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Study on solids-to-liquid and alkaline activator ratios on kaolin-based geopolymers

C.Y. Heah a,, H. Kamarudin a, A.M. Mustafa Al Bakri a, M. Bnhussain b, M. Luqman a, I. Khairul Nizar c,C.M. Ruzaidi a, Y.M. Liew a

a Centre of Excellence Geopolymer System Research, School of Materials Engineering, Universiti Malaysia Perlis (UniMAP), P.O. Box 77, D/A Pejabat Pos Besar, 01000 Kangar,

Perlis, Malaysiab King Abdulaziz City Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabiac School of Environmental Engineering, University Malaysia Perlis, P.O. Box 77, D/A Pejabat Pos Besar, 01000 Kangar, Perlis, Malaysia

h i g h l i g h t s

" Solids-to-liquid and activator ratios

affect significantly the compressive

strength.

" Strength of kaolin-based

geopolymers increased with the

ageing time.

" The geopolymer do not destroy in

water proving the presences of

geopolymer bondings.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 3 February 2012

Received in revised form 19 April 2012

Accepted 29 April 2012

Keywords:

Kaolin

Geopolymer

Alkaline activator

Geopolymerization

Solids-to-liquid

Activator ratio

a b s t r a c t

Kaolin and alkali activator were mixed with the solids-to-liquid ratios in range of 0.601.20 (Al 2O3/Na2O

molar ratio of 0.631.27). Sodium silicate and sodium hydroxide ranged between 0.16 and 0.36 (SiO2/

Na2O molar ratio of 3.193.67) were mixed together to prepare alkali activator. The results concluded

that compressive strength was affected by both S/L and Na2SiO3/NaOH ratios and strength increased with

ageing day. Both these ratio also influenced the workability of the mixes. Besides, the kaolin geopolymers

showed good volume stability in water. Compressive strength was highest at S/L and Na2SiO3/NaOH

ratios of 1.00 and 0.32, respectively. In term of molar ratios, optimum was achieved at Al 2O3/Na2O of

1.09 and SiO2/Na2O molar ratios of 3.58. Microstructures showed that kaolin particles were slightly acti-

vated with large part of unreacted raw materials remained in the system. Geopolymer sample reduced in

peak intensities over time as presented by XRD analysis and the presence of crystalline peaks in the kao-

lin geopolymers was Zeolite X. FTIR analysis showed the presence of geopolymer bonding increased over

age. In overall, kaolin geopolymers does not undergo complete geopolymerization and showed slow

strength development. Vast research works have to be carried out to further improve the properties of

kaolin geopolymers.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Glukhovsky and later by Krivenko in the 1950s have developed

on the alkali-activated system containing the calcium silicate hy-

drated (CSH) and aluminosilicate phases. Due to the various cat-

astrophic fires in France in 19701973, Davidovits started research

on the non-combustible and non-flammable plastic materials. In

developing new inorganic polymer materials, Davidovits found

that the synthesis of both plastic and heat resistant feldspathoids

and zeolites was controlled by the similar hydrothermal condition

and required high PH, concentrated alkali, atmospheric pressure

and thermoset at temperature below 150 C. The development of

this technology was based on the geosynthesis of aluminosilicate

kaolinite with caustic soda at 100150 C which was then polycon-

densed into a hydrated sodalite or hydrosodalite [1].

In 1978, Davidovits created the name geopolymers meaning

the mineral polymers resulting from geochemistry or geosynthesis.

The geopolymer chemistry concept was invented in 1979 [1]. Geo-

polymerization is a reaction that chemically integrates minerals

0950-0618/$ - see front matter 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.conbuildmat.2012.04.102

Corresponding author. Tel.: +60 12 5711154.E-mail address: [email protected] (C.Y. Heah).

Construction and Building Materials 35 (2012) 912922

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involving silicoaluminates sources. Source of alumina in and silica

acts as a source of precursor that readily dissolved in the alkaline

solution, and synthesized by alkaline and/or silicate activation

which lends itself to the process of geopolymerization [2]. This re-

sults in a amorphous to semi-crystalline three dimensional silico-

aluminate structure [1] that has significant similarities to zeoliteprecursor gels [3]. The SiO4 and AlO4 tetrahedra are linked alterna-

tively by sharing all the oxygens. The negative charges of Al 3+ in

the IV fold coordination are charge-balanced by the positive ions

(Na+, K+, Li+, Ca2+, Ba2+, NH4+, H3O+). Geopolymers have the empir-

ical formula of: Mn{(SiO2)zAlO2}wH2O, where M is a cation (K+,

Na+ or Ca2+); n is a degree of polycondensation and z is 1, 2, 3 [4].

