Effect of calcium leaching on the pore structure, strength, and chloride penetration resistance in concrete specimens

Effect of calcium leaching on the pore structure, strength, and chloride penetration resistance in concrete specimens

Nuclear Engineering and Design 259 (2013) 126–136 Contents lists available at SciVerse ScienceDirect Nuclear Engineering and Design journal homepage...

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Nuclear Engineering and Design 259 (2013) 126–136

Contents lists available at SciVerse ScienceDirect

Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes

Effect of calcium leaching on the pore structure, strength, and chloride penetration resistance in concrete specimens Yoon Suk Choi, Eun Ik Yang ∗ Department of Civil Engineering, Gangneung-Wonju National University, 7, Jukheon-gil, Gangneung-si, Gangwon-do 210-702, South Korea

h i g h l i g h t s     

The volume of pores with diameters ranging from 50 nm to 500 nm is increased. The Pores larger than 200 nm in size increase in number during the initial time. The residual strength of the leached part with OPC ranges from 35% to 60%. The residual strength of the mineral admixture replacement ranges from 23% to 50%. The chloride diffusion coefficient of leached concrete is increased 2–5 times.

a r t i c l e

i n f o

Article history: Received 23 August 2012 Received in revised form 8 February 2013 Accepted 10 February 2013 Keywords: Radioactive waste repository Calcium leaching Pore structure Chloride penetration Concrete degradation

a b s t r a c t In radioactive waste repositories constructed underground and on shorelines, concrete members can remain in contact with groundwater for a long period. However, even pure water creates concentration gradients which lead to the diffusion of Ca ions from the pore water and the degradation of the underground concrete. Therefore, the purposes of this study are to investigate not only the alteration of the pore structure and the loss of compressive strength associated with dissolution but also the characteristics of chloride penetration after leaching-related degradation. The results show that as the leaching period increases, the volume of pores with diameters ranging from 50 nm to 500 nm is greatly increased. Also, pores larger than 200 nm in size rapidly increase in number during the initial leaching time, while those smaller than 200 nm increase in number only gradually. Furthermore, the residual strength of the leached part with OPC ranges from 35% to 60%. In addition, those of the mineral admixture replacement ranged from 23% to 50%. The chloride diffusion coefficient measured by the chloride profile increased two-to-five-times with the leaching duration. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The radioactive waste repositories are scheduled to be constructed deep underground and on shorelines in South Korea. Also, these facilities require long-term durability of the concrete, as the concrete members will likely be in contact with groundwater for a long period (Faucon et al., 1997). However, pure water creates concentration gradients which lead to the diffusion of Ca ions from the pore water and the subsequent degradation of underground concrete. Therefore, the main degradation factors are the gradual leaching of cement hydrate in pore water and chloride attack by sea water. This causes the concrete’s porosity to increase and its strength to decrease (Haga et al., 2005; Yang and Choi, 2011; Carde and Franc¸ois, 1999). In addition, the chloride diffusion coefficient

∗ Corresponding author. Tel.: +82 33 640 2418; fax: +82 33 646 1391. E-mail address: [email protected] (E.I. Yang). 0029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2013.02.049

will increased due to the change in the pore size distribution (Yang, 2006). Therefore, it is important to investigate the degradation factors related to the leaching of cement hydrate. These include the pore structure and strength as well as the chloride diffusion coefficient. Many studies have investigated the leaching of cement constituents from cement hydrate. Atkinson and Guppy (Atkinson et al., 1987; Atkinson and Guppy, 1987) as well as Berner (1998) developed cement paste dissolution models to evaluate long-term changes in the compositions of liquid phase. Faucon et al. (1997) studied the alteration of hardened cement paste by dissolution and indicated that hydrate phases and the structures of hydrates change with the progress of dissolution. Meanwhile, Buil (1992) and Carde et al. (1997) considered diffusion as the transport mechanism of leached constituents in their experiments. Haga et al. (2005) also studied the effects of the pore volume in hardened cement paste on dissolution phenomena, and Saito et al. (Saito and Nakane, 1999; Saito and Deguchi, 2000) developed a new acceleration test method for the leaching degradation of cement hydrate.

