w i t h time f o r v a r i o u s s i l i c a f u m e c e m e n t m i x t u r e s .

d e c l i n e d from 13.5% a t 28 days t o 11.9% a t 180 days, S i m i l a r r e s u l t s a r e o b t a i n e d f o r m o r t a r s made a t w / ( c + s f ) of 0.60. P a s t e s made a t a w/(c+sf) of 0.6 a l s o have shown much l e s s change i n non-evaporable w a t e r c o n t e n t between 10 and 90 days w i t h s i l i c a fume c o n t e n t s from 8 t o 20% ( 6 ) . O t h e r s have found s i m i l a r e f f e c t s ( 7 ) . With v a r i o u s f i b r e s i n c l u d e d

i n

p d e t e s , t h e d e g r e e of h y d r a t i o n i s 25% l o w e r t h a n t h e r e f e r e n c e p a d t e (8).

The specimens a t w/(c+sf) of 0.25 show s i m i l a z ' t r e n d s t o t h o s e o b s e r v e d f o r t h e h i g h e r r a t i o , a l t h o u g h changes between 28 and 180 days a r e g r e a t e r . At one d a y , non-eva o r a b l e w a t e r c o n t e n t f o r

_P

S' and p t O a r e s i m i l a r , w h i l e i t i s much lower f o r P3O9 presumably due t o t h e l a r g e r d o s e of Melment and low

H

w a t e r c o n t e n t ( 9 ) .

sH,

py0 and P30 a r e , however, a l l v e r y s i m i l a r a f t e r one day.

Rate of h e a t e v o l u t i o n

The e f f e c t of s i l i c a fume c o n t e n t on t h e r a t e of h e a t e v o l u t i o n of cement a t a w/ ( c + s f ) of 0.60 i s shown i n Fig. 4. There a r e t h r e e p e a k s e v i d e n t , e s p e c i a l l y w i t h 1 0 , 20 and 30% s i l i c a fume mixes. The r e s d l t s a r e p l o t t e d on t h e b a s i s of t h e cement c o n t e n t f o r comparison; t H e r e i @ c o n s i d e r a b l e i n c r e a s e i n t h e r a t e of h e a t e v o l u t i o n and t h e o c c u r r e n c e of t h e p e a k s i s a c c e l e r a t e d , t h e g r e a t e r t h e s i l i c a flrme c o n t e n t . However, t h e r e i s l i t t l e e f f e c t on t h e dormant p e r i o d which o c c u r s a t a b o u t 1175 h , an o b s e r v a t i o n r e p o r t e d p r e v i o u s l y (10). The t o t a l h e a t e v o l v e d a t v a r i o u s t i m e s from t h e s t a r t of t h e h y d r a t i o n r e a c t i o n i s p l o t t e d i n F i g s . 5 and 6; t h e dependence of h e a t e v o l u t i o n on s i l i c a fume c o n t e n t i s c l e a r l y shown. I f t h e s e r e s u l t s a r e p l o t t e d a g a i n s t

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Huang Cheng-yi and R.F. Feldman Vol. 1 5 , No. 4 A - 0 % S I L I C A F U M E B - 1 0 % S I L I C A F U M E C - 20% S I L I C A F U M E D - 3 0 % S I L I C A F U M E 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 15 16 17 1 8 19 2 0 2 1 2 2 2 3 24 T I M E , h FIG. 4 Change i n r a t e of h e a t e v o l u t i o n of c e m e n t - s i l i c a fume p a s t e s w i t h t i m e . T I M E , h FIG. 5 T o t a l h e a t e v o l v e d by c e m e n t - s i l i c a fume b l e n d s d u r i n g h y d r a t i o n w i t h t ime

.

