Investigation of relations between porosity, pore structure, and C1− diffusion of fly ash and blended cement pastes

CEMENT and CONCRETE RESEARCH. Vol. 16, pp. 749-759, 1986. Printed in the USA. 0008-8846/86 $3.00+00. Copyright (c) 1986 Pergamon Journals, Ltd.

INVESTIGATION OF RELATIONS BETWEEN POROSITY, PORE STRUCTURE,

AND

CI- DIFFUSION OF FLY ASH AND BLENDED CEMENT PASTES

Shiqun Li and Della M. Roy* Materials Research Laboratory, The Pennsylvania State University University Park, PA 16802

(Refereed) (Received June 26, 1986) ABSTRACT By-product fly ash from coal combustion influences the properties of cement pastes significantly. A study has been carried out to contribute to understanding the behavior and function of fly ash and other substituents in cement pastes. Comparisons are made with the properties of pastes of as-recelved portland cement and preliminary comparisons made with granulated blast furnace slag cement and silica fume-cement. Relations between the median pore size, porosity and C1diffusion of fly a s h - c e m e n t pastes, and the i m p l i c a t i o n s of observations for diffusion mechanisms and the probable pore structure of the blended cement pastes are discussed. Measurements include none v a p o r a b l e water (Wn), porosity and pore structure by mercury porosimetry, C1- transport by a rapid method under the influence of an applied e l e c t r i c field, and the p e r m e a b i l i t y to water. X-ray diffraction phase characterization has been made before and after C1diffusion.

Introduction Blended cements containing appropriate amounts of a suitable siliceous modifier such as fly ash, glassy blast-furnace slag, silica fume, or natural p o z z o l a n exhibit s u b s t a n t i a l l y more resistance to water and c h l o r i d e ion penetration than does corresponding portland cement type-I. The prospects for making use of the property of combining with calcium hydroxide, which is possessed by m a t e r i a l s a v a i l a b l e to the cement industry, appear promising. This study focused on fly ash as a cement substituent, not only to improve byproduct utilization, but also essentially because of the positive effects this material may have on the r e s p e c t i v e binders. An understanding of the pore structure of cement pastes and its influence also is of practical importance from many viewpoints. Numerous studies have been made of the influence of w/s ratio and curing temperature on the penetration of C l - and water through pastes (1-6), but the effect of the amount of fly ash and its chemical composition on the pore structure and CI- diffusion are not tully known. The *Also affiliated,

Department of Materials Science and Engineering. 749

750

Vol. 16, No. 5 Shiqun Li and D.M. Roy

current study attempted to i m p r o v e the l e v e l of u n d e r s t a n d i n g of the relationship between the amount of fly ash addition, and the porosity and C1diffusion. Experimental Methods and Characterization Results (1) Sample Preparation and Curing: The chemical a n a l y s e s and physical properties of the essential m a t e r i a l s are shown in T a b l e s 1 and 2 , respectively. Type I portland cement and low-calcium fly ash were mixed in weight p r o p o r t i o n s of 100:00, 80:20, 70:30, and 60:40 w l t h w / s ratios of 0.3. 0.5, and 0.6, respectively. Cement and slag were mixed in the weight ratio of 35:65 with w/s of 0.3 and 0.5, respectively; and cement and silica fume, 90:10 with w/s ratio of 0.5. The sample catalog is shown in T a b l e 3. After being mixed thoroughly, all the samples above were molded, and de-aerated by means of vibration for 1 minute, then cured under saturated deionized water vapor for 28 days at 38eC. (2) Non-Evaporable Water: Because no simple crying procedure is capable of separating chemically combined from physically adsorbed water, the term, non-evaporable water is adopted to mean the amount retained by a sample after it has been subjected to a drying procedure intended to remove all the free water. Samples were first dried at 105eC for 24 hours, then ignited in an electric furnace from 950e-1000eC for 20 minutes and until constant weight. (3) Permeability to Water: Cylindrical samples for testing permeability were cut into a half-inch length from the mlddle of the cured speclme~ The water pressure for testing permeability was from ambient atmosphere to 1500 psi zor five uays. ( 4 ) Poroslty and Pore Size Distribution: Samples were freeze-dried and then i n v e s t i g a t e d by mercury intrusion porosimetry, carried out with a

Table 1.

Chemical Analysis of Starting Materials (percent by weight).

composition SiO 2 AI203 TIO 2 Fe203 MgO CaO MnO SrO BaO Na20}

silica rume B59 (82-352)

2.59

(950~1000eC)

Totals on

3.70 .

