Change in crystal polymorphism of CaCO3 generated in cementitious material under various pH conditions

Construction and Building Materials 188 (2018) 1–8

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Change in crystal polymorphism of CaCO3 generated in cementitious material under various pH conditions Heesup Choi ⇑, Masumi Inoue, Risa Sengoku Kitami Institute of Technology, Japan

h i g h l i g h t s
Crack in concrete structures significantly increases ion diffusivity and permeability inside the material and allows chloride ions.
When the aqueous environment, self-healing can occur where a part of the crack is filled by rehydration of the cement particles and precipitation of

CaCO3.
Change in the crystal forms of CaCO3 generated in the hardened cement paste through pH adjustment were monitored for the generation of denser

CaCO3 crystals.
The pH conditions enabling the generation of the denser vaterite CaCO3 are presented herein.

a r t i c l e

i n f o

Article history: Received 19 May 2018 Received in revised form 5 August 2018 Accepted 10 August 2018

Keywords: Micro-crack Cementitious materials Self-healing CaCO3 CO2 pH Vaterite

a b s t r a c t Although concrete is the most universal material in construction, its tensile strength is markedly lower than its compressive strength, resulting in the unavoidable occurrence of cracks in concrete structure. Since infiltration of harmful substances such as chloride, carbon dioxide, and sulfate can occur easily through such cracks, they grow larger and deeper due to repeated infiltration and the concrete structure can eventually suffer catastrophic damage. When the crack widths are small in an aqueous environment, self-healing can occur where a part of the crack is filled by rehydration of the cement particles and precipitation of CaCO3. In addition, self-healing performance can be maximized through control of the crystal forms of CaCO3 by adjusting temperature and pH. Therefore, in this study, the crystal forms of CaCO3 generated in the self-healing process were examined, and changes in the crystal forms of CaCO3 generated in the hardened cement paste through pH adjustment were monitored for the generation of denser CaCO3 crystals. Based on this, the pH conditions enabling the generation of the denser vaterite CaCO3 are presented herein. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Concrete is the most universal material in construction, and is generally irreplaceable. However, since its tensile strength is markedly lower compared with its compressive strength, the formation of large and small cracks in a concrete structure are unavoidable [1]. Infiltration of harmful substances such as chloride, carbon dioxide, and sulfate easily occurs through these cracks and as they grow larger and deeper due to repeated infiltration, the concrete structures can eventually be critically damaged [2]. In Japan, cracks smaller than the allowed crack width 0.05 mm are considered to not cause large problems with regards to waterproof performance of concrete [3]. However, while concrete micro-cracks do not ⇑ Corresponding author. E-mail address: [email protected] (H. Choi). https://doi.org/10.1016/j.conbuildmat.2018.08.045 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

immediately degrade the safety performance of the structure, deterioration factors can infiltrate from fine cracks into the interior of the concrete and cause degradation of water tightness as a crude measure of durability [4]. In addition, these deterioration factors can repeatedly infiltrate, accelerating the deterioration of the concrete due to the expansion of crack widths [5,6]. Thus, the safety performance of the concrete structures is likely to decrease significantly. Therefore, for all concrete structures, including waterproof concrete prepared using cement-based composite materials that require water tightness, strategies for preventing micro-cracks at earlier stages are needed. When crack widths are particularly small in the concrete in an aqueous environment, self-healing has been observed where a part of the crack is filled due to the rehydration of cement particles and precipitation of CaCO3 [7]. Self-healing products usually contain hydrates such as C-S-H hydrate, ettringite, and calcium hydroxide

2

H. Choi et al. / Construction and Building Materials 188 (2018) 1–8

(Ca(OH)2), which precipitate along with CaCO3 at the newly generated crack surfaces [8,9]. In the self-healing mechanism of concrete, CaCO3 is produced as a carbonic acid that is not readily dissolved in water due to the reaction of the Ca2+ in concrete with CO2 3 dissolved in water [10]. In addition, the cracks are restored by this phenomenon, and cracks smaller than 0.1 mm are reportedly restorable [10–12]. The crystal reaction Eqs. (1)–(3) of CaCO3 are shown below [10].