I I I I . / - F 80 - /.' ,/ / ' / 70

-

- 3% GROUND SAND - 39% FLY A S H IEDM.1 0 1 0 20 3 0 4 0 5 0 T I M E . h FIG. 6 T o t a l h e a t e v o l v e d by v a r i o u s cement b l e n d s d u r i n g h y d r a t i o n w i t h t ime

.

t h e t o t a l weight of t h e b l e n d , an i n c r e a s e i n t o t a l h e a t e v o l v e d o v e r t h e cement w i t h o u t s i l i c a fume a d d i t i o n would s t i l l b e r e c o r d e d u p t o 20 h of h y d r a t i o n f o r a l l t h r e e a d d i t i o n s . T h i s r e s u l t i s c o n t r a r y t o e a r l i e r o n e s

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Vol. 15, No. 4 591

CEMENT, S i 0 2 FUME, BLENDS, HYDRATION, CALORIMETRY

(10) which were o b t a i n e d a t a w/(c+sf) of 0.5 and 20°C, i n c o n t r a s t t o t h e

p r e s e n t work which was performed a t a w/(c+sf) of 0.60 and 25OC. F u r t h e r h e a t

e v o l u t i o n measurements p r e s e n t e d i n Fig. 6 compare v a r i o u s a d d i t i o n s r e p l a c i n g

30% cement. The ground s i l i c a sand h a s o n l y a v e r y s l i g h t e f f e c t , b u t t h e f l y

a s h h a s a c o n s i d e r a b l y g r e a t e r e f f e c t . Whether t h i s i s due t o a c c e l e r a t i o n of

t h e cement h y d r a t i o n o r e a r l y p o z z o l a n i c r e a c t i o n i s n o t known. However,

d u r i n g t h e f i r s t 12 h , t h e s i l i c a fume, even a t 10% a d d i t i o n , c a u s e s t h e l a r g e s t a c c e l e r a t i o n of t h e cement h y d r a t i o n r e a c t i o n .

Discussion

1 . The e f f e c t of an amorphous s i l i c a ( A e r o s i l , . w i t h a s u r f a c e a r e a of

200 m2Ig) on t h e h y d r a t i o n of C S and cement h a s been r e p o r t e d p r e v i o u s l y

(10). It was found t h a t t h e Ca4+ and OH- i o n c o n c e n t r a t i o n i s g r e a t l y

reduced w i t h i n t h e f i r s t 10 minutes. The h y d r a t i o n of C3S i s a c c e l e r a t e d

because a r e t a r d i n g h y d r a t e l a y e r t h a t forms on t h e s u r f a c e of t h e C3S

g r a i n a t e a r l y t i m e s i s c o n v e r t e d more r a p i d l y i n t h e l o w e r ~ a + + and OH-

s o l u t i o n t o a second, l e s s r e t a r d i n g l a y e r . I n a d d i t i o n , i t was s u g g e s t e d

t h a t i n cement t h e r e t a r d i n g c o a t i n g of e t t r i n g i t e on C3A was n o t formed

a s e f f i c i e n t l y , a l s o because of t h e lower ~ a + + c o n c e n t r a t i o n i n t h e e a r l y

p e r i o d s . It a p p e a r s t h a t l a r g e amounts of s i l i c a fume, a l t h o u g h n o t

p o s s e s s i n g a s l a r g e a s u r f a c e a r e a a s A e r o s i l , impose s i m i l a r e f f e c t s on

C3S i n t h e cement h y d r a t i o n . Before e i g h t h o u r s h y d r a t i o n , l a r g e r

q u a n t i t i e s of Ca(0H) r e l a t i v e t o t h e cement a l o n e were p r e s e n t , p r o b a b l y

due t o t h e n u c l e a t i n g i n f l u e n c e of t h e s i l i c a fume s u r f a c e . A f t e r e i g h t

h o u r s , t h e Ca(OH)2 c o n t e n t i s lower t h a n would b e produced by normal

cement h y d r a t i o n , and i t i s p r o b a b l e t h a t a t t h i s p o i n t ~ a + + i o n s have

r e a c t e d w i t h t h e s i l i c a fume. Heat e v o l u t i o n r e s u l t s would i n d i c a t e t h i s ,

s i n c e ground s i l i c a s a n d , which a l s o a p p e a r s t o a c c e l e r a t e t h e cement

h y d r a t i o n (Ca(0H) c o n t e n t r e s u l t s ) g i v e s a r e l a t i v e l y low h e a t e v o l u t i o n

curve. S i l i c a fume, from i t s e f f e c t on peak 3 of Fig. 4 , a l s o a f f e c t s t h e

h y d r a t i o n r a t e of C3A.