Types of geopolymers includes poly(sialate), poly(sialate-siloxo)

and poly(sialate-disiloxo) as shown as follows [1]:

Formerly, two alkali-activated binding systems were estab-

lished: (1) the alkali activation of blast furnace slag (Si + Ca) with

mild alkaline solution and (2) alkali activation of metakaolin and

Class F fly ash (Si + Al) with medium to high alkaline solution [5].

According to Palomo et al. [5], the alkali activation of blast furnace

slag has calcium silicate hydrate (CSH) as main products. For alkali

activation of metakaolin and fly ash, the reaction products were

the zeolite-like polymer. Later, plenty researches have been carried

out to investigate the physical properties, mechanical properties,

characterization and the durability of the geopolymers by using

the source materials such as metakaolin [6], fly ash [7] and slag

[8]. Geopolymers exhibit a wide variety of properties and charac-

teristics depending on the raw material selection and processing

conditions. Several well-known benefits of these materials include

excellent mechanical strength, long term durability [9], lowshrink-

age [10], fast setting [9], acid resistance [10], fire resistance [9] and

low thermal conductivity [10].

Alonso and Palomo [11] found that the rate of polymer forma-

tion was influenced by parameters such as curing temperature, al-

kali concentration, and initial solid content, among others. In

particular, the source materials, as well as the curing regime for

the geopolymers were important factors that must be taken into

consideration during geopolymers synthesis [12]. In addition, the

water content, the kaolinite ratio, and the type of metal silicate

used also have a substantial effect on the final properties of the

geopolymers. Criado et al. [13] also suggested that the activation

rate and the chemical composition of the reaction product de-

pended on the particle size, chemical composition and type of

the alumino-silicate source, activator concentration and etc. Even

so, the activation mechanism was independent of the variables sta-

ted above at any given time.

Focusing on the solids-to-liquid (S/L) ratio, which correspondsto the aluminosilicate-to-activator solution ratio, Yao et al. [14] re-

ported that high S/L ratios resulted in low viscosity of the slurry

and the lower S/L ratios increased the geopolymerization period.

Zuhua et al. [15], on the other hand, stated that low S/L ratios could

accelerate the dissolution of source materials, however, it was not

beneficial to polycondensation process at high sodium hydroxide

(NaOH) concentration. Provis et al. [3] observed that geopolymers

with very high S/L ratios generally do not achieve high strength

due to low extent of binder formation.There is limited research on the effect of alkaline activator,

especially sodium silicate-to-sodium hydroxide (Na2SiO3/NaOH)

ratio on geopolymer synthesis. However, Hardjito and Rangan

[16] reported that compressive strength increased when Na2SiO3/

NaOH ratio was increased. Both S/L ratio [17] and Na2SiO3/NaOH

ratios [18] influenced the workability of geopolymers.

In early study, researchers have focused on metakaolin as the

aluminosilicate source [19,20]. Later, the utilization of fly ash as

aluminosilicate source became major study areas [21,22]. The use

of raw kaolin itself in geopolymer has not been studied in detailed.

It is important to study on the basic material (kaolin) to address

how much kaolin canstandalone in theproductionof geopolymers.

Moreover, the use of basic material led to easy in the interpretation

of result and eliminated complex interpretation of results due tothe utilization of complex raw material, such as fly ash with lots

of impurities. which may affect the data interpretation.

Kaolin geopolymer is well known by its characteristics of being

lightweight and capability to bind materials which makes it ideal

for industrial applications such as cementitious materials. If suc-

ceeded, this kaolin geopolymers would have wide range of the

applications especially in the low strength precast materials field,

not limited to cement board, architectural products, decorative

wall or countertop, plasterboard, drains or pipe, wall panel, roof

tiles, road barrier and many more. These binders are still in the

early stages of development; hence, they need further research to

become technically and economically viable materials. In this pa-

per, important parameters, that is, S/L ratio and Na 2SiO3/NaOH ra-

tio on the synthesis of kaolin geopolymers were discussed. The

effect of S/L and Na2SiO3/NaOH ratios was also discussed in term

of relation of various oxide molar ratios (e.g. SiO2/Al2O3, SiO2/

Na2O, H2O/Na2O and Al2O3/Na2O molar ratios). Physical observa-

tion, bulk densities as well as the workability of the kaolin geo-

polymers were reported. In addition, the SEM, XRD and FTIR

analyses were performed.

