ACI Materials Journal-Comparison Between Diffusion and Electrochemical - By Hiroshi Saito and Sun

8/2/2019 ACI Materials Journal-Comparison Between Diffusion and Electrochemical - By Hiroshi Saito and Sun

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ACI MATERIALS JOURNAL TECHNICAL PAPERTitle no . 96-M27

Comparison between Diffusion Test and Electrochemical

Acceleration Test for Leaching Degradation of Cement

Hydration Productsby Hiroshi Saito and Sunao Nakane

Nuclear waste repositories are required to be e f fec troe Jor more than

1000years. Thus, where concrete is used as a barrier and/or struc-

tural material, it is necessary in the design ifthesej(lCl"litiesto predict

the degradation ifconcrete over this range. Acceleration tests are use-

Jul in quantitatively predicting this degradation. The authors have

been studying an electrochemical test method to clarify its usefulness

in predicting the chemical degradation ifconcrete in contact with

water. This report describes results ifdiffusion and acceleration tests

and shows the usefulness of the method by comparing the quantity if

Ca2+ions dissolved in water and the degree ifdegradation obtained

by electron probe micro-analyzer (EPMA).

Keywords: cements; diffusion; hydration; leaching.

INTRODUCTIONSome situations, such as nuclear waste repositories, require

long-term durability of concrete in contact with ground water.

Nuclear waste repositories will be constructed deep under-

ground. Therefore, the main degradation factors are gradual dis-

solution of cement hydrate in water and chemical attack by

components contained in water that are harmful to concrete.

Generally, the latter degrades concrete more than the former.

However, repository sites may be chosen where there is a low

concentration of such harmful components. Thus, it is important

to investigate degradation due to dissolution of cement hydrate.This study aimed at establishing an electrochemical acceler-

ation test method to clarify the degradation mechanism by

leaching, and to make a model to simulate the long-term deg-

radation behavior of concrete. In this method, the speed of Ca 2+

ion movement toward the cathodewas fixed by applying a con-

stant potential gradient across the mortar specimens in contact

with water, thus accelerating the dissolution of cement hy-

drate. Preliminary tests Ishowed that this method accelerated

cement hydrate dissolution in water and reproduced the degra-

dation of cement hydrate in a CaO-SiOQ-HQO (C-S-H) system

containing a large quantity of water. It also showed that dis-

solution speed increased with a higher potential gradient and

the method reproduced the naturally-occurring degradation

of cement hydrate, which progresses gradually from the sur-

face to the interior. Q Other researchers ':" have also helped clar-

ify the feasibility of using an electric field in degrading cement-

based materials in contact with water.

RESEARCH SIGNIFICANCEThere remain two main tasks to verify the usefulness of an

electrochemical acceleration method. One is to remove the

doubt that degradation obtained by this method differs from

that occurring under natural conditions. The other is to esti-

mate the acceleration rate of this method. It is very important

to clarify these tasks for establishing this method as an accel-

eration test method. This paper describes the differences in ce-

ment hydrate degradation and acceleration rate based on

diffusion and acceleration test results.

208

DISSOLUTION MECHANISM OF CEMENTHYDRATE AND ACCELERATION PRINCIPLE OF

ELECTROCHEMICAL METHODThere are chemical equivalents between main cement hy-

drate Ca(OH)Q and C-S-H, and their component ions in pore

water, as described by the following chemical reactions

(1 )

(2 )

Dissolution of cement hydrate causes a shift in the chemical

equilibrium toward the right-hand side of these reactions. If

Ca2+ or OH- concentration in the pore water decreases, the

chemical equivalent moves to the right-hand side and dissolu-

tion of cement hydrate occurs. Brown" showed the chemical

equivalent relationship between Ca(OH)2' C-S-H, and each

component ion in the pore water from the viewpoint of pl~ase

equilibrium; there is a chemical equivalent in the pore solutl~n.

IfCa2+ concentration in the pore water decreases, Ca(OH)2 dIS-

solves, thus supplying additional Ca(OH)2 ions to maintain the

chemical equivalent. After the Ca(OH)2 is completely dissolved,

the Ca2+ ions supplemented from C-S-Hstart to dissolve and

C-S-H finally degrades to Si02 gel. As described above, the two

dissolution reactions do not occur at the same time; Ca(OH)Qdissolves first, then C-S-H.

