Modification of cement pore fluid compositions by pozzolanic additives

CEMENT and CONCRETE RESEARCH. Vol. 18, pp. 165-178, 1988. Printed in the USA 0008-8846/88 $3.00+OO. Copyright (c) 1988 Pergamon Press plc.

] [ I D I F I C I T I O ! OF c i r m ~ 7 POR~ FLUID CONPt~ITIQNS BY PQZZOLLilC LDDITIVBS F . P . G l a a a e r , [ . Luke a n d ][.J. £ n ~ s D e p a r t m e n t o f Chemi~rtry, U n i v e r s i t y o f £ b e r d e e n , h e . - t o n Yalk, Old £ b e r d e e n , kB9 2UE, S c o t l a e d .

(refereed) (Received Jan. 14; in final form Nov. 17, 1987)

ABSTRACT The l z q ~ c t o f b l e n d i n g a g e n t s on t h e i n t e r n a l e n v i r o n m e n t of cement s y s t e m s i s a s s e s s e d by c h e m i c a l a n a l y m l s o f t h e p o r e fluid in cured blends. The s h o r t - t e r m b e h a v t o u r o f PF£ b l e n d s is co~licated by t h e p r e ~ e n c e o f s o l u b l e a l k a l i . In general, pore fluid sodium levels are n o t much a l t e r e d by PP£ w h i l e Pota~tum is reduced. SI02 fume has a r a r e izmedtate txpact on t h e P o r e f l u i d c h e m i s t r y : 10-20% a d d i t i o n s c a n l e a d to order-of-megnttude reductions, e~pectally In ceestua content. The p o t e n t i a l o f c e m e n t t o u p t a k e c h l o r i d e a n i o n s Is asse~d: the correlation w i t h t h e sum o f t r t c a l c t u m a l u m i n a t e ( C ~ ) a n d ferrtte phases i s more s i g n i f i c a n t t h a n w i t h C~£ a l o n e . Blast furnace sla~ can markedly affect the internal redox p o t e n t i a l , which d e c r e a s e d f r o m ÷lOOmY i n P o r t l a n d cement to minus 200-2~mV in sla~-rich blends.

E f f o r t s t o model t h e b e h a v i o u r o f c e m e n t syb-tems a n d p r e d i c t t h e i m p a c t o f e n v i r o n m e n t a l c o n d i t i o n s on i t s d u r a b i l i t y a r e h a n d i c a p p e d by t h e o f t e n poor q u a l i t y o f t h e d a t a b a s e . S t u d i e s i n t h i s a r e a have been conducted alon 8 parallel l i n e s a n d some o f t h e p r i n c i p a l r e ~ l t s presented here. New t n s t g h t s i n t o t h e c h e m i c a l e n v i r o n m e n t i n s e t c e m e n t s have b e e n o b t a i n e d from s e l e c t e d pore f l u i d a n a l y s e s i n P o r t l a n d and P o r t l a n d b l e n d e d c e m e n t s , a ~ d a t 20"~-40°C f o r up t o 90d. Developments in four areas are described: (t) application of the technique to determining the partition of r a d i o a c t i v e waste s ~ e c i e s between pore f l u i d and s o l i d p h a s e s i n s e t c e m e n t , i n t h i s came u s i n g 1~4Cs, ( i i ) t h e n a t u r e o f t h e c h e m i c a l c o n t r o l s o f t h e e a r l y ( 1 - 1 4 d ) s t a g e s of h y d r a t i o n r e s u l t i n g f r o m the p r e s e n c e of b l e e d i n g a g e n t s , e . g . c l a ~ F PF£, ( t i t ) t h e c h l o r i d e b i n d i n g c a p a c i t i e s o f t h e s o l i d h y d r a t i o n p r o d u c t s o f OPC a s r e l a t e d t o the aluminate and ferrite c o n t e n t s of the c l i n k e r and ( i v ) the s p e c i a l c h e m i c a l f e a t u r e s o f s l a g c e m e n t s , f o r which we r e p o r t f o r t h e f i r s t time ~, (redox potential) and p o i s i n g c a p a c i t i e s , a s d e f i n e d by t h e Ce''/Ce 3- couple.

165

166

Vol. F.P.

Glasser,

18, No. 2

et al.

EESULTSA]~DI~U~I~ (I) l~l~vlour of Caeelum

(Ce ~) in C~m~ut and Blen¢]~l C~mpnt Kmtrlcee

lany radioactive waste species are somewhat soluble in cement Pore fluids: Cs i s a n e x a m p l e . P a r t l y f o r t h i s r e a s o n , Cs i s d i f f i c u l t to t~btllze i n c e m e n t mtrlces. During leaching Ca, which I s h e l d mainly in the pore fluid, is inhibited from e x c h a n g i n g w i t h l e a c h a n t solutions. T h i s I s due t o t h e p h ] m l c a l b a r r i e r s t o m i g r a t i o n l ~ d by t h e p r e s e n c e of t n t e r l o c k i n 8 s o l i d p h a s e s which g r e a t l y i n c r e a s e t h e tortuooity of diffusion paths. The p r e s e n c e o f e s i l i c e o u s a d d i t i v e , e . g . c l a s s F PF£, I s known t o r e d u c e C6 l e a c h r a t e s , b u t no c l e a r c u t distinction c a n be made i n t h e c o u r s e o f e x t s t l n 8 e x p e r i m e n t a l s t u d i e s between p h y s i c a l and chemical m e c h e n t s n s of r e t e n t i o n . Deter,rinationo of the pore fluid compositions in blended relents have b e e n u s e d t o improve our u n d e r s t a n d i n g of the 6 p e c i f l c chemical e f f e c t s occurring w i t h i n cement m a t r i c e s . M ~ t ~ _ r t l n ~ a l and ~ l t ~ C y l i n d e r s i n t e n d e d f o r p o r e f l u i d e x t r a c t i o n were - - d e from o r d i n a r y P o r t l a n d c e m e n t t o BS 12 ( d e s i g n a t e d OPC, T a b l e 1); i t c o n t a i n e d on analysis,

0.19% ] [ a ~ ) and 0.49% [ 2 0

(vt.)

