Carbonation of granulated blast furnace slag cement concrete during twenty years of field exposure

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Carbonation of granulated blast furnace slag cement concrete during

twenty years of field exposure

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no.

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Institute for

lnstitut de

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2 Research in

recherche en

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Construction

construction

Carbonation of Granulated

Blast Furnace Slag Cement

Concrete during Twenty

Years

of Field Eicposum

I

by G.G. Litvan and A. Meyer

_{A N , ~ L \ [ Z E D}

Reprinted from

"Fly Ash, Silica Fume, Slag, and Natural Pouolans

in Concrete"

Proceedings Second International Conference

Madrid, Spain, 1986

ACI, SP-91, Vol. 2, p. 1445-1462

(IRC Paper No. 1398)

Price $2.00

NRCC 26203

1

N!?C

-

CISTI

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g r a n u l e (CLG), o n t t c o n s t r u i t e s dans des c o n d i t i o n s soigneusement c o n t r a l 6 e s e t documentks. AprBs 20 ans d ' e x p o s i t i o n , d e s c a r o t t e s o n t 8 t B a n a l y s e e s e t une c a r b o n a t a t i o n importante s u r 40 ma de profondeur a 6 t 6 d6cel6e p a r a n a l y s e thermogravimgtrique e t par a n a l y s e chimique. F a i t p l u s important, on n ' a guBre trouvd de Ca(OH)* dans l e beton de CLG,

B

quelque niveau que c e f a t , de s o r t e que t o u t e armature d ' a c i e r d e v r a i t 8 t r e c o n s i d Q C conme a u j e t t e

B

_corrosion.

D'aprBs l e s r d s u l t a t e obtenus grace au porosim8tre

B

_mercure,

l a p o r o s i t e du bdton de CPO a diminu6 a p r e s

l a

c a r b o n a t a t i o n mais c e l l e du beton de CLG n'a pas changd.

De

p l u s , l e grossissement des pores a e n t r a f n 6 un accroissement de l a p e r k b i l i t g du beton de CLG, a p r s s c a r b o n a t a t i o n , e t l a r e s i s t a n c e

B

l a t r a c t i o n de l a zone de s u r f a c e - ~ a diminus

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Blast Furnace Slag Cement Concrete

During Twenty Years of Field Exposure

by

G .

_{Litvan and A. Meyer}

S y n o p s i s : Two e x p e r i m e n t a l h o u s e s , one of o r d i n a r y p o r t l a n d cement (OPC) c o n c r e t e and t h e o t h e r o f g r a n u l a t e d b l a s t f u r n a c e s l a g cement (GBFSC) c o n c r e t e , w e r e b u i l t u n d e r c a r e f u l l y c o n t r o l l e d and documented c o n d i t i o n s . A f t e r 20 y e a r s of e x p o s u r e , c o r e s were a n a l y s e d and s i g n i f i c a n t c a r b o n a t i o n t o 40 mm i n d e p t h was d e t e c t e d by TGA and t h e w e t c h e m i c a l method. More s i g n i f i c a n t l y , l i t t l e Ca(OH)2 was f o u n d i n t h e GBFSC c o n c r e t e a t a l l l e v e l s , s o t h a t a n y r e i n f o r c i n g s t e e l would h a v e t o be c o n s i d e r e d s u s c e p t i b l e t o c o r r o s i o n . According t o Hg p o r o s i m e t r y r e s u l t s , t h e p o r o s i t y o f OPC c o n c r e t e d e c r e a s e d a f t e r c a r b o n a t i o n b u t t h a t of GBFSC remained unchanged. I n a d d i t i o n , i n c r e a s e d p e r m e a b i l i t y o f GBFSC c o n c r e t e w i t h c a r b o n a t i o n was i n d i c a t e d by c o a r s e n i n g of t h e p o r e s , and t h e t e n s i l e s t r e n g t h of t h e s u r f a c e r e g i o n s u f f e r e d a l a r g e d e c r e a s e . Keywords: b l a s t f u r n a c e s l a g ; c a r b o n a t i o n ; c h e m i c a l a n a l y s i s ; e x p o s u r e ; f i e l d t e s t s ; p o r t l a n d c e m e n t s ; p o r t l a n d s l a g cements; r e i n f o r c e d c o n c r e t e ; t e n s i l e s t r e n g t h .

