Cite this article

Research Article

Inazumi S, Kuwahara S, Jotisankasa A and Chaiprakaikeow S
Improvement mechanism of sodium carbonate on traditional composite filler.
Proceedings of the Institution of Civil Engineers – Ground Improvement,
https://doi.org/10.1680/jgrim.19.00014

Paper 1900014
Received 18/02/2019;
Accepted 22/07/2019

Keywords: dredging & reclamation/foundations/grouting

ICE Publishing: All rights reserved

Ground Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Shinya Inazumi

Apiniti Jotisankasa

Professor, Department of Civil Engineering, Shibaura Institute ofTechnology, Tokyo, Japan (corresponding author: inazumi@shibaura-it.ac.jp)(Orcid:0000-0002-5639-5254)

Associate Professor, Department of Civil Engineering, Kasetsart University,
Bangkok, Thailand

Shuichi Kuwahara

Lecturer, Department of Civil Engineering, Kasetsart University,
Bangkok, Thailand

Managing Director, Japan Association for Pulling-out Existing Piles,
Tokyo, Japan

Susit Chaiprakaikeow

Certain issues may be encountered when traditional composite fillers are mixed with cement and bentonite (CF-CB)and applied to cavities in the ground. These issues include the bleeding and separating of water, cement andbentonite, and the long time needed for the strength development. In a series of studies, it has been shown that the
suppression of bleeding, earlier gelling and earlier development of strength can be expected by mixing a suitableamount of sodium carbonate in the CF-CB, that is, creating a CF-CBS. The mechanisms of the various properties of CFCBS and CF-CB were investigated by physical analyses. The following points were clarified from the results of thiswork. Earlier development of strength and gelling are brought about by the earlier formation of ettringite
accompanying the promotion of the cement hydration reaction. The suppression of bleeding can be accomplished byearlier gelling due to the bentonite taking the Na+ ionised from sodium carbonate as an interlayer cation. The
increase in strength is attributed to the calcium carbonate produced by CO32− ionising from the sodium carbonate
bonding with the Ca2+ dissolved from the interlayer cations of the cement or bentonite.

1.

Introduction

As the number of obsolete infrastructures being rebuilt has been
increasing in recent years, it has become necessary to improve
the quality of the fillers used as backfill in cavities in the groundformed during the removal of existing underground structures,such as piles. The role of the filler is to increase the recoverabilityof the target ground (Bauer et al., 2016; Ghadimi et al., 2017;
Han, 2015). If proper treatment by filling is not carried out
and/or the physical properties of the ground and the filling site
differ greatly, then phenomena such as settlement may occur inthe surrounding ground (Granata et al., 2015; Inazumi et al.,2017; Kuwahara and Inazumi, 2019).

suppressing bleeding and bringing about earlier gelling, earlier
strength development and an increase in strength could be
realised by mixing sodium carbonate (Na2CO3) with CF-CB.
An attempt was thus made to clarify the mechanism by which
the above properties are exhibited by a composite filler comprising cement, bentonite and sodium carbonate (CF-CBS).
For this purpose, typical physical analyses (X-ray diffraction(XRD) analysis, scanning electron microscopy (SEM) observations and energy-dispersive X-ray (EDS) analysis) werecarried out.

2.
A composite filler comprising cement and bentonite (CF-CB)
has fluidity and self-hardness and does not require compaction; therefore, it is a typical filler widely used for filling in
narrow spaces and spots that would be difficult to compact(Aqil et al., 2005; Mikkelsen, 2005). The mechanical qualities
of CF-CB are easy to produce and CF-CB can be mixed
according to the conditions of the construction by performing
mixing studies in most laboratories. However, some difficulties
can be encountered, such as taking several hours for the CFCB to develop in strength and not being able to control the
CF-CB only by adjusting the blending (mixing). Due to theseissues, an uneven development of strength is generated due tomaterial segregation (bleeding).
The aim of this study was to overcome the above-mentioned
issues and to develop a filler capable of ensuring a predetermined strength quickly and reliably. Hashimoto et al. (2017a,
2017b) and Inazumi et al. (2019) revealed that the effect ofEarlier studies on CF-CBS