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Table 1 Concrete mix proportion. W/B (%)

S/a (%)

40 50

43 45 45 45

Unit weight (kg/m3 ) Water

Cement

Sand

Gravel

Fly ash

Blast furnace slag

170 173 173 173

425 345 311 242

721 782 776 778

934 933 927 929

– – 35 –

– – – 104

As mentioned in the previous paragraphs, these studies focused on modeling cement paste alterations and structural changes of hardened cement paste as associated with dissolution. Also, the mortar specimen size for the leaching test was small. Above all, the material used in radioactive waste disposal facilities is concrete, and the concrete used is not homogeneous in comparison with the mortar. Specifically, concrete has many interfacial transition zone (ITZ) between the aggregates and the cement paste. Most concrete properties are affected by the ITZ characteristics. In ordinary concrete, the ITZ has less crack resistance than either hydrate cement or the mortar, and so fractures occur preferentially in the ITZ. Even before any load has been applied, a large number of micro-cracks exist in the ITZ of sound concrete. Moreover, calcium-leaching may occurs mainly in the ITZ because there is a higher proportion of portlnadite and ettringite compared to bulk paste (Mindess, 2003). Therefore, it is desirable to evaluate the degree of degradation using a leached concrete specimen, directly. However, literature data (Nguyen et al., 2007; Sellier et al., 2011; Marinoni et al., 2008) pertaining to the calcium leaching of concrete are not enough. Thus, in this study, a strongly concentrated ammonium nitrate solution was used to assess the leaching of cement hydrate in concrete. The pore-size distribution and compressive strength of the degraded parts were then measured and the deteriorations of resistance against chloride penetration were compared. And effect of calcium leaching on the pore structure, strength, and chloride penetration resistance in concrete was discussed. 2. Experiments 2.1. Materials and concrete mix proportions Table 1 show the mix proportions of the concrete specimens. Ordinary Portland cement (OPC) (ASTM Type I) was used in all of the mixtures. The Crushed gravel was used as a coarse aggregate with a maximum aggregate size Gmax of 20 mm. The Specific gravity and absorption ratio of the coarse aggregate were 2.54 and 0.63%, respectively. The specific gravity, absorption ratio, and the fineness modulus (FM) of the fine aggregate were 2.59, 1.07%, and 2.65, correspondingly. In addition, to investigate the effect of mineral admixtures on leaching damage, part of the cement mixture was replaced by mineral admixtures at a water/binder (W/B) ratio of 0.5. The replacement ratios were 30% for blast-furnace slag (BFS) and 10% for fly Ash (FA). The physical and chemical compositions of the cement and mineral admixtures are shown in Table 2. After the removal of the mold, all specimens were cured for 28 days in saturated limewater at a temperature of 20 ± 3 ◦ C. 2.2. Leaching acceleration tests In this test, instead of slow leaching kinetics obtained by means of de-ionized water or an electrochemical method, an ammonium nitrate solution was used to degrade the concrete specimens. The ammonium nitrate (NH4 NO3 ) reacts initially with the calcium hydroxide (Ca(OH)2 ) in the cement hydrates, leading to the appearance of a very soluble calcium nitrate (Ca(NO3 )2 ) and the

emanation of gaseous ammoniac NH3 (Carde et al., 1996). Unfortunately, these phenomena do not occur under natural conditions. Under natural conditions, there is a chemical equivalence between the Ca(OH)2 , C S H, and each ion component in the pore solution from the viewpoint of phase equilibrium. If the Ca2+ concentration in the pore water decreases, the Ca(OH)2 dissolves, thus supplying additional Ca(OH)2 ions from the cement hydrates to maintain the chemical equivalence. After the portlandite is completely dissolved, the Ca2+ ions supplemented from C S H start to dissolve and C S H finally degrades to SiO2 gel. Nevertheless, these differences between the artificial condition and the natural condition do not change the mechanism of calcium leaching from the concrete (Carde et al., 1997). For the leaching acceleration test, a 6 mole concentration of an ammonium nitrate solution was used and the concrete specimens were totally immersed for a predetermined period (15, 30, 60, 90, 180 and 365 days). Also, the leaching acceleration tests were performed in a thermostatic chamber at 20 ◦ C (Gawin et al., 2009). After a specified period, the specimens were cut and shaped into cubes of 10 × 10 × 10 mm in size (a surface part 0 mm to 10 mm) using a diamond cutter for using in the MIP test. Also, the other sample pieces were used for calcium amounts. To investigate the amount of leached calcium, the relative calcium amounts were measured by means of atomic absorption spectroscopy for leached specimens and for reference specimens cured during same periods in limewater. Meanwhile, the depth of degradation thickness was measured optically by the phenol-phthalein test (total thickness of specimen: 100 mm). This substance turns red-pink if applied onto a material whose pH is higher than 10. Thus, the color of phenol-phthalein applied to a degraded zone would not turn red. 2.3. Main test items and test procedure To investigate the long-term mechanical properties and durability as a function of the leaching kinetics, the experiments in this study are divided into two groups. For the evaluation of the long-term properties, pore size distribution, bulk density, porosity, and compressive strength tests were conducted. For durability, the rapid chloride permeability test (RCPT) (ASTM C 1202, 1997;