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G.G. L i t v a n i s a s e n i o r r e s e a r c h o f f i c e r 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. His r e s e a r c h d e a l s w i t h d u r a b i l i t y problems of b u i l d i n g m a t e r i a l s . Dr.-Ing. A. Meyer i s p r o f e s s o r , Dept. C i v i l E n g i n e e r i n g , Technische Hochschule Darmstadt, and D i r e c t o r of Research and Development, H e i d e l b e r g e r Zement AG, H e i d e l b e r g , West Germany. His r e s e a r c h i n t e r e s t i s i n t h e a r e a of b u i l d i n g m a t e r i a l s , c o n c r e t e technology, f i b r e c o n c r e t e , t i m b e r c o n s t r u c t i o n and b u i l d i n g p h y s i c s .

INTRODUCTION

I n a s u r v e y of t h e long-term behaviour of c o n c r e t e made from g r a n u l a t e d b l a s t f u r n a c e s l a g cement (GBFSC), Schr6der and Smolczyk (1) r e p o r t e d c o n f l i c t i n g c o n c l u s i o n s i n t h e p e r t i n e n t l i t e r a t u r e . On t h e one hand, no s i g n i f i c a n t d i f f e r e n c e s were observed between r a t e and e x t e n t of c a r b o n a t i o n of o r d i n a r y p o r t l a n d cement (OPC) and GBFSC. On t h e o t h e r , a tendency was observed f o r GBFSC t o c a r b o n a t e a t a h i g h e r r a t e t h a n OPC, p a r t i c u l a r l y a t f i r s t . They e x p l a i n e d t h e disagreement a s f o l l o w s :

"a. So f a r we have t o o few long-time c a r b o n a t i o n s t u d i e s w i t h c o n c r e t e a v a i l a b l e , s o t h a t we t o o o f t e n e x t r a p o l a t e t h e r e s u l t s of few y e a r s on t o much l o n g e r t i m e s p a n s . A l l we know about c a r b o n a t i o n f u n c t i o n s s o f a r can only be regarded a s rough approximations and s o q u i t e i n a d e q u a t e f o r e x t r a p o l a t e d c o n c l u s i o n s and t h e e x a c t d e f i n i t i o n of d i f f e r e n t behaviour.

"b. The e x p e r i m e n t a l c o n d i t i o n s of many t e s t s a r e not s t r i c t l y comparable. Since c o n c r e t e q u a l i t y , e.g., i t s d e n s i t y , i s a d e c i s i v e f a c t o r , i t must be k e p t uniform i f we want t o measure t h e i n f l u e n c e of chemical cement composition."

I n t h e l a s t decade i n t e r e s t i n t h e t o p i c h a s i n c r e a s e d

c o n s i d e r a b l y a s use of a d d i t i o n a l cementing m a t e r i a l s such a s GBFSC a s a replacement f o r OPC h a s been made even more a t t r a c t i v e by t h e e s c a l a t i n g c o s t of energy. But i n s p i t e of i n t e n s e a c t i v i t y i n t h e f i e l d , few long-term e x p e r i m e n t s d e s i g n e d t o c l a r i f y u n d e r s t a n d i n g of t h e d u r a b i l i t y of GBFSC c o n c r e t e have been performed.

A t t h e time of t h e SchrGder and Smolczyk s t u d y (1) a f i e l d exposure program had been i n p r o g r e s s f o r some s i x y e a r s i n Germany. Those e x p e r i m e n t a l h o u s e s , b u i l t under c o n t r o l l e d c o n d i t i o n s , have now been monitored f o r 20 y e a r s . An a n a l y s i s of t h e accumulated r e s u l t s i s p r e s e n t e d h e r e .

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EXPERIMENTAL

The r e i n f o r c e d c o n c r e t e house w i t h OPC was b u i l t 2 3 September 1963 and t h a t w i t h GBFSC on 17 September 1964, b o t h i n Beckum, W e s t p h a l i a , F.R.G. No i n d u s t r y o t h e r t h a n a cement p l a n t c o u l d have a f f e c t e d t h e s u r r o u n d i n g atmosphere. These s m a l l houses had n o windows and were n e i t h e r h e a t e d n o r i n h a b i t e d . The p r o p e r t i e s of t h e cements and c o n c r e t e s a r e l i s t e d i n T a b l e s 1 and 2. TWO c o r e s of 15-cm d i a m e t e r were d r i l l e d from t h e n o r t h w a l l of e a c h e x p e r i m e n t a l house, l e n g t h s of 15 cm (GBFSC) and 17 cm (OPC) r e p r e s e n t i n g t h e e n t i r e t h i c k n e s s of t h e w a l l s . The c o r e s were h a l v e d l e n g t h w i s e i n t h e l a b o r a t o r y by means of a water-cooled diamond saw and b o t h e n d s of t h e s e m i - c i r c u l a r p i e c e s were c u t i n t o t e n 4-m t h i c k s l i c e s . The s e c t i o n s were t h e n d r i e d i n vacuo a t room t e m p e r a t u r e and s t o r e d i n a d e s i c c a t o r . Before t e s t i n g , t h e c o n c r e t e was c r u s h e d w i t h a p e s t l e i n a n a g a t e m o r t a r and any c o a r s e and i d e n t i f i a b l e f i n e a g g r e g a t e s were removed.