The components of CF-CBS are water, bentonite, cement andsodium carbonate. By mixing 50−90% water, 1−10% bentonite, 10−40% cement and 0·5−3% sodium carbonate, basedon the total weight of CF-CBS, the CF-CBS will be effective
in suppressing bleeding and bringing about earlier gelling,earlier strength development and an increase in strength.
Furthermore, bentonite, cement and sodium carbonate are
introduced in this order into a mixer to which water is added
in advance, and sufficient stirring is carried out every timeeach material is added. In this way, bleeding is mostly suppressed and gelling occurs early (Hashimoto et al., 2017a,
2017b; Inazumi et al., 2019).
The strength properties of CF-CBS have been evaluated by
the uniaxial compressive strength according to the Japaneseindustrial standard (JIS A 1216) ‘Uniaxial compression test
method of soil’ (Hashimoto et al., 2017a, 2017b; Inazumi
et al., 2019). The initial strength of CF-CB is increased by1Ground Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Inazumi, Kuwahara, Jotisankasa andChaiprakaikeow

Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution

Table 1. Components of ordinary Portland cement used
Silicondioxide: %21·4

Aluminiumoxide: %Ferricoxide: %Calciumoxide: %Magnesiumoxide: %Sulfurtrioxide: %

4·6

2·5

63·0

0·8

1·7

mixing sodium carbonate. This indicates that the strength
development can be accelerated. With regard to the hardening
action at a material age of 1 d, CF-CB requires about 6 h togel from the time the sample is mixed and stirred, but CF-CBS
gels in about 1 h. Therefore, sodium carbonate promotes
earlier gelling of CF-CB. Immediate gelling and earlier
strength development have useful effects such as stabilising thetarget ground quickly and shortening the construction period.
Moreover, even for the long-term strength (over 28 d), theincrease in strength by mixing sodium carbonate has been confirmed. Thus, mixing sodium carbonate not only results inearly gelling, but also has the effect of increasing the strengthof CF-CB.

It is considered that the mechanism for bleeding suppression iscaused by an increase in the swelling property of bentonite.
The nature of bentonite differs depending on the type of interlayer cation, and the sodium (Na) type with many sodium ions(Na+) is more swellable than the calcium (Ca) type with alarge amount of calcium ions (Ca2+) (Onikita, 2007). Whensodium carbonate and bentonite are mixed in water, thesodium carbonate dissolves in the water and ionises to generate
Na+, which are ion exchanged with Ca2+ contained in theinterlayer cations of the bentonite. It is thought that thisincreases the swelling property by strengthening the propertiesof the sodium type and contributes to the suppression of
bleeding.

The problem of component material segregation has been evaluated based on the bleeding ratio according to the Japan
Society of Civil Engineers standard (JSCE F 522) ‘Bleeding
ratio and expansion ratio test method for injection mortarof prepacked concrete (polyethylene bag method)’ (Hashimoto
et al., 2017a, 2017b; Inazumi et al., 2019). CF-CBS
almost never causes bleeding and the component material segregation of CF-CB is suppressed by adding sodium carbonate.
In addition, bleeding is suppressed even with a small amountof bentonite by using bentonite with a high swelling ability.

It is thought that the increase in strength is due to the formation of calcium carbonate. Calcium carbonate is also produced by the interlayer cations Ca2+ and CO32− being bonded
in the reaction between CO32− and calcium hydroxide, as
described above, and the cation-exchange reaction of bentonite.
Calcium carbonate is a water-insoluble substance and its molecules are bonded to each other to form a hardened body.
Therefore, it is thought that the generation of calcium carbonate may bring about an increase in the initial and long-term
strength.

The flowability has been evaluated by the flowing time of the
P-funnel according to the Japan Society of Civil Engineers
standard (JSCE F 521) (JSCE F 521) ‘Flowability test method
for injection mortar of prepacked concrete (method by Pfunnel)’ (Hashimoto et al., 2017a, 2017b; Inazumi et al.,2019). There is no change in the flowability of CF-CB whenmixing it with sodium carbonate; its high fluidity is maintained. Having high flowability without component material
segregation shows that filling can be reliably performed evenfor irregularly shaped cavities and that it is possible to expressa predetermined strength homogeneously at the filling point.
Earlier gelling and earlier strength development of CF-CBS is
thought to be due to the promotion of the hydration of thecement reaction with sodium carbonate and the formation of
calcium carbonate (CaCO3). Calcium hydroxide (Ca(OH)2)
produced immediately after the hydration of cement is carbonated by carbonate ions (CO32−) generated by the ionisation of
sodium carbonate to produce calcium carbonate. The production of calcium carbonate promotes the hydration reaction
by accelerating the consumption of calcium hydroxide, and it
is expected to promote the gelling of cement and to enhance
the initial strength.
23.