Table 2 Physical and chemical composition of materials. Properties

Physical Specific gravity Blaine (cm2 /g) Chemical (%) SiO2 Al2 O3 Fe2 O3 CaO MgO SO3 LOI

Material OPC

FA

BFS

3.15 3200

2.25 3400

2.89 4300

21.36 5.03 3.31 63.18 2.89 2.30 1.40

49.89 29.99 7.42 5.01 0.98 – 4.31

33.54 15.22 0.51 43.88 2.62 2.54 0.01

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Table 3 Experimental variables. Item

Content

W/B ratio Mineral admixture Specimen size

0.4, 0.5 FA (10%), BFS (30%) Diameter 100 mm, height 100 mm (leaching test) Diameter 100 mm, height 200 mm (compression test) Diameter 100 mm, height 50 mm (chloride diffusion test) 28 day, water curing (20 ± 3 ◦ C) Immersion NH4 NO3 solution (6 M) 0 (Initial), 15, 30, 60, 90, 180 and 365 days Degraded thickness Quantity of leached calcium ions Pore-size-distribution, pore volume, bulk density Sound concrete, Ca2+ leached concrete Rapid chloride permeability test (RCPT) Nord Test NT Build 443

Curing condition Leaching method Leaching period Phenolphthalein method Atomic absorption spectroscopy Mercury intrusion porosimetry Compression test Chloride diffusion test Total number of specimens

MIP & AAS 120

Tang and Nilsson, 1992), and a chloride diffusion test (NT Build 443, 1995) were performed. Test variables and the scheme are listed in Table 3 and Fig. 1, respectively. A mercury intrusion porosimetry (MIP) test based on ASTM D 4284 (2003) was conducted to measure the pore size distribution, bulk density, and porosity. For the compressive strength tests, a cylinder (diameter 100 mm, height 200 mm) was prepared based on ASTM C 39 (2003) and tests were carried out to determine the properties of the degraded zones and the sound zones. The compressive strength tests for the reference and leached specimens

Compression test 120

RCPT 168

NT Build 443 96

were conducted with leaching periods 30, 60, 90, 180 and 365days. The compressive load was supplied by a universal testing machine with a capacity of 1000 kN. RCPT tests were performed after the acceleration leaching test. In this test, a water-saturated, 50-mm thick, 100-mm diameter concrete specimen was subjected to 30 V of applied DC voltage for eight hours using the apparatus shown in Fig. 1. In one reservoir was a 3.0% NaCl solution, and in the other reservoir was a 0.3 M NaOH solution. For diffusion tests using the NT BUILD 443, cylindrical concrete specimens 100 mm in diameter and 50 mm height

Fig. 1. Scheme of the calcium leaching test.

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100

W/C 0.4 W/C 0.5 W/B 0.5 (FA10%) W/B 0.5 (BFS30%)

25

Ca leached quantity (%)

20 15 10

2+

Degraded thickness (mm)

30

5

129

W/C 0.4

W/C 0.5

FA10%

BFS30%

80

60

40

20

0

0 0

5

10

15

0

20

50

100

0.5

were used. First, leaching acceleration tests were conducted for 30, 60, 90, and 180 days, after which the leached specimens were exposed to a chloride solution (3.0% NaCl) for 6 months. For the measurement of chloride ions, samples with a fixed thickness (10 mm) were cut from the specimens to obtain the concentration profile along the depth. After pulverizing the samples, the only powders that passed through a No. 40 sieve were used. Chloride

(a) W/C 0.4 and W/C 0.5

3

Bulk density (kg/m )

3

Bulk density (kg/m )

Calcium leached W/C 0.4 W/C 0.5

50

100

Water curing (W/B 0.5) FA10% BFS30%

2600

2000

0

150

350

2400 2200 2000 Calcium leached (W/B 0.5) FA10% BFS30%

1800 1600

200

250

300

350

400

0

50

Leaching period (days)

100

150

200

250

300

350

400

Leaching period (days)

Fig. 4. Relationship between leaching period and bulk density (measured range: surface part 0–10 mm).