2. Experimental method

2.1. Materials

The NaOH powder used was of caustic soda micropearls, 99% purity with brand

name of Formosoda-P, made in Taiwan.

A technical grade sodium silicate (Na2SiO3) solution was supplied by South Pa-

cific Chemicals Industries Sdn. Bhd. (SPCI), Malaysia. The chemical compositionscomprised of 30.1% SiO2, 9.4% Na2O and 60.5% H2O with modulus SiO2/Na2O of

3.2, specific gravity at 20 C = 1.4 g/cm3 and viscosity at 20 C = 0.4 Pa.s.

The kaolin was supplied by Associated Kaolin Industries Sdn. Bhd., Malaysia.

The general chemical composition is tabulated in Table 1 which obtained from X-

ray fluorescence (XRF) analysis. The physical form of kaolin used was of powder

Table 1

Chemical composition of kaolin.

Chemical Wt (%)

SiO2 54.0

Al2O3 31.7

Fe2O3 4.89

TiO2 1.41

ZrO2 0.10K2O 6.05

MnO2 0.11

LOI 1.74

C.Y. Heah et al. / Construction and Building Materials 35 (2012) 912922 913


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type and has minimum 40% of particle size less than 2 lm and maximum 2% of

moisture content. It was used as SiAl cementitious materials. Distilled water

was used throughout.

2.2. Sample preparation

NaOH solution having the concentration of 8 M was prepared in volumetric

flask and was allowed to cool down to room temperature. NaOH solution was

mixed with Na2SiO3 solution with Na2SiO3/NaOH ratio ranged from 0.16 to 0.36

to prepare alkali activator solution 24 h prior to use. The kaolin powder and alkali

activator solution were mixed with the solids-to-liquid ratio ranged from 0.60 to

1.20 and were stirred well for few minutes by using mechanical mixer. The detail

of mixture proportions were given in Table 2. The fresh paste was then rapidly

poured into 50 50 50 mm steel molds and the samples were compacted

approximately one half of the depth (about 1 in. or [25 mm]) of the mold in the en-

tire cube compartments and the paste was tamped in each cube compartment at

each layer as described in ASTM C109 [23]. Lastly, the samples were put into the

oven at temperature of 80 C for 24 h for curing purpose. The samples were sealed

with thin plastic at the exposed portion of the mold during the curing stage.

2.3. Qualitative observation

The kaolin geopolymers were soaked in water up to 180 days. The observation

of these kaolin geopolymers were recorded at 1, 7 and 180 days to determine the

soundness of the geopolymers produced.

Workability of the fresh kaolin geopolymers were measured using mini-slump

cone as proposed by Kantro [24]. Based on Kantro [24], the test method corre-

sponded to the slump test (ASTM C143) used for concrete mixes. For neat cement

paste, this mini-slump test has been devised. The mini-slump cone was placed at

the center of the glass plate and filled with geopolymer paste. Gently, the cone

wasliftedand thediameter of thepat formed wasmeasured. Thetwo perpendicular

diameters were recorded and the average diameter was calculated [25,26]. Bulk

densities of kaolin geopolymers were determined by measuring the mass and the

dimensions of the kaolin geopolymers in accordance to BS EN12390-7 [27].

2.4. Compressive test

Compressive strength tests of allspecimens were evaluatedby using theInstron

machine series 5569 Mechanical Tester. The specimens were taken out from oven

after 24 h of curing and wereput inroomtemperatureuntil the day of testing. Com-

pressive test was carried out to evaluate the strength development of the speci-

mens. The samples were tested up to 180 days. Three specimens were tested for

each ratio.

2.5. Scanning Electron Microscope (SEM)

JSM-6460LA model Scanning electron microscope (JEOL) was performed to re-

veal the microstructure of kaolin geopolymers at different S/L and Na 2SiO3/NaOH

ratios. The specimens were prepared and coated by using Auto Fine Coater; model

JEOL JFC 1600 prior to examination. The chemical composition of the reaction prod-

ucts was determined by an Energy Dispersive X-ray Spectroscopy (EDX).

2.6. X-ray diffraction (XRD)

Specimens were prepared in powder form and undergone XRD examination.

XRD-6000, Shimadzu X-ray diffractometer equipped with auto-search/match soft-

ware as standard to aid qualitative analysis was used.