a n d was L t x e d

to a water:solid

r a t i o of 0.50. The c y l i n d e r s were c a s t i n a = F e r s p e x " mold; a f t e r a 24h initial s e t , t h e y were p u t i n t o v a p o u r - t t g h t s e a l e d p l a s t i c bags and s t o r e d i n h u m i d i t y c h a m b e r s a t 98% ]~H a t e i t h e r 20 ° o r 40°C f o r 28d. CA;, added a s t h e ' = ' ~ s i s o t o p e t o t h e , d x i n 8 w a t e r , was f i x e d a t a t o t a l radtonucltde concentration b e t w e e n 2 - 5 • 10 = c o u n t s s - 1 . ABed cylinders were tested to determine the partition o f Cs b e t w e e n a q u e o u s

and s o l i d phases. S i n c e no s p e c i f i c c h e ~ r l c a l mechanic,-= f o r f t x i n 8 Cs in the solid p h a s e s was p r e s e n t , Its content i n the pore f l u i d s increased as curing progreeemd. Experiments were also undertaken in parallel, under essentially identical conditions, to determine the ] ~ d i f y i n 8 e f f e c t on Cs p o r e f l u i d l e v e l s o f v a r i o u s a d d i t i v e s . . Three a d d i t i v e s were u s e d ; a ][orwegtan f e r r o 6 i l t c o n - s a m l t e r condensed silica fume c o n t a i n i n g " 98% S i 0 2 , which c o n s i s t s o f amorphous, h i g h - s u r f a c e area particles; a c l a s s F PFA f r o m F i d d l e r s F e r r y power s t a t i o n (UK), and b l a s t f u r n a c e s l a g (BFS) f r o m S c u n t h o r p e , E n g l a n d . Petrographic e x a m i n a t i o n of t h e s l a g d i s c l o s e d t h a t i t had a h i g h g l a s s c o n t e n t , e x c e e d i n g " 98%, whose m a j o r e l e m e n t c o x p o s i t l o n m 33.83 Si02, 39.6 CaO, 10.2 A12(~, 7 . 9 ] ~ , 0 . 7 4 TtO=, 0 . 5 5 K20, 0 . 7 4 XnO, 0 . 3 2 |a=O and 1.31 S ( a l l v t %); X-ray diffraction r e v e a l e d t h a t t r a c e s of ~ e l t l i t e s o l i d s o l u t i o n had c r y s t a l l i z e d . B l e n d s were p r e p a r e d a t w e l l - s p a c e d c o m p o s i t i o n a l i n t e r v a l s up t o 50% c e m e n t r e p l a c e m e n t by s i l i c a fume o r PFA, a n d up t o 90Z r e p l a c e m e n t by s l a g . The c o u r s e o f r e a c t i v i t y of the s l l l c a fume and PFA were followed indirectly, by d e t e r n t n t n g t h e Ca(OH)z c o n t e n t s o f m a t u r e d blends using TGA. T a b l e 2 r e c o r d s Ca(OIl)2 c o n t e n t s a c h i e v e d a f t e r a 28d c u r e a t 20°C and 981; BH. Cylinders intended for these measurements were s p i k e d w i t h Ca, b u t ' o r d i n a r y ' I . e . n o n - r a d i o a c t i v e Cs was u s e d i n p l a c e of l ~ ' C s . I t i s n o t b e l i e v e d t h a t t h e p r e s e n c e o f Cs a l t e r s significantly t h e h y d r a t i o n b e h a v i o u r a t t h e low C6 c o n c e n t r a t i o n s u s e d , ca 1000 ppm. The p r o p o r t i o n o f s l a g r e a c t e d was e s t l n n t e d by s e l e c t i v e chemical dt~olutton, u s i n g KDT£, t o be a b o u t 251; s l a g r e a c t e d i n 28d and 3 5 - 4 0 Z i n 84d. Ye have r e s e r v a t i o n s a b o u t t h e a b 6 o l u t e r e l i a b i l i t y o f d i s s o l u t i o n methods, b u t t e n t a t i v e l y c o n c l u d e t h a t a t t h e end of t h e

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167

PORE FLUID, COMPOSITION,

tests rather l e ~

than half of the slag h a d reacted.

TABLE I Mlneralogical Composition of Cements

~us=

OPC

zaec

A l i t e (Oa=S) C~S C=A Ferrite

73.5 7.5 3 13

68 10.5 12.5 6.0

Total

FUME, SLAG, FLY ASH

SILICA

2

3

99

100

Cs pore fluld concentrations are shown in Figs. 1-3. Fig. 1 shows the effect of SIO~ fume. This is an especially c l e a r c u t example because acts relatively

U n t t s : m e s ~ ~. A n a l y s i s by Z - r a y diffraction, c o u r t e s y o f V.A. G u t t e r i d g e , Cement and C o n c r e t e A s s o c i a t i o n , Vexham S p r i n g s , S l o u g h , England. Chemically determined alkali contents of t h e s e cements a r e g i v e n i n Table 6. HAPC i s u s e d t o designate a high-alkali Portland cement.

SiO=

rum

re-

rapldly with OPC, s o t h a t a s t e a d y s t a t e i s n e a r l y o b t a i n e d a f t e r o n l y 28d c u r e , e s p e c i a l l y a t 40°C. The initial c o n c e n t r a t i o n o f C6 i n t h e m i x i n g w a t e r i s shown by a broken horizontal line. After curing, its concentration r i s e s because of the absence of any Jechanism of incorporation into the solid phases, coupled with the decrease in v o l u m e of t h e aqueous p h a s e as free water is consumed to satisfy the hydration demands of the cement. The presence o f only IOZ fume has a drastic effect on lowering pore fluid Cs but larger increments are less effective. 60

w

,

Table 2 Ca(f~)2 contents of Blended Natured Cylinders Used for Pore Fluid Extraction Xlx Proportions,

by ~elght

~ Ca(OH)2

100% r h ]C 'E

2C 100

-

-

90

10

-

17.2

80 70 60 50

20 30 40 50

-

12.2 7.4 5.2 3.7

i00 90 80 70

-

-

-

I0 20 30

-

-

-

• 28d cure .20°[ o 28d cure ,~O*C

[nlhoI - C 0¢1£ in ~x~r~ wOter

~- . . . . . . . . . . . . . . \

31.8

26.7 17.9 9.5 negligible

10 ~ u.