H2 O þ CO2 () H2 CO3 () Hþ þ HCO3 () 2Hþ þ CO2 3 2þ

Ca

þ

CO2 3

() CaCO3

ðpH water > 8Þ

Ca2þ þ HCO3 () CaCO3 þ Hþ

ð1Þ 2.2. Experimental methods

ð2Þ

ð7:5 < pH water < 8Þ

ucts according to the hydration reaction of water [10]. Cement paste specimens (u10  30 mm) with a water-cement ratio of 0.4 were produced using ordinary Portland cement (C, density: 3.16 g/cm3, average particle diameter 10 lm) according to the ASTM C 150. To prevent dissipation of water in the specimens, suture curing was conducted at a constant temperature and humidity of 20 ± 1 °C and 60%, respectively, from immediately after dispensing for 1 day. Subsequently, the demolded specimens were subjected to underwater curing in a water tank at 20 ± 1 °C for 28 days under various pH conditions for the specimens cut to sizes of u10  3 mm, as shown in Fig. 2. Fig. 2 shows a schematic diagram of the preparation of the hardened cement paste specimens used in this study.

ð3Þ

Among the many studies related to self-healing involving bacteria, crystalline admixtures, superabsorbent polymers, and Engineered Cementitious Composites (ECC) with Fly Ash [13–29], Choi et al (2017) reported that carbon dioxide was produced as ultrafine nano-sized bubbles due to the self-healing mechanism under aqueous conditions. The CO2 precipitated large amounts of CaCO3 in the surface layer and inside the micro-cracks due to the self-healing of cement-based composite materials, with most of the precipitates identified as vaterite [30,31]. Representative crystals of CaCO3 can be classified into 3 types including calcite, vaterite, and aragonite, and almost all CaCO3 produced as Ca(OH)2 in the hardened specimen of cement combined with CO2 in the pore 3 water can be classified as calcite [32,33]. Since vaterite has low density and large volume as a hexagonal crystal, it exhibits excellent pore-filling effects, superior to the other CaCO3 crystals [33,34]. Thus, it can contribute to improved water tightness and strength of concrete, allowing self-healing substances with a denser crystal structure than calcite to form [30,32]. The generation of the 3 types of CaCO3 crystal forms can be controlled by pH [32,33]. According to previous studies, control of polymorphism scale formation is possible at room temperature of 20 °C with vaterite, aragonite, and calcite at pH 9.0, 10.5, and >11, respectively [32]. In this study, for cement-based composite materials with a waterproof layer, the crystal forms of CaCO3 generated in the self-healing process were examined and clarification of the effects of pH on the crystal form of CaCO3 as a product of self-healing were determined. To produce denser CaCO3 crystals, the possibility of changing the crystal forms of CaCO3 generated in the hardened cement paste through pH adjustment was confirmed with determination of pH conditions allowing generation of vaterite. In addition, to identify the effects of Ca2+ on the generation of self-healing materials according to changes in pH, optimum conditions for selfhealing were determined using Ca(OH)2 and NH3 aqueous solutions. Fig. 1 shows a schematic of the self-healing process studied herein.