.

---

0.20

.

.

0.59 . . 1.65 . .

1.20

2.22

100.30

100.45

slag B64 (82-588)

94,54 0.10 <0.05 0.20 0.26 <0.05 0.007 <0.05 <0.05 O.05

---

303 S Total c a r b o n Insoluble residue

aGain SO 3 .

fly ash B92 (85-382)

20.50 53.20 6.00 26.00 --1.38 2.10 7.95 2.90 0.97 62.80 3.57 --0.041 . . . . . . . . . . . . 0.90 0.29

K20 P205

L.O.I.

portland cement Type I (I-15)

.

.

.

.

.

.

0.45 0.09

0.55 <0.05

0.44

--1.28

. (750eC)

.

32.70 10.00 0.48 0.86 11.00 42.04 0.476 0.05 0.05 0.25

.

.

.

.

.

.

.

.

3.57 99.66

ignition indicates the existence of sulfur as S or S 2- rather

* 99.59

than

Vol. 16, No. 5

751 POROSITY; PORE STRUCTURE, C1

Table 2 .

DIFFUSION, FLY ASH PASTE

Physical Properties of Essential Materials. materials portland cement (I-15) Type I

properties

fly ash 85-328 B92

silica fume 82-352 B59

slag 82-588 B64

density (Kg/dm 3)

3.18

2.35

2.12

2.90

specific surface area BET (m2/k~) Blaine (mZkg)

1112 497.3

921.0 305.2

21,400 ---

1039.9 486.0

Table 3.

Sample Catalog; Porosity and Surface Area Determined by Mercury Poroslmetry. porosity ( c c / c c )

Sample Number 8500 8501 8502 8520 8521 8522 8530 8531 8532 8540 8541 8542 8511 8560 8561

Mix Proportion (by weight) CIF.A. CIF.A. C/F.A. C/F.A. C/F.A. C/F.A. C/F.A. C/F.A. C/F.A. C/F.A. C/F.A. C/F.A. C/S.F.

C/S. C/S.

C = Cement, Slag.

F.A.

100:0 100:0 100:0 80:20 80:20 80:20 70:30 70:30 70:30 60:40 60:40 60:40 90:10 35:65 35:65

w/s

Before ClDiffusion

After CIDiffusion

0.3 0.5 0.6 0.3 0.5 0.6 0.3 0.5 0.6 0.3 0.5 0.6 0.5 0.3 0.5

0.134 0.300 0.383 0.176 0.357 0.406 0.183 0.400 0.471 0.276 0.467 0.527 0.322 0.138 0.316

0.130 0.321 0.379 0.184 0.365 0.420 0.213 0.400 0.477 0.230 0.468 0.506 0.340 0.162 0.302

= Fly Ash,

Surface Area (m2/g) Before CIDiffusion 28.0 37.0 51.0 44.0 55.0 60.0 50.0 59.0 60.0 56.0 64.0 66.0 56.0 40.0 83.0

After CIDiffusion 20.4 45.0 49.0 42.0 58.0 59.0 44.0 61.0 65.0 51.0 62.0 65.0 67.0 44.0 80.0

S.F. = Silica Fume, S = Granulated Blast Furnace

Quantachrome Autoscan Porosimeter. Table 3 shows ~he porosities and surface areas, as measured by mercury porosimetry, before and after CI- diffusion. (5) C h l o r i d e Ion Diffusion: The method of rapid d e t e r m i n a t i o n of chloride permeability (7) used water-saturated samples which were measured up to 5 hrs. under an applied electric potential of 60 volts across 5-cm samples. Figure 1 s c h e m a t i c a l l y i l l u s t r a t e s the e x p e r i m e n t a l a r r a n g e m e n t for measurement of chloride ion diffusion under the action of an electric field with the resistance m o d i f i e d from that used in standard FHWA tests (7). The p r i n c i p l e of this method is to d e t e r m i n e the resistance ~o transport of the chloride ion through the sample under the potential of an electric field, by comparing the charge passed after a fixed period of time. Although there are a number of factors which influence such a process, a proportionality has been found b e t w e e n the charge passed and the ease of transport of C l - ions, the

752

Vol. 16, No. 5 Shiqun Li and D.M. Roy

j ll II

MC I ROVOLTDMM

l

POWER SUPPLY 0 D,C. 0 160 v,

more mobile species, through the sample. (6) X-ray Diffraction Phase Characterization has been carried out on all of the s a m p l e s before and a f t e r C I diffusion.