2O

U)

% S*lfco Fume

Fig. I Control of Pore Fluid Cs Contents by Silica Fume Additions.

Figs. 2 and 3 compare the effect of a broad spectrum of the concentration of Cs in the pore fluid. Fig. 2 data relatively hlgh Ce spikes: spiking level - 21 mg d r 3. the pore fluid Cs concentration increases upon ageing neat reasons explained previously. But PFA and BFS are lowering pore fluid levels: the effectlvenes~ of BFS

additives on are based on As expected, OPC, for the effective in continues to

168

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Glasser,

18, No. 2

eta].

r~,

32001

2"0Ct t E

~ 32 E

zq"

2~ 2

tel

1~1

T0~9

c_ 2¢ 2 ~

8 &

'ii i Control of by Various Relatively BFS = B l a s t

Fill. 2 Pore Fluid C~ Contents Blending Agents at High L e v e l s o f Added Cs. Furnace S l a g

"8 '~.

.~

~

,1

°

.

FIG. 3 Control of Pore Fluid Cs Contents by Various Blending Agents at Relatively Low Levels of Added Cs. "SiO~" refers to Si02 fume, PFA to class F Ash

improve upon prolonging the cure duration from 28 to 48d. Fig. 3 shows very similar features to those in Fig. 2, but the effectiveneso of additives in removing pore fluid C~ tends to rise as total Cs in the system is decreased, suggesting that the solid phases tend towards saturation and Ic~e efficiency with rising total C6. The presence of xxlifiers has a substantial effect on reducing soluble Cs relative to the neat OPC control, indicating that their presence enhances the retentive properties of the solld matrix for Cs.

The p r e s e n c e o f m o d i f i e r s v e r y s i g n i f i c a n t l y e n h a n c e s the a b i l i t y of the solid cement h y d r a t i o n p r o d u c t s t o r e t a i n Cs. The ' n o r m a l ' p r o d u c t s o f c e m e n t h y d r a t i o n - Ca(OH)2, l i m e r i c h C-S-H, AF. and AFt p l m s e e - have b e e n r~hown s e p a r a t e l y t o h a v e l i t t l e u p t a k e f o r Os. The b e h a v t o u r o f c e m e n t s i s t h e r e f o r e t h a t w h i c h would be e x p e c t e d f r o m a m i x t u r e o f p h a s e s , e a c h o f which h a s a low u p t a k e . However, t h e p r e s e n c e of s i l i c e o u s additives introduces two a d d i t i o n a l factors. Firstly, s i l i c e o u s a d d i t i v e 6 have e o r p t i o n p o t e n t i a l i n t h e i r own r i g h t a n d t h u s s o r b C~s f r o m a l k a l i n e s o l u t i o n s (I). Secondly, siliceous a d d i t i v e s r e a c t w i t h Ca(OH)= a n d l i m e r l c h C-S-H p r o d u c i n g a low C/S ratio product. The Cs s o r p t l o n p o t e n t l a l o f C-S-H d e p e n d ~ m a r k e d l y on I t s C/S r a t l o , s o r p t l o n b e i n g e n h a n c ~ l a t t h e l o w e r r a t l o 8 ( 2 ) . PFA's potentially contrtlmte i l t o t h e C-S-H p r o d u c t , which may a l s o be a f a c t o r i n e n l ~ D c i z ~ C~ e o r p t i o n i n t o b e r m o r i t e - l i k e phases.

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169 PORE FLUID,

COMPOSITION,

SILICA FUME,

SLAG, FLY ASH

Quantitative comparisons between the different additives is handicapped by several factors. It should be recalled In this context that the PFA contains less reactive Si02 per unit than fume (about 7DZ vs 98~) and, on account of the lower specific surface of PFA end Its somewhat inhomogeneous distribution on a micrometer scale, its blends with OPC will to reach a degree of mlcrostructural maturity comparable wlth blends containing fume over the tlmescale used in this study. However, it is anticlpitated that the blends with PFA will continue to improve in performance with time, eventually approaching but probably not equalling those containing the same weight proportion of Si02 fume. Thus differences in performance between the two types of system are considered to arise from t ~ factors. One is the dilution of hydraullcally-actlve material in PFA by essentially inert material, so it contributes less SiO~ per unit mess than fume. The other factor is kinetic; diffusion to and from the coarser siliceous PFA particles is slower than with fume, even assuming a good particle distribution, so changes in the mineralogy of the calcium silicate matrix occur at a slower rate. It is lnterestlng to compare the efficiency of the solid phases In cement with respect to their ability to incorporate different alkalis. Sodium, potassium and caesium differ significantly. ](ass balance calculations, taking into account the amount of pore fluid as well as its composition and the total |a and K contents of the system, dlgcloee that in neat OPC much of the |a is incorporated in the solid phases, while K m l n l y concentrates in the pore fluid. Ce continues this trend, concentrating almost quantitatively in the pore fluid (3). In commercial OPC, Cs Is present in negligible amounts and we need consider only la and K with respect to alkali-aggregate reactions. The evidence suggests that I especially concentrates in the pore fluid, so the K content of cements may be more important than lie In promoting reactions at short ages with alkali-susceptlble aggregates. On the other hand, the presence of siliceous additives alters the distribution pattern of the alkalis. In general, all siliceous blending agents appear to leave the |a retention characteristics of the s y s t e m l a r g e l y u n a f f e c t e d but enhance the K r e t e n t i o n of the m e t r t x . The m a g n i t u d e o f t h e a l k a l i p o r e f l u i d r e d u c t i o n t h u s a c h i e v e d i s r o u g h l y p r o p o r t i o n a l t o t h e amount of silica i n t r o d u c e d by t h e b l e n d i n g a g e n t , p r o v i d e d c o m p a r i s o n s a r e mede a f t e r e q u i v a l e n t f r a c t i o n s o f t h e b l e n d i n g a g e n t have r e a c t e d . S t u d i e s o f t h e s o r p t l o n o f C8 on l a b o r a t o r y C-S-H p r e p a r a t i o a s have shown t h a t t h e s o r p t i o n p o t e n t i a l on C-S-H h a v i n g a f i x e d C/S r a t i o i n c r e a s e s i n t h e same o r d e r . Nnreover, the magnitude of the 6orption from a s o l u t i o n h a v i n g I n i t i a l l y a c o n s t a n t ( o r n e a r l y c o n s t a n t ) C/S r a t i o i s e n h a n c e d a s t h e mean C/S r a t i o o f t h e s o l i d d e c r e a s e s . Thus agpreelent is obtained between laboratory studies and the chemically more complex c e m e n t p a s t e s , s u ~ e s t t n g that explanations derived from simple l a b o r a t o r y models are e s s e n t i a l l y c o r r e c t w i t h r e s p e c t t o PPA, s i l i c a fume a n d o t h e r p o z z o l a n s . However, s l a g ~ a p p e a r t o d i f f e r f r o m thugs Just described. Some o f t h e s p e c i a l f e a t u r e s o f 8 l a g - r i c h c e m n t blends are featured in a subsequent section. (ll)