P h e n o l p h t h a l e i n T e s t

2% p h e n o l p h t h a l e i n i n e t h y l a l c o h o l was u s e d t o d e t e r m i n e t h e d e p t h of c a r b o n a t i o n ( 2 ) on t h e c o r e s immediately a f t e r t h e y were sawn l e n g t h w i s e ( s p l i t ) .

Thermogravimetry

lOO-mg specimens were h e a t e d i n a Dupont Model 1090 i n s t r u m e n t a t 20°C/min i n a s t r e a m of n i t r o g e n gas.

Chemical A n a l y s i s of C a r b o n a t e s

Approximately 0.5 g was d i g e s t e d i n 3 _M_{H2S0,+ and t h e}

l i b e r a t e d C02 gas f l u s h e d by N2 s t r e a m was absorbed i n 0.1 N Ba(OH)2. The amount of n e u t r a l i z e d Ba(OH)2 was d e t e r m i n e d by t i t r a t i o n w i t h 0.1 N HC1.

Pore S i z e A n a l y s i s

P o r o s i t y and p o r e s i z e d i s t r i b u t i o n were d e t e r m i n e d by mercury i n t r u s i o n p o r o s i m e t r y . The maximum p r e s s u r e a p p l i e d i n t h e AMINCO i n s t r u m e n t was 410 MPa ( 6 0 000 p s i ) .

Compressive S t r e n g t h

P r e p a r a t i o n and t e s t i n g of t h e specimens were c a r r i e d o u t i n c o n f o r m i t y w i t h German S t a n d a r d D I N 1048.

T e n s i l e S t r e n g t h

A SATTEC M a t e r i e l i n s t r u m e n t was u s e d t o measure t h e f o r c e r e q u i r e d t o b r e a k , i n t e n s i o n , t h e s u r f a c e r e g i o n of t h e c o n c r e t e below a s t e e l p l a t e (35-m diam) a f f i x e d t o t h e specimen w i t h epoxy r e s i n . T e n s i l e s t r e n g t h of t h e specimens was determined i n August 1982, i . e . , a t 19 and 18 y e a r s , r e s p e c t i v e l y .

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Freezing and Thawing

Specimens 10 mm wide and 25 mm l o n g were c u t from t h e 4-mm t h i c k s e c t i o n s . Length changes were determined d u r i n g t e m p e r a t u r e c y c l e s (+8 and -lg°C) i n a homemade e x t e n s o m e t e r ( 3 ) p l a c e d i n a microcomputer-governed t e m p e r a t u r e l h u m i d i t y t e s t chamber

(Thermotron Model SM.8C).

RESULTS

The p h e n o l p h t h a l e i n - t r e a t e d c o n c r e t e s u r f a c e s were t r a c e d from c o l o u r photographs f o r e a s e of r e p r o d u c t i o n and a r e shown i n Fig. 1. Thicknesses of t h e c a r b o n a t e d l a y e r s a t t h e two e n d s of t h e specimens a r e not t h e same, presumably owing t o d i f f e r e n c e s i n humidity and Cop c o n c e n t r a t i o n of t h e e x t e r i o r and i n t e r i o r

atmospheres t o which t h e house was exposed. The w i d t h of t h e c o l o u r l e s s l a y e r s i s 6 mm and 3 mm, r e s p e c t i v e l y , f o r OPC c o n c r e t e , and 22 mm and 13 mm, r e s p e c t i v e l y , f o r GBFSC c o n c r e t e .

The r e s u l t s of t h e r m o g r a v i m e t r i c a n a l y s i s a r e g i v e n i n Fig. 2. The c o n c e n t r a t i o n of CaCOJ and l i m e was determined by i n t e g r a t i o n of t h e a r e a s below t h e peaks (Fig. 3). The, peak of t h e d e r i v a t i v e o c c u r r i n g between 100 and 200°C i s caused by l o s s of w a t e r from t h e c a l c i u m s i l i c a t e h y d r a t e s . That between 450 and 550°C i s due t o decomposition of Ca(OH)2 i n h y d r a t e d C S ( 4 ) . That a t

3

approximately 780°C s i g n i f i e s decomposition of CaC03 ( 5 ) . The f o l l o w i n g o b s e r v a t i o n s were made:

- a t a l l d e p t h s t h e l i m e c o n t e n t of t h e OPC c o n c r e t e was f a r

g r e a t e r t h a n t h a t of t h e GBFSC c o n c r e t e ;

-

t h e r e was no lime i n an a p p r o x i m a t e l y 20-mm t h i c k l a y e r below t h e i n t e r i o r and none i n a 10- t h i c k l a y e r below t h e e x t e r i o r s u r f a c e of t h e GBFSC c o n c r e t e ;

-

even a t t h e c e n t r e of t h e GBFSC c o n c r e t e o n l y a v e r y s m a l l amount of l i m e was p r e s e n t ;

-

t h e calcium c a r b o n a t e c o n t e n t of t h e OPC c o n c r e t e was s i g n i f i c a n t i n t h e v i c i n i t y of t h e exposed s u r f a c e ; t h e c e n t r e , 38 mm below t h e s u r f a c e , was e s s e n t i a l l y c a r b o n a t e f r e e ;

-

CaC03 was d e t e c t e d i n a l l s l i c e s of GBFSC c o n c r e t e e x c e p t two, which o r i g i n a t e d 40 mm and 70 mm (marked c e n t r e on Fig. 2) below t h e i n t e r i o r s u r f a c e ;

-

i n both c o n c r e t e s c a r b o n a t i o n p r o g r e s s e d from t h e e x t e r i o r s u r f a c e a t a s l o w e r r a t e t h a n from t h e i n t e r i o r .

, Chemical A n a l y s i s

CaC03 c o n c e n t r a t i o n s a t v a r i o u s d e p t h s from t h e exposed s u r f a c e s a r e shown i n Fig. 4 f o r GBFSC c o n c r e t e and i n Fig. 5 f o r t h e r e f e r e n c e OPC c o n c r e t e . Agreement between chemical a n a l y s i s and TGA r e s u l t s can be c o n s i d e r e d good i n view of t h e inhomogeneity of t h e samples, caused by t h e f i n e a g g r e g a t e t h e y c o n t a i n e d . The weight of t h e cement i n t h e g r o s s sample was not w e l l d e f i n e d and i s t h e r e f o r e a major s o u r c e of d i s c r e p a n c y . When, i n s e v e r a l

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i n s t a n c e s , t h e same method of a n a l y s i s was r e p e a t e d , d e v i a t i p n s of 10 t o 15% were found ( s e e , f o r example, Fig. 4 . ) . I n a n a t t e m p t t o overcome such u n c e r t a i n t y , specimens were d i s s o l v e d i n a c i d and t h e c o n c e n t r a t i o n s e x p r e s s e d a s p e r c e n t a g e s of t h e a c i d s o l u b l e f r a c - t i o n of t h e specimen. These v a l u e s a r e shown i n F i g s . 4 and 5.

Pore S i z e D i s t r i b u t i o n

R e p r e s e n t a t i v e r e s u l t s f o r t h e two t y p e s of p a s t e ( a t v a r i o u s d e p t h s from t h e exposed s u r f a c e s ) a r e p r e s e n t e d a s h i s t o g r a m s (Fig. 6a-d). T h e i r r e p r o d u c i b i l i t y i s q u i t e good, a s may be s e e n from t h e v a l u e s i n Table 3, which l i s t s t h e f r a c t i o n of p o r o s i t y i n s e l e c t e d r a n g e s of p o r e s i z e ; i n s e v e r a l i n s t a n c e s two o r more specimens from t h e same l e v e l were analysed.

Because t h e p o r o s i t y v a l u e s a r e e x p r e s s e d a s p e r c e n t a g e s o f sample volume, which a l s o c o n t a i n s t h e non-porous sand a g g r e g a t e , t h e t o t a l p o r o s i t y v a l u e s l i s t e d i n Table 3 mst be c o n s i d e r e d nominal and a r e g i v e n o n l y f o r comparison. With i n c r e a s i n g

c a r b o n a t i o n t h e t o t a l p o r o s i t y of GBFSC c o n c r e t e does n o t a p p e a r t o be a f f e c t e d , b u t t h a t of OPC c o n c r e t e a p p e a r s t o d e c r e a s e

(Table 3).

The p o r e s i z e d i s t r i b u t i o n of GBFSC c o n c r e t e undergoes changes w i t h i n c r e a s i n g c a r b o n a t i o n , mainly i n t h r e e p o r e s i z e ranges: p o r e d i a m e t e r 3 s m a l l e r t h a n 0.009 pm; t h o s e between 0.019 and 0.35 pm; and t h o s e g r e a t e r t h a n 18

um.