200

D,C,

)

2 A,

lol n

DIFFUSION CELL ~

/

~.~

C LL z

3.0%

~

~

~' . ~

=~,~

Results CELL I f

0.3N

~

1

Figure

/NaClcSOCC LcUTO I }N 4~~/~/~ 0 N 0aO 4H 0S 0OLUTION/IJ

2

shows the coulomb aCharge a S f u passed n c t i o n of

porosity oz the specimens, ii lustrating the disPLATINUM ELECTRODE tinct relationships establlshed for each maFIG. I. Schematic diagram of rapid chloride permeability terlal, e.g., test cell. 100% cement, 80:20 cement:fly ash, etc. A several-fold decrease in chloride transport rate is evident in the blended cements compared with the control specimens, pure portland cement pastes. Figure 3 shows a log-linear plot of the water permeability, indicating also a unique and somewhat different type of relationship for each material. There /

~,,--- lo cm

.08cm~ /

--~

X.C

,5OO0

~ a X . ' ~ , / F . A . (80/20) e.C./F.A, (70~0) / =.C,/F,A, (60/qO) /

cn 4000 OQ ~E

/

i0-10

*.C./S.F, (90/10) ~C,IS, (35165)

1o-11

''ZO~ lnnn 0 0.0

LI

0.Z

0.3

0.4

0.5

o.8

,E" no

2.

Coulomb charge transported as a function of porosity (Hg porosimetry). C = cement, FA = fly ash, SF = silica fume, S = granulated blast-furnace slag.

L3

L,

Ls

POROSITY (cclcc)

POROSITY (cclcc) FIG.

i

nl

FIO. 3.

Water permeability vs. porosity; specimens as in Fig. 2.

Vol. 16, No. 5

753 POROSITY, PORE STRUCTURE, CI- DIFFUSION, FLY ASH PASTE

. . . . . .

50r

X.C,

b

m. C,/F,A, (80•20) • ,. C,/F,A, (70/30) A. C,/F,A, (60/qO)

40

PORE SIZE DIST'RIBUI:ION ..... x.C

40 [.

•.

30 L

x

v.

C/S 35/65

IE x@.

m E

•

vv

X~x$_

2O

@ $

•

20 L

Xx X x

v

b GJc X x

I0

N

~@

1o [ x x

10-3 FIG.

4.

r

(urn)

Ol lO-2

size distribution of pastes: cement and slag.

Pore

;

• x

vv •

~1~ m

. . . . . .

0

Xx

10

i

i

r ~um)

i

i

i

i

i

10-2

(a) cement and cement-fly ash; (b)

are inadequate data available for slag and silica r u m e mixtures, but they each appear a l s o to b e h a v e d i s t i n c t l y differently. The n o n - e v a p o r a b l e water content (not shown) increases as the porosity increases, not ooinoidentally since in high w/c materials more water is available both to form cementltious h y d r a t i o n product, and to remain f i l l i n g the pores, and is r e f l e c t e d in the porosity (or as evaporable water, not measured). Because the r e l a t i o n s h i p b e t w e e n porosity (measured Dy Hg porosimetry) and the other parameters was not totally straightforward, other relationships were also sought. Figure 4 gives differential plots of the pore volume with respect to radius, indicating the u s u a l l y finer pore structure with the blended pastes. Generally similar relationships were also observed for higher w/s m a t e r i a l s (w/s = 0.50). Since no simple r e l a t i o n s h i p s were yet found to a p p l y to a l l the materials, other r e l a t i o n s h i p s were examined. It is b e l i e v e d that in such materials the water/solid ratio expressed according to volume ratio is more r e l e v a n t than the weight ratio. Indeed, Fig. 5 expresses the porosity as a function of w/s (volume) ratio. This figure shows that pure portlanG cement pastes have higher w/s (volume) for a given wlc (weight) than the comparative blended pastes. One family of curves represents each w/s (weight) ratio, and another family compares similar materials, i.e., 100% cement, 6 ~ cement, 40% fly ash (weight), etc., with varying wls (weight) ratios. This indicates that fly ash substituted materials have significantly lower wls (volume) ratios, which ~hould help in decreasing permeability; however, at the same time the measured porosity is higher. Also, each m a t e r i a l has a distinct property variability with changing w/s. O n l y w h e n the m e a n pore radius is used as a aependent parameter is a r e l a t i o n s h i p found w h i c h flts most of the data (Fig. 6). W h e n the mean pore radius is plotted as a function of w/s (volume) ratio, a linear flt with some scatter is obtained for all the data, showing that the m e a n pore radius is strongly controlled by the initial w/s (volume) ratio, for all the materials. Figure 7 g i v e s the c o u l o m b s charge passed (~ c h l o r i d e permeability) as a function of w/s (volume) ratio. Again, as with the porosity by Hg porosimetry two ~amilies of curves are found, one showing the effect on C1- transport of changing w/s (volume) within a given material, and the other showing the real effect of keeping a constant w/s (weight) ratio. Figure B is a plot of water