I n f l u e n c e o f PFA o n Pm-e F l u i d s a t K a r l v

I n r e c e n t y e a r s , s e v e r a l p a p e r s have r e p o r t e d t h a t PF£ i n f l u e n c e s the early stages of hydration in ble~4~d cements. General a~reement e x i s t s , b a s e d on c a l o r i m e t r i c a n d SEM i n v e G t i g a t i o n s , t h a t P F I a c t s a s a

170

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18, No. 2

F.P. Glasser, e t a ] .

retarder both on C~A (4, 5) and on CoS ( 6 - 8 ) . Retardation effects could perhaps be explained by determining the influence of PFA upon the con~c~ition of the pore fluid developed during the early hydration stages of blended cements: some data exist but only at longer ages (9). It r e m l n ~ unclear why PFA - supposedly inert during the first few days - should exert s~ch a substantial retarding effect. Ye report here the influence of PFA on pore fluid compositions in the early stages of hydration. FJrnert r e n t a l

] ~ t hccl

Two c o m m e r c i a l c e m e n t s , d a t a f o r which a r e shown i n T a b l e 1 ~ e r e used. The b l e n d l n g a g e n t s c o n s i s t e d o f two c o m m e r c i a l P F A ' s from two d i f f e r e n t s o u r c e s , namely Eggbrough and L o n g a n n e t . These were c h o 6 e n as repre~ntative o f h i g h and low a l k a l i c l a ~ F ashes: their alkali c o n t e n t s a s w e l l a s t h o 6 e o f t h e c e m e n t s shown I n T a b l e 3, were d e t e r m i n e d by f l a m e p h o t o m e t r y f o l l o w i n g LiB(~ f u s i o n of r e p r e s e n t a t i v e sawples. Soluble alkali concentrations of t h e two P F A ' s , shown i n T a b l e 4, were o b t a i n e d by s l m k t n g l g ~ a m p l e s i n 0 . 1 c l ~ o f d e t o n i z e d w a t e r f o r 10 m i n u t e s and f i l t e r i n g o f f t h e s o l u t i o n u n d e r vacuum: the filtrate was a n a l y z e d with a Pye Untcam SP9 a t o m i c a b s o r p t i o n spectrophotometer. TABLE 3

TABLE 4

A l k a l i C o n t e n t (~t •)

OPC H£PC PFA E~borough Longannet

0.19 0.15

0.48 1.22

1.14 0.28

3.73 1.39

Soluble Components of PFA/lg l O - I d ~

Eggborough Longannet

1.25 0.88

3.75 (1.00

6.02 1.03

Neat c e m e n t a n d 7 0 : 3 0 w e i g h t p e r c e n t b l e n d s o f cement a n d PFI were idxed at water:solid r a t i o 6 o f 0 . 6 , c a s e i n t o 42 nm s p l i t c y l i n d r i c a l • o l d s and a g e d a t " 98-100~ rh a t 1 8 ~ . At p r e d e t e r m i n e d t i m e s c y l i n d e r s were demolded a n d p o r e s q u e e z e d . The f l u i d volume c o l l e c t e d r a n g e d b e t w e e n 7 t o 25 c i P , d e p e n d i n g on t h e deKree o f h y d r a t i o n , at early s-tasq~s o f c u r i n g , n o t a b l y 0.25 and 0.5 days, the ~01utions contained particulate mtertal which ~ removed sutmequent to collection by r e i n J e c t l n 8 t h e p o r e f l u i d t h r o u g h a s y r i n p filter of 0 . 2 2 ~ua p o r e s i z e . The p a r t i c u l a t e p o r t i o n was s u b 6 e q u e n t l y shown by IRD t o c o n s i s t l a r g e l y o f u n r e a c t e d c e m e n t c l i n k e r p h a s e s . ila a n d mr c o n t e n t s o f t h e p o r e 8 o l u t i o n B were d e t e r x i n o d w i t h a Pye Unicam SP9 a t o m i c a t m o r p t i o n s ~ p e c t r o p h o t o m e t e r . F i ~ . 4 a n d 5 show t h e results.

goNm,lts a~cl D t ~ u ~ t a n The d i f f e r i n g b e h a v i o u r of | a and K I s shown by c o ~ r i a 8 Fl~s. 4 and 5. I t i s e v l d e n t t h a t the a d d i t i o n o f 30 weight percent PFA, from e i t h e r s o u r c e , lower~ t h e ~ t u m c o n t e n t o f t h e p o r e f l u i d by a b o u t 301L T h i s c o u l d i m p l y t h a t a t s h o r t affe8 t h e c e m e n t m J p p l l u a l l t h e p o t a s s i u m , t h e PFA a c t i n g p r l m r i l y as a diluent. This Interpretation m i g h t a p p e a r t o be supported by compartn 8 t h e HAPC a n d K ~ C / P F A b l e n d R ,

Vol.