(a) W/C 0.4 and W/C 0.5

(b) W/B 0.5 (FA10%) and W/B 0.5 (BFS30%)

Calcium leached W/C 0.4 W/C 0.5

30

25

Pore volume (%)

Pore volume (%)

Calcium leached (W/B 0.5) FA10% BFS30%

30

25 20 15 10 Water curing W/C 0.4 W/C 0.5

5

20 15 10 Water curing (W/B 0.5) FA10% BFS30%

5

0

0 0

50

100

150

200

250

400

ions from concrete powder were extracted according to ASTM C 114 (2004) and measured using a selective ion electrode. In addition, to investigate the diffusion coefficient, Fick’s second law was used. Meanwhile, to improve the reliability of test results, measured values of each test are the average of three specimens. Also, the total number of specimens for each test is described in Table 3.

2200

1600

300

(b) W/B 0.5 (FA10%) and W/B 0.5 (BFS30%)

Water curing W/C 0.4 W/C 0.5

1800

250

Fig. 3. Quantity of leached calcium in specimen (measured range: surface part 0–10 mm).

Fig. 2. Degraded thickness (total thickness of specimen: 100 mm).

2400

200

Leaching period (days)

Square root of time (days )

2600

150

300

Leaching period (days)

350

400

0

50

100

150

200

250

300

Leaching period (days)

Fig. 5. Relationship between leaching period and pore volume (measured range: surface part 0–10 mm).

350

400

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(a) W/C 0.4

(b) W/C 0.5 0.18 Initial : 28day water curing 30 days 60 days 180 days 270 days

0.16 0.14

L o g d iffe r e n tia l in tr u s io n (m l/g )

L o g d iffe r e n tia l in tr u s io n (m l/g )

0.18 90 days 365 days

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 10

1

2

10

3

10 10 Pore size diameter (nm)

4

10

(c) W/B 0.5 (FA10%)

90 days 365 days

0.12 0.10 0.08 0.06 0.04 0.02 1

10

2

3

4

10 10 Pore size diameter (nm)

10

5

10

(d) W/B 0.5 (BFS30%) 0.18

Initial : 28day water curing 30 days 60 days 180 days 270 days

0.16 0.14

L o g d iffe r e n tia l in tr u s io n (m l/g )

0.18 L o g d iffe r e n tia l in tr u s io n (m l/g )

0.14

0.00 0 10

5

10

Initial : 28day water curing 30 days 60 days 180 days 270 days

0.16

90 days 365 days

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 10

1

10

2

3

10 10 Pore size diameter (nm)

4

10

0.14

90 days 365 days

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 10

5

10

Initial : 28day water curing 30 days 60 days 180 days 270 days

0.16

1

10

2

3

4

10 10 Pore size diameter (nm)

10

5

10

Fig. 6. Pore size distribution of the sound concrete specimen under water curing (measured range: surface part 0–10 mm).

(a) W/C 0.4

(b) W/C 0.5 0.18 30 days 180 days

0.16

60 days 270 days

90 days 365 days

L o g d iffe r e n tia l in tr u s io n (m l/g )

L o g d iffe r e n tia l in tr u s io n (m l/g )

0.18

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 10

1

10

2

3

10 10 Pore size diameter (nm)

4

10

(c) W/B 0.5 (FA10%)

0.12 0.10 0.08 0.06 0.04 0.02 1

10

2

3

4

10 10 Pore size diameter (nm)

10

5

10

0.18 15 days 30 days 180 days

0.16 0.14

60 days 270 days

L o g d iffe r e n tia l in tr u s io n (m l/g )

L o g d iffe r e n tia l in tr u s io n (m l/g )

90 days 365 days

(d) W/B 0.5 (BFS30%)

0.18 90 days 365 days

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 10

60 days 270 days

0.14

0.00 0 10

5

10

30 days 180 days

0.16

1

10

2

3

10 10 Pore size diameter (nm)

4

10

5

10

15 days 30 days 180 days

0.16 0.14

60 days 270 days

90 days 365 days

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 10

1

10

2

3

10 10 Pore size diameter (nm)

Fig. 7. Pore size distribution of the leached specimen for 365 days (measured range: surface part 0–10 mm).