I n t h e c e n t r e s e c t i o n , where t h e p a s t e i s c a r b o n a t e d l e a s t , 6.5% of t h e volume, o r 55% of t h e t o t a l p o r o s i t y , i s i n p o r e s of d i a m e t e r (0.009; i n t h e s u r f a c e l a y e r 0.6% of t h e volume, o r 3% of p o r o s i t y , i s i n p o r e s of d i a m e t e r (0.009 Dm (Table 3). A s t h e volume f r a c t i o n of s m a l l p o r e s d e c r e a s e s w i t h i n c r e a s i n g p r o x i m i t y t o t h e exposed s u r f a c e , t h e volume f r a c t i o n of medium s i z e pores i n t h e 0.019-0.35 pm r a n g e and of t h o s e w i t h d i a m e t e r s >18 pm

i n c r e a s e s . T h i s i s t r u e f o r b o t h e x t e r i o r and i n t e r i o r exposed s u r f a c e s .

The v a l u e s f o r p o r e s i z e d i s t r i b u t i o n i n OPC c o n c r e t e , a s l i s t e d i n Table 3 and p l o t t e d i n Fig. 6, r e v e a l s i m i l a r t r e n d s , b u t o n l y t o t h e e x t e n t t h a t t h e s m a l l p o r e f r a c t i o n ((0.009 pm)

d e c r e a s e s w i t h i n c r e a s i n g c a r b o n a t i o n . Behaviour of OPC c o n c r e t e d i f f e r s from t h a t of GBFSC c o n c r e t e i n two i m p o r t a n t r e s p e c t s : 1 ) t h e change i n p o r e s i z e d i s t r i b u t i o n i s much less d r a m a t i c , and 2) t h e t o t a l p o r o s i t y d e c r e a s e s w i t h i n c r e a s i n g c a r b o n a t i o n . Thus, t h e e f f e c t s of c o a r s e n i n g of t h e pore s t r u c t u r e i n r e l a t i v e terms a r e m i t i g a t e d by t h e s m a l l e r o v e r a l l p o r e volume.

Compressive S t r e n g t h

The OPC and GBFSC c o n c r e t e c o r e s o b t a i n e d i n t h e c o u r s e of m o n i t o r i n g a t 12 and 11 y e a r s of age, r e s p e c t i v e l y , were t e s t e d and found t o be 40.4 and 52.8 MPa.

(9)

T e n s i l e S t r e n g t h

Table 4 l i s t s t h e v a l u e s f o r t h e s u r f a c e r e g i o n of t h e two t y p e s of c o n c r e t e core.

F r e e z i n g and Thawing

R e s i d u a l l e n g t h changes of t h e exposed s u r f a c e and c e n t r e s e c t i o n s of t h e GBFSC and OPC c o n c r e t e specimens a r e shown i n Fig. 7 a s a f u n c t i o n of t h e number of f r e e z e - t h a w c y c l e s . These r e s u l t s must be c o n s i d e r e d a s merely a n i n d i c a t i o n of a t r e n d owing t o u n c e r t a i n t i e s i n t r o d u c e d by t h e v a r y i n g amounts of c o a r s e

a g g r e g a t e i n t h e specimens. Aggregates do n o t normally c o n t r i b u t e t o e x p a n s i o n and t h e r e f o r e have a d i l u t i n g e f f e c t . N e v e r t h e l e s s , i t seems t o be c l e a r t h a t t h e s l o p e s of r e s i d u a l e x p a n s i o n v e r s u s number of freeze-thaw c y c l e r e l a t i o n s a r e s i g n i f i c a n t l y g r e a t e r f o r GBFSC samples (20015 = 42) t h a n f o r OPC samples (12015 = 24). T h i s behaviour i n d i c a t e s v e r y much g r e a t e r freeze-thaw s u s c e p t i b i l i t y f o r GBFSC p a s t e s t h a n f o r OPC p a s t e s .

DISCUSSION

The p h e n o l p h t h a l e i n test i n d i c a t e d g r e a t e r c a r b o n a t i o n i n GBFSC t h a n i n OPC c o n c r e t e s , i n agreement w i t h some e a r l i e r r e p o r t s

( 2 , 6 ) b u t a t v a r i a n c e w i t h a t l e a s t one s t u d y ( 7 ) . T h i s method h a s l i m i t e d p r e c i s i o n , a s m a n i f e s t e d by t h e wide s c a t t e r of r e s u l t s i n t h e t e s t s e r i e s on t h e two e x p e r i m e n t a l h o u s e s o v e r a n 1 8 y e a r p e r i o d (Fig. 8 ) . It a l s o s u f f e r s from i n s e n s i v i t y , a s demonstrated by t h e TGA r e s u l t s ( F i g . 2) which i n d i c a t e s i g n i f i c a n t c a r b o n a t i o n of b o t h t y p e s of c o n c r e t e w e l l beyond t h e d e p t h d e t e c t e d by t h e i n d i c a t o r .