18, No. 2

171 PORE FLUID, COMPOSITION,

007

006 ~-'g 00S

-

00} 002

~~

SILICA FUME, SLAG, FLY ASH

HAPCIEGG APC APC/LONG

0 -

0

'E

•o

0

~

HAPCILONG

0

001

I

t

T,mel~y) to9 ~nle T~me (Ooylloq scole 02~

00S -

00£.

0 20

oo3

'g 0;s 010

~002 00~

x

~

00S I

[

o~sos T,II~eldoy) IOg stole

F1G. 4 Pore Fluid la Concentrations of Hlgh alkali Cement with Blending Agents (top) and Low Alkali Cements (bottom) as a function of time. Egg. and Long. identify two different class F fly ash sources from Longannet end Eggborough power stations (OK).

--O~/LONG

I

~

~

") ~',.

~

T,me [ck~)l~ s~'ate

FIG. 5 Evolution of Pore Fluid K Concentrations of High Alkali Cement with Blending Agents (top) and Low Alkali Cements (bottom) as a function of time. Egg. and Long. identify two different class F fly ash sources from Longannet and Eggborough power stations (~).

where similar trends are observed but with potassium ion concentrations differing by a factor of - 2.5, which corresponds closely to the ratio of total potassium in the two cements, Table 3. Previous studies (10) indicate that the cement releases most of its Potassium to the Pore fluid within 30 days giving potas61um in concentrations for RAPC = 0.551 moles dm -~ and OPC = 0.22 moles dm -3. This work shows that comparable concentrations are attained by 14 days. In comparison to potassium, sodium behaves rather differently; PFA obviously contributes sodlum to the system with Eggborough contributing significantly more than Longannet PFA. Blends with Longannet 81ve pore fluids with m,ch lower sodium contents than neat cement, but which are appreciably greater than would be expected by simple dilution. From solubility data on the alkali components of these two PFa's (Table 4) It is evident that PFA gives rlse to two sodium contributions, luch of the la appearing In pore fluid Is obtained from a relatively soluble fractlou0 e.g. surface alkali. It Is perhaps difficult to relate the analytlcally-determlned alkali contents in Table 4 to conditions relevant to cement hydration, but If PFA were mixed wlth water to a w/s ratio of 0.6, the resulting solution from E ~ b o r o u g h PFA would be 0.013N with respect to |a and O. O05K with respect to K. This readlly-soluble sodium is essentially all In solution by 14 days: sodium released from PFA at longer ages comes from the second source, namely the glassy

172

Vol. F.P. G ] a s s e r ,

18, No. 2

et a l .

matrix. Longer-term curing suggests that the sodium content reaches a steady state within 90 days ( 5 ) . Inexplicably the HA/~, which contained slightly less sodium than the OPC0 gave a slightly higher sodium concentration. One criticism made of pore fluid analyses Is that the squeezing proces~ removes the aqueous phase selectively from the larger Pores and, on account of thermodynamic differences arising as a function of pore radius, the analyes are unrepresentative o f the fluid contained in finer pores. Presently-available t e c h n i q u e s c e r t a i n l y remove o n l y 10-20~ of the t o t a l pore water, ltouever, the real situation i s q u i t e complex. In a l l p o r e s q u e e z e r s u s e d t o d a t e t h e s a m p l e h a s c y l i n d r i c a l g e o m e t r y , a force belng~ applied uniaxially. The actual pressure distribution achieved within the cylinder, by analogy wlth ceramic processing and tablettlng technology, is very unequal. The h i g h e s t p r e s s u r e s a r e achieved along t h e faces and walls of t h e c y l i n d e r , and it is l i k e l y t h a t a ~uch h i g h e r e x t r a c t i o n e f f i c i e n c y i s a c h i e v e d from t h o ~ e P o r t i o n s of t h e c y l i n d e r . Thus, a n o v e r a l l e x t r a c t i o n efficiency is only significant in this context if a homogeneous pressure distribution is achieved. In an attempt partially to resolve this question, Incremental pressures have been used,, and aliquots of pore fluid collected. These remained essential constant in composition from lowest to highest applied force, approximately 100 tonnes on a 42mm cy I i nder. Despite these difficulties of i n t e r p r e t a t i o n , pore f l u i d e x t r a c t i o n is the only practical method which h a s t h u s f a r b e e n d e v i s e d t o determine the internal constitution of the aqueous f r a c t i o n . Moreover, some f o r m s o f c h e m i c a l a c t i v i t y in concrete e.g. corrosion, are likely t o be i n i t i a t e d in the vicinity of l a r g e r p o r e s , r a t h e r t h a n I n t h e d e n s e r p a s t e f r a c t i o n , so aqueous phase c o ~ 0 6 1 t t o n s d e t e r m i n e d by pore squeezing are directly relevant.

The p r e s e n t f i n d i n g s on S o l u t i o n a l k a l i l e v e l s i n PFA b l e n d s a c c o r d ~ e l l with the r e s u l t s o f O r u t z e c k e t a l ( 9 ) . Potassium:sodium -nlar r a t i o s I n t h e aqueous pha~e a r e h i g h , on t h e o r d e r o f 10. It might thus appear t h a t PFA's kave i n g e n e r a l n e g l i g i b l e e f f e c t on s o l u t i o n a l k a l i contents. However, we b e l i e v e t h i s i s n o t n e c e s s a r i l y a l w a y s the case. In t h e s h o r t t e r n , t h e p r i n c i p a l a l k a l i s o u r c e s a r e on t h e s u r f a c e s o f grains; b o t h c l i n k e r a n d PFA a r e i n c o n t a c t w i t h h i g h - t e m p e r a t u r e g a s stream from which a l k a l i s c a n c o n d e n s e on s o l i d p a r t i c u l a t e matter. Essentially a l l of t h i s a l k a l i i s r e l e a s e d t o t h e pore f l u i d s ; its coxplete dissolution certainly occurs within the first 14 d a y s and p r o b a b l y much more r a p i d l y , w i t h i n t h e f i r ~ few h o u r s . Much o f t h i s a l k a l t r e m a i n s i n t h e p o r e f l u i d , where i t i s a v a i l a b l e t o i n f l u e n c e t h e hydration rate of c l i n k e r . Ye b e l i e v e t h a t t h e r e l a t i v e alkali c o n t r i b u t i o n s a r i s i n g f r o m c e m e n t a n d PFA c o n d i t i o n t h e p o r e f l u i d and influence the rate o f cement h y d r a t i o n . The l i t e r a t u r e is in d t s a g r e e l e n t on t h e r o l e o f PF£: some I n v e s t i g a t o r s c l a i m r e t a r d a t i o n , other acceleration. Ye d e v e l o p a t h e o r y i n v o l v i n g c h e m i c a l b a l a n c e s i n t h e s y s t e m which a i m s t o u s e p o r e f l u i d d a t a t o e x p l a i n why b o t h a c c e l e r a t i o n a n d r e t a r d a t i o n c a n be o b 6 e r v e d i n a p p r o p r i a t e c o n d i t i o n s .