In Table 1, the experimental factors and conditions are listed. By referring to the existing literature for self-healing conditions [32,33], saturated Ca(OH)2 (solubility: 0.082 g/100 mL) and NH3 (solubility: 54 g/100 mL) aqueous solutions were used to identify the effects on the change in crystal forms and porosity of CaCO3 as a function of pH (Table 1). A pH control device with terephthalic acid (C8H6O4, solubility: 0.0015 g/100 mL) for the Ca(OH)2 aqueous solution and ammonium chloride (NH4Cl, solubility: 37.2 g/100 mL) for the NH3 aqueous was used as a solvent for adjusting the pH (Fig. 3). In addition, the experiments were conducted at 20 °C by setting the pH conditions for generation of each crystal form of CaCO3 based on the literature to be pH 9.0, 10.5, and 12.0 [32,33], and by setting the self-healing period to 7 days (Table 1). The specimens were cut to the size of u10  3 mm and dried for 1 day in a drying oven at 105 °C to measure the absolute dry weights before self-healing. Subsequently, the specimens were wetted by immersion in water for 1 day to measure the underwater and surface dried weights before self-healing (Step A). Afterwards, the specimens were immersed for 7 days in pH adjusted Ca(OH)2 and NH3 aqueous solutions. Subsequently, specimens were immersed in acetone for more than 4 h to stop the hydration reaction, followed by the measurement of absolute dry, surfacedried, and underwater weights after self-healing (Step B) using the same method as before self-healing (Table 2). The absolute dry density, water absorption ratio, and porosity were then calculated using the weights of specimens before (Step A) and after self-healing (Step. B) to compare and evaluate changes in physical characteristics due to self-healing.

30mm

10mm

- Specimen : 3mm

Specimen

Cement paste, W/C 0.4 20

, RH100 , 28day

10mm 2. Experimental Fig. 2. Schematic diagram of the preparation of the cement samples.

2.1. Materials and specimen preparation In the self-healing of cement-based composite materials, evaluating the change in crystal forms of CaCO3 produced in the hardened cement paste through pH adjustment was performed. The major reacting species depend on hydration prod-

Ca2+

pH control Ca2+ + CO32-

CO3

CaCO3 H2O

Specimen: Hardened cement paste (water/cement ratio 0.4) Temperature: 20 °C

Calcite Vaterite Aragonite

2Fig. 1. Process of the self-healing by pH control.

Table 1 Experimental factors and conditions.

Self-healing condition [24,25]

pH: 9.0, 10.5, and 12.0 (constant temperature of 20 °C) Self-healing period: 7 days

pH 9.0 (CH-9, NH-9), pH 10.5 (CH-10, NH-10), pH 12.0 (CH-12, NH-12)

Note: CH: Ca(OH)2 aqueous solution; NH: Ammonia (NH3) solution; CH NH-9: each aqueous solution at pH 9.0; CH NH-10.5: each aqueous solution at pH 10.5; CH NH12.0: each aqueous solution at pH 12.0.

H. Choi et al. / Construction and Building Materials 188 (2018) 1–8

pH electrode

pH controller Specimen

solvent

agitator Fig. 3. Schematic of the pH controller.

Table 2 Experimental procedure and evaluation. Step

Experimental sequence

Subject and method of evaluation Physical property

Self-healing substances

A

Before self-healing

Dry density

B

After self-healing

Water absorption ratio Porosity

Porosity reduction by carbonation MIP SEM analysis

Note: Step A: Before self-healing; Step B: After self-healing.

Meanwhile, since the hardened cement paste exhibited a porous structure with large amounts of voids, the reduction ratio of the voids due to the changes in weight and water absorption ratio before and after self-healing was measured. To calculate the change in the number of voids, the void reduction ratio was determine through correlation with the theoretical model for generation and porosity of hydration products developed by Papadakis [35]. In addition, by comparing the reduction ratio of voids and the change in the actual water absorption ratio, self-healing filling effects due to the changing crystal forms were evaluated. In addition, pore structure was measured using a mercury intrusion porosimetry (MIP). In addition, to identify control of polymorphism scale formation to vaterite, which is denser than calcite [32,33], SEM analysis was performed for each specimen as a function of pH. However, to determine the change in number of voids, MIP and SEM analyses were conducted using only the CH series in Ca (OH)2 aqueous solution based on the physical performance data including absolute dry density and water absorption ratio.