2.7. Fourier transform infrared spectroscopy (FTIR)

Perkin Elmer FTIR Spectrum RX1 Spectrometer was used to evaluate the func-

tional group of the sample. Small amount of potassium bromide (KBr) and geopoly-

mer powder were put into a mold. By using cold press machine, mold which

contains powder and KBr was pressed at 4 ton for 2 min to produce specimens

for examination.

3. Results and discussion

3.1. Qualitative observation

In order to observe the soundness of the kaolin geopolymers,

the samples were soaked in water up to 180 days as shown in

Fig. 1. It was observed that the kaolin geopolymers with S/L ratio

of 0.60 (Mix 1) disintegrated in water at 7 days. This was probably

because of the very weak structure as shown by the very low com-

pressive strength reported (Fig. 4). However, kaolin geopolymers

with S/L ratio of 1.00 (Mix 3) did not collapse when put in water

even after 180 days. No cracks were observed. This kaolin geopoly-

mers was stable in water. Based on Davidovits [1], activation ofkaolinite clay produced three dimensional network structure that

was stable in water. Even though this kaolin-based geopolymers

achieved quite a low strength in range of 56 MPa, however the

geopolymer bonding (Figs. 1012) existed in the system made it

stable and did not collapse in water [28]. The same observation

was recorded for kaolin geopolymers with Na2SiO3/NaOH ratios

of 0.16 (Mix 5) and 0.32 (Mix 8). This proved that this kaolin geo-

polymers formation was relied on the geopolymerization process

but not merely relied on the drying and hardening of the water-

glass binder.

Fig. 2 showed the workability of kaolin geopolymers measured

by using mini-slump cone. Workability is a property of fresh binder

which measures the ease of the fresh paste can be mixed, placed,

consolidated and finished. The slump values suggested the work-ability decreased with increasing S/L and Na2SiO3/NaOH ratios.

As S/L ratios were increased, there were more solid contents than

the fluid medium within the mixes. Thus, the mixes became stiffer.

At this time, the workability was also strongly depended on the

particle shape of the solid materials. The kaolin particles decreased

the workability due to the plate-like structure which resulted in

the inter-particle friction. On the other hand, with the increased

ofthe Na2SiO3/NaOH ratio, the sodiumsilicate content in the mixes

increased. Workability decreased as result of the highly viscous

property of sodium silicate than the NaOH solution [29].

Fig. 3 displayed the bulk densities of kaolin geopolymers at 7

and 28 days. The kaolin geopolymers showed bulk densities be-

tween 1250 kg/m3 and 1500 kg/m3. The kaolin geopolymers

showed no significant difference in bulk densities among them-selves; however kaolin geopolymers with optimum strength

exhibited slightly greater bulk densities. This supported that the

bulk density increased with the increasing mechanical strength

[30]. At 28 days, the bulk densities dropped which were partly re-

sulted from the loss of water due to drying [31].

3.2. Effect of solids-to-liquid ratio on compressive strength

Fig. 4 showed the compressive strength of kaolin-based geo-

polymers at various S/L ratios with constant Na2SiO3/NaOH ratio

of 0.24 up to 180 days. Overall result showed that compressive

strength increased with the ageing time. However, the strengths

achieved were quite low. This was because kaolin-based geopoly-

mers required more time to react in order to become hard solid.A weak structure was formed initially because of its nature of

low reactivity which contributes to the low rate of reaction devel-

opment [9,32].

Table 2

Detail of mixture proportions.

Mix no. NaOH molarity (M) S/L ratio Na2SiO3/NaOH ratio

1 8 0.60 0.24

2 8 0.80 0.24

3 8 1.00 0.24

4 8 1.20 0.24

5 8 1.00 0.16

6 8 1.00 0.207 8 1.00 0.28

8 8 1.00 0.32

9 8 1.00 0.36

914 C.Y. Heah et al. / Construction and Building Materials 35 (2012) 912922


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Compressive strength increased linearly from day 1 to day 3 [2].

