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Effects of sodium hydroxide and sodium silicate solutions

on compressive and shear bond strengths of FA–GBFS geopolymer

Tanakorn Phoo-ngernkham a, Akihiro Maegawa b, Naoki Mishima c, Shigemitsu Hatanaka c,Prinya Chindaprasirt d,⇑

a Program in Civil Engineering, Faculty of Engineering and Architecture, Rajamangala University of Technology Isan, Nakhon Ratchasima 30000, Thailandb Mie Prefecture Industrial Research Institute, Mie 514-0819, Japanc Dept. of Architecture, Faculty of Engineering, Mie University, Mie 514-8504, Japand Sustainable Infrastructure Research and Development Center, Dept. of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand

h i g h l i g h t s

 FA–GBFS geopolymer activated with sodium silicate (cured at 23 C) resulted mainly in amorphous gel.

 Activated with sodium hydroxide resulted in significant crystalline CSH.

 Incorporation of GBFS enhanced compressive strength and microstructure of FA geopolymer pastes.

 Shear bond strength depended on strength and amount of NASH gel of FA–GBFS geopolymer.

a r t i c l e i n f o

 Article history:

Received 27 December 2014

Received in revised form 16 March 2015

Accepted 1 May 2015

Available online 16 May 2015

Keywords:

Geopolymer

Granulated blast furnace slag

Fly ash

Compressive strength

Shear bond strength

Repair material

a b s t r a c t

This article investigated the effects of sodium hydroxide and sodium silicate solutions on the properties

of fly ash (FA)–granulated blast furnace slag (GBFS) geopolymer. Three types of geopolymer pastes viz., FA

paste, FA + GGBS paste and GGBS paste were tested. They were activated with three types of alkaline

solutions viz., sodium hydroxide solution (NH), sodium silicate solution (NS), and sodium hydroxide plus

sodium silicate solution (NHNS). NH with 10 molar concentration, alkaline liquid/binder ratio of 0.60 and

curing at ambient temperature of 23 C were used for all mixes. The results indicated that the reaction

products and strengths of geopolymer depended on the types of source materials and alkali activators.

The use of NH and NHNS solutions resulted in the formation of crystalline calcium silicate hydrate

(CSH) which co-existed with amorphous gel. Whereas the use of NS solution resulted in mainly the

amorphous products with only a small amount of crystalline CSH in GBFS paste. The increase in GBFS

content enhanced the compressive strength and microstructure of geopolymer pastes due to the

formation of additional CSH. The shear bond strength between Portland cement concrete substrate and

geopolymer paste was found to relate to both compressive strength and amount of NASH gel of geopoly-

mer paste.

 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The manufacturing of Portland cement (OPC) results in high

emission of carbon dioxide (CO2) to atmosphere causing green-

house effect [1]. To solve this problem, the use of geopolymer as

an alternative binder for application in concrete industry is recom-

mended   [2–5]. Geopolymeric material is formed using source

materials containing silica (SiO2) and alumina (Al2O3) such as fly

ash (FA), calcined kaolin or metakaolin and granulated blast

furnace slag (GBFS) activated with alkali solutions   [6–10].

Specifically, sodium hydroxide based geopolymer is more attrac-

tive as it gives lower carbon footprint than sodium silicate based

geopolymer. For normal strength concrete, Turner and Collins

[11]   showed that the CO2   footprint of a sodium silicate based

geopolymer concrete was approximately 9% less than comparable

OPC concrete.

Fly ash is a by-product from coal burning in thermal power

plants. Both low calcium fly ash   [6,12]  and high calcium fly ash

[8,13]   are suitable source materials due to their high SiO2   and

Al2O3   contents. One obstacle with the use of fly ash is the low

http://dx.doi.org/10.1016/j.conbuildmat.2015.05.001

0950-0618/  2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +66 4320 2355.

E-mail address: prinya@kku.ac.th (P. Chindaprasirt).

