Improving efficiency of calcium oxide expansive

Magazine of Concrete Research Volume 68 Issue 20 Improving efficiency of calcium oxide expansive additives by polylactic acid film Wang, Tian, Zhang et al.

Magazine of Concrete Research, 2016, 68(20), 1070–1078 http://dx.doi.org/10.1680/jmacr.15.00529 Paper 1500529 Received 11/12/2015; revised 04/02/2016; accepted 22/02/2016 Published online ahead of print 18/04/2016 Keywords: admixtures/cement paste/shrinkage ICE Publishing: All rights reserved

Improving efficiency of calcium oxide expansive additives by polylactic acid film Rui Wang

Anqun Lu

State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Research Institute of Building Science, Nanjing, People’s Republic of China; Jiangsu Sobute New Materials Co., Ltd, Nanjing, PR China

State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Research Institute of Building Science; Jiangsu Sobute New Materials Co., Ltd, Nanjing, PR China

Qian Tian

Jun Cheng



Shouzhi Zhang

Lei Li



Hua Li

State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Research Institute of Building Science; Jiangsu Sobute New Materials Co., Ltd; College of Materials Science and Engineering, Southeast University, Nanjing, PR China (corresponding author: [email protected])


Jiaping Liu

Calcium oxide-based expansive additive (CEA) has the disadvantages of a too-fast reaction rate when mixed with water and lower storage stability when exposed to the atmosphere, which reduces its expansion efficiency for shrinkage-compensating concrete. This paper introduces a modification technology by encapsulating CEA particles with polylactic acid (PLA) polymer to form a protective membrane. Experimental results indicate that the encapsulating PLA membrane could build an inductive period in the hydration process of CEA and delay the initiation of hydration when mixed with water, thus effectively reducing the invalid hydration of CEA during the plastic stage. Restrained and free deformation testing results demonstrate that the expansion of the cement paste incorporating modified CEA increases by 25% and 35% over that of the paste with the original CEA, respectively. The weathering rate, assessed under conditions of 25°C and 60% humidity to evaluate the storage stability, was slowed down by 30%. This paper offers an original method to improve the expansion efficiency of additives so that sufficient varieties of polymer can be utilised.

Introduction Calcium oxide (CaO)-based expansive additives (CEAs) are widely used for the mitigation of cracking in concrete structures, for example, by compensating for the drying shrinkage strain of concrete or providing chemical pre-stress (Collepardi et al., 2005, 2008; Gao et al., 2006; Mo et al., 2014; Serris et al., 2011; Yan and Qin, 2001). However, this type of additive has a very fast reaction rate when mixed with water. The hydration of CEA during the plastic stage of fresh concrete is not useful for expansion, and substantially lowers the concrete’s expansion efficiency. Moreover, the very high sensitivity of CEA to water also lowers the storage stability of the product when exposed to the atmosphere, and reduces its effectiveness in actual applications as well. In order to ensure an effective expansion and shrinkage compensating effect, it is necessary to reduce the hydration of CEA before the formation of the 1070

cement matrix, and subsequent large expansion can be achieved if the reaction occurs after the period when plastic properties are dominant. Methods have been adopted to optimise the use of CEA. Higuchi et al. (2014) reported that high-temperature carbonation to generate a calcite film surface on the expansive additive can increase expansion and improve stability to weathering. Lee and Ryou (2014) found that granulated expansive agent with a polyvinyl alcohol (PVA) film can control the time of autogenous healing and can prevent water migration by closing cracks. However, problems such as efficiency and stability of the modified CEA still need to be studied further. It is difficult to control the calcite film in a quantifiable way by the carbonation method. The PVA used was a water-soluble polymer, and the expansion efficiency of the granulated

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Magazine of Concrete Research Volume 68 Issue 20

Improving efficiency of calcium oxide expansive additives by polylactic acid film Wang, Tian, Zhang et al.

expansive agent with a PVA film was not discussed. In the current paper, a novel approach has been developed to improve the expansion efficiency of CEAs by particle encapsulation with a degradable polymer membrane.