4

10

5

10

Y.S. Choi, E.I. Yang / Nuclear Engineering and Design 259 (2013) 126–136

(a) W/C 0.4

(b) W/C 0.5 Degraded ratio : Ad/At

0.19

0.31

0.33

water curing

0.57

0.69

calcium leached

60 50

0.44

50.87

51.19

51.88

43.52

42.10

41.90

53.27

54.92

53.72

46.08

40

35.04

30

32.91 28.98

20 10 0

Initial

30

60

90

180

270

Compressive strength (MPa)

Compressive strength (MPa)

Degraded ratio : Ad/At 0

70

365

0

70

0.22

0.41

0.61

0.77

calcium leached

50 38.28

40

20

33.04

33.49

24.69

25.31

38.65

40.48

41.90

24.94 20.81 16.40

10 0

9.48

Initial

30

0.78

50 40

37.16

37.67

38.29

27.73

27.51

39.66

41.10

42.53

32.82

30

31.85

20

23.51 19.13

10 0

Initial

30

60

90

180

15.68

270

365

Degraded ratio : Ad/At 0.72

60

90

180

270

Compressive strength (MPa)

Compressive strength (MPa)

0.41

60

30.31

0.67

(d) W/B 0.5 (BFS30%)

water curing

30

0.59

calcium leached

60

Degraded ratio : Ad/At 0.21

0.5

Leaching period (days)

(c) W/B 0.5 (FA10%) 0

0.39

water curing

Leaching period (days)

70

131

365

70

0

0.23

0.37

0.37

water curing

0.56

0.64

0.77

calcium leached

60 50 40 30

40.28

40.44

41.26

30.99

30.38

29.83

41.77

41.99

44.58

32.03 25.07

20

21.21

17.84

10 0

Initial

Leaching period (days)

30

60

90

180

270

365

Leaching period (days)

Fig. 8. Result of compressive strength (specimen size: diameter 100 mm, height 200 mm).

(b) W/C 0.5 Strength of sound concrete Maximum strength Minimum strength

60

S tr e n g th o f d e g r a d e d z o n e (M P a )

Strength of degraded zone (MPa)

(a) W/C 0.4

50 40 30 20 10 0 0

50

100

150

200

250

300

350

Strength of sound concrete Maximum strength Minimum strength

60 50 40 30 20 10 0

400

0

50

Leaching period (days)

S tr e n g th o f d e g r a d e d z o n e (M P a )

Strength of degraded zone (MPa)

50 40 30 20 10 0 50

100

150

200

250

300

350

400

(d) W/B 0.5 (BFS30%)

Strength of sound concrete Maximum strength Minimum strength

0

150

Leaching period (days)

(c) W/B 0.5 (FA10%) 60

100

200

250

300

350

400

Strength of sound concrete Maximum strength Minimum strength

60 50 40 30 20 10 0 0

50

Leaching period (days) Fig. 9. strength of degraded part.

100

150

200

250

300

Leaching period (days)

350

400

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calcium leached

20

16

12

8

4

0 0

50

100

150

200

250

300

24

water curing

2

water curing

-12

-12

(b) W/C 0.5 Coef. of Chloride diffusion ( 10 m /s)

24

2

Coef. of Chloride diffusion ( 10 m /s)

(a) W/C 0.4

350

20

16

12

8

4

0

400

0

50

100

Leaching period (days)

Coef. of Chloride diffusion ( 10 m /s)

16

12

8

4

0 100

150

200

250

300

24

water curing

2 -12

-12

2

Coef. of Chloride diffusion ( 10 m /s)

calcium leached

20

50

200

250

300

350

400

350

400

(d) W/B 0.5 (BFS30%)

water curing

0

150

Leaching period (days)

(c) W/B 0.5 (FA10%) 24

calcium leached

350

400

Leaching period (days)

calcium leached

20

16

12

8

4

0 0

50

100

150

200

250

300

Leaching period (days)

Fig. 10. Chloride diffusion coefficient of leached concrete using RCPT (total thickness of specimen: 50 mm).