Construction and Building Materials 91 (2015) 1–8

Contents lists available at   ScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t

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strength development with ambient temperature curing   [6]. A

number of researchers  [14–16]  have incorporated additives con-

taining calcium to enhance the strength development of fly ash

geopolymer. The presence of calcium has a positive effect on the

mechanical properties of geopolymeric material   [9]   by forming

additional calcium silicate hydrate (CSH) which coexists with the

geopolymer products  [17,18]. However, a certain amount of cal-

cium can lead to a decrease on compressive strength of geopoly-

mers   [19,20].   Pacheco-Torgal et al.   [21]   reported that when

calcium hydroxide percentages above 10% are used, strength

decrease after curing for 14 days is noticed.

GBFS is usually utilized as a binder in concrete industry due to

its high content of CaO; and SiO2 and Al2O3 in an amorphous state

[22,23]. Therefore GBFS shows pozzolanic and binding properties

in an alkaline medium   [24]. A number of researchers [6,24,25]

found that the incorporation of GBFS to FA geopolymer resulted

in additional calcium in the system and the mechanical properties

and microstructure of geopolymer were improved.

Several researchers indicate that sodium silicate and sodium

hydroxide based activators are suitable considering the mechanical

properties of GBFS and FA geopolymers   [6,25,26]. The main reac-

tion products formed as a result of alkali activation of GBFS are

CSH and/or CASH gels similar to those of PC [27], whereas the main

product of alkali activation of FA is NASH gel   [28]. As GBFS is a

glassy phase material, it is, therefore, easier to activate than fly

ash. Fly ash contains a larger portion of crystalline phase and usu-

ally requires temperature between 40 and 85 C to accelerate the

reaction   [24]. Previous studies suggested that the incorporation

of GBFS results in the enhancement of strength development of 

fly ash geopolymers  [29,30]. Bond strength is one of the most

important properties of high performance binder for application

as a repair material. Several tests i.e. pull-out, splitting, flexural,

and slant shear are used to evaluate the bond strength of repair

materials [3]. The slant shear test was successfully used to study

the shear bond strength of concrete repair [31] and bond between

concrete substrate and geopolymer [3,4].

Therefore, the use of GBFS to improve the strength of FAgeopolymer is very attractive. The objective of this study is to

investigate the mechanical properties viz., compressive strength

and shear bond strength, and microstructure of FA–GBFS geopoly-

mer with types of alkaline solutions. The obtained results should

be beneficial to the understanding and to the future applications

of FA–GBFS geopolymer.

2. Experimental details and testing analysis

 2.1. Materials

The materials used in this research were fly ash (FA) from Hekinan power plant

and ground granulated blast furnace slag (GBFS) from Nippon Steel & Sumitomo

Metal Corporation. 10 M sodium hydroxide (NH) and sodium silicate (NS) with

11.67% Na2O, 28.66% SiO2, and 59.67% H2O were used as activators.The chemicalcompositions andphysical properties of FA andGBFSare shownin

Tables 1 and 2. The specific gravity of FA and GBFS were 2.20 and 2.91. The median

particle sizes of FA and GBFS were 12.3 and 12.4 lm with the corresponding Blaine

finenesses of 3100 and 4950 cm2/g, respectively. Fig. 1 shows the scanning electron

micrographs (SEM) of FA and GBFS. The FA consisted of spherical particles with

smooth surface, while the GBFS consisted of irregular and angular particles similar

to the previously published results [32]. TheXRDs of FAand GBFS as shown in Fig.2

showed that as-received FA mainly consisted of amorphous phase as shown by a

hump around 18–28 2theta with some crystalline phases of mullite (Al6Si2O13),

quartz (SiO2), magnesioferrite (Fe3O4) and calcium oxide (CaO), while as-receivedGBFS mainly consisted of amorphous phase as shown by a hump around 25–35 2-

theta [27] with a small amount of magnetite.