It h a s t o be remembered t h a t p h e n o l p h t h a l e i n i s a n a c i d - b a s e i n d i c a t o r , and t h a t c o l o u r change s i g n i f i e s n o t s o much t h e p r e s e n c e of c a r b o n a t e a s t h e a b s e n c e of l i m e . The r e d c o l o u r o f t h e p a i n t e d a r e a must n o t be i n t e r p r e t e d a s e v i d e n c e of no c a r b o n a t i o n .

The d e p t h of c a r b o n a t i o n of t h e GBFSC c o n c r e t e does n o t d i f f e r t o o much from t h a t of OPC c o n c r e t e a c c o r d i n g t o t h e r e s u l t s of TGA and c h e m i c a l a n a l y s i s ( F i g . 3). Any d e v i a t i o n below t h e e x t e r i o r s u r f a c e i s p r o b a b l y more a p p a r e n t t h a n r e a l , presumably caused by s c a t t e r of t h e r e s u l t s . An i m p o r t a n t f e a t u r e of t h e OPC c o n c r e t e c a r b o n a t i o n p r o c e s s i s t h a t d e p t h , X, i n c r e a s e s w i t h t h e s q u a r e r o o t of t i m e , t. Thus AX, t h e i n c r e m e n t a l c a r b o n a t i o n p e r u n i t t i m e , d e c r e a s e s w i t h a g e ( 8 , 9 ) because a l o g a r i t h m i c r e l a t i o n e x i s t s between r a t e and p a t h l e n g t h i n d i f f u s i o n - c o n t r o l l e d p r o c e s s e s . The v a r i o u s s e c t i o n s of t h e c o r e s a r e a s o u r c e of i n f o r m a t i o n , n o t o n l y a b o u t s p a t i a l v a r i a t i o n of t h e p r o p e r t i e s a t t h e time of t e s t i n g b u t a l s o a b o u t time-dependent changes (e.g., t h o s e due t o d i f f u s i o n of CO ) t h a t have o c c u r r e d . The c e n t r e s e c t i o n h a s p r o p e r t i e s s i m i f a r t o o r a p p r o a c h i n g t h o s e of t h e o r i g i n a l

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non-carbonated concrete, while the surface sections are in the

final stage of the carbonation process.

The spatial distribution of the carbonate content found in OPC

concrete is consistent with the predictions of the logarithmic

relation; the relatively high degree of carbonation at the surface

decreases rapidly in the region below. In contrast, in GBFSC

concrete the carbonate content decreases gradually from the surface

towards the centre, indicating a relation between

X

and

t

that is

almost linear in the first 20 years of exposure. It is thus quite

possible that significant carbonation of GBFSC concrete will occur,

even in the central region, well within the expected service life

of the concrete. Data shown in Fig. 8 also point to this.

The reason for the difference in rates of carbonation of the

two types of concrete is suggested by the pore size distribution

analysis. The values in Table 3 indicate that carbonation of OPC

concrete caused a decrease in total porosity, from a nominal 15% to

11.55%, and a sharp reduction in pores with diameter less than

0.009

urn.

At the same time the volume of large

( > l a

pm) and

mid-size pores remained relatively constant. Carbonation rate

therefore decreased because of increased resistance to diffusion

and longer path length.

On

the other hand, the total porosity of

GBFSC did not decrease and the volume fraction of the large

(>18 pm) and mid-size (0.35-0.019 pm) pores increased in both

relative and absolute terms. This change more than compensates for

the longer path length.

The cumulative pore size distribution of the surface and

centre sections of the two types of concrete is shown in Fig. 9.

Although it does not have much greater total porosity than the

GBFSC centre section, the OPC centre section is quite permeable

owing to its large pores. After carbonation of its surface

section, the OPC concrete becomes more impermeable, although not to

the same extent as pristine GBFSC; the carbonated GBFSC has more

pores than the pristine OPC in almost all pore diameter regions.

Thus, the advantage of low permeability of GBFSC concrete is lost

with carbonation.

Carbonation of concrete is, in general, undesirable because

this reaction decreases alkalinity and the high pH that provides

good protection against corrosion of reinforcing steel is lost

(9-10).