Vol. 18, No. 2

173 PORE FLUID, COMPOSITION, SILICA FUME, SLAG, FLY ASH

(lli) Chloride Blndin¢v C a ~ c l t v

of Cement Pastes

The role of C I - , chloride, is important in concretes where it is known to lower the threshold electrochemical potential for corroelon of steel (11). Chloride may be present in cement-making raw materials, or deliberately introduced as an accelerator, or taken up from the environment as occurs upon immersion in sea water or when salt I$ applied as a de-iclng agent. At high chloride contents, C1- i s increaslngly incorporated in the paste in the form of chloroalumin~tes. Their compositions are shown in Table 5. Since chloroaluminntes require essential Ca and A1 for their formation It might be supposed that clinkers rich In these components would exhibit enhanced chlorlde binding capacity. According to this hypothesis, favourable conditions for chloride binding might be encountered in cement clinkers rich in C~A. Recent work has shown that C3A Is an excellent chloride binder (6). The present investigation was to study the CI- partition between solid and aqueous phases using two cements differing markedly in C~A content.

Chloride-Containlng Solid Aluminates |ature of Phase Formed Ca(OH)2, CaSO, and dilute C~C12 with aluminates

C.AB,~ - Type: C3A.~W'-.~(OB),CI)2. N~C~USO,.12B20 C3A.~,.12B=O solid solution

Ca(OB)=, CaSO. and concentrated CaCI= wlth aluminates.

a. ~-C i.CaCI2.1OB20 and CsA.3C,CI2.3OH20 (below - IO°C)

Raw Raterials and ~rocedures Two ordinary Portland cements, OPC and HAPC, were used. Table 1 records the mineraloglcal compositions of ;hese cements, from which it can be seen that the principal differences between the two lle in their C3A and ferrlte contents, the OPC being Low In C3A while the BAPC is very hlgh in C~A. Both cements were mixed neat to a constant water:cement ratio of 0.40 using mlxln 8 water whose composition ~ms

500C

I

~'I.OR~IE

FIG. 6 Pore Fluid Cl levels shown Function of Celent Type Tables for Properties of Cements) Cure, Duration Initial CaCI2 Concentration,

as a (see the and

HAPC-1&d " , ~ ' s " "

f

c ZO0~ m

ss I

s SS

,

1

Added CoClz ~ %

z

174

Vol.

18, No. 2

F.P. G l a s s e r , eC a l .

analytically adjusted in steps to contain b a t t e n 0.0 and 3.0 wt % as Cl as CaCI2. The mixes were cast into ca 42 mm diameter cylinders weighing - 200g, using split 'Perspex' molds. After they had set, the cyjllnders were stored in sealed vapour tight bags, to which had been added a few extra drops of water. Two ageing periods were used; 14d and 107d, after which the cylinders were squeezed and the pore fluid analyzed chemically for CI- content.

Re~ The two cements, despite their dlfferln 8 C~A contents, do not differ significlantly in their 14d chloride binding capacity, OPC having pore fluids only slightly lower in chloride than HAPC. b~nat is perhaps more significant is t h a t ageing lowers the pore fluld content in both cases suggesting that chloride ion removal occurs slowly. However, the extent to which the pore fluid is depleted in Cl- is most marked for the low C~Aclinker, OPC. These results suggest that the CaO and AI~{~ (or Fe20~), which, together with CI-, are necessary for chloroaluminate formation may be furnished by either or both C~A and ferrite phases. In sulphate-retarded clinkers, ferrite hydrates more slowly than C~A, so its influence on the hydrated phases is likely to become more apparent with increasing age. Ye therefore conclude that at longer ages, > 100d, the capacity of the hydrate products to bind chloride correlates better with the sum of the aluminate and ferrite phases than with either taken separately. It is noteworthy to mention that the chloride binding capacity is also influenced by the fineness and alkalinity of the cement and the chloride cation type (12, 13). The results are best appreciated when presented graphically, as in Fig. 6. Both "blanks', i.e. cements to which no Cl- had been deliberately added, lay close to the origin of the Figure. At CI- additions of up to 2% the relationship between (CI added) and (CI in pore fluid) is nearly linear although some suggestion of departure from linearity occurs in both cements after ageing for i07d.

(iv)

The use of blast furnace slags is long established, especially in central Europe where slag-based c e m e n t s c o n t a i n i n g up t o - 70% r e p l a c e m e n t o f c e m e n t have b e e n e m p l o y e d f o r many d e c a d e s . Experience shows t h a t t h e b e s t h y d r a u l i c a c t i v i t y is obtained using slags with a h i g h g l a s s c o n t e n t a n d t h a t t h i s g l a s s s h o u l d have a h i g h l i m e c o n t e n t . Slags are normally activated w i t h OPC t o improve t h e e a r l y s t r e n g t h , a l t h o u g h |aOH a n d ][OH a r e a l s o good a c t i v a t o r s . Despite the substantial CaO a n d SiO~ c o n t e n t s o f b l a s t f u r n a c e s l a g s - t y p i c a l l y 40% CaO and 30% SiO~ - t h e y d i f f e r from c e m e n t i n C/S r a t i o s and m o r e o v e r , have a math higher al~)~:Fe~Oy ratio, typically ) 1 0 , , and a g r e a t e r ~80 c o n t e n t , o f t e n " 8-12~. Moreover, s l a g s c o n t a i n a p p r e c i a b l e S i n c h e m i c a l l y r e d u c e d form a s s u l p h i d e (S ~ - ) i o n s . I t would seem r e a s o n a b l e t o suppose that substantial slag additions would a p p r e c i a b l y m o d i f y t h e c o m p o s i t i o n of cement pore f l u i d ~ , , particularly at longer ages such that appreciable hydration of the slag had o c c u r r e d . Little i n f o r m a t i o n i s a v a i l a b l e on t h e c o m p o s i t i o n o f s l a g - c e m e n t p o r e f l u i d s . T u u t i (14) e x a l i n e d 70% s l a g b l e n d s a n d p a r t i a l l y analyzed the pore

Vol.