 2.2. Mix proportion and curing 

The mix proportions of geopolymer pastesare shownin Table 3. Constant liquid

alkaline to binder ratio of 0.60 was used for all mixes. Three types of geopolymer

pastes viz., FA paste, FA + GGBS paste and GGBS paste and three types of alkaline

solutions viz., sodium hydroxide solution (NH), sodium silicate solution (NS), and

sodium hydroxide plus sodium silicate solution (NHNS) were used. The NHNS solu-

tion was prepared with NS/NH ratio of 2.0. For the other series the NH and NS solu-

tions were used directly.For themixing of pastes, FA andGBFSwere drymixed until

the mixture was homogenous. Right after, the liquid solution was added and the

mixing of paste was done for another 3 min.

 2.3. Testing and analysis

 2.3.1. Compressive s trength of geopolymer pastes

After being mixed, the fresh geopolymer pastes were placed into cylindrical

moulds of 50 mm in diameter and 100 mm in height. They were covered with vinyl

sheet and placed in ambient temperature curing (23C). The samples were

demoulded at theage of 1 dayand immediately wrapped with vinyl sheet to protect

moisture loss and kept in the 23 C controlled room until the testing age. The com-

pressive strengths were testedat theages of 7, 28, and 60 days. The reported results

were the average of three samples.

 2.3.2. X-ray diffraction (XRD) and scanning electron microscopy (SEM) of geopolymer 

 pastes

At the age of 28 days, the geopolymer paste samples were broken and the mid-

dle portion was collected and ground to fine powder. The XRD scans were per-

formed at 5–60 2theta with an increment of 0.02/step and a scan speed of 

0.5 s/step. The amorphous phases of geopolymer pastes were determined by quan-

titative XRD analysis using Bruker’s TOPAS software. The specimen was placed on a

brass stub sample holder with double stick carbon tape. The specimen was driedusing infrared light for 5 min and then coated with a layer of gold using a blazer

sputtering coater. The micrographs were recorded at 20kV and 1000

magnification.

 2.3.3. Shear bond strength between concrete substrate and geopolymer paste

The shear bond strength was evaluated using the slant shear test of concrete

substrate and geopolymer paste. The slant angle of 30  to the vertical is recom-

mended by ASTM C882   [33]  because the failure stress is close to the minimum

stress   [31]. However, stiffer angle of 45   is also officially used for the standard

 Table 1

Chemical compositions of FA and GBFS (by weight).

Materials SiO2   Al2O3   Fe2O3   CaO MgO K2O Na2O SO3   LOI

FA 52.31 27.04 6.85 3.32 1.23 1.29 1.15 0.99 1.60

GBFS 30.53 13.67 0.33 46.00 5.09 0.36 0.24 – 0.22

 Table 2

Physical properties of FA and GBFS.

Materials Specific gravity Median particle

size (lm)

Blaine fineness

(cm2/g)

FA 2.20 12.3 3100

GBFS 2.91 12.4 4950

(a) FA (b) GBFS

Fig. 1.  SEM of FA and GBFS.

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evaluation of epoxy bond with concrete [34]. Preliminary test showed that the 45concrete substrate was easier to cut with less chipping and cracking than the 30

concrete substrate. The slant angle of 45  was, therefore, used in this study. The

results could be compared within the context of this work and the results should

thus be interpreted with care.

The preparations of concrete specimens and the casting of slant shear speci-

mens were based on the previous study  [3,4]. The concrete substrates were pre-

pared using the aged slant shear concrete prisms by cutting in half at 45  line to

the vertical. The saw cut surface was used as it was shown to be suitable substrate

concrete surface for shear bond strength assessment [35]. The compressive strength

of concrete substrate was 35.0 MPa. The paste was then cast into the mold with the

other half filled with concrete substrate. After the casting of samples, they were

covered with vinyl sheet to protect moisture loss and kept in the 23 C controlled

room until the testing ages. The shear bond strength was calculated as the ratio

of maximum load at failure and the bond area. The shear bond strength was mea-

sured at the age of 28 days with a constant loading rate of 0.30 MPa/s. The reported

results were the average of three samples.