The pH of concrete is 13.5, and in this medium corrosion

rate is negligible. Strictly speaking, concern is not so much for

the quantity of CaC03 formed but for the quantity of Ca(OH)2

remaining after carbonation. Obviously, regardless of degree of

carbonation, a substantial lime reserve is what is needed. In this

respect the results shown in Fig. 3 are of concern. While OPC

concrete contains significant amounts of Ca(OH)2

even in carbonated

areas, the GBFSC is practically devoid of it. Clearly, no

protection against corrosion exists in a 23-30-mm thick layer,

depending on type of environment (interior or exterior), and the

small amount of lime still present in the central region may well

be converted before very long.

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It has been known, of course, that slag containing cements

suffers from this potential problem owing to the small initial

concentration of Ca(OH)2.

Lime is produced during hydration of

portland cement and its concentration in blended cements, in which

a large fraction of cement is replaced by slag, is greatly reduced.

Furthermore, slag on curing does not produce lime; in fact, it

reacts with lime to result in a very small lime reserve. This, on

carbonation, can easily be exhausted.

The low permeability of pristine GBFSC concrete might be

viewed as a factor compensating for the potentially bad effects of

low lime content. The fact that carbonation of this type of

concrete not only reduces lime concentration but also increases

permeability defeats this argument.

The development of pores with diameter in the

0.35-2

pm range

in the process of carbonation may imply that the freeze-thaw

resistance of GBFSC increases with age, as pores of this size.

were

shown (11) to protect the paste from damage.

The tensile strength results (Table 4) show a very dramatic

decrease due to carbonation of GBFSC. Further studies have to be

carried out to confirm the results.

Resistance to freezing and thawing could not be evaluated with

sufficient precision to assess the effect of carbonation.

CONCLUSIONS

Examination of GBFSC and OPC concrete from experimental houses

exposed in the field for

20

and

21

years, respectively, leads to

the following conclusions:

1. The rate of carbonation in GBFSC significantly exceeds that in

OPC concrete.

2.

Carbonation increases the permeability of GBFSC because the

small pores originally present become larger.

3. Carbonation seriously decreases the tensile strength of GBFSC

concrete.

4. After

20

years of exposure very little lime is left in GBFSC

concrete.

5.

Of the methods for determining extent of carbonation, TGA and

chemical analysis are far superior to the phenolphthalein

test.

ACKNOWLEDGEMENT

The chemical analysis of the cement, long-term monitoring of

experimental houses, and tensile strength determinations were

carried out by Zemlabor, Dr. Werner Loch. The authors gratefully

acknowledge this effort. They are also indebted to Messrs.

H. Schultz and G. Polomark for carrying out the measurements so

competently. This paper is a contribution from the Division of

Building Research, National Research Council of Canada.

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REFERENCES

1. Schrader, F. and Smolczyk,

H.G.

"Carbonation and protection

against steel corrosion." Principal Paper: Blast Furnace

Slags and Slag Cements. Proc., Fifth International Symposium

on the Chemistry of Cement, Tokyo, Vol. 5, 1968, p. 188.

2. Meyer,

A. "Investigations on the carbonation of concrete,"

Proc. Fifth International Symposium on the Chemistry of

Cement, Tokyo, Supplementary Paper 111-52, 1968, p. 394.

3. Litvan,

G.

"Phase transitions of absorbates. 111. Heat

effects and dimensional changes in nonequilibrium temperature

cycles." J. Coll. Interface Sci., Vol. 38, No. 1, 1972,

pp. 75.

4. Ramachandran, V.S.

"Differential thermal method of estimating

calcium hydroxide in calcium silicate and cement pastes."

Cem. Concr. Res., Vol. 9, No.

6, 1979, pp. 677.

5. Duval, C.

"Inorganic thermogravimetric analysis," Elsevier,

Amsterdam, 1963, p. 278.

6. Hamada,

M. "Neutralization (carbonation) of concrete and

corrosion of reinforcing steel." Principal paper, Vol. 111,

Proc., Fifth International Symposium on the Chemistry of

Cement, Tokyo, 1968, p. 343.

. .

7. Kleinschmidt,

H.-J.

"Investigation of the progress of

carbonation in concrete structures." Deutscher Ausschuss fiir

Stahlbeton, Vol. 170, 1965, p. 25.

8. Smolczyk,

H.G.

Discussion of "Carbonation of Concrete."

Proc., Fifth International Symposium on the Chemistry of

Cement, Vol. 111, Tokyo, 1968, p. 369.

9. Uhlig,

H.H.

and Revie, R.W.

"Corrosion and corrosion

control."

(Third Ed.) Wiley, 1985, p. 96.

10. Tuutti, K.

"Corrosion of steel in concrete." Swedish Cem&nt

and Concrete Research Institute, Stockholm, Sweden, Research

Report Fo 4.82, 1982.