Furthermore, cement hydrate dissolution seems to corre-

spond to movement of related ions. This suggests that if disso-

lution speed is determined by ion movement speed, then

dissolution speed would be accelerated by increasing ion move-

ment speed. Thus, a potential gradient was supplied to increase

ion movement speed. According to electrochernical principles,

ions in solution move toward opposite electrodes under a direct-

current potential gradient; i.e., cations move toward the cath-

ode and anions move toward the anode. In an infinite dilute

solution, the moving speed under 1VIcm is defined as mobility,

and each ion has its own characteristic value.

Fig. 1 shows the experimental apparatus and mass-transport

concept applied in this test. The specimen was placed betweentwo glass vessels, each containing 1 I of ion-exchanged water.

Each vessel also had a small pipe for exhausting gas produced

by electrolysis. One vessel had an anode (Pt) and the other had

a cathode (stainless steel) connected via an electric DC power

source to provide a potential gradient across the specimen. Ca 2+

ions in the pore solution moved rapidly to the cathode, and thus

hastened cement hydrate dissolution. In this electrochernical

ACl Materials Journal, V. 96, No. 2, March-April 1999. .., .Received November 18 1997, and reviewed under Institute publication policies.

Copyright 1999, American Concrete Institute. All rights n:,served, including the

making of copies unless perrrussion IS obtained from the copynght propnetors. Perti-nent discussion will be published in the January-February 2000 ACI Materials Journal

if received by October I, 1999.

ACI Materials Journal/March-April 1999


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Hiroshi Saito is a deput general manager in the consulting department ofthe Tedmtcai

Re se ar ch In sti tu te at th e Ob ay asl ii Co rp ., To/em, J a p an . H e r e c e i v ed h i s D E ng Fo m G u nn ra

U niv e rs it y in 1.998. H i s r e s e ar c h i n t e r e st s i n c lu d e c o n c r e te m i c r o s t ru c t ur e a n d d u r a b il it y .

AC I me mb er Sunao Nakane i s a gene ra l m anager in the co ns ult ing depar tm ent cfthe Techni -

ca l Res earch Ins ti tu te a t t he O bav as hi Co rp . H i s re s earch in t e re s t s i nc lude co nc re t e enginee ring .

Exper imental apparatus (Accelerat ion test)

Pt e"

Fe

Solution on anode side Solution on ca1hode side

React ion at anode side

H2

0 ..2H+ + 1/202

+ 2e"

React ion at ca1hode side

2H20+ 20" .. 20W + H

2

Mass transport

Fig. I-Experimental apparatus and mass transport in test.

reaction, cations in the pore solution, such as Na+, K+, and

CaN, moved toward the cathode, while anions, such as OH-,

moved toward the anode. Ca2+ and OW ions produced by dis-

solution move in the same direction under natural conditions.

Therefore, the movement direction of OH- is different from

that under natural conditions. Further, a small current (several

mAl flowed during the acceleration test, so water electrolysis

occurred at the same time at both electrodes, forming small H+

and OH- ions and producing HQand O2 gases. These phenom-

ena do not occur under natural conditions. Nevertheless, these

differences between the artificial condition and the natural con-

dition do not change the dissolution mechanism, but only influ-

ence the movement efficiency of ions and solubility because ofthe different pH condition. Consequently, this method of'accel-

eration with an electric field produces leached samples in a

short time without altering the assumed degradation mecha-

nism, as given in the reference material.':" for the leaching of

cement-based materials in water.

EXPERIMENT

Diffusion and acceleration tests were conducted in a room

air-conditioned to approximately 25 C. The test specimens

were cylindrical pieces of mortar with a water-cement ratio (wl c)

of 0.65 and a sand-cement ratio of 2.0. The pieces were 5 cm

(1.97 in.) in diameter and 1 cm (0.394 in.) thick. Ordinary

portland cement and Toyoura standard sand [solid volume

Pump

Fig. 2-E.r:perimental apparatus (diffusion test).

ratio is 57.2 percent and particle size distribution is from

0.105 to 0.3 mm (4.14 x 1O-'~to 1.18 X 10-2 in.)J were used in

the specimens. Each specimen was tested after curing in 20 C

water for four weeks.

Diffusion tests were conducted for 66 months under zero po-

tential gradient. Fig. 2 shows the experimental apparatus. One 1

of ion-exchanged water was circulated at 10 ml/min. The solu-

tion in the tank was exchanged the second and third day, once a

week from the fourth to 60th day, then every tenth day after that.

The exchanged solution was poured into a 1-1flask and its CaN

concentration was determined by atomic absorption analysis.