In addition, two available commercial repair material products (Epoxy A andEpoxy B) were tested for shear bond strengths with concrete substrate and

compared with those of geopolymer pastes. Epoxy A was a general purpose

non-shrink grout mortar and Epoxy B was a multi-purpose non-shrink grout. The

recommended water to binder ratio of 0.15 was used for Epoxy-A and the 28-day

compressive strength was 62.0 MPa. For Epoxy-B, the recommended water to

binder ratio of 0.14 was used and the 28-day compressive strength was 70.0 MPa.

The preparation of the slant shear prism was the same as those of the geopolymer

slant shear test.

3. Results and discussions

 3.1. Compressive strength

The results of compressive strengths of geopolymer pastes are

shown in   Table 4. The compressive strength of pastes obviouslyincreased with increasing GBFS content for all NH, NHNS and NS

series. For example, the 28-days compressive strengths of FA,

FA + GBFS and GBFS pastes with NHNS were 42.8, 114.5, and

171.7 MPa, respectively. The readily available free calcium ions

reacted with silica and alumina and formed C(A)SH gel which

coexisted with the geopolymer gels  [27,36]. In addition, the reac-

tion of GBFS and alkali solutions was an exothermal process and

generated heat which promoted the geopolymerization process.

The increase in GBFS content, therefore, led to high compressive

strength geopolymer pastes.

According to  Table 4, the mixes with 100% FA gave low early

strengths for all NH, NHNS and NS series. The NH solutionis impor-

tant for dissolving of Si4+ and Al3+ ions from raw materials and sub-

sequent geopolymerization process  [17]. However, the strengthdevelopment of FA paste activated by either NH or NS at ambient

Q = Quartz (SiO2), M = Mullite (Al6Si2O13), C = Calcium oxide (CaO), F = Magnetite (Fe3O4)

0 10 20 30 40 50 60

2 theta (degree)

   I  n   t  e  n  s   i   t  y   (

  c  o  u  n   t  s   )

GBFS

FA

M

Q

M

Q

Q

M

Q QQC

M

MM

C

F

Fig. 2.  XRD of FA and GBFS.

 Table 3

Mix proportions of geopolymer pastes.

Mix No. Source material Activator FA (g) GBFS (g) NaOH (g) Na2SiO3  (g) SiO2/Al2O3  ratio CaO/SiO2  ratio

1 FA NH 100 – 60 – 1.94 0.06

2 FA + GBFS NH 50 50 60 – 2.04 0.60

3 GBFS NH – 100 60 – 2.23 1.51

4 FA NHNS 100 – 20 40 2.36 0.05

5 FA + GBFS NHNS 50 50 20 40 2.56 0.47

6 GBFS NHNS – 100 20 40 3.07 1.10

7 FA NS 100 – – 60 2.57 0.05

8 FA + GBFS NS 50 50 – 60 2.88 0.42

9 GBFS NS – 100 – 60 3.49 0.96

 Table 4

Compressive strength of geopolymer pastes.

Mix No. Source material Activator Compressive strength (MPa)

7 days 28 days 60 d ays

1 FA NH 0⁄ 1.5 4.4

2 FA + GBFS NH 6.5 18.3 26.2

3 GBFS NH 15.5 27.1 34.4

4 FA NHNS 0⁄ 42.8 52.9

5 FA + GBFS NHNS 84.9 114.5 127.2

6 GBFS NHNS 144.4 171.7 176.4

7 FA NS 0⁄ 0⁄ 1.1

8 FA + GBFS NS 38.7 54.9 62.4

9 GBFS NS 119.6 173.0 197.1

0⁄ = very low strength.

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temperature was very low [17,37]. The use of NHNS showed signif-

icant improvement in the strength development at later ages. For

the FA paste with NHNS, although the 7-day compressive strength

was very low, the 28-day and 60-day compressive strengths were

relatively high at 45.0 and 52.9 MPa, respectively. The low strength

at early stage of FA paste was due to low reaction process at ambi-

ent temperature, however, the strength development of FA paste

over time is similar to that of Portland cement concrete [15].

For the mix with increased GBFS content (FA + GBFS paste), the

paste with NH gave lower strength than those with NHNS and NS.