11. Litvan,

G.G.

"Air enterainment in the presence of

superplastizers." American Concrete Institute

J., No. 7-8,

1983, p. 326.

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Table 1

Properties of Cements

-

GBFSC

OPC

Blended slag,

Ordinary portland

interground

Cement type,

PZ 275

HOZ 275

Ignition loss,

%

Chemical analysis, %

:S3

Fe O3

ca8

MgO

S03

F 6 O

~ P ~ u l f

ide)

Slag content

Glass content

Setting time, Vicat

Initial

h,

min.

Final

Table 2

Mix Design and Properties of Concrete

OPC

GBFSC

Cementitious material content, kg/m3

lb/cu yd

Sand content, kg/m3

lb/cu yd

Cement

:water:sand

Weight proportion

Compressive strength at 28 days

20

x

20

x

20- cubes

MPa

(psi)

Compressive strength in Sept. 1975

(at 12 and 11 yr, respectively)

MPa

(psi)

(14)

Table 3

Total P o r o s i t y and Fraction o f Pore Volume i n S e l e c t e d S i z e s of GBFSC and OPC Concrete a t Various Depths

from Exposed Surf ace

-

-Fraction o f volume i n p o r e s , diarn Depth Total

mm p o r o s i t y > 1 8 pm 0.35-0.019 pm t0.009 0.35-2 pm

GBFSC

(15)

Table 4

T e n s i l e Strength of t h e Surface Region of GBFSC and OPC Concrete a t 19 and 18 y e a r s , r e s p e c t i v e l y , MPa

Exposure

OPC GBFSC

Ext Int Ext Int

Avg 4.96 6.6 0.86 2.7

Fig. 1--Trace of photographs depicting ~henol~hthalein-treated

concrete surfaces. Hatched area represents purple coloured

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T E M P E R A T U R E , " C

Fig. 2--Derivatives of thermogravimetric analysis curves of

sections of OPC (left) and GBFSC (right) concrete core samples.

Numbers indicate distance in mm between specimen and nearest

exposed surface of the wall

LllIN OPC 0-0

GBFSC .----a

GBFSC

DISTANCE FROM INTERIOR DISTANCE FROM EXTERIOR

SURFACE, mm SURFACE. rnm

Fig. 3--Concentration of CaC03 and Ca(OH)2, determined by TGA, of

OPC and GBFSC concrete specimens at various depths from exposed

surf

ace

(17)

9

-

_I _I _I _I _I _I _I _I _I 8 - 0-0 TGA - 0 C H E M I C A L A N A L Y S I S 7

-

R E R U N eP- I .----a E X P R E S S E D A S P E R C E N T A G E

I

O F S O L U B L E C O N T E N T I

-

I 1 I

-

L I I

-

I I 4 0

/"-

; I , ,

, ,

0 2 0 4 0 20 0 D I S T A N C E F R O M I N T E R I O R

'

D I S T A N C E F R O M E X T E R I O R S U R F A C E , m m S U R F A C E , mm

Fig 4--CaCO3 concentration of GBFSC concrete specimen, determined

by chemical analysis, as a function of distance from exposed

surface. Results obtained by TGA shown for reference purposes

0-0 T G A C H E M I C A L A N A L Y S I S E X P R E S S E D A S P E R C E N T A G E O F : &--a S A M P L E &----a S O L U B L E C O N T E N T D I S T A N C E F R O M I N T E R I O R D I S T A N C E F R O M E X T E R I O R S U R F A C E , m m S U R F A C E , m m

Fig. 5--CaC03 concentration of OPC concrete specimen, determined by

chemical analysis, as a function of distance from exposed surface.

Results obtained by TGA shown for reference purposes

(18)

LA, z

(19)

(c) OPC concrete interior exposure

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I

I I I I I

_-

Ibl - S U R F A C E 0-0 0-0

-

a-a - - C E N T R E a-. L

-

N U M B E R O F C Y C L E S

Fig. 7--Residual expansion of specimens a s a function of number of freeze-thaw cycles, (a)

GBFSC

(b)

OPC

concrete

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8 1 2 16 20 24 E X P O S U R E , y e a r s

Fig. 8--Depth of carbonation determined b y phenolphthalein test as a function of years of exposure

I I I I t I r I l OPC CENTRE 60 mm GBFSC SURFACE 3 mm

----

GBFSC CENTRE 65 mm

*

...

_{OPC SURFACE 3}_mm

....

...

....

...

348.8 2.9 1.3 0.58 a35 a055 0.m 0.016 0.012 0 . m P O R E D I A M E T E R , pm