18, No. 2

175 PORE FLUID, COMPOSITION,

SILICA FUME, SLAG, FLY ASH

fluids obtained after ageing up to 21 months; S 2- concentrations up to 300 mg dm --~ were obtained. The modification of pore fluid compositions was considered to be significant because some evidence was obtained, using synthetic pore fluids, that the presence of sulphide inhibited pitting of mild steel induced by chlorides. Ve report here studies aimed at characterizing the chendcal environment in slag cement blends. In addition to chemistry, two concepts are relevant. One is the Eh, or redox potential, the other is the poising capacity. The redox poising capacity, p = dC~/d~, is analogous to the buffering capacity in the pH system; it is evaluated as a function of the concentration, C, of the redox couple, the number of electrons required for its oxidation or reduction and the difference between the potential, E, of the solution and the formal potential of the couple.

Slag cement blends were made using cement OPC, Table I, and blast furnace slag. The slag was examined petrographically and found to contain > 98Z glass; it also had 1.31Z S. Cylinders formulated with O-90Z slag and OPC as well as two llme-actlvated compositions at 95 and 97.5 slag (balance, Ca(OH)=) were also made. the cylinders were aged 25d at both 20oC and 40~C. The fraction of slag reacted, determined by chemical dissolution in EDTA and weighing the slag thus recovered, was about 25Z at both temperatures. The cylinders aged at 20°(; were used for pore fluid extraction. During curing, the cylinders were stored in individual sealed polythene begs which inhibit but probably do not totally prevent oxygen transport. The pore fluid was, however, collected with the exclusion of air and showed no signs of precipitation during handling. In these preliminary studies we determined the composition of S species, ~ . and Poising capacities of the pore fluid. The content of S =- was determined using a S =- selective agS electrode against standards prepared to the same pH as the pore fluid; $20~ =- and S04 -2- were determined by ion chromatography. The ~ was determined using a Pt electrode with a calomel electrode as a reference standard. Poising capacity has no exact definition for cement s y s t e m : we therefore adopted as a measure of poising capacity the ability of the pore fluid to reduce Ce =" using the couple Ce ~- + e- d Ce =°. The pore fluid can be titrated directly, but solid or wet cement solids require di~lution in some medium which is neutral with respect to redox reactions; EDTA is satlsfactry in this respect. The end point of the reaction is determined potentlometrlcally, and the results expressed as moles of electrons per gram of solid. Ye e~plmslze that pore fluid expression is an essential step in determining r~; measurements on solid cement matrices are fraught with uncertainties owing to the high resistivity of the matrix. However, poising capacities c~u be determined on the solid hydration products, on the pore fluld, or on both. eesults The ~ of several commercial, Portland cements were measured and found to lie between +75 and +I00 mV. These numerical values, corre~pondlng to mildly oxidizing conditions, were not unexpected in view of the thermal and redox equilibirla which obtain during the normal firing of cement clinker. The properties of slag cement blends,

176

Vol. F.P. Glasser,

containing 4.

18, No. 2

et al.

in this instance an 85Z slag,

15Z cement,

are shown

in Table

TABLE 6 The pore fluid of slag blends i s slightly lower than that achieved by neat OPC, Some Physlochemical Properties of Slag - Cement Pore Fluids which at L>Sd gave a pR ~ 13.4. Ageing Conditions However, the most notable Function 35d. 20°C 25d. 40">C difference between the pH 12.6 12. b chemical make-up of the PL~ -227 mV -202 mV pore fluids is the presence [E ~-1 120m~ dm -~ 145m~ dm --~ of S species, includlng [S~Oc,~- ] 3 1 0 ~ dm -~ 320m~ d ..... two - S~032- (thiosulphate) [SO, =-] 297m~ dm-:' 198rag dm -= and S =- (sulphide) - which are not normally present in OPC pore fluids. The presence of these species and the slightly lower pH enhance sulphate solubility which, at 200-300rag dm -~ in slag rich blends, Is significantly higher than in OPC controls at the sa~. ages. The p o i s i n g capacity of the cements are compared in Table ?. Since polsln S capacity relates to electron transfer, it is most conveniently expressed in terms of moles of electrons available per unit mass of substance. Several other cement types are included for comparison, and the poising capacity of blend components, determined separately by total dissolution, is found to increase in the order OPC < HAC ~ natural pozzolan < BFS. It might be supposed that the carbon content of PFA would influence the E~. but in practice it is largely inert, hence the poising capacity of PFA is relatively low despite the presence of carbon. Blast furnace slags, however, have poi~In S capacities typically an order of magnitude higher than OPC.

Although much remains to be learned about chemical differences between the envlronments in slag cements and those in OPC, the present evidence shows that major differences do exist. ~reover, these differences are such as are likely to yield significant differences in the performance of materials contained in cement. Flrstly, the sulphur rich chemistry of the pore fluids in high slag blends is likely to alter signlflcantly the corrosion potential of meta111c materials. It is not known how, for example, steel will behave in such environments. Tuutl (14) assumed that the presence of S =- in slag concretes would be relatively ephemeral, and this is probably correct when atJ~spherlc oxygen can penetrate; under such conditions, chemlcally-reduced S will be oxldlzed to S042-. However, well-made slag concretes have low permeabilities and their substantial poising capacity will retard the progress of the 'oxidized' zone. Xoreover, many concretes are used in service conditions where they are permanently in contact with oxygendepleted environments and under these circumstances it is difficult to see how the F~ can change.