As mentioned, NaOH is needed for the leaching of silica and alu-

mina. However, the leaching and subsequent reaction is low at

ambient temperature around 23 C   [38]. The use of NS alone or

in conjunction with NH resulted in additional silicate in the system

and this helped speed up the geopolymerization leading to

increased compressive strength   [26]. In addition, the FA + GBFS

paste with NHNS showed higher strength than those of NH and

NS for all of curing age. The FA + GBFS blend with NHNS was the

most effective alkali activated binder and high strength mixes were

obtained. Similarly, high strength mixes were also reported with

the GBFS geopolymer with sodium sulfate  [39].

For the GBFS paste, the paste with NS showed the highest com-

pressive strength at the ages of 28 and 60 days due to the reaction

between CaO from GBFS and SiO2  with subsequent formation of 

CSH  [27].   Ismail et al.   [27]   reported that the alkali activation of 

GBFS involved the dissolution of Ca and participation of Si and Al

to form CSH and CASH gel and thus led to high mechanical

strength. Furthermore, the compressive strength of geopolymer

depends primarily on SiO2/Al2O3 mole ratio [40]. Previous research

reported that SiO2/Al2O3 around 3.50 gave a high strength high cal-

cium geopolymer [41]. It should be noted here that the SiO2/Al2O3

ratio of GBFS paste with NS was 3.49 (see   Table 3). The 28-day

compressive strength of this paste cured at ambient temperature

was very high at 171.7 MPa.

 3.2. XRD analysis

The results of XRD patterns of geopolymer pastes are shown in

Figs. 3–5. As shown in Fig. 3 for the NH series, the FA paste con-

sisted of glassy phase as indicated by the hump at 28–35 2theta

[41]   and crystalline phases of quartz, mullite, magnetite and

hydrosodalite (Na4Al3Si3O12(OH)). These crystalline phases were

mainly from the remain of non-reacted elements from FA. The

presence of sodium alumino-silicate zeolite in the form of hydroso-

dalite indicated some geopolymerization, but the strength of 

geopolymer paste was still low  [17]. For the mix with increased

GBFS content (FA + GBFS paste), the XRD pattern of paste with

NH showed both amorphous and crystalline phases. The peak of 

the crystalline phases viz., quartz, mullite, magnetite and hydroso-

dalite were reduced and additional peak of calcium silicate hydrate

(CSH) was present. This indicated the reaction of calcium to form

CSH and also the presence of NASH gel with reduced hydrosodalite

resulting in an increase in the strength of paste. For the GBFS paste

with NH, the major phase of paste was amorphous with additional

CSH and calcite with no appearances of quartz, mullite, magnetite

and hydrosodalite. The high CaO content contributed to the devel-

opment of CSH and the excess CaO reacted with CO2  and formed

calcite [9]. The CSH gel and calcite coexisted with the NASH gel

[27]   and reasonable 28-days compressive strength of 27.1 MPa

was obtained.

For the NHNS mixes as shown in Fig. 4, the FA paste consisted of 

glassy phase as indicated by the hump at 28–35 2theta and the

crystalline phases of quartz, mullite and magnetite with no

hydrosodalite. The strength came mainly from the NASH gel and

relatively high 28-days strength of 42.8 MPa was obtained. For

the FA + GBFS paste with NHNS, additional CSH was formed with

reduced amount of quartz and mullite. Additional CSH was respon-

sible for the increase in strength and the high 28-days compressive

strength of 114.5 MPa was obtained. For the GBFS paste with

NHNS, the XRD pattern showed a large amount of amorphous

phase with additional CSH and only a small amount of magnetite

(from GBFS). With only GBFS, the amorphous phases were easily

detected as broad hump around 28–35 2theta due to the forma-

tion of amorphous components in geopolymer gel. Calcium ele-

ment also reacted to form CSH which coexisted with geopolymer

gel   [27]   and high 28-days compressive strength of 171.7 MPa

was obtained. The presence of CSH and relatively high compressive

strength is in line with the previous research  [6]. For mixes with

NSNH, the incorporation of GBFS provided calcium to react andform CSH in the geopolymer system. Without GBFS, there was

insufficient calcium and no CSH was, therefore, detected similar

to the mixes with NH solution only.