754

Vol. 16, No. 5 Shiqun Li and D.M. Roy 3O 1

I

I

xC o C/FA (80120) $ C/FA (70/30) A C/FA (60140)

w/sT0.6wt ~%~ / ~ w/s=

0.5 -

%./

//#'~

~

~

I

0.5wt

* C/SF

I

I

\

(90/10)-

,C/S

I

\

I

A\

(35/65) 20_

ii 0

O.4

\

//

\ \ o o.3

\ \ \

I0

\

Q2

0.1 2.0 FIG. 5.

I

I

I

I /

1.75

1.5

1.25

1.0

water/solid

(vol)

I

Porosity as a f u n c t i o n of w/s ( v o l u m e ) r a t i o ; abbrev i a t i o n s as in Fig. 2.

Xw/s:O.6wt ~-- ~ ~

1 t

I w/s =05wt

/

~*SF

I

1.25 r a t i o (vol)

Mean p o r e r a d i u s ( v o l . ) ratio.

I

\

\

T E

\

1

FIG, 6,

I

1.5 water/solid

1.0

vs,

w/s

I

I

i0-?

4000

1

I

1.75

I

/ ",, 1 \ / "

/

2.0

0.75

o

.=--

\ ~w,'_s=

I0 - e

\

\

iO-e

.~ I0 -'o

x o.3. II

E i0_11 _

2000

\

/

10_12 iO'-Is

2.0

FIG. 7.

1.5 water/solid

1.0

0.75

10-14

(vol)

Coulomb charge transported vs. w/s (vol.) ratio.

A

-

FIG. 8.

I 1.75

I I 1.5 1.25 water/solid (vol)

X o4~

I 1.0

Water permeability vs. (vol.) ratio.

0.75

w/s

permeability (log linear plot) vs. w/s (volume) for which most of the data fit a single curve; there are some u n e x p l a i n e d odd data which, it is believed, reflect the difficulty of this type of measurements. By tJ~p meth_o~ used it is d i f f i c u l t to make v a l i d m e a s u r e m e n t s b e l o w ~ 1 0 - ~ x - 1 0 -Iz cm's -. Figure 9 expresses water permeability instead as a function of mean pore radius (loglinear plot). The r e l a t i o n s h i p is linear for a l l the m a t e r i a l s with some scatter and one c o m p l e t e l y a n o m a l o u s data point which, again, r e f l e c t s the difficulty in making linear permeability measurements of very low permeability materials. Since it might be expected that a r e l a t i o n s h i p exists between the

Vol. 16, No. 5

755 POROSITY, PORE STRUCTURE, CI- DIFFUSION, FLY ASH PASTE x.~l

6000

I

I

I

I

I

I

\ \ \ \ \ \

i0 - ?

T

4000

io-e

/

Io-e i0_~o

io-" i0 -m

2000

/

/ ,,// I

FIG. 9.

-I[-

/ / . /

\ \x

c.)

o E io-"

\ \

E

.

A

I I I I0 20 Mean Pore Radius (nrn)

I

30

Water permeability vs. mean pore radius.

lO-" lo-e lo~ io-© lo~' lO-" lo-" Water Permeability cm-s"1

FIG. 10.