3. Results and discussion 3.1. Evaluation of degradation by leaching Test results of the degraded thickness over a period of one year and the quantity of leached calcium in the specimens are shown in Figs. 2 and 3. Fig. 2 shows the linear variation between the degraded thickness and the square root of time. Carde et al. (1997) found that the leaching degradation is governed by a diffusion mechanism and can be described by Fick’s law, which relates the degraded thickness to the square root of time. On the other hand, the degraded thickness is a linear function of the square root of the immersion time. The tendency of leaching degraded thickness in this study using the concrete specimens similar to the results of another research (Carde et al., 1997) based on mortar or cement paste. The degraded thickness increases with the leaching period. Also, the degradation thickness was influenced by the W/B ratio of concrete specimen. For example, the degraded thickness with a higher W/B ratio is greater than that with a lower W/B ratio in Fig. 2. Thus, it seems that lower W/B ratio makes more difficult to calcium leaching of cement hydrate, because the cement matrix of concrete with lower W/B ratio is more dense than the higher W/B concrete.

To compare the difference between degraded and sound concrete, Fig. 3 shows the quantity of Ca2+ ions in the specimens after the leaching test. The quantity of leached Ca2+ ion increased sharply up to 90days after which the quantity of leached Ca2+ ion gradually lessened because of the quantity of leached Ca2+ was measured at a fixed surface part 0 mm to 10 mm. On the other hand, the degraded thickness according to the phenol-phthalein method was compared in terms of the total depth (100 mm). Thus, the quantity of leached Ca2+ does not correspond to the degraded thickness. However, the influence of W/B ratio is also appeared in results of the leached calcium quantity. As shown in Fig. 3, when the quantity of leached Ca2+ ions is compared with the W/B ratio, the measured value of W/B 0.4 is smaller than W/B 0.5. These results also show that the abundant cement hydrate makes leaching difficult. 3.2. Bulk density and porosity The relationship between the leaching period and the bulk density is shown in Fig. 4. The bulk density of leached concrete shows a decrease upon longer leaching periods. The decrease in the bulk density of the leached concrete at W/B 0.4 and 0.5 was relatively rapid until the 90th day. Comparing the sound and leached concrete, the bulk density of the sound concrete is mostly unchanged after the 28th curing day. This means that the weight reduction

Y.S. Choi, E.I. Yang / Nuclear Engineering and Design 259 (2013) 126–136

(a) W/C 0.4

(b) W/C 0.5

24

24

16

Leaching

Curing

20

3

20

Period 30 days 60 days 90 days 180 days

Chloride content (kg/m )

Curing

3

Chloride content (kg/m )

Leaching

12

8

4

0

16

Period 30 days 60 days 90 days 180 days

12

8

4

0 0

10

20

30

40

50

60

0

10

Depth (mm)

20

30

40

50

60

Depth (mm)

(c) W/B 0.5 (FA10%)

(d) W/B 0.5 (BFS30%)

24

24

3

20

16

Period 30 days 60 days 90 days 180 days

Leaching

Curing

20

3

Curing

Chloride content (kg/m )

Leaching Chloride content (kg/m )

133

12

8

4

0

16

Period 30 days 60 days 90 days 180 days

12

8

4

0 0

10

20

30

40

50

60

Depth (mm)

0

10

20

30

40

50

60

Depth (mm)

Fig. 11. Profile of chloride content (NTBuild 443) (total thickness of specimen: 50 mm).

due to leaching degradation was mostly completed during 90 days. However, the location of measurement for porosity and bulk density was a surface part 0–10 mm. Thus, it is considered that if the measuring range becomes larger, it takes a long time to completely achieve the leaching deterioration. Meanwhile, regarding the mineral admixture replacement, the decrease rate until the 30th day for BFS, FA replacement is greater than that of leached concrete with non-displacement. This is caused by the short reaction duration and because the reaction of the mineral admixture is slower than the hydration reaction. Therefore, the curing duration is important when considering the leaching degradation of concrete-blended mineral admixture. The change in the porosity with an increase in the leaching period is shown in Fig. 5. The porosity of the leached concrete becomes greater with the leaching periods. After the 90th day, the increase in the porosity rate became moderate. Also, the porosity in the sound concrete for all mixtures was approximately 5–15% less than those obtained at the initial times. Based on the results obtained by the bulk density and porosity tests, it was found that the quantity of leached Ca2+ ions is affected not only by the quantity of cement hydrate, but also by the quality of hydrate. If the curing period is not sufficient for the concrete blending the mineral admixture, the mineral admixture does not become more efficient against leaching degradation.