Experiments Materials Ordinary Portland cement (OPC) and CEA were used in this investigation; the chemical composition and physical properties of the OPC and CEA are shown in Table 1. CEA and modified CEA were sieved according to a diameter of 160 μm before the experiment; Chinese ISO standard sand with a fineness modulus of 3·0 and tap water were used. The PLA polymer was donated by Professor Xiabin Jing from Changchun Institute of Applied Chemistry Chinese Academy of Sciences.

There has been a growing interest in modifying surface properties of inorganic oxides and other solid substrates with organic molecules or polymers, such as polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP) or polymer vinyl acetate (PVAc) (Karde et al., 2015; Ron and Cohen, 2001; Sudam et al., 2006). In all of the applications, information regarding surface character is essential in assessing the surface behaviour, which depends on the hydrophobicity or the interaction between the polymeric films with the atmosphere. Ideally, for CEAs, hydration during the plastic stage should be prevented or minimised, but once the cement matrix is formed there should be continuous hydration, creating expansive forces. Polylactic acid (PLA) is an aliphatic polyester made up of lactic acid (LA) building blocks and degradable under alkaline conditions (James and Matthew, 2012; Lim et al., 2008). As shown in Figure 1, PLA was purposely selected to encapsulate CEA by the soluble method to compress the early hydration by a water-insoluble PLA protective membrane and delay the hydration process of the CEA through the degradation of PLA to form water-soluble lactic acid (LA). This paper describes the expansive properties and hydration of PLA-encapsulated CEAs.

Calcium-oxide-based expansive additive encapsulated by PLA (CEA/PLA) was prepared as follows: 2 g PLA polymer was dissolved in dichloromethane with a concentration of 20 mg/ml, then 98 g CEA was added and stirred for 5 min; the solvent was then removed by vacuum rotary evaporation. The solid produced in this way was milled and sieved by a diameter of 160 μm.

Methods Characterisation of the expansive additives CEA and CEA/PLA PLA content in the expansive additives was calculated by the weight of polymer and CEA added. A scanning electron

–

OH

–

OH

H2O OH

OH

H2 O OH

O

OH

CH3 O CH3 PLA (water insoluble) –

OH

CH3 LA (water soluble) –

H2O

H2O

OH

OH

OH

H 2O –

CEA particle by PLA encapsulation

H2O –

H2O

OH

OH

–

OH

H2O

OH

–

H2O

–

OH

–

OH

H2O

H2O

OH

H2O

H2O –

–

OH

–

H2O

– –

Hydration products of expansive additives

HO

OH

n

– H2O OH

O OH–/H2O

O

H 2O

OH

OH

H2O

H2O

–

–

O

HO

H2O

H2O

–

OH

H3C

–

OH

–

H2O

– OH H2O H2O – OH – OH

H2O

–

H2O

CEA particle O

OH

H2O

–

OH

–

–

H2O

H2O

H2O

–

OH

OH

H2O

–

OH

LA

The hydration process of expansive additives

Figure 1. Schematic representation of the prolonged hydration of CEA particles by PLA encapsulation


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Material

OPC CEA


Chemical composition Calcium oxide (CaO)

Aluminium oxide (Al2O3)

Silicon dioxide (SiO2)

Iron (III) oxide (Fe2O3)

Magnesium oxide (MgO)

Sulfur trioxide (SO3)