3

each specimen. In this section, using the results of the specimen performed at the first data of Table 3, comparison and evaluation of absolute dry density, water absorption ratio and porosity of CH and NH series were conducted. Figs. 4–7 show the absolute dry density and water absorption ratio of each specimen as a function of pH under various immersion conditions in Ca(OH)2 and NH3 aqueous solutions. As shown in Figs. 4 and 5, the absolute dry density after self-healing (Step B) of the CH series slightly increased compared with before selfhealing (Step A) while the water absorption ratio decreased by approximately 1% irrespective of pH. Particularly, the dry density increased by approximately 1.4% at pH 9.0, 1.5% at pH 10.5, and 2.4% at pH 12.0. Meanwhile, as shown in Figs. 6 and 7 for the NH series, the absolute dry density after self-healing (Step B) was slightly reduced compared with before self-healing (Step A), while the water absorption ratio showed a slight increase. The dry density of the NH series showed opposite tendencies to that of the CH series with a reduction of approximately 3.6% at pH 9.0, 1.2% at pH 10.5, and 4.5% at pH 12.0. Figs. 8 and 9 show the calculated porosity of the CH and NH series, respectively. For the CH series, a reduction of porosity was observed by approximately 7.1% at pH 9.0, 7.4% at pH 10.5, and 6.9% at pH 12.0. For the NH series, an increase in porosity was observed by approximately 11.0% at pH 9.0, 9.6% at pH 10.5, and 7.8% at pH 12.0, showing a similar trend as the water absorption ratio. Based on the above results, under immersion conditions in Ca (OH)2 and NH3 aqueous solutions, the change in the absolute amount of Ca2+ supplied is considered to have significant effects on the generation of CaCO3 after self-healing. This is likely the cause of differences in the absolute dry density, water absorption ratio, and porosity. For the CH series using a Ca(OH)2 aqueous solution compared to a NH3 aqueous solution (NH series), the generation of CaCO3 as a precipitate of self-healing increased as a large number of Ca2+ ions infiltrated into the specimen [31,39]. Meanwhile, for the NH series, the reduction of absolute dry density and increased water absorption ratio and porosity as compared to the CH series can be attributed to the elution of Ca from the hardened cement paste in the aqueous solution [36]. 3.2. Micro-analysis of the CaCO3 materials formed by self-healing

3. Results and discussion 3.1. Change in physical properties upon self-healing To identify changes in the physical characteristics of the hardened cement paste according to the type of aqueous solution and pH, the absolute dry, surface-dried, and underwater weights before and after self-healing were used to calculate the absolute dry density qd (g/cm3), water absorption ratio A(%), and porosity P(%) according to Eqs. (4)–(6).

Wd qd ¼ ðW s  W w Þ=qw

ð4Þ

A¼

Ws  Wd  100 Wd

ð5Þ

P¼

ðW s  W d Þ=qw  100 ðW s  W w Þ=qw

ð6Þ

where Wd is the absolute dry weight [g], Ws is the surface-dried weight [g], Ww is the underwater weight [g], and qw is the density of water at 20 °C (0.99821[g/cm3]). Table 3 shows the results of absolute dry density, water absorption ratio and porosity of CH and NH series with three times of

3.2.1. Changes in porosity To understand the filling effects of CaCO3, actual water absorption ratios of before and after self-healing of the CH series under various pH conditions were substituted into Eq. (7) to obtain the reduction ratio due to carbonation after self-healing, DP[%].

DP ¼

PA  PB  100 PA

ð7Þ

where PA is the porosity before self-healing (A) and PB is the porosity after self-healing (B). The microscopic filling effects of CaCO3 were examined using the actual weight changes under the same conditions as the void reduction ratio. Because of the water absorption ratio, the reduction ratio of voids due to carbonation after self-healing was obtained using the theoretical model proposed by Papadakis regarding the generation of hydration products and porosity [35]. When Ca(OH)2 (74 [g/mol]) was converted to CaCO3 (100 [g/mol]), 26 g of weight is gained per mol [36]. Based on Papadakis’ theoretical model for the generation and porosity of hydration products [35], the overall weight increase Wtotal[g] and reaction ratio R[%] when all Ca(OH)2 is changed to CaCO3 after selfhealing of the CH series can be obtained using Eqs. (8) and (9).