3.3. Distribution of the micro pore size Figs. 6 and 7 show the change in the pore size distribution between the sound and leached concrete from surface part 0 mm to 10 mm. In Fig. 6, the pore size of the sound concrete mainly ranges from 30 nm to 100 nm and shows a nearly constant trend regardless of the curing duration. This tendency appears the W/B 0.4, and 0.5 and with FA replacement. In case of BFS replacement, however, a different distribution was evident, That is, the pores, from 30 nm to 100 nm, were nearly filled with BFS particles or pozzolan reaction substance, so the peak of the pore size disappeared. Meanwhile, Fig. 7 shows that as the leaching period became longer, the pore volume within 200–500 nm in diameter is also greatly increased. Also, the volumes of pores larger than 200 nm rapidly increase during the initial leaching period while those below 50 nm gradually increase regardless of the W/B ratio. That is, the leaching of Ca2+ from calcium silicate hydrate (C S H) was not easy, unlike the leaching of portlandite (Ca(OH)2 ). Also, this tendency explained that the dissolution of calcium bound in the cement matrix occurs first with the leaching of portlandite, after which the progressive decalcification of C S H occurs. In other words, the primary hydrate phases are C S H gel and portlandite in OPC concrete. According to another study (Haga et al.,

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(a) W/C 0.4

(b) W/C 0.5 9.7

Degraded thickness (mm)

12

2

5.2

calcium leached

40 30 20 10

30

60

90

50

5.9

180

30 20 10 0

30

90

180

Degraded thickness (mm)

17.8

2

14.5

Coef. of Chloride diffusion ( 10 m /s)

calcium leached

40 30 20 10

60

90

50

4.9

180

10.1 water curing

-12

-12

2

Coef. of Chloride diffusion ( 10 m /s)

12 water curing

30

60

(d) W/B 0.5 (BFS30%)

Degraded thickness (mm)

0

15.2

Leaching period (days)

(c) W/B 0.5 (FA10%) 50

12.2 calcium leached

40

Leaching period (days)

6.1

8.8 water curing

-12

water curing

0

Coef. of Chloride diffusion ( 10 m /s)

50

4.7

-12

2

Coef. of Chloride diffusion ( 10 m /s)

Degraded thickness (mm)

11.2

13.2

calcium leached

40 30 20 10 0

30

60

90

180

Leaching period (days) Fig. 12. Chloride diffusion coefficient of leached concrete using NTBuild 443 (total thickness of specimen: 50 mm).

2005; Daimon et al., 1999) of involving a mortar leaching test, the changes in the pore volume assumed that pores 50 nm or smaller were attributable mainly to the C S H gel, while pores larger than 200 nm were attributable mainly to the Ca(OH)2 . From these results of another research, it is appears that much portlandite (calcium hydroxide) is removed by leaching and that the part of the C S H is also removed. Similar results were noted in the concrete specimen. Although the similar voids increased it is considered that the effect due to calcium leaching is more serious in the concrete specimen due to the interfacial transition zone (ITZ) near the particles in the course aggregate. In general, the concentrations of crystalline compounds such as calcium hydroxide are larger in ITZ (Mindess, 2003).

3.4. Compressive strength of leached concrete Fig. 8 shows the compressive strength of sound concrete according to the water curing age and that of degraded concrete according to the leaching period. The ratio of the degraded area to the total area is also shown in Fig. 8. According to Fig. 8, the compressive strength at 365 days for sound concrete showed a higher value than that at 28 days for all mixtures. In contrast, the compressive strength of the leached specimen gradually decreased with an increase in the degraded ratio. The leached specimen, however, was composed wholly of sound parts and leached parts. Therefore, it is necessary to determine the residual strength of the only leached

part. To do this, numerical analyses were performed based on the following equation. Fd = c (At − Ad ) + d Ad Here, Fd is the maximum load of the test specimen and  c is the strength of the sound part of leached the concrete. Also, and At are Ad the area of gross section and degraded section, respectively. At this time, we do not have any information about the strength of the sound parts in leached concrete specimens. Consequently, we assumed that the concrete of the sound part could have a strength value between the initial strength before leaching at a minimum and the strength developed during water curing at a maximum. Thus the calculated strength of leached part is individually presented as the maximum strength or the minimum strength in Fig. 9. From Fig. 9, the residual strength of the leached part with OPC was in the range 35–60%. Also, the mineral admixture replacement case, with the blending of BFS, the residual strength of the degraded zone was in the range 23–50%. Particularly, the residual strength of FA concrete was in the range 25–30%. According to previous research (Saito and Deguchi, 2000; Carde and Franc¸ois, 1997) which studied leached mortar, the fully leached mortar has a minimum strength of 24% or 20–60% of the strength of sound mortar. In addition, the residual strength of mortar is reduced by nearly 50% compared to the cement paste. This is due to the effect of the ITZ.