Loss on ignition

64·02 82·31

4·57 0·26

22·13 10·64

3·05 1·2

2·00 1·26

2·44 2·05

1·79 2·28

Density: g/cm3

Blaine fineness: cm2/g

3·16 3·00

3200 2800

Table 1. Chemical composition and physical properties of materials

microscope (SEM) was employed to identify the morphological features of CEA and CEA/PLA under high vacuum, and element analysis was conducted by energy dispersive spectrometer (EDS) (SEM, Quanta250, FEI Company, Czech Republic). Compressive strength, setting time and bulk density of the cement paste The specimens (40mm 40mm 160 mm) of the cement pastes with CEA or CEA/PLA were prepared according to the Chinese standard GB 23439-2009; the water–cement ratio of the pastes was 0·35, and 6% (by weight) of the OPC was replaced by expansive additives. The compressive strength of the cement paste specimens was evaluated in accordance with Chinese standard GB 17671. Tests were conducted at 0·5, 1, 3, 5 and 7 d. The setting time of fresh pastes was measured according to ISO 9597. The bulk density of fresh pastes was tested according to Chinese standard JGJ 70-90. A 1-litre container was used during the testing. After the fresh paste had been placed into the container, it was vibrated for 10 s before the mass was measured. Each record was measured three times and an average was then taken. Expansion of unrestrained specimens Expansion of unrestrained specimens incorporating expansive additives was measured using a dilatometer based on the corrugated tubes testing methods proposed by Jensen and Hanse (1995). The deformation was monitored by a non-contact displacement measuring system and measurements were acquired at 1 min intervals. The ambient temperature for the measurement was 20°C. Expansion of restrained specimens Expansion deformation of restrained cement pastes including additives was measured on specimens (40 40 140 mm) according to Chinese standard GB 23439-2009, where the cement paste was encapsulated by polyethylene film and tinfoil, commencing at the final setting time. The dilatometric deformation was monitored by a laser displacement sensor and measurements were acquired at 1 min intervals. The ambient temperature for the measurement was 20°C. 1072

Accelerated storing stability test of expansive additives Accelerated storing stability of CEA and CEA/PLA were tested as follows: 2·5 g of sample was put into a 30 ml glass cup, then two samples were uncovered and exposed to 25°C and 60% humidity in a temperature and humidity chamber. Tests were conducted at 1, 3, 5, 7, 12, 22, 33, 44 and 70 d. Heat of hydration The heat of hydration was measured to investigate the effects of the PLA-encapsulating treatment of the expansive additives. Equal masses of water and powder were mixed, and the heat flow and the heat were monitored using isothermal heat conduction of a multichannel thermal activity monitor (Tam Air, TA Instruments, USA). Data were acquired at 1 min intervals and the ambient temperature for the measurement was 30°C. The first few minutes of heat flow were not taken into account because of a slight disturbance of the signal caused by the opening of the calorimeter. Measurement of hydration ratio of expansive additive A Bruker-AXS D8 Discover X-ray diffractometer equipped with a LynxEye array detector was used by adding alphaaluminium hydroxide (α-Al2O3) as an internal reference at a concentration of 10 wt% using a planetary ball mill, at a work condition of Cu target, 40 kV operating voltage, 30 mA operating current, 4·0° sola slit, 7–80° 2θ, 0·02° step size and 0·3 s/step. The expansive additive and cement were mixed, followed by moulding in polystyrene bottles at a water–powder ratio of 0·35 (by weight) at 20°C and curing under sealed conditions. Hydration of the samples was halted by dry ethanol flushing. Vacuum drying was applied for 10 h at 50°C in a vacuum oven. The powder was compressed onto a glass sample plate specially designed for the diffractometer; after the instrument parameters had been adjusted, the glass sample plate was inserted into the sample holder and the testing was carried out. For each sample, three trial records were prepared, each record was measured three times and then an average was taken. Measurement of loss on ignition (LOI) was conducted after burning the samples for 30 min at 1000°C. The amount of calcium hydroxide was determined for the prepared samples to quantify the reaction rate. Calcium hydroxide content was determined according to the weight loss during heating




from 350 to 450°C using the thermogravimetric–differential thermal analysis (TG–DTA) apparatus. The content of calcium hydroxide generated was also determined by Rietveld analysis of the X-ray diffraction analysis (XRD) patterns. Both of the contents obtained were corrected for the LOI (Li et al., 2014).

the carbon proportion and carbon–calcium ratio corresponds to a better encapsulation of PLA on the CEA particles’ surface. The carbon proportion and carbon–calcium ratio of the virgin surface of the CEA particle were 1·45% and 0·027, respectively, and in the case of 0·5% PLA encapsulation, these values increased to 6·36% and 0·195, respectively. When the polymer addition rate increased to 5%, the carbon proportion rose to 13·72% and the carbon–calcium ratio went up to 0·361. The experimental results indicated that the addition of a small amount of PLA polymer effectively encapsulated the CEA particles and formed a continuous protective cover on the surface of the virgin CEA particles. Because the material is difficult to mill when the ratio of PLA to CEA is above 2·0%, the content of PLA in CEA/PLA was set as 2% in the experimental tests described below.