Kaolin geopolymers with S/L ratio of 1.20 (Mix 4) recorded the

highest compressive strength while kaolin geopolymers with S/L

ratio of 0.60 (Mix 1) showed the lowest compressive strength. La-

ter strength gain after day 3 presented that the compressive

strength of kaolin geopolymers with S/L ratio of 1.00 (Mix 3) was

most ideal in terms of the strength and the workability of the geo-

polymer slurry. At high S/L ratio, the geopolymer slurry was less

workable [2]. Mix 4 (S/L ratio of 1.20) always has higher compres-

sive strengths than Mix 3 (S/L ratio of 1.00). Even so, S/L ratio of

1.20 was not chosen for the next experiment investigating the ef-

fect of Na2SiO3/NaOH ratios. This was owing to the very low work-

ability of the geopolymer slurry (Fig. 2a) which resulted in

difficulty in compaction during the molding process [14,17]. Forsamples with S/L ratio of 0.60 (Mix 1), the geopolymer slurry

was highly workable (Fig. 2a). The samples displayed no or very lit-

tle strength increment up to 180 days. In addition, the samples

appeared peeled off. This may probably due to the high amount

of activating solution which hinders the geopolymerization pro-

cess. The result concluded here was in agreement with the previ-

ous related studies conducted. In the study of Kong et al. [17]

based on metakaolin-based geopolymer, S/L ratio of 0.80 was used

in investigation as this ratio gave nearly optimum strength and

provided good workability. Higher S/L ratio than 0.80 has very

low workability and will deteriorate the properties of the paste

produced. In contrast, according to Xu and van Deventer [32], the

S/L ratio, by mass should be approximately 3 to produce fly ash-

based geopolymers to allow for geopolymerization process and

alkaline activator solution formed a thick gel instantaneously upon

mixing with source material.

On the other hand, kaolinite, the major mineral of kaolin, con-sisted of dioctahedral (1:1) layer silicate. The kaolinite layer com-

prised of Si2O52n sheet and Al(OH)3 sheet linked by sharing

bridging oxygen atom. The kaolinite layers were linked together

Fig. 1. Performance of kaolin geopolymers in water for (a) Mix 1, 1 day, (b) Mix 1, 7 days, (c) Mix 3, 180 days; (d) Mix 5, 180 days and (e) Mix 8, 180 days.

Fig. 2. Mini-slump flow values for kaolin geopolymer mixes at various (a) S/L ratios and (b) Na2SiO3/NaOH ratios.

Fig. 3. Bulk densities of kaolin geopolymers with various (a) S/L ratios and (b) Na2SiO3/NaOH ratios at 7 and 28 days.



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by hydrogen bonds [33]. Based on Davidovits [34], the alkaline at-

tack started from the outer surfaces of the kaolinite crystal and

continued layer by layer and from outside to inside. The attack

started by the division of Al-OH bond by the OH ions. So, when

low S/L ratio was employed, there was more fluid medium then

the solid content in the mix, the contact between the activating

solution and the reacting materials was far and limited due to

the large volume of fluid medium. The dissolution of aluminosili-

cate was believed slow. This further explained the reason for the

low compressive strength of kaolin geopolymers with S/L ratio of

0.60 (Mix 1). In contrast, when higher S/L ratio was used, the solid

contents increased. The contact between the activating solution

and the reacting materials were believed improved and hence

showed increase in the compressive strength measured.Therefore, 1.00 was chosen as the best S/L ratio and the S/L ratio

of a mixture design influenced the workability of the geopolymer

slurry and thus the compressive strength measured.

3.3. Effect of sodium silicate-to-sodium hydroxide ratio on

compressive strength

Fig. 5 represented the compressive strength of kaolin geopoly-

mers with various Na2SiO3/NaOH ratios at constant S/L ratio of

1.00. Sinusoidal fluctuated curve was observed. The strength re-

sults fluctuated as observed in day 1 to day 3 due to the different

reaction rate and formation of the structure within the tested ratio

mainly due to the fact that kaolinite has comparatively lower reac-

tivity which needed sufficient time for interactions to occur.The rate of reaction of kaolin depended on the supply of Si and

Al to the reacting gel [35]. The reaction of the kaolin could be

determined by the compressive strength measured. After 7 days,

a nearly stable parabolic curve observed. In overall, compressive

strength increased as the Na2SiO3/NaOH ratio was increased until

0.32 (Mix 8). Mix 8 showed steady and higher strength increment

among other ratios. As the Na2SiO3/NaOH ratio increased, the con-

tent of the waterglass liquid increased. This waterglass liquid was

required for the geopolymerization process and acted as binder, al-

kali activator and dispersant or plasticizer [9]. However, at higher

Na2SiO3/NaOH ratio than 0.32, it was believed that the high

amount of waterglass liquid may inhibit the geopolymerization

process.

Fig. 4. Compressive strength of geopolymer samples with various solids-to-liquid (S/L) ratios and constant Na2SiO3/NaOH ratio of 0.24.