Coulombs charge transported vs. water permeabillty.

t r a n s p o r t r a t e of I c h l o r l d e ions (reflected in coulombs 6000 charge passed) and liquid transport (water permeabll ity) the two are p l o t t e d in Fig. 1 0 . Again, a sequence of indi4000 vidual relationships E is f o u n d f o r the 0 different materials; X O in addition, there u are a couple of anomalous points. Sur2000 prisingly, the silica fume material exhlbited higher c h l o r i d e transport not c o n s i s t e n t with the low w a t e r per~t I I I meability; however, I0 20 30 not enough s a m p l e s Mean Pore Radius (rim) of s i l i c a t'ume (or slag) were i n v e s t i FIG. II. Coulombs charge transported vs. mean pore gated to m a k e v a l i d comparisons with the radius. o t h e r b l e n d e d materlals. Finally, a series of linear relationships is also found between c o u l o m b s charge transported and mean pore radius (Fig. 11) for type I cement paste and for each of the three levels of rly ash substltutlo~ Within each material, the chloride transport rate increased linearly wlth mean pore size. There were not enough data for slag and silloa zume to establish clear relationships.

756

Vol. 16, No. 5 Shiqun Li and D.M. Roy

D!scussion Porosity, mean pore radius and pore size d i s t r i b u t i o n in cementitious m a t e r i a l s are very important m i c r o - s t r u c t u r a l characteristics; they are r e l a t e d to a series of properties of materials, such as f l e x u r a l strength, fracture toughness, durability, and r e s i s t i v i t y to ionic diffusion. The results presented here reveal the fact that the pore structure of materials in q u e s t i o n is e s s e n t i a l l y affected by the fineness, w/s ratio, chemical composition, and p o z z o l a n i c reaction. The m a t e r i a l s i n v e s t i g a t e d were relatively mature, having been cured 28 days at 38oc; therefore, they would be expected to h a v e d e v e l o p e d fine pore structures. In the d e s c r i p t i o n which follows it is assumed that diffusion only takes place in the aqueous phase and that CI- ion adsorbed onto or adsorbed into the solid phases and can only move by desorption into the liquid with which it is in equilibrium. Both CI- ion and water penetration clearly confirm that the permeability to various species of various w a t e r - s a t u r a t e d cement pastes is a strong function of chemical c o m p o s i t i o n at fabrication; of course, it is a function of w/s ratio, e s p e c i a l l y of the w/s (volume) ratio. B l e n d e d cement pastes exhibit much lower chloride transport rates, as reflected in the transport under an imposed electric field. With increased fly ash replacement ratio and decreased w/s ratio, the p e r m e a b i l i t y to water and, e s p e c i a l l y , to c h l o r i d e ion, is dramatically decreased although the porosity is increased. For instance, in the case of 40~ r e p l a c e m e n t fly ash cement pastes of w/s ratio of 0.3, the porosity (measured by Hg porosimetry) is 0.276 cc/cc, which is twice that of the corresponding pure cement paste (0,134), but the charge passed, for the former, is 170 coulombs, which is only 1/18 as great as that of 3100 coulombs of the latter. Kumar and RoY ,(4-6) found the f o l l o w i n g c o m p a r a t i v e CId i f f u s i o n rates, D (10 - 1 3 m s- I ) under an imposed c o n c e n t r a t i o n gradient, measured at 38oc, for low w/c pastes: slag cement 35:65

pc

slllca fume 90:10

fly ash 70:30

w/s

0.35

0.30

0.35

0.30

0.35

0.35

0.30

D

156

87

18

9

7.6

13.5

13.4

The d i f f u s i o n rates were an order of m a g n i t u d e lower for all the blended materials than for portland cement, roughly comparable to the magnitude of the differences found in the current study under an applied electric field. The most important control ling physical parameter, however, appears to be the mean pore radius. Accompanying changes, the pore surface area increases as the mean pore radius decreases, generating greater pore surface area to impede transport of species. The total porosity measured by Hg porosimetry appears not to be the most significant measure, as it must include closed pores (including possibly hollow fly ash spheres) broken and intruded under the very nigh Hg pressures; while instead the median pore size may reflect the rate-controlling pores for the transport of species. In fact, a linear-log relation was found between mean pore radius and water permeability. In contrast, no simple r e l a t i o n was found between the coulombs charge passed and mean pore radius, but instead a family of linear relationships for each material was found. Thus, the ionic transport under an applied field is dependent on the chemical as well as physical nature of the host medium. The differences in the chemical nature are contrasted in Fig. 12: (a) shows

Vol. 16, No. 5

757 POROSITY, PORE STRUCTURE, CI

DIFFUSION, FLY ASH PASTE

A. C a l c i u m A t u m i n a t e H y d r a t e Gel

A.