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In general, capillary pores larger than 50 nm, referred to as macro-pores in modern literature, are more influential in determining the strength and impermeability characteristics, whereas pores smaller than 50 nm, referred to as micro-pores, play an important role in drying shrinkage and creep (Mehta, 2005). In this study, the volumes of pores larger than 200 nm increased due to hydrate leaching. That is, a loss of strength was caused mainly by a change in the pore size for pores larger than 200 nm. Therefore, when a leaching attack is expected in a concrete member, it is needed to correct the design strength of the concrete. When the mineral admixture is blended into concrete, it is expected to increase the resistance against calcium leaching. However, the effect of the mineral admixture was minor in this study. Contrary to our expectations, the loss of strength in the concrete blend with the mineral admixture was larger than the non-replacement result. Thus, it appears that the leaching speed was faster than the pozzolanic reaction (leaching started at 28 days). Therefore, a sufficient curing duration must be allowed for concrete blended with a mineral admixture to prevent a loss of compressive strength due to leaching.

3.5. Resistance of chloride penetration in calcium-leached concrete

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The conclusions obtained from this study are summarized below: (1) When the leaching period increases, the pore volume for pores 200–500 nm in diameter also increase. Also, the volumes of pores larger than 200 nm increase during the initial leaching period while those below 50 nm increase gradually regardless of the water-binder ratio. (2) The residual strength of the leached part with OPC was in the range 35–60%. In the mineral admixture replacement case, upon blending with BFS, the residual strength of the degraded zone was in the range 23–50%. Specifically, the residual strength of FA concrete was in the range 25–30%. (3) In the case of RCPT, the chloride diffusion coefficient of leached OPC was increased by approximately 1.5–2.5 times. Also, coefficients of leached concrete blended with FA and BFS showed three- to six times increases, respectively. The chloride diffusion coefficient as measured by the chloride profile was greatly increased two- to five times with the leaching duration. (4) In this study, use of the mineral admixture did not show useful results. Therefore, a sufficient curing duration must be allowed for concrete blended with a mineral admixture to resist leaching degradation. Acknowledgments

Fig. 10 shows the result of the chloride diffusion coefficient as measured by RCPT at each leaching period. According to the results, the chloride diffusion coefficient decreased clearly with an increase in the curing time and decreased at a lower W/B ratio. However, the diffusion coefficient of chloride after the leaching test greatly increased with the leaching period and the degradation depth. In the case of OPC, the diffusion coefficient increased by approximately 1.5 times to 2.5 times. Specifically, the diffusion coefficients of FA and BFS concrete showed increases of three- to six-times. Fig. 11 present the chloride profiles as measured by NT Build 443, while Fig. 12 show the coefficient of chloride diffusion as determined by Fick’s second law on the basis of the profiles. As shown in Fig. 11, the chloride content according to the leaching depth was greater than that of sound concrete. From Fig. 12, the diffusion coefficient was greatly increased two-to five-times with the leaching duration. In this study, chloride diffusion coefficient of leached concrete calculated by the averaging the sound and leached part of specimens. Thus, the chloride diffusion coefficient of leached concrete is not enough to evaluate the chloride diffusion coefficient of fully leached concrete. However, the increase of chloride penetration is clearly appeared by the leaching attack. Thus, it is desirable to increase the cover depth of a concrete member exposed to leaching attack to improve its resistance of chloride penetration. Generally, when admixture such as FA and BFS are blended into concrete, the chloride diffusion is lower than of normal concrete. The use of FA or BFS increases the chloride binding capacity due to improved chemical and physical binding. However, if the concrete has not had a sufficient curing period in regards to the reaction with portlandite, even when the mineral admixture is mixed with cement paste, the mineral admixture is not efficient against both leaching degradation and chloride attack.

4. Conclusions A study was performed to evaluate the degradation condition caused by the calcium leaching of concrete. This study is based on concrete with various W/B ratios. Also, the pore size distribution, compressive strength and chloride penetration were measured.

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