Acceleration tests were conducted for 2,6,8, and 12 months

under three potential gradients: 2, 5, and 10 VIcm (5.1, 12.7,

and 25.4 Vlin.). Specimens were set up in the experimental ap-paratus shown in Fig. 1.The solution, on the cathode and anode

side was ion-exchanged water. Each solution around the cath-

ode and anode was exchanged every day for three days at the

beginning, then once a week after that. The solution around the

cathode was poured into a 1-1flask and its CaN concentration

was determined by atomic absorption analysis.

The specimens were removed after the test and cut with a

diamond cutter to observe their cross sections. The profile of

the cement hydrate structure for each specimen was observed,

and the length of the degraded thickness, determined by visual

inspection to have obviously become coarse, was measured.

Two specimens were selected to compare the differences in

degradation of the cement hydrate. One had been tested for 66

months under zero potential gradient, and the other for' 2months under 5 VIcm (12.7 Vlin.) potential gradient. In these

two specimens, the cumulative quantity of CaN dissolved in

water and the degraded thicknesses were similar. The differ-

ences in degradation of the cement hydrate were measured by

the following methods: mineralogical analysis of paste (X-ray

diffraction method); and distribution Ca-to-Si molar ratio

[electron probe micro-analyzer (EPMA)J.

RESULTS AND DISCUSSION

Comparison of Ca2T dissolving speed andacceleration rate

The cumulative quantity of CaN ions dissolved in water can

be determined by comparing the Ca2+ dissolution speed and the

ACI Materials Journal/March-April 1999 20 9


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2000

1500 ............; _--_ . .: . - - - _ ~ - - - _ ..-

. . . .T . . - : -. .1000

500

oo 500 1000 1500 2000 2500

Test period [days]

Fig. 3-Cumulative quantity if CaN dissolved in water (diffusiontest, 1mg = 2.2X 10-6 lb).

'5 10000.5 .

+ B O O ON

a s0 6000'0

~4000. .c:

a s: : 2er

2000C D. 2 :(tj

0'SE 0: : 20

.: 2[V/vm]

: 5[V/cm] --" '1 '7 .. .'

.. : 10[V/cm]

- t- ,..' r ~'~ ..- ,..j"= ..:::.: .Al .. "---: ~ : . - - :- - _ _ - _ + . . . " . . ~ , . . " . . , _ . . , - . ; . , _ .~ ~ , . - e

. . . . ~ ' . ~ ~ , + _ . . , , .

-r- : j

100 200 300 400

Test period [days]

Fig. 4--Cumulative quantity of Ca?" dissolved in water (accelera-

tion test).

acceleration rates in Fig. 3 and 4. The former was determined

from the diffusion test and the latter from the acceleration test.

In the diffusion test, the amount of Ca?" ions increased gradu-

ally, up to 1600 mg (3.52 X 1O'3lb) at the 66th month. Applying

a potential gradient to the specimen significantly accelerated

the dissolution of CaN with the cumulative quantity reaching

3658 mg (8.06 X 10'3 lb) at the isrh month under 'lV/cm

(5.08V /in.), 6104 mg (1.35 X 1O'2lb) under 5 V/cm (12.7 V!in.)

and 8337 mg (1.87 X 1O'21b) under 10 V/cm (25.4 V!in.).

For each result obtained for each acceleration test, the hori-

zontal axis, i.e., the test period, was expanded to make it similar

to the diffusion test result. These results are shown in Fig. 5.

Based on the expansion rate, the acceleration rate was 14 times

greater at 2 V/cm (5.08 V!in.), 31 times greater at 5 V/cm(12.7 V/in.) and 63 times greater at 10 V/cm (25.4V!in.). Dis-

solving speed obtained in the diffusion test may correspond to

the rapid water flow condition because of exchanging water

periodically. Whereas the groundwater would be stagnant in a

nuclear disposal site, so the acceleration rate obtained in the ac-

celeration test may be greater under natural conditions. A

more accurate estimation of acceleration speed is dependent on

the site conditions.

Comparison of cement hydrate

To compare the degradations of cement. hydrate, Fig. 6

shows cross-sectional photographs of the specimens after the

tests. In the case of the acceleration test, degradation can be

clearly distinguished by visually comparing the anode and cath-

210

. . . . 2000C)

.

+ 1500N

a s0

'0 1000

~:;::l

a 500:: 2

0

0

0

. . . . . . . . . . .j . . . . .

. .t .... .. ..i- .. --__i

, ~/cm 1 _

40 80

Test period

120 160

[days]

'0)2000..

t ta s

1500

o'0 1000

~

a: : 2o

. .. .-.-.-.- -.-..-.-_.----- _ - _ _ .. ,

! 1

500 ~5v_/_cm~L__.,

o

o 20 40 60

Test period [days]

80

'5 2000.5 .