Experimental programme and devices

The components of the CF-CBS and CF-CB produced for thisstudy were water, bentonite, ordinary Portland cement andsodium carbonate. Tables 1 and 2 respectively show the components of the cement and the bentonite used. For the bentonite, slightly swollen bentonite with a swelling degree of 4 wasused, as recommended by the American Colloid Company(ACC) method (Kawatra and Ripke, 2001). For a comparisonof different types of bentonite, highly swollen bentonite with a
Table 2. Components of slightly swollen and highly swollen
bentonites

Silicon dioxide: %Aluminium oxide: %Ferric oxide: %Magnesium oxide: %Calcium oxide: %Sodium oxide: %Potassium oxide: %Loss on ignition: %Slightly swollen
bentonite

Highly swollen
bentonite

67·1
16·8
3·7
3·3
2·7
2·2
1·3
2·9

68·4
18·9
4·6
2·2
1·7
2·1
0·6
1·7

Ground Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Inazumi, Kuwahara, Jotisankasa andChaiprakaikeow

Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution

Table 3. Mix proportions
Specimen
12
3
4

Slightly swollen
bentonite: kg/m3

Highly swollen
bentonite

Cement:kg/m3

Sodiumcarbonate: kg/m3

Water:
kg/m3

w/c

50
0
50
0

0
20
0
20

240
240
240
240

20
20
0
0

897
908
905
916

374
378
377
382

swelling degree of 20 was also used. Ordinary CF-CB is used
in a weight ratio of cement to bentonite of approximately 4/6
and a weight ratio of water to cement (w/c) of approximately
300/500. In addition, from the results of preliminary tests, it
was confirmed that adequate fluidity, strength and material
separation resistance are exhibited by using 50−90% water,
1−10% bentonite, 10−40% cement and 0·5−3% sodium
carbonate based on the total weight of CF-CB or CF-CBS(Hashimoto et al., 2017a, 2017b; Inazumi et al., 2019). Table 3
shows the mixing conditions of the specimens used in thephysical analyses. The specimens used in the physical analyses
were mixed so as to have the same fabrication conditions andshow the same useful properties as the fillers described in
Section 2 and used by Hashimoto et al. (2017a, 2017b) andInazumi et al. (2019). In Table 3, specimen 1 is the CF-CBS
specimen using slightly swollen bentonite, specimen 2 is the
CF-CBS specimen using highly swollen bentonite, specimen 3is the CF-CB specimen using slightly swollen bentonite andspecimen 4 is the CF-CB specimen using highly swollen
bentonite.
3.1
SEM
SEM was performed to observe the reactions and products
occurring on the surfaces of the specimens. In the SEM observations, specimens 1, 2 and 3 were used to compare the difference depending on the presence or absence of sodium
carbonate and the type of bentonite. The SEM equipmentused was a JSM-6010LA instrument (Jeol Company Ltd.),
and uncoated specimens were observed at an acceleration
voltage of 15 kV.
3.2
XRD analysis
XRD analysis was performed to investigate the reaction of
CF-CBS and CF-CB as well as the change in the hydration
products generated in the specimen by adding sodium carbonate. In this analysis, a powder XRD method was carried out
using Smart Lab manufactured by Rigaku Corporation. The
X-rays used were CuKα (λ = 1·5418 Å).
3.3
EDS analysis
In the EDS analysis, specimens 2 and 4 of CF-CBS were used
for comparing and examining the element distribution of thegenerated substances and the content ratio of the constituent
elements. Also, because the type of bentonite depends on theinterlayer cations and because there is no difference in theconstituent elements of the minerals, a comparison was madewith fillers using highly swollen bentonite with outstanding
physical properties. In this analysis, a JSM-7600F device (JeolCompany Ltd.) was used and uncoated specimens wereobserved at an acceleration voltage of 15 kV.

4.