Results and discussion Encapsulation and characterisation of expansive additives particles Surface modification of an inorganic powder by organic molecular substances based on the physical adsorption behaviour or hydrogen bonding has been one of the popular methods to widen the applications or improve the properties of the material. PLA was chosen to encapsulate CEA because PLA is a renewable resource and degradable under alkaline conditions, which tally with the internal environment of cement concrete. In addition, the carboxylate group (–COOH) at the end of the polymer chain offers the benefit of anchoring onto the CEA surface through hydrogen bonding. Figure 2 shows SEM images of CEA and CEA/PLA. The surface of the CEA particles (Figure 2(a)) was observed to be angular with a lot of grinding edges, whereas the surface of the encapsulated-CEA particles (Figure 2(b)) was much smoother. Table 2 shows the experimental results for the carbon proportions and carbon–calcium (C–Ca) ratio of the surface of different encapsulated-CEA particles analysed by EDS. Because the PLA film covering the surface of the CEA is readily recognisable by the carbon element, an increase of

Setting time and bulk density of the cement paste Setting times and bulk density of fresh pastes containing CEA or CEA/PLA are listed in Table 3. The initial time and the final time for paste incorporating CEA were 330 min and 360 min, respectively, and the corresponding results for paste with CEA/PLA were 345 min and 375 min. Bulk densities of pastes with CEA and CEA/PLA were 1927 g/l and 1931 g/l, respectively. The similarities of the results demonstrate that the modification of CEA by PLA did not influence the setting time and bulk density.

Expansion of unrestrained cement pastes Different test methods for the determination of expansion can be found in the literature (Aveline et al., 2011; Dulu and Peter,

4 µm

4 µm

(a)

(b)

Figure 2. Scanning electron microscopy images of expansive additives: (a) CEA; (b) CEA/PLA


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Ratio of PLA to CEA: %, by weight Carbon proportion: % Carbon–calcium ratio


0

0·5

2

5

1·45 0·027

6·36 0·195

8·84 0·224

13·72 0·361

Table 2. The carbon proportion and carbon–calcium (C/Ca) ratio of different additives particles (SEM/EDS)

Samples

Setting time: min

CEA CEA/PLA

Bulk density: g/l

Initial

Final

330 345

360 375

1927 1931

Table 3. Properties of pastes including CEA or CEA/PLA

2002; Jensen and Hanse, 1995; Jose and Odler, 2002). The method invented by Jensen and Hanse (1995) can measure the whole process of deformation and avoids the defect of the choice of the time-zero. The expansion curves for unrestrained cement pastes containing CEA or CEA/PLA are shown in Figure 3; they start from the very beginning of expansion. The expansion of both the CEA/PLA pastes and the CEA pastes increases rapidly within 24 h. The CEA/PLA paste expanded to 5050ε significantly faster than the CEA paste, which only reached 3700ε during this time. From 24 h to 140 h, both curves show slow growth and neither of the samples expanded further until 11 d after casting (the detailed data are not shown). The expansion strain of unrestrained cement paste incorporating modified CEA was 35% greater than for the

CEA paste, which indicates that treatment by encapsulation using PLA can improve the expansion efficiency of CEAs remarkably.

Expansion of restrained cement pastes To acquire the whole process of the expansion of the restrained cement pastes containing additives, a laser displacement sensor was utilised to monitor the matrices from final setting time. The expansion properties of restrained cement pastes as a function of age are shown in Figure 4. Three distinct stages in the strain response can be observed in both of the two curves. In the first stage, within 24 h, the expansive strain increases rapidly due to hydration of the expansive additives and reaches a value of 400ε and 300ε, respectively. It should be noted that the slope of expansive strain for CEA/PLA paste is significantly higher than that of the CEA pastes, which is exactly consistent with the result for unrestrained cement pastes. During the second stage, from approximately 24 h to 130 h, the expansive strains increase slowly to the maximum values of 436ε and 351ε. In the final stage, beyond 130 h, the expansive strain decreases gradually at a much slower rate, which is different to the unrestrained results. The expansion strain of restrained cement paste incorporating modified CEA was 25% more than the CEA paste at the age of 235 h. This result demonstrates that the encapsulation by PLA favours an improvement in the expansion efficiency of CEAs under the restrained condition as well.