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H. Choi et al. / Construction and Building Materials 188 (2018) 1–8

Table 3 Result of the absolute dry density, water absorption ratio and porosity of CH and NH series. Type

pH

CH series

NH series

1st

2nd

3rd

1st

2nd

3rd

Step. A

Step. B

Step. A

Step. B

Step. A

Step. B

Step. A

Step. B

Step. A

Step. B

Step. A

Step. B

Dry density(g/cm3)

9.0 10.5 12.0

2.01 2.00 2.03

2.04 2.03 2.08

2.04 2.01 2.01

2.08 2.04 2.05

2.01 2.00 2.04

2.03 2.04 2.08

2.00 2.02 2.09

1.93 2.00 1.99

1.94 2.11 2.04

1.88 2.06 2.02

2.06 2.00 2.04

2.02 1.97 2.01

Water absorption ratio (%)

9.0 10.5 12.0

10.88 10.79 10.52

9.96 9.84 9.56

10.48 10.65 11.02

9.88 9.80 10.13

10.93 10.09 10.52

10.01 9.27 9.77

11.35 10.97 10.73

13.23 12.27 12.17

10.24 10.71 10.66

12.54 13.01 12.77

10.55 11.02 11.21

13.22 12.95 13.08

Porosity(%)

9.0 10.5 12.0

21.88 21.58 21.36

20.32 19.98 19.88

21.33 21.94 21.57

20.42 20.12 20.99

21.66 22.11 20.86

20.31 21.16 20.14

22.71 22.16 22.40

25.53 24.50 24.29

21.99 22.41 22.13

24.71 24.46 23.99

20.89 21.48 21.53

24.01 25.11 25.34

Note: CH: Ca(OH)2 aqueous solution; NH: Ammonia (NH3) solution; Step A: Before self-healing; Step B: After self-healing.

20

Dry density (g/cm3)

Step. A 2.01 2.04

2

Step. B 2.03 2.08

2.00 2.03

1

0

Water absorption ratio (%)

3

Step. A 13.23

12

10.5

11.35

12

9.0

30

Step. B

25

10.88

9.96

10.79

9.84

10.52

9.56

8

Porosity (%)

Water absorption ratio (%)

Step. A 16

21.88 20.32

20

21.58 19.98

10

4

5

0

0

Step. A

9.0

10.5

21.36 19.88

15

12

9.0

Step. B

10.5

12

pH

pH

Fig. 8. Porosity (CH series).

Fig. 5. Water absorption ratio of the CH series.

30

3

2.00 1.93

Step. B

2.02 2.00

25 2.09 1.99

1

Porosity (%)

Step. A

Dry density (g/cm3)

12

Fig. 7. Water absorption ratio of the NH series.

20

25.53 22.71

9.0

10.5

pH Fig. 6. Dry density of the NH series.

12

24.50 22.16

24.29 22.40

20 15 10 5

0

10.5

pH

Fig. 4. Dry density of the CH series.

2

12.17 10.73

4

pH

12

12.27 10.97

8

0

9.0

Step. B

16

0

Step. A 9.0

Step. B

10.5

pH Fig. 9. Porosity (NH series).

12

5

H. Choi et al. / Construction and Building Materials 188 (2018) 1–8

DW d  100 W total

ð9Þ

where Wd is the weight before self-healing (A) [g] and DWd is the increased/reduced weight after self-healing (B) [g]. Based on Eqs. (10) and (11) proposed by Papadakis [35], the concentration of Ca(OH)2 can be calculated as follows [37,38].




 3 1 CaðOHÞ2 ¼ ½C 3 S0 F C 3 S þ ½C 2 S0 F C2 S  4½C 4 AF o F C 4 AF 2 2 h  i  ½C 3 A0 F C 3 A þ C S H2 0

F i ðt Þ ¼ 1 

 1=ð1ni Þ ½i ¼ 1  1  kH;i t ð1  ni Þ ½i0

ð10Þ ð11Þ

where Fi(t) is the reaction ratio of substance i for time t; [i] and [i]0 are the concentration of substance i for time t [mol/m3] and its and initial concentration; kH,i is the reaction rate constant of substance i at 20 °C [1/s]; and ni is the experimental value. Lastly, to obtain the reduction ratio of voids due to carbonation after self-healing using actual weight change DecB, Papadakis’ Eq. (12) accounting for porosity due to carbonation was used [35].