O. C a l c i u m A l u m i n a t e $ | 1 1 c a t e Hydrate

B. CEMENT

70:30

ACH

C. C a l c i t e CH

O. Ca4A1207 " nH20 E, A zope o f SlO 2 g e l

Qz. Quartz m.

C2P

Muiltte

CH

I . . . . 15

i 20

. . . .

i 25

....

J .... 30

C3A

AFt

i 35

" ~

. . .

, . . 40

, ]0

AFt

CAF A F t

15

CH C /C~S

AFt

i

I

,

20

25

30

. . . .

i

, .

35

28 CuKa

FIG. 12.

(a) XRD patterns of 70:30 C:FA pastes with w/s = 0.3, 0.5, 0.6, r e s p e c t i v e l y , from bottom to top. (b) XRD of p o r t l a n d cement pastes; w/s as in (a).

decreasing Ca(OH) 2 and also a silica-rich Eel formed wlth 30% fly ash; thus, part of the Ca(OH) 2 reacts w i t h the fly ash to form C-S-H, but part of the product is a s i l i c a - r i c h Eel, with a broad diffuse peak near 22.5@20; (b) shows the similar portland cement sequence with increasir~ w/c, from bottom to top, the main difference being more anhydrous cement phases and less Ca(OH) 2 in the low w/c specimen There is no singular relation apparent between porosity or even mean pore size, and transport properties of all materials under an applied field. An important difference b e t w e e n m o l e c u l a r d i f f u s i o n and ionic d i f f u s i o n is relative importance of the additional charge interaction between anion and the other ions in the pore solution and on pore surfaces (the electrlcal doublelayer). The results above sugEest that a reduction in chloride transport rate occurs a l s o due to the change of chemical c o m p o s i t i o n beyond that caused by pb~ysical factors. It can be assumed that the activation energy for chloride ion diffusion In blended cements, including ash cements, must be considerably in excess of values for the activation energy for ehlorlde in diffusion in OPt pastes. To e x p l a i n the fact, a h y p o t h e s i s is made: because of Dhe p o z z o l a n l c reaction, it is assumed that there is, on the one hand, a signlficant amount of gel produced during hydration in ash cement pastes; the critical point is that the Eel-type hydrates are malnly located to block the pores instead of just form as a surface layer on the solid pore surfaces. Figure 13 shows this model s c h e m a t i c a l l y . A c c o r d i n E to Steopol et al. (10), SiO 2 Eel d i s p l a y e d very low permeability for chloride ion, and Craciunescu (11) reported that the d i f f u s i o n of C l - ion was affected by the type of cation. The authors calculated that the sum of the cone~ntratign of cations in the pore solution of hydrated pastes studi~dhere, Ca +2, A1 +s, and Si +4 (the A1 probably was a complex ion such as AIOH ~T and Si was a complex ion or multimeric species) in

758

Vol. 16, No. 5 Shiqun Li and D.M. Roy

e ~ 4RRI

Hydration Products of OPCParticles with I~ater

| /

A Thin Layer of H~cl;~t,~shPr~cWtst er

Impermeeb|e Gelatinous Hydrates of {alctum Silicate and Calcium A]umtnate ~Pozzolantc Reaction> and Some SIOZ Gel

FIG. 13.

Model (schematic) showing impermeable gel plug formir~ in pore.

FIG. 14.

Model of pore tortuosity and ions inhibiting CItransport.

fly ash cement paste (w/s = 0.5) pore solution is about 2.5 times as high, and the c o n c e n t r a t i o n of K + is twice as low as that in normal p o r t l a n d cement paste pore solutiom The former ions have lower diffusion rates and restrict the m o b i l i t y of the coexisting CI- ion as well, whereas the K + ion, on the contrary, increases the Ci- ion mobility (12). Therefore, this might render a higher resistance to CI- ion diffusion for fly ash cement pastes. Meantime, the type of C-S-H in a hydrated f l y ash cement is different from that occurring with OPC pastes (9), and the Ca(OH) 2 content is lower, being replaced Dy the C-S-H reaction product, which causes decreased permeability ot the former pastes. This phenomenon explains the better resistance to chemical attack. The ultra-fine structure C-S-H increases in proportion (9) with the ash content at the expense of a part of the portlandite. The lesser amounts of Ca(OH) 2 in pozzolanic-cement pastes, and their higher silica and alumina contents are of benefit for these kinds of cementitious materials to minimize the penetration of chloride i o m On the other hand, the pore channels in ash cement pastes appear to have very high tortuosity. Figure 14 schematically shows this model of the tortuous pore channel, in which the CI- ion may be inhibited by i n t e r a c t i o n with other ions as it passes through the channel. The ions may exist in the pore channel itself, or as part of the e l e c t r i c double layer. It is not known with certainty what additional effects minor chemical components present in the fly ash may have on the e l e c t r i c a l p o t e n t i a l - a c c e l e r a t e d ionic transport, but all the b l e n d e d cements h a v e substantially lower Cl- transport rates than the portland cement at comparable w/s (volume) ratios or mean pore radii, suggesting that the minor components are not of great importance, and that the increased Si (and AI) are the prime chemical components responsible for the diminished transport rates. Finally, it should be emphasized that fly ash containing pastes are much more mature w h e n cured at 38°C, as in the present study, and the same phenomena observed in the present study would only occur at much later ages under normal ambient curing. Conclusions 1.