Accelerated storing stability test of expansive additives It is necessary to minimise the weathering of expansive additives, as they lose their properties when stored in a moist or humid environment. Figure 5 shows the results of the accelerated storing stability test of two samples at 25°C and 60% humidity. In the accelerated storage period of the expansive

6000 400 Expansive strain: × 10–6

Expansive strain: × 10–6

5000 4000 3000

CEA CEA/PLA

2000 1000

CEA CEA/PLA

200

100

0

0 0

20

40

60

80 100 Time: h

120

140

160

180

Figure 3. Expansion of unrestrained cement pastes containing expansive additives

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300

0

20 40 60 80 100 120 140 160 180 200 220 240 Time: h

Figure 4. Expansion of restrained cement pastes containing expansive additives




0·6

120 100 Heat flow: mW/g

Water absorption: g

0·5 0·4 0·3 0·2

CEA CEA/PLA

60 40

0·1

20

0

0 0

10

20

30

40

50

60

CEA CEA/PLA

80

0

70

2

4

6

Time: d

Figure 5. Accelerated storing stability test of expansive additives

10

12

14

10

12

14

700 Cumulative heat of hydration: J/g

additives during 70 d of exposure, the moisture uptake of the CEA increased rapidly to 0·4 g at 12 d, while the CEA/PLA was 0·3 g during the same time, which was reduced by 30%. Water absorption of both samples was increased further slowly after 22 d. The results demonstrate that the PLA film on the surface of the CEA can reduce the weathering rate under conditions of high humidity, which implies that the encapsulation can delay the hydration of the original CEA.

8 Time: h (a)

600 500 CEA CEA/PLA

400 300 200 100 0

Heat of hydration Calorimetry is a useful method to obtain information on the kinetics of hydration (Jansen et al., 2012; Kong et al., 2015; Shi et al., 2002). Figure 6 shows the heat evolution rate and cumulative heat of both samples during hydration with the equal amount of water added as additives. For CEA, heat evolution was generated immediately and it took only 0·5 h to reach the exothermic peak of 120 mW/g; after that the speed of heat evolution slowed down drastically. In contrast, the curve of the heat evolution rate of CEA/PLA appeared an induction after water was added and the delayed exothermic peak of 100 mW/g occurred at 1·18 h. No significant differences in the total heat flow accumulated were observed between the two samples, as shown in Figure 6(b). It can be seen that the hydration of CEA was rapid, and much expansion may be wasted in the plastic stage. On the other hand, the encapsulation of CEA by PLA generates a protective induction and retards its hydration, which reduces the loss of hydration at the plastic stage and subsequently increases the amount of CEA available to release more expansion during the hardening period. The curves of heat flow and the heat accumulated for the cement pastes containing CEA or CEA/PLA are shown in Figure 7. To recognise the different peaks, the hydration of the individual cement was achieved under the same conditions. As regards the rapid reaction between CEA and water shown in Figure 6, for the paste consisting of CEA and cement, the first

0

2

4

6

8 Time: h (b)

Figure 6. Heat of hydration where equal amounts of additives and water are used at 30°C

peak of 60 mW/g occurred 0·5 h after the water was added, and the second peak accounting for the hydration of the cement occurred with a value of 11 mW/g at 10 h. For the cement paste containing CEA/PLA, the first peak, with a value of 30 mW/g, was much lower and smaller than for of CEA paste. The latter peak of 32 mW/g in the curve of CEA/PLA:cement in Figure 7(a) was much higher and larger than the result of CEA:cement, which indicates the major attribute of the combination of expansive additive and cement. This indicates that the treatment investigated can also suppress and delay the hydration of CEA in cement paste without changing the cumulative heat of hydration, as shown in Figure 7(b).