+ 1500N

a so

-10000

~:;::l

a 500: : 20

0

0

10V/cm I .

30

[days]4010 20

Test period

Fig. S-Expandedjiguresftr calculating acceleration rate.

ode side regions. The former is dense, while the latter is coarse.

There is a degradation front at approximately 8.5 mm (0.33 in.)

from the cathode surface of the specimen. In the case of the dif-

fusion test, degradation can also be clearly distinguished by visual

inspection. There is a degradation front at approximately 8 to 11

mm (0.32 to 0.43 in.) from the surface in contact with solution.

These photographs suggest that the degradation front moved

toward the interior by dissolution ofCa'".

Fig. 7 shows x-ray diffraction patterns of the specimens after

the tests. The specimens were divided into two pieces at their

degradation boundary. The undegraded region is inside the deg-

radation boundary and the degraded region is outside the degra-

dation boundary. The cement hydrates in each specimen were

separated using a bromoform-ethanol solution (specific gravity

= 2.32). The main cement hydrates, Ca(OH)2 and C-5-H, are

within the undegraded region in both specimens. However,



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Surface in contact

with water

Cathorde side

surface

J O O O

CH ;Ca(OH)2

e-S-H; x Ca O ySi02 zH 20

1000CH

1000

CH

Undegraded region

CH

CH

[ Diffusion test] [ Acceleration test] 20 30

[ Diffusion test]

40 50

Fig. 6~Degradation ifspecimens.

there is only C-S-H and no Ca(OH)2 in the degraded region.

These data show that Ca(OH)2 had completely dissolved and

corresponded to results!" reported previously. Thus, it is clear

from the x-ray diffraction pattern that applying the potential

gradient makes no difference in degradation of cement hydrate.

Fig. 8 shows the Ca-to-Si molar ratio in the cement hydrate

structure measured by EPMA, and Fig. 9 shows the same data

displayed schematically. In both specimens, the Ca-to-Si molar ra-

tio was over 2.0 in the undegraded regions and under 2.0 in the de-

graded regions. The degradation boundary can be also

discriminated from these data. The Ca-to-Si molar ratio in the de-

graded region shows the Ca-to-Si molar ratio of C-S-H, because

there is only C-S-H in the degraded region measured by the X-ray

diffraction pattern. Therefore, this is one indicator of C-S-H deg-

radation.

In the case of the diffusion test, the Ca-to-Si molar ratio

distribution within the degraded region is clearly separated

into two regions: one from the surface to a depth of 2 mm

(0.079 in.), with a Ca-to-Si molar ratio of 0.8 to 1.2; and the

other inside the former, with a Ca-to-Si molar ratio of1.6 to

2.0. In the acceleration test, the region with the Ca-to-Si mo-

lar ratio of 0.8 to 1.2 is from the surface to the neighborhood

ofthe boundary. The region which shows a Ca-to-Si molar ra-

tio of 1.6 to 2.0 is very narrow, and is in the neighborhood of

the degradation boundary. The Ca-to-Si molar ratio changes

gradually beyond the degraded region.

These results show that the degradations of cement hydrate

are similar, though C-S-H is slightly more degraded in the ac-

celeration test than in the diffusion test. It is particularly inter-

esting that the Ca-to-Si molar ratios are very similar in the

neighborhood of the surface in contact with water. These results

show that the acceleration method reproduces degradation sim-

ilar to that produced by diffusion. Therefore, it is concluded

that the electrochemical method is very useful for accelerating

leaching degradation of cement hydrate in contact with water.

Fig. 10 shows reflection electron images on cement hydrate

obtained by EPMA. The upper row shows the data obtained

from the diffusion test and the lower row shows those obtained

from the acceleration test. Comparison of these data show that

they are similar in the undegraded regions. However, the area

in the neighborhood of the dissolution surface is dark com-

pared to that in the acceleration test, indicating that it is more

degraded at that region. In the middle of the degraded region,

the acceleration test specimen has more dark parts than the dif-

fusion test specimen, so it seems more degraded in that region.

It is easy to distinguish the degradation boundary for the accel-

eration test specimen, but not for the diffusion test specimen.


Undegraded region

Degraded region

28 10 20 30 40

[ Acceleration test]

50

Fig. 7-X-ray diffraction patterns.

SUrface in contact

with water

Cathorde side

surface

( Diffusion test I [ Acceleration test I

Fig. a-Ca/Si molar ratio'distribution of cement hydrate.