D2C B ¼

R D2C  100  100 20  D2H ðtÞ

ð12Þ

where Ƞ0 is the initial porosity of fresh concrete, D2H(t) is the porosity due to the hydration reaction, and D2c is the reduction ratio of voids due to carbonation. The pore reduction ratio due to carbonation of the CH series as a function of pH is shown in Fig. 10. The water absorption ratio was used for the experimental value (DP), while weight change and Papadakis’ model [35] were used for the calculated value (D2CB). The experimental results shows that the difference was not large, although the pore reduction ratio was increased by approximately 1.0, 1.2, and 0.7% at pH 9, 10.5, and 12.0, respectively, irrespective of pH when the experimental value (DP) after self-healing was compared to the calculated value (D2CB). Therefore, at W/C 0.4, predicting the change in the number of voids due to carbonation after self-healing is possible using the experimentally determined weight change and Papadakis’ model. Similar results for the change in the number of voids were obtained using the change in water absorption ratios before and after self-healing. However, when the theoretical model of hydration products and porosity proposed by Papadakis [35] was applied to this study, evaluations were conducted assuming that only Ca(OH)2 contributed to the change in voids due to carbonation in the specimens with selfhealing. Consequently, the difference of approximately 1% was confirmed when compared with the change in number of voids

calculated using the actual water absorption ratio. In the future, quantitative reviews will be required for the change in the number of voids due to carbonation of hydration products other than Ca (OH)2 such as C-S-H gel, ettringite, and Friedel’s salt after selfhealing. 3.2.2. Structural changes of the voids by MIP In Figs. 11 and 12, the void distributions before and after selfhealing of the CH series as measured by the MIP are shown. The longitudinal axis of the graph shows the void content and accumulated void content (%), while the lateral axis represents the pore diameter. According to the measurement results for the void amount in Fig. 11, a slight shift to a smaller distribution of fine pores was confirmed irrespective of pH within the range of 0.1–1 lm after selfhealing (Step B) compared with before self-healing (Step A). In addition, at approximately 0.1–0.01 lm in the fine pore diameter (capillary pore), a slight increase in the fine pore amount was observed after self-healing (Step B). In addition, according to the accumulated void content shown in Fig. 12, differences according to pH change were not large, although the void content was reduced by approximately 4, 2.2, and 2.1% at pH 9.0, 10.5, and 12.0 after self-healing (Step B) compared with before self-healing (Step A). The absolute amount of Ca2+ ions present in the Ca(OH)2 aqueous solution increased during self-healing compared with before self-healing. This is considered to have contributed to an increase in CaCO3 generation inside the hardened cement paste, resulting in void filling. 3.2.3. Crystallographic change in CaCO3 as a function of pH To examine the generation and control possibilities of vaterite as a denser CaCO3 compound according to pH control, SEM analysis

6

Void Vol.%

R¼

ð8Þ

Porosity reduction by carbonation (%)

2

0.01

0.1

1

10

100

1000

m

Fig. 11. Comparison of the void sizes.

40

Experimental value

6.93 6.18

6.32

6 4 2

9.0

Step. A Step. B_9.0 Step. B_10.5 Step. B_12.0

Calculated value

7.41

7.13 6.11

0

4

Diameter

10 8

Step. A Step. B_9.0 Step. B_10.5 Step. B_12.0

0 0.001

10.5

12

pH Fig. 10. Porosity reduction by carbonation of the CH series.

Cumulative void Vol.%

W total


 ¼ ð100  74Þ  CaðOHÞ2  ðW d þ DW d Þ

30

20

10

0 0.001

0.01

0.1

1

Diameter

10

100

m

Fig. 12. Comparison of the cumulative void size.