Chloride ion diffusion under an applled electric field

is affected

Vol. 16, No. 5

759 POROSITY, PORE STRUCTURE, CI- DIFFUSION,

FLY ASH PASTE

not o n l y by the pore structure of the materials, but a l s o by the diffusion mechanism. 2. Fly ash blended-cement has a high potential to prevent diffusion of c h l o r i d e ions, and the optimum proportion of cement:fly ash found in this study Is 60:40 percent (by weight). This is aue, on the one hand, to the clogging of pores by gel produced from pozzolanic reaction~ and on the other hand, to the tortuoslty, and the action of multlvalent cations on chloride 1on diffusion in the pore solutio~ This type of materlal offers the cement and concrete industry a more efficient material for p r e v e n t i n g some chemical attack. S. The method of rapid determination of chloride ion diffusion under the action of an e l e c t r i c f i e l d is convenient; a l t h o u g h strict e q u i v a l e n c e to normal dlffusion under a concentration gradient may not be established, the r e l a t l v e trends appear v a l i d ; i.e., the c h l o r i d e diffusion r a t e is unquestionably much lower than in normal cement pastes. 4. It w o u l d be beneficial to perform slmilar experiments at longer curir~ age~ use different temperatures~ compare the effects of different types of fly ash and other b l e n d i n g materials; and to analyze the composition of pore solutions, in order to ascertain more f u l l y the mechanism of chloride diffusion in blended cement pastes and further to understand the behavior of fly ash and other blending admixtures in hardened cement pastes. References 1.

2. 3.

4. 5.

6.

7. 8. 9. 10. 11. 12.

D.M. Roy and K.M. Parker, Proc. C A N M E T / A C I First Intl. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, Vol. I, Ed. V.M. Malhotra, pp. 397-414; ACI SP-79, Detroit (1983). Seishi Goto and Della M. Roy, Cem. Concr. Res. ~, $7S-579 (1981). C.L. Page, N.R. Short, and A. E1 Tarras, Cem. Conor. Res. I, 395-406 (1981). A. Kumar and D.M. Roy, J. Am. Ceram. Soc. 69 (4), 356-360 (1986). D.M. Roy, A. Kumar, and J.P. Rhodes, F I X Ash, S__i~ica Fume. ~ l a ~ and N a t u r a l P o z z o l a n s i_~nConcrete, Vol. 2, SP-91, Ed. V.M. Malhotra, pp. 1423-1444, American Concrete Institute, Detroit (1986). A. Kumar and D.M. Roy, Pore Structure and Ionic D i f f u s i o n in Admixture Blended Portland Cement Systems, Proc. 8th Intl. Congr. C h e • Cement (in press). D. Whiting, Rapid Determination of the Chlorlde Permeability or Concrete, Report No. FHWA/RD-81/119, August 1981, NTIS, DB No. 82140724. B.K. Marsh, R.L. Day, and D.G. Bonner, Cem. Concr. Res. 1_55, 1027-1038 (1985). R. Sersale, Proc, ~th Int.l. Congress on the Chemistry o__~fCement, Vol. 1, Prlncipal Reports, IV-1/3 1/18, Septima, Paris (1980). A. Steopol and L. Vaicum, Zement-Kalk-Gips 8, 348-3S1 (1961). L. Craciunescu, Zement-Kalk-Glps 23, 182-183 (1969). H. Ushlyama, S. Goto, S u p p l e m e n t a r y Paper, Section II, Proo. VI Intl. Congress on the Chemistry of Cement, September 1974, Moscow.