Measurement of hydration ratio of expansive additive Figure 8 shows the XRD patterns of portlandite contents by hydration in cement pastes containing additives. As shown in Table 4, the portlandite contents of the cement pastes at 6 h


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1800 CEA:cement = 3:1 CEA/PLA:cement = 3:1 Cement

Heat flow: mW/g

40

1500 Intensity: ×104 counts/s

50

CEA 30 Cement 20 10

1200 CEA 900 Calcium hydroxide

600

Calcium hydroxide

300 CEA/PLA 0

0 0

5

10 Time: h (a)

15

15

20

20

25

40 30 35 2θ: degrees (a)

45

50

55

1500 400

Intensity: ×104 counts/s

Cumulative heat of hydration: J/g

1800 500

CEA:cement = 3:1 CEA /PLA:cement = 3:1 Cement

300 200 100

1200

CEA

900 600

Calcium hydroxide

Calcium hydroxide

300

0

CEA /PLA 0 0

5

10 Time: h (b)

15

Figure 7. Heat of hydration where equal amounts of powders (additive:cement = 3:1, w/w) and water are used at 30°C

were 6·21% and 5·17% for the CEA and 5·30% and 3·38% for the CEA/PLA according to TG–DTA and XRD, respectively. The table shows that the amount of portlandite generated in CEA/PLA was smaller than in CEA for at least the first 6 h after water was added. The amounts of portlandite in the CEA/PLA paste and CEA paste narrowed as time elapsed to 12 h. The setting time of a paste with the same mix proportions was approximately 6 h; this implies that the hydration of the CEA/PLA was delayed in contrast with CEA in the plastic stage, which corresponds to the results for heat of hydration in Figure 7. In general, any hydration of the expansive additive before the formation of the cement matrix is considered to be a loss. This means that the PLA encapsulation reduces the loss of hydration of CEA in the plastic stage, and the compensatory amount of expansion additives results in an increase in expansion during the expansion period. This was also the reason why the expansions of restrained and unrestrained cement pastes containing CEA/PLA were more than those of CEA pastes. 1076

15

20

20

25

30 35 40 2θ: degrees (b)

45

50

55

Figure 8. Diffraction patterns of the expansion pastes containing CEA and CEA/PLA at (a) 6 h and (b) 12 h

Time: h

6 12

TG–DSC

XRD

CEA

CEA/PLA

CEA

CEA/PLA

6·21 7·24

5·30 7·07

5·17 5·66

3·38 5·17

Table 4. The amount of calcium hydroxide in the pastes containing CEA and CEA/PLA from XRD and TG–differential scanning calorimetry (DSC) methods (%)

Compressive strength of the cement paste Compressive strength development is shown in Figure 9. Both the treated CEA pastes and the CEA pastes showed similar strength increases with age, indicating that the modification of CEA has no effect on the strength of cement paste, although the treatment could reduce the weathering rate, improve the expansion efficiency and suppress the hydration of the virgin CEA.




Collepardi M, Borsoi A, Collepardi S, Olagot J and Roberto T

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(2005) Effects of shrinkage reducing admixture in shrinkage compensating concrete under non-wet curing conditions. Cement and Concrete Composites 27(6): 704–708.

Compressive strength: MPa

60 50 40

Collepardi M, Troli R, Bressan M, Liberatore F and Sforza G

CEA CEA /PLA

30 20 10 0 0

1

2

3 4 Time: d

5

6

7

Figure 9. Compressive strength of pastes containing expansive additives

Conclusions (a) A CEA was encapsulated by PLA polymer to form a protective membrane, and its expansion efficiency was examined in restrained and unrestrained cement pastes. (b) The protective membrane did not affect the setting time of the pastes. (c) Restrained and unrestrained deformation test results demonstrate that the expansion of the cement paste incorporating the modified CEA increases by 25% and 35% over that of the paste including the original CEA. (d) The delayed hydration of the modified CEA was monitored by an isothermal calorimeter. The hydration product portlandite generated from CEA/PLA in cement pastes was less than that of CEA at 6 h after casting and the amounts of portlandite narrowed as time elapsed at 12 h according to XRD analysis. (e) The storage stability of CEA/PLA under 25°C and 60% humidity was improved by 30% and the PLA modification of CEA has no effect on the strength of cement paste.

Acknowledgements This work was financially supported by the National Nature Science Foundation of China (51408275) and National Outstanding Youth Science Foundation Program (51225801).

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