1000

6

H. Choi et al. / Construction and Building Materials 188 (2018) 1–8

Preparation of SEM specimens Cleavage Self-healing

Measuring surface of SEM

Specimen

10mm

Specimen

Fig. 13. Preparation of SEM specimens.

at each pH condition was conducted on the CH series. Self-healing was conducted using Ca(OH)2 aqueous solution and the SEM images of specimens for each of the 3 pH conditions including

9.0, 10.5, and 12.0 are shown in Figs. 14–16, respectively. SEM was performed for the cleavage surface (inside the specimen) of the u10  3 mm specimens (Fig. 13), and crystal form and size of

Fig. 14. Self-healing substances at pH 9.0 (CH series).

Fig. 15. Self-healing substances at pH 10.5 (CH series).

Fig. 16. Self-healing substances at pH 12.0 (CH series).

H. Choi et al. / Construction and Building Materials 188 (2018) 1–8

CaCO3 from the literature [32,33] were compared with the crystal structures obtained in this study for determination of the crystal form of CaCO3. Therefore, from the above comparison of the crystal form and size of CaCO3, the crystal structure of CaCO3 obtained in this study can be guessed. According to the SEM analysis, Ca(OH)2 was almost not detected in the cement hydration products at pH 9.0 and 12.0 with vaterite dominant at pH 9.0 and calcite at pH 12.0. In addition, Ca(OH)2 was observed to a small extent along with aragonite at pH 10.5. Thus, control of vaterite, aragonite, and calcite in the crystal forms of CaCO3 produced in the cement matrix is possible when selfhealing is conducted at adjusted pH. Particularly, the effective generation and control of vaterite as a denser CaCO3 crystal is possible by adjusting the pH to approximately 9.0 at 20 °C. 4. Conclusions In this study, changing the crystal forms of CaCO3 generated in hardened cement paste was achieved by adjusting pH to generate denser crystals. The pH conditions allowing for the predominant generation of vaterite among the crystals of CaCO3 were also established. In addition, the optimum self-healing conditions according to the change in absolute amount of Ca2+ can be summarized as follows. 1) When self-healing is conducted in Ca(OH)2 aqueous solution, the generation of CaCO3 inside the hardened cement paste increased after self-healing irrespective of pH, since the absolute amount of Ca2+ was larger than in the NH3 aqueous solution. Thus, more effective self-healing was possible as the absolute dry density increased and the water absorption ratio and porosity decreased. 2) Based on the differences in the void reduction ratio obtained by actual water absorption ratios and weight changes, evaluation of void changes according to self-healing and prediction of filling effects were possible by using the actual weight change and applying a theoretical model for the generation of hydration products and porosity proposed by Papadakis. 3) For self-healing using Ca(OH)2 aqueous solutions, the change in crystal forms of CaCO3 generated in the cement matrix produced mainly vaterite at pH 9.0, aragonite at pH 10.5, and calcite at pH 12.0. Particularly, the possibility of effective generation and control of vaterite as a denser CaCO3 crystal was confirmed by adjusting pH to 9.0 at 20 °C. Conflict of interest The authors declared that there is no conflict of interest. References [1] H.S. Choi, M. Inoue, S.M. Kwon, H.G. Choi, M.K. Lim, Effective crack control of concrete by self-healing of cementitious composites using synthetic fiber, J. Mater. 9 (2016) 1–14. [2] Japan Concrete Institute, Practical Guideline for Investigation, Repair and Strengthening of Cracked Concrete Structure, Japan Concrete Institute, Tokyo, Japan, 2013 (In Japanese). [3] S. Jacobsen, Effect of cracking and healing on chloride transport in OPC concrete, Cem. Concr. Res. 26 (1996) 869–881. [4] D. Romildo, T. Filho, Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibers, Cem. Concr. Compos. 27 (2005) 537–546. [5] K. Wang, D.C. Jansen, S.P. Shah, Permeability study of cracked concrete, Cem. Concr. Res. 27 (1997) 381–393. [6] R.P. Khatri, V. Sirivivatnanon, Role of permeability in sulfate attack, Cem. Concr. Res. 27 (1997) 1179–1189. [7] A.M. Neville, in: Properties of Concrete, Person Education Limited, London, UK, 1995, p. 328.

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