Results and discussion

4.1
SEM observation and XRD analysis
Figure 1 shows SEM images and Figure 2 shows the results of
the XRD analysis.
In Figure 1, unreacted cement particles can only be confirmed
at the age of 3 h for specimen 3, while hydration products
can be confirmed around the cement particles for specimens 1
and 2. In particular, acicular crystals with a size of
approximately 2 μm often observed, and ettringite (3CaO.
Al2O3.3CaO4.32H2O) was considered in view of the shape ofthe crystals. Ettringite is a substance produced by thehydration reaction of the aluminate phase from the beginningof the test, and the hydration reaction of cement is accelerated
by mixing sodium carbonate (JCA, 2012). Although the same
reactions can be confirmed even at 1 d of age, in the case of
specimens 1 and 2, the periphery of the cement particles wascompletely covered with a hydrate of low crystallinity, which isconsidered to be calcium silicate hydrate (CSH). It is supposedthat the hydration reaction of alite (3CaO.SiO2) is promoted.
Compared with the results of the XRD analysis shown in
Figure 2, the peaks of erythrite are small in specimens 1 and 2
at 1 d of age, whereas sharp peaks of CSH appeared. This isconsistent with the results of the SEM observations (seeFigure 1). In specimens 1 and 2, calcium aluminate hydrate(4CaO.Al2O3.xH20), similar to the composition of ettringite
caused by the hydration reaction of the aluminate phase, canbe confirmed from 1 d of age. This agrees with the viewpointof promoting the initial hydration reaction due to thehydration of the aluminate phase.
At 7 d of age, the hydration reaction was found in specimens
1, 2 and 3 and it was possible to observe the formation of amore dense and hardened body and the presence of some crystals (see Figure 1). In the XRD analysis, the peak of calcium
hydroxide appeared strongly, and it is considered that calcium
hydroxide was formed with a high degree of crystallinity (seeFigure 2).
3

Ground Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Inazumi, Kuwahara, Jotisankasa andChaiprakaikeow

Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution

Ettringite
Unhydrated
cementEttringite
Unhydrated
cementUnhydrated
cement5·0 µm

5·0 µm

5·0 µm

(a)

(b)

(c)

CSH

Ettringite
CSH
Unhydrated
cementEttringite

Ettringite

5·0 µm

5·0 µmUnhydrated
cement(d)

5·0 µm(e)(f)
Unhydrated
cement
CSH

Calcium hydroxide (CH)

Ettringite
CH

CSH

CH

Ettringite

CSH

Ettringite
5·0 µm

5·0 µm

5·0 µm(g)

Ettringite

(h)

(i)

Ettringite

CH

CSH

CSH

CH
CH
CSH

Ettringite

5·0 µm

5·0 µm(j)

5·0 µm(k)(l)

Figure 1. SEM images of CF-CBS. (a) Specimen 1 (sodium carbonate, low swelling bentonite), 3 h. (b) Specimen 2 (sodium carbonate,high swelling bentonite), 3 h. (c) Specimen 3 (no sodium carbonate, low swelling bentonite), 3 h. (d) Specimen 1 (sodium carbonate,
low swelling bentonite), 1 d. (e) Specimen 2 (sodium carbonate, high swelling bentonite), 1 d. (f) Specimen 3 (no sodium carbonate,
low swelling bentonite), 1 d. (g) Specimen 1 (sodium carbonate, low swelling bentonite), 7 d. (h) Specimen 2 (sodium carbonate, highswelling bentonite), 7 d. (i) Specimen 3 (no sodium carbonate, low swelling bentonite), 7 d. ( j) Specimen 1 (sodium carbonate,
low swelling bentonite), 28 d. (k) Specimen 2 (sodium carbonate, high swelling bentonite), 28 d. (l) Specimen 3 (no sodium carbonate,
low swelling bentonite), 28 d

A large amount of ettringite and CSHs are produced in specimens 1 and 2 at the age of 28 d, and several crystals were
packed and bonded (see Figure 1). In specimen 3, it can be
confirmed that a hydrate was formed around a large crystal,4

but when compared with specimens 1 and 2, the amount of
ettringite and CSH were both small (see Figure 2). That is,when sodium carbonate is added, a hydrate is densely formed.
In the XRD analysis (see Figure 2), clear and sharp peaks canGround Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Inazumi, Kuwahara, Jotisankasa andChaiprakaikeow

Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distributionEA

10 000E

AE

15 000E

10 000

E

20

30
40
2θ : deg

50

60

0

70

C A

E

5000

10

E
A

0

10

20

30
40
2θ : deg(a)