Ca/Si;;;;Z.O Ca/Si=20-1.6 Ca/Si=1.2-0.8 Ca/Si;;;;20 Ca/Si=2.0-0.8

6mm 2mm

.Degradation boundary8.5mm

Degradation boundary

Fig. 9-Ca/Si molar ratio distribution ifcement hydrate displayed

schematically.

Relationship between quantity of Ca2+ dissolved inwater and degraded thickness

The degraded thickness corresponds to the quantity ofCa2+

dissolved in water. The cumulative Ca2+ measured in the cath-

ode compartment (TCa) can be linked to the degraded thick-

ness (DL) of the sample. The data obtained in these tests are

plotted in Fig. 11. DL increases in proportion to TCalSand

211


5/5

E30

..25

e n

20e nQ)

c..ll'

15(,,)

;S

i10

"i 5 . .0)

~ 00

neighborhood of degradation

boundary

middle region surface in contact with water

neighborhood of degradation

boundary

middle region cathode side surface

Fig. 1O-Riflection electronimages.

o Accele la llon test D Iffu sion te al

50

Quantity of Ca2

+ dissolved in water perunit area [m glcm 2]

Fig. 11-Relationship between degraded thickness and quantityof CaN dissolved in water (1 mm = 0.0394 in.; 1 mg/cm? =2.04 X 10-3 lblft).

the relationship between TCal Sand DL, described by a re-

gression Eq. (s), was obtained from the acceleration tests

DL = 0.07 X TealS

where

DL degraded thickness (mm)

TCal S= quantity of Ca2+ dissolved in water per unit area

(rug/ern")

The data obtained from the diffusion test are a little different

from the above line and are close to the dotted line, which is the

line obtained by assuming that DL depends only on the quan-

tity of dissolved Ca(OH)2' In the diffusion test, these data show

that the Ca2+ ions dissolved in water comprise mainly those

produced by Ca(OH)2 dissolution. However, in the acceleration

test, they show that they comprise those produced not only by

Ca(OH)2, but also by C-S-H dissolution. This difference corre-

sponds to C-S-H degradation obtained by EPMA.

212

Future prospects

It has become clear that the proposed acceleration test can

reproduce the actual degradation process of concrete in contact

with water within a short time. The programs to estimate the

change of physical and mechanical properties of concrete due to

degradation are being carried out and a model for predicting a

long-term degradation will be made hereafter .

CONCLUSIONS

The electrochemical test method was proposed for accelerat-

ing cement hydrate dissolution in concrete in contact with wa-

ter by diffusing Ca2+ ions. Diffusion tests and acceleration tests

were carried out using the same mortar specimens and test re-

sults were compared. Conclusions are as follows:

1. The amount ofCa'" ions dissolved in water was increased by

applying a potential gradient. The acceleration rate was calculat-

ed to be about 60 times greater at 10 VIcm (25.4 V/in.) by com-

paring the cumulative amount ofCa2+ ions dissolved in water.

2. Based on the Ca-to-Si molar ratio distribution obtained by

EPMA, the cement hydrate degradations were similar, espe-

cially those in the neighborhood of the surface in contact with

water, but C-S-H degradation of the acceleration test specimen

was more than that in the diffusion test specimen.

s. Based on Result 1 and 2, it is concluded that the electrochem-

ical method is very useful for accelerating leaching degradation.

(s)

REFERENCESI. Saito, H.; Nakane, S.; and Fujiwara, A., "Preliminary Experimental

Study on the Deterioration of Cementitious Materials by AccelerationMethod," Nuclear Engineering and Design, V 138, No. 2, 1992, pp. 151-155.

2. Nakane, S., and Saito, H., "Importance of Prediction Techniques on

Durability in the Durability Design for Concrete Structure," Integrated

Des ign and Environmental Issues in Concrete Technology, K. Sakai, ed., E & FN

Span, London, 1996, pp. 69-82.

3. Gerard, B., "Contribution des couplages menique-chimie-transfert

dans la tenue a long terme des ouvrages de stockage de dehets radioac-

tifs,' these de doctorat ENS Cachan/Univ. Laval-Quebec, 1996. (in

French.)

4. Wittmann, F. H., "Corrosion of Cement-Based Materials under the Influ-

ence of an Electric Field," Materials Science Forum, V.247, 1997, pp. 107-126.

5. Brown, P. W, and Clifton, 1. R, "Mechanism of Deterioration in

Cement-Based Materials and in Lime Mortar," Durability of Building Materi-

als, V 5, 1988, pp. 409-420.


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