For the NS mix as shown in Fig. 5, the XRD of FA paste showed

the amorphous phase of NASH gel with hump around

0 10 20 30 40 50 60

2 theta (degree)

   I  n   t  e  n  s   i   t  y

   (  c  o  u  n

   t  s   )

FA with NH

FA+GBFS with NH

GBFS with NH

MH

Q

H

Q

MM

FM M M

QQ

Q

S

S

M MMQ Q Q

HM Q H

Q

M

Q = Quartz (SiO2), M = Mullite (Al6Si2O13), H = Hydrosodalite (Na4Al3Si3O12(OH)), F=Magnetite (Fe3O4),

C= Calcite, S= Calcium Silicate Hydrate

F

F

C

C

CC C

Fig. 3.  XRD of geopolymer pastes with NH.

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18–32 2theta with crystal of quartz, mullite and magnetite. With

the incorporation of 50% GBFS (FA + GBFS paste), the paste with NS

showed also amorphous phases, however, with a shift in the loca-

tionof the humpto a broad humparound 28–35 2theta with asso-

ciated decreases in quartz and mullite peaks. The reductions of 

crystalline phase were similar to the behaviors of NHNS paste.

The shift in the broad hump was significant indicating the

increased disorder of the glass phase and the presence of some

CASH compound   [42].   While, the XRD of GBFS paste with NS

indicated the existence of amorphous gel and a small amount of 

crystalline CSH with only the crystalline phases of magnetite.

Mixes with NS only should also have a high pH condition initially,

therefore a small amount of crystalline CSH could thus be formed

[43]. The lowering of pH condition in the mix favored the forma-

tion of the NASH gel  [41] while the crystalline phase of magnetite

from GBFS remained. This resulted in high 28-day compressive

strength of GBFS paste with NS of 173.0 MPa.

 3.3. SEM analysis

The results of SEM analyses of fracture surfaces of geopolymer

pastes are shown in   Fig. 6. In general, the FA pastes had rather

loose matrix as shown in Fig. 6a, d and g. The FA pastes with NH

and NS showed less dense and loose matrix while the SEM of FA

paste with NHNS showed a larger number of non-reacted and/or

partially reacted fly ash particles embedded in a continuous

matrix. The FA paste with NHNS appeared to be slightly denser

than those NH and NS. This is in line with the previous research

which indicated that the use of NH in conjunction with NS could

accelerate the geopolymerization process [38].

Noticeable difference was found with the FA + GBFS mix com-

pared to that of FA paste. The micrograph showed less number of 

unreacted fly ash particles and the matrices appeared denser than

those of FA pastes. FA is relatively slow to react at ambient temper-

ature and temperature curing is needed to accelerate the strength

0 10 20 30 40 50 60

2 theta (degree)

   I  n   t  e  n  s   i   t  y   (

  c  o  u  n   t  s   )

FA with NHNS

FA+GBFS with NHNS

GBFS with NHNS

MQ

M M

Q

M

Q

M

M

Q

S

S

Q

QM

M MMQ Q Q

Q

F

Q = Quartz (SiO2), M = Mullite (Al6Si2O13), F=Magnetite (Fe3O4), S= Calcium Silicate Hydrate

F

F

Fig. 4.  XRD of geopolymer pastes with NHNS.

0 10 20 30 40 50 60

2 theta (degree)

   I  n   t  e  n  s   i   t  y

   (  c  o  u  n   t  s   )

FA with NS

FA+GBFS with NS

GBFS with NS

M Q

M

M

Q

M M MQ Q Q

M QM

QM

M

QM

M

Q

Q = Quartz (SiO2), M = Mullite (Al6Si2O13), F=Magnetite (Fe3O4), S= Calcium Silicate Hydrate

S

F

F

F

Q

Fig. 5.  XRD of geopolymer pastes with NS.