10 000
5000
0

0

10

20

30

40
2θ : deg

50

60

25 000
20 000Intensity: counts

intensity: counts

15 000

15 000E

10 000

C4AHx

E

5000
0

70

15 000C4AHx
E

10 000

E

5000
0

E

0

10

20

30

Intensity: counts

25 000
C4AHx

40

CH E

10

20

30
40
2θ : deg

E

50

0

10

60

0

10

A

20

30

CH
A
C
A

CH
A

15 000
10 000

C4AHx
E

5000

E

0

E

CH
A

30
40
2θ : deg

50

60

70

65 00060 00055 00050 00045 00040 00035 00030 00025 000
20 000
15 000
10 000
5000
0

10

E

60

CSH
C CSH
AA

20 000

20

C

C4AHx
E
0

10

E: Ettringite
C: Calcium
carbonate
CH: Calcium
hydroxide
A: Alite
C4AHx: Calcium
aluminatehydrate

20

(j)

CH

CH CH
C

30
40
2θ : deg

50

(k)

60

70

E: Ettringite
C: Calcium carbonate
A: Alite
CSH
E
A

10 000

CH
A

E
5000

0

10

20

CSH

C

E

E

30

40

50

60

70

60

70

50 00045 00040 00035 00030 00025 000
20 000
15 000
10 0005000
0E: Ettringite
C: Calcium carbonate
CH: Calcium hydroxide
A: Alite
C4AHx: Calcium aluminate
hydrate

CH

C4AHx
E

E

C CH
A A
A

E

E
A

CH
C

CH
A
A

0

10

20

30

40
2θ : deg

50

60

70

(i)

E

E

50

CH
20 000
15 000
C4AHx

10 000

E

70

0

E

A

E

5000

60

E: Ettringite
CH C: Calcium carbonate
A CH: Calcium hydroxide
A: Alite
C4AHx: Calcium aluminate
hydrate
C
A
CH
C CH

25 000

E: Ettringite
C: Calcium carbonate
CH: Calcium hydroxide
C4AHx: Calcium aluminate
hydrateCH

50

15 000

70

A

(h)

CH
C

0

CH
C

A
E

40
2θ : deg2θ : deg

CH

70

A
20

CH
A

(f)

(g)

E

5000
0

CH

50

25 000
20 000

CH
C

E
E

5000

0
0

30
40
2θ : deg

15 000
10 000

E CF

0

70

E
A

C

CH

10 000(c)2θ : deg

CH E: Ettringite
A C: Calcium carbonate
CH: Calcium hydroxide
A: AliteCA
C4AHx: Calcium aluminate
hydrate30 000

20 000

E: Ettringite
C: Calcium carbonate
CH: Calcium hydroxide
C
A: AliteA
C4AHx: Calcium aluminateCH
hydrate
A
CH
C
A
E
A
CH
A
A

Intensity: counts

Intensity: counts

20 000

15 000(e)Intensity: counts

CH60

E: Ettringite
C: Calcium carbonate
CSH
CH: Calcium hydroxide
C
A: AliteA
C4AHx: Calcium aluminate
CSH
hydrate
CH
A
CSH
A
CH
E
A
A CH
C
A
CSH

(d)
25 00050

A

E: Ettringite
C: Calcium carbonate
CH: Calcium hydroxide
A: Alite
CF: FerriteC
A A CHA(b)

CSH CSH E: Ettringite
C
CH C: Calcium carbonate
CH: Calcium hydroxide
A: Alite
C4AHx: Calcium aluminateA
C4AHx
hydrate
CH
CSH
E
E
E
CH
A
C
E
CSH A

20 000C

Intensity: counts

0AA

Intensity: counts

5 000
0

C

20 000

20 000Intensity: counts

E

A

C
A

25 000Intensity: counts

A
15 000

E: Ettringite
C: Calcium carbonate
A: Alite

30 000Intensity: counts

A

20 000Intensity: counts

E: Ettringite
C: Calcium carbonate
A: AliteCA

25 000

0

10

20

30

40
2θ : deg

50

60

70

(l)