The alkali cycle in cements is, In the short term, influenced by blendlng agents in two ways. Firstly, they may contribute readily soluble alkali which adds to that released from hydrating cement.

VoI. 18, No. 2

177 PORE FLUID, COMPOSITION, SILICA FUME, FLAG, FLY ASH

TABLE 7 Poising Capacity,

Showing O x i d i z a b l e Content o f V a r i o u s Unhydrated Cements

~erlal OPC BAC PFA <'~ Natural Pozzolan c2~ BFS <3~ BFS <4' |otes: Italy.

Oxldisible Content, in eouivalents 8-' solid 9.59 x 10 - s 2.30 x I0-" 3.43 X i0 -a 1.73 x I0-" 1.14 X 10 - 3 1. I0 x I0 -~

(I) Class F Fly ash; Eggborough,~. (2) Pozzolanico Grade 325, (3) Ravenscraig, Scotland. (4) Frodingham, England.

Secondly, and in the somewhat longer term, they decrease the Ca:Si ratio of the C-S-H phase and thereby improve its uptake Potential for alkali. the alkali partition into C-S-H is most effective for Cs, and decreases progressively for K and |a. Sufficient differences occur between K and Na, so that the common practice of combining alkali as 'equivalent Na~O' is of questionable value in assessing, for example, the potential for alkali-aggregate reactlonin blended cement systems. Silica fume, on account of its rapid reaction and intimate dispersion, is more effective in controlling alkali than class F PFA's, although substantial differences can occur between different PFA's: the reasons for this are not fully understood. Free chloride ions in cement paste increase the potential risk of steel corrosion. The binding power of the cement paste for chloride ions has been assessed, using two cement types, differing in C3A content. These do not differ significantly in chloride binding capacity at longer ages, and it is suggested that chloroaluminates can be formed from either or both C~A and ferrite clinker phases, hence the Poor correlation with C:,A alone. Slags are shown to develop a vigorous sulphur chemistry in cements. The reduced S species, S~-, ~ 0 ~ 2- etc. lower the redox potential and increase somewhat the solubility of S042-. Roreover, blgh slag contents buffer the redox potential at low values, minus 200-250 mY and possibly even lower. Much ~ r k yet remains to be done on assessing more fully the evolution of Pore fluid compositions with time and temperature, with the S partition between solid and aqueous phases and on relating chemstry to microstructural changes. The a d d i t i o n of blending agents, each different in composition, introduces great diversity into cement systems. This diversity affects the internal environment. The relationship between amount of blending agent and the modification to properties is not yet fully quantified: indeed, in some instances we are still seeking appropriate parameters to characterize the system. I f t h e b e h a v i o u r o f c e m e n t s y s t e m s i s t o be m o d e l l e d and p r e d i c t e d , i t w i l l be n e c e ~ m _ r y t o add t o and e n h a n c e t h e quality of the data base. But t h e p r e s e n t p a p e r r e p o r t s e n c o u r a g i n g progress in this direction, and i t s c o n c l u s i o n s a r e i n good g e n e r a l a g r e e m e n t w i t h thorpe o f o t h e r s t u d i e s .

178

Vol.

18, No. 2

F.P. G l a s s e r , et a l .

The financial support of the Science and Engineering Research Cuncil (OK) is acknowledged in respect of Dr. Luke's contribution. The Department of the Environment (UIr) provided financial support for Dr. Angus. Sis contributions w i l l be used in the formulation of Government policy, but at this stage they do not necessarily represent Government policy.

1.

2. 3. 4.

5. 6.

7.

8. 9. 10. 11.

12. 13. 14.

F.P. G l a s s e r , J. K a r r , " E f f e c t of S i l i c a , PFA and S l a g A d d i t i v e s i n t h e C o m p o s i t i o n o f Cement Pore F l u i d s ' , pp 2 3 9 - 2 4 2 i n " A l k a l i s in Concrete', Proc. 6th SyIp. on Alkali-Aggregate Reactions, Copenhagen DBF, 1983. C.E. llcCulloch, R.¥. Crawlord, M.J. Angus, F.P. Glasser and A.A. R a h m n , ](Ineralogical Rag., 49, 211-221 (1985). F.P. Glasser and J. Raft, 11 Cemento Iio. 82, 85-94 (1985), C. Plo,rJan and J.O. Cabrra, Effects of Ply Ash Incorporation in Cement and Concrete, Ed. S. Diamond, p 71 Pro<:. N.R.S. Resting Boston, Ra (1981). C. Plowman and IIJ.G. Cabrera, Cem. Concr. Res. 14, 238 (1984). I. Jawed and J. Skalny, Effects of Fly Ash Incorporation in Cement and Concrete, Ed. S. Diamond, p 60, Proc. I(.R.S. Meeting, Boston IU (1981). A. Ghose and P.L. Pratt, Effects of Fly Ash Incorporation in Cement and Concrete, Ed. S. Diamond, p 82, Proc. X.R.S. l~etlng, Boston RA (1981). ¥. FeJon, I(.¥. Grutzeck, D.X. Roy, Cem. Concr. Res. I~, 174 (1985). ICY. Grutzeck, ¥. FaJun, D.M. Roy, Rat. Res. Soc. Sy~p. Proc. Vol. 43 (1985). F.P. Glasser and J. Rarr, The Chemistry and Chemically Related Properties of Cement, Brit. Cer. Proc. Ho. 35 pp 419-429 (1984). ¥.R. l~olden, C.L. Page and H.R. Short, "The Influence of Chlorides and SuIphates on Durability = Chapter 9, pp 14-150. "cororosion of Reinforcement in Concrete Colnstruction=. Ed. A.P. Crane: Society of Chemical Industry, Ellis Horwood, Chichester, England, 1983. Z. Byfors, C. Raneson and J. Tritthart, Cem. Concr. Res. 16 760 (1986). G. Bluak, P. Gunkel, H.G. Smolszyk, Proc. 8th Int. Cong. Chemistry of Cements i 85-90 (1986). K. Tuuti, =Corrosion of Steel in Concrete = Swedish Cement and Concrete Research Institute, Stockholm ISB| 0346-9606 pp 469 1982.