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development [44,45]. The increasing in GBFS content enhanced the

reaction. Also, the reaction of GBFS with alkali solution is an

exothermal process and liberated heat and thus formation of addi-

tional CSH and CASH led to the overall strength development

[7,27]. According to   Fig. 6, the FA + GBFS paste with NHNS

appeared denser than those of with NH and NS. It appeared that

the activation with NHNS solution enhanced the geopolymeriza-

tion of the FA–GBFS geopolymer paste compared with the

activation with single activator as reported by previous research

[46]. The relatively high 28-day strength of 114.5 MPa was

obtained with the FA + GBFS mix with NHNS.

For the GBFS mix, the matrix appeared denser and more

homogenous than the pastes with 50% and 0% GBFS. As mentioned,additional CSH from the reaction of GBFS and alkali solution was

formed   [27]. This additional CSH modify the microstructure of 

paste. In addition, mixes with sodium silicate i.e. pastes with

NHNS (see Fig. 6f) and NS (see  Fig. 6i) appeared denser than the

mix with NH (see   Fig. 6c). The silicate in the system promoted

the reaction between calcium and silicate and formed calcium sil-

icate products.

 3.4. Shear bond strength between concrete substrate and geopolymer 

 pastes

The results of slant shear capacity of Portland cement concrete

substrate (compressive strength of 35 MPa) and geopolymer pasteat 45 of interface line to the vertical are shown in Fig. 7. The shear

bond strengths tended to increase with the increasing GBFS con-

tent for all three series.

For the NH series, the strengths increased with increasing GBFScontent. The shear bond strengths of FA, FA + GBFS and GBFS pastes

Fig. 6.  SEM of geopolymer pastes.

0

5

10

15

20

25

30

35

40

   F   A  w   i   t   h   N   H

   F   A   +   G   B   F   S  w   i   t   h   N   H

   G   B   F   S  w   i   t   h   N   H

   F   A  w   i   t   h   N   H   N   S

   F   A   +   G   B   F   S  w   i   t   h   N   H   N   S

   G   B   F   S  w   i   t   h   N   H   N   S

   F   A  w   i   t   h   N   S

   F   A   +   G   B   F   S  w   i   t   h   N   S

   G   B   F   S  w   i   t   h   N   S

   E  p  o  x  y   A

   E  p  o  x  y   B

   S   h  e  a  r   b  o  n   d  s   t  r  e  n  g   t   h   (   M   P  a   )

 NA

Fig. 7.   Shear bond strength between concrete substrate and geopolymer paste or

epoxy with interface line at 45  to the vertical.

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were 3.0, 7.0 and 12.0 MPa, respectively. The increase in shear

bond strength was directly related to the increase in compressive

strength of geopolymer paste. It had been reported that the tensile

strength and bonding of geopolymer were higher than those of 

conventional Portland cement system  [47]. In addition, the high

CaO content from GBFS promoted the formation of additional

CSH and/or CASH gel, therefore, the bonding at the interface tran-

sition zone between old concrete and geopolymer was enhanced

[3,4].

For the NS series, the compressive strength tended to increase

with the increase in GBFS content. However, the shear bond

strength at high 100% GBFS content (GBFS paste with NS) started

to drop compared with that of 50% GBFS mix (FA + GBFS paste with

NS). For the FA + GBFS mix, the paste contained predominantly

NASH gel with some CSH/CASH products. At high 100% GBFS con-

tent, with the presence of a large amount of calcium, CSH and/or

CASH gel formed in a substantial quantity in addition to NASH

gel. This is considered somehow offsetting the excellent bondingbehavior resulting in a slight drop in the shear bond strength at

this high level of GBFS content. It is worth mentioning that the

shear bond strength does not only depend on the strength of 

geopolymer, the strength of a weaker portion i.e. concrete

substrate also has a large influence.

For the NHNS series, the trend of the result was similar to those

of NS series. The optimum shear bond strength was obtained with

the 50% GBFS content mix, and at high 100% GBFS content, the

bond strengths were slightly lowered. However, the shear bond

strengths of this series were higher than those of the other series.

The high shear bond strengths generally corresponded to the high

compressive strengths of geopolymer pastes.