2. The r a t e s of h y d r a t i o n i n m o r t a r s and p a s t e s c o n t a i n i n g s i l i c a fumes a r e

somewhat d i f f e r e n t . Hydration p r o c e e d s f a s t e r i n p a s t e s w i t h s i l i c a fume,

r e g a r d i n g b o t h Ca(OH)2 and non-evaporable w a t e r c o n t e n t s ; t h e h y d r a t i o n r e a c t i o n s 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 t e r m i n a t e e a r l i e r w i t h r e g a r d

t o t h e above two p a r a m e t e r s . I n an e a r l i e r p a p e r (51, i t was shown t h a t

t h e sand-paste i n t e r f a c e i n m o r t a r s a f f e c t e d p o r e s t r u c t u r e and s t r e n g t h .

The i n t e r f a c e around v a r i o u s t y p e s of i n c l u s i o n s i s n o r m a l l y l i m e r i c h ,

and i n t h e p r e s e n c e of s i l i c a fume, i t i s i n f l u e n c e d d u r i n g t h e r e a c t i o n .

Reduction of Ca(OH)2 c o n t e n t d u r i n g t h e r e a c t i o n a l s o g r e a t l y r e d u c e s t h e p e r m e a b i l i t y of t h e s e b o d i e s , even w i t h r e g a r d t o t h e m o b i l i t y of i o n s

s u c h a s CI1- and O F ( 1 ) . Large q u a n t i t i e s of sand i n m o r t a r s a p p e a r t o

a c t a s a s i n k f o r c r y s t a l l i z e d Ca(OH)2. I f , a t e a r l y p e r i o d s of h y d r a t i o n

( w i t h i n 14 d a y s ) , sand p a r t i c l e s l i m i t t h e amount of Ca(OH)2 t h a t i s

d e p o s i t e d i n t h e p a s t e m a t r i x , t h e p a s t e i t s e l f may become l e s s permeable and a c c e s s t o cement g r a i n s by H20 may be reduced; t h i s may r e s u l t i n l i m i t e d h y d r a t i o n r e l a t i v e t o mixes w i t h o u t sand.

Conclusions

1. S i l i c a fume a c c e l e r a t e s h y d r a t i o n r e a c t i o n s i n p o r t l a n d cement by

p r o v i d i n g n u c l e a t i o n s i t e s f o r Ca(OH)2 w i t h i n minutes a f t e r r e a c t i o n

commences, and a l s o by r e a c t i n g w i t h ~ a + + i o n s . Reduced ~ a + + i o n a f f e c t s

t h e n a t u r e of h y d r a t i o n p r o d u c t s .

2. The d e g r e e of h y d r a t i o n of p a s t e s and m o r t a r s of s i l i c a fume-cement b l e n d s

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Vol. 1 5 , No. 6 Huang Cheng-yi and R.F. Feldman

a s i n k f o r w e l l c r y s t a l l i z e d Ca(OH)2 and l o w e r s t h e p e r m e a b i l i t y o f t h e p a s t e phase of t h e mortar.

Acknowledgement

The a u t h o r s w i s h t o r e c o g n i z e t h e work of G.W. Chan and G.M. Polomark, who h e l p e d perform t h e experiments. T h i s p a p e r 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 Canada, and i s s u b m i t t 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 .

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T h i s paper. 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 . 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 of C a n a d a , O t t a w a , O n t a r i o , K1A OR6. Ce document e s t d i s t r i b u e s o u s forme d e t i r e - a - p a r t p a r 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 blltiment. Les d t 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 d t 6 d e l ' e d i t e u r o r i g i n a l . Ce document n e p e u t O t r e r e p r o d u i t en t o t a l i t s ou en p a r t i e s a n s l e consentement d e l ' e 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 G t r e o b t e n u e en e 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 l 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 , K1A OR6.