Figure 2. XRD patterns of CF-CBS. (a) Specimen 1 (sodium carbonate, low swelling bentonite), 3 h. (b) Specimen 2 (sodium carbonate,high swelling bentonite), 3 h. (c) Specimen 3 (no sodium carbonate, low swelling bentonite), 3 h. (d) Specimen 1 (sodium carbonate,
low swelling bentonite), 1 d. (e) Specimen 2 (sodium carbonate, high swelling bentonite), 1 d. (f) Specimen 3 (no sodium carbonate,
low swelling bentonite), 1 d. (g) Specimen 1 (sodium carbonate, low swelling bentonite), 7 d. (h) Specimen 2 (sodium carbonate,high swelling bentonite), 7 d. (i) Specimen 3 (no sodium carbonate, low swelling bentonite), 7 d. ( j) Specimen 1 (sodium carbonate,
low swelling bentonite), 28 d. (k) Specimen 2 (sodium carbonate, high swelling bentonite), 28 d. (l) Specimen 3 (no sodium carbonate,
low swelling bentonite), 28 d

be confirmed in specimens 1 and 2, indicating that substanceswith high crystallinity were generated. This point seems to contribute to the increase in strength of the CF-CB by adding
sodium carbonate which has been clarified in earlier studies(Hashimoto et al., 2017a, 2017b; Inazumi et al., 2019).

4.2
EDS analysis
Figure 3 shows the element mapping obtained by the EDS
analysis, while Figure 4 shows the results of the quantitative
analysis in the EDS analysis.

A comparison based on the presence or absence of sodium
carbonate at the age of 3 h shows that the ratio ofaluminium (Al) and silicon (Si) to the content of all the constituent elements was relatively high and the proportion of
calcium (Ca) was relatively low in specimen 2 mixed withsodium carbonate. In, as cement hydrates, aluminium is contained in the ettringite (3CaO.Al2O3.3CaO4.32H2O) producedby the hydration reaction of the aluminate phase. It can be
judged that aluminium becomes an index of ettringite, eventhough a large amount of aluminium is present on the crystal
surface in the element mapping. Silicon is contained in CSH,
5

Ground Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Inazumi, Kuwahara, Jotisankasa andChaiprakaikeow

Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution

Ettringite Unhydrated
cementEttringiteCHCSH
5·0 µm

5·0 µm

(1)

(2)

(1)

(3)

(4)

(3)

(5)

(2)

(4)

(5)
(a)

(b)

Unhydrated
CementEttringite
Unhydrated
CementCSH
5·0 µm

5·0 µm

CSH

(1)

(2)

(1)

(2)

(3)

(4)

(3)

(4)

(c)

(d)

Figure 3. Digital mapping of each constituent element by EDS analysis. (a) Specimen 2 (sodium carbonate, high swelling bentonite), 3 h.
(b) Specimen 2 (sodium carbonate, high swelling bentonite), 7 d: (1) SEM, (2) aluminium, (3) calcium, (4) silicon and (5) sodium.
(c) Specimen 4 (no sodium carbonate, high swelling bentonite), 3 h: (1) SEM, (2) aluminium, (3) calcium and (4) silicon. (d) Specimen 4(no sodium carbonate, high swelling bentonite), 7 d: (1) SEM, (2) aluminium, (3) calcium and (4) silicon

which is a hydrate of alite (3CaO.SiO2); it was widely distributed among the crystals and therefore silicon can be judged to be
an indicator of CSH. From this point, it is believed that the6amount of ettringite and CSH in CF-CB increased by adding
sodium carbonate, which is consistent with the results obtained
by SEM observations and XRD analysis. Moreover, because

Ground Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Inazumi, Kuwahara, Jotisankasa andChaiprakaikeow

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Na

Mg

Al

Si

K

Ca

Fe

Increase
Swellinswellingg

S
2Na +Ion exchange(a)Bentonite
Water

(b)

CementNa2CO3

Na +

CO32–

(c)
Promotion ofofhydration
hydration(d)Hydrate

0

20

40

60

80

100

wt%

Figure 4. Results of quantitative analysis by EDS analysis.
(a) Sodium carbonate; 3 h, (b) sodium carbonate; 7 d,(c) no sodium carbonate; 3 h, (d) no sodium carbonate; 7 d. Na,
sodium; Mg, magnesium; Al, aluminium; Si, silicon; K, potassium;
Ca, calcium; Fe, iron; S, sulfur