The shear bond strengths of two available commercial repair

material products (Epoxy A and Epoxy B) and concrete substrateare also shown in Fig. 7. The shear bond strength between concrete

substrate and Epoxy A and Epoxy B were 20.6 and 26.2 MPa,

respectively. The shear bond strengths of 5 mixes i.e. all three

NHNS mixes viz., FA, FA + GBFS and GBFS pastes with NHNS; and

the other two NS mixes viz., FA + GBFS and GBFS with NS were

comparable to those of the epoxy mixes. This indicated that the

FA–GBFS geopolymer pastes should be look into more extensively

for the application as repair materials.

The failure patterns of the concrete sample and samples from

the shear bond tests (slant angle of 45) are givenin Fig. 8. The fail-

ure pattern of concrete sample was in a monolithic failure mode

(Fig. 8a). For the high shear bond strength of FA + GBFS paste with

NHNS (Fig. 8b), the monolithic failure mode was also observed.

Namely, the geopolymer and concrete substrate acted as one unitand the crack formed and run in the vertical direction passed

through the slant bond area similar to the failure mode of normal

concrete specimen (Fig. 8a). This confirmed the excellent bonding

between geopolymer and concrete substrate. The failure patterns

of FA paste with NS, Epoxy A and Epoxy B are shown in  Figs. 8c –

e. The failure modes were similar and were between the combined

failure with monolithic failure and bond failure. Crack path cov-

ered the concrete substrate, bonding surface and geopolymer

paste. This was considered due to the similar shear bond strengths

of the FA paste with NS and the two epoxy samples (Fig. 7). The

results in   Figs. 7 and 8   thus indicated the suitability of the

FA + GBFS mix with NHNS for high shear bond strength

requirement.

The 28-day shear bond strengths of usable geopolymer pastes

obtained from this work were approximately 21–31 MPa which

were higher than the previously published results   [3]. This was

due to the use of slant angle of 45 to the vertical in this study com-

pared to 30   in the previous work. Similar trend of result was

reported in the previously published work [31].

4. Conclusions

The effect of using sodium hydroxide and sodium silicate solu-

tions as liquid portions in the mixture on properties of FA–GBFS

geopolymer was investigated in this study. The FA paste contains

amorphous NASH gel and some crystalline phases of the remain

of fly ash. The increase in GBFS content enhanced the compressive

strength and microstructure of geopolymer pastes due to the for-

mation of additional CSH. The use of NH and NHNS solutions

resulted in crystalline CSH and amorphous gel, whereas the use

of NS solution resulted in mainly the amorphous products.

For the FA and FA + GBFS pastes, the use of NH solution or NSsolution alone gave low strengths when cured at ambient temper-

ature. Better strength development was obtained with the use of 

NHNS solution. For the GBFS paste, the presence of silicate

enhanced the strength development and thus pastes containing

NS solution performed better. Relatively high 28-day compressive

strengths of 171.7 and 173.0 MPa were obtained for GBFS pastes

with NHNS and NS solutions, respectively.

The shear bond strength (slant angle of 45) between concrete

substrate and geopolymer paste was increased with the increase

in compressive strength and amount of NASH gel of geopolymer

paste. The highest 28-day shear bond strength of 31.0 MPa was

obtained with FA + GBFS paste with NHNS solution. This indicates

that it may be possible to use FA–GBFS geopolymer pastes as a

repair material. However, additional tests are required to confirmthis.

Fig. 8.  Specimen failure modes.

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 Acknowledgements

The authors gratefully acknowledge the financial supported

from Higher Education Research Promotion and National

Research University Project of Thailand, Office of the Higher

Education Commission, through the Advanced Functional

Materials Cluster of Khon Kaen University; Khon Kaen University

and the Thailand Research Fund (TRF) under the TRF SeniorResearch Scholar, Grant No. RTA5780004. The authors also would

like to acknowledge the support of the Mie Prefecture Industrial

Research Institute and Department of Architecture, Faculty of 

Engineering, Mie University, Japan.

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