calcium is contained in almost all components of cement,
calcium is not suitable for specifying ettringite or CSH. With
regard to the cause of the difference in the proportion of
calcium, calcium was not observed, as it was ionised in the
middle of the hydration reaction in specimen 2, while specimen
4 contained a large amount of unhydrated cement and a great
deal of calcium was observed. From the above it is clear thatthe hydration reaction of cement is seen to progress by adding
sodium carbonate, and the formation of ettringite and CSH is
promoted.
A comparison based on the presence or absence of sodium carbonate at the age of 7 h revealed that the ratios of aluminiumand calcium were high and the proportion of silicon was low
in specimen 2 mixed with sodium carbonate. Comparing theresults obtained at 3 h and 7 d of age, the proportion of
calcium increased in specimen 2, but decreased in specimen 4.
It is considered that the calcium decreased because the selectivity of Ca2+ is as high as the interlayer cation of bentonite(Christidis and Koutsopoulou, 2013), and Ca2+ may not be
observed due to the incorporation as ions in the bentonite
during the process of the cation-exchange reaction. The
increase in calcium during the addition of sodium carbonate isthought to be caused by the formation of calcium carbonate
by Ca2+ contained as an interlayer cation of the bentonite andCO32− from sodium carbonate in the process of the cationexchange reaction. Also, sodium was observed only in specimen 2 mixed with sodium carbonate, and a comparison at 3 h
and 7 d of age showed a decrease as time elapsed. It is thoughtthat the sodium carbonate or sodium, which was crystallised,
was taken in the bentonite as Na+ by the cation-exchange

Ca2+

Ca2+
CaCO3
Increase
Increased
Strength
strength

Figure 5. Mechanisms of various properties of CF-CBS based on
SEM observations, XRD and EDS analyses. Na2CO3, sodium
carbonate; CaCO3, calcium carbonate

reaction. It is considered that the cation-exchange reaction of
Na+ from sodium carbonate and Ca2+ from bentonite occurs
due to the above increase and decrease in calcium and sodium.
Calcium carbonate was also present in the results of the XRD
analysis, so calcium carbonate was considered to have been
generated.
Figure 5 summarises the mechanisms by which various properties of CF-CBS develop based on the results of the SEM
observations, XRD analysis and EDS analysis. Na+ is generated by ionising sodium carbonate in water and Ca2+ from theinterlayer cations of bentonite that undergo a cation-exchange
reaction, so that Na+ contributes to increasing the swellability
as an interlayer cation of bentonite. Increasing the swellability
improves the effect of suppressing bleeding. Ca2+ binds with
the CO32− generated by the ionisation of sodium carbonate
to form calcium carbonate, which contributes to the increasein strength. Regarding the production of calcium carbonate,sodium carbonate is also produced by carbonating calcium
hydroxide produced by the hydration of Ca2+ dissolved in
cement at the beginning of the test. This leads to the expeditious consumption of calcium hydroxide and promotes thehydration reaction. As ettringite forms quickly, it causes earlier
gelling and earlier strength development. In addition, rapid
gelling suppresses sedimentation of the cement particles, which
also leads to the suppression of bleeding.

5.

Conclusions

The mechanisms by which various properties of CF-CBS
develop was investigated using SEM, XRD and EDS analyses.
The findings are as follows.
(a) Calcium hydroxide is carbonated by adding sodium
carbonate to produce calcium carbonate. This contributes
to the increase in strength of CF-CBS.
7

Ground Improvement

Improvement mechanism of sodium
carbonate on traditional composite filler
Inazumi, Kuwahara, Jotisankasa andChaiprakaikeow

Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution(b) As the consumption of calcium hydroxide is accelerated
by the addition of sodium carbonate, the hydration
reaction is promoted and ettringite is generated promptly.
This contributes to earlier gelling and earlier strength
development of CF-CBS.
(c) As Ca2+ from the interlayer cations of bentonite and Na+from sodium carbonate generate an ion-exchange
reaction, Na+ as interlayer cations increase andthe swelling property of the bentonite increases.
This contributes to the suppression of bleeding of
CF-CBS.
(d) In the process of the cation-exchange reaction, calcium
carbonate is formed by Ca2+ from the interlayer cationsof the bentonite and CO32− from sodium carbonate. This
contributes to an increase in the strength of CF-CBS.
As a future prospect, it will be necessary to verify theapplicability of CF-CBS as a cavity-filling material by implementing field tests and attempting a practical example.

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