Fig. 5. Compressive strength of geopolymer samples with various Na2SiO3/NaOH ratios (at constant S/L ratio of 1.00).

Fig. 6. SEM micrograph of kaolin.



7/12


Due to the low reactivity of the kaolin itself, kaolin faced prob-

lem owing to low dissolution of Al and Si ions into the system.

Thus, insufficient Al and Si ions were to be taken into reaction to

form a rigid network of geopolymers. As the Na 2SiO3/NaOH ratio

increased, high content of waterglass liquid caused the geopolymer

slurry became very sticky due to the viscous nature of the water-glass liquid. This could be clearly showed by the decreased in

slump values as the Na2SiO3/NaOH ratio was increased (Fig. 2b).

These even worsen the condition where the dissolved Al and Si

ions were unable to contact to each other. Thus, 0.32 was chosen

as the best Na2SiO3/NaOH ratio for kaolin geopolymers. In previous

research, the Na2SiO3/NaOH ratio by mass of 0.24 was reported for

metakaolin-based geopolymers [36,37]. However, a higher Na2-SiO3/NaOH mass ratio of 1.50 and 2.50 were reported by Chinda-

prasirt et al. [7] and Hardjito and Rangan [16], respectively for fly

ash-based geopolymers. It was believed that the Na2SiO3/NaOH ra-

tio chosen depended strongly on the workability of the mixes. Even

though both kaolin and metakaolin have plate-like morphology,

metakaolin-based geopolymers required lower Na2SiO3/NaOH ra-

tio than kaolin geopolymers due to metakaolin has higher waterdemands [38] as result of the increased in the surface are after cal-

cination [26,39]. Thus, metakaolin geopolymers required only a

lower mass ratio of Na2SiO3/NaOH (lower Na2SiO3 content) as high

water demands in addition to the increased in the sodium silicate

content declined significantly the workability of the mixes due to

the sticky nature as stated above. Nevertheless, higher ratio was

used for the fly ash-based geopolymers owing to the fly ash could

achieved better workability than kaolin and metakaolin. Spherical-

shaped particles of fly ash enhanced the workability of mixes by

reducing the inter-particle friction [40]. In addition, the spherical

shape minimized the particles surface to volume ratio, and thus

reducing the water demands [26]. Thus, increased in the sodium

silicate content would not affect significantly the workability of

fly ash geopolymers and hence, higher Na2SiO3/NaOH ratio could

be applied for fly ash geopolymers.

3.4. Effect of solids-to-liquid and sodium silicate-to-sodium hydroxide

ratios based on various molar ratios

Table 3 represented the summary of the compressive strength

of kaolin geopolymers based on SiO2/Al2O3, SiO2/Na2O, H2O/Na2O

and Al2O3/Na2O molar ratios. By concerning the S/L ratios, the

Al2O3/Na2O molar ratios increased when S/L ratios were increased.

In other words, the aluminosilicate sources increased (or alkali

activator liquid decreased) when the S/L ratios were increased.

The Al2O3 came from the aluminosilicate source while the Na2O

came from the alkali activator solution. In geopolymer chemistry,

every Al atom required one Na atom to reach equilibrium [6]. Atlow S/L ratio, Al2O3/Na2O molar ratio decreased, excess Na content

in the geopolymer system may weaken the structure formed. Con-

versely, the Na content was insufficient in the geopolymer system

at high S/L ratios (high Al2O3/Na2O molar ratio). The Na2O content

was very important in geopolymerization process. Na2O content

improved the solubility of the aluminosilicate source. However, ex-

cess in the Na2O content may degrade the strength of the geopoly-

mers produced.

In the contrary, increment in the Na2SiO3/NaOH ratios increasedthe SiO2/Na2O molar ratios. The compressive strength optimized at

SiO2/Na2O molar ratio of 3.58 and decreased for any further rise in

the SiO2/Na2O molar ratio. This meant that the SiO2 content was

important to provide the silicate species to allow for the rapid ex-

change and oligomerization reaction between the aluminate and

silicate species from the kaolin and the silicate species from the

activating solution [41]. At low SiO2/Na2O molar ratio (low Na2-SiO3/NaOH ratio), the Na2O was believed to be in excess as in the

case of low S/L ratio (low Al2O3/Na2O molar ratio). Thus, from

the study here, the optimum Al2O3/Na2O and SiO2/Na2O molar ra-

tios were 1.09 and 3.58, respectively. These optimum values fell

into range proposed by Davidovits [42] who stated that the compo-

sition of reactant mixture, in term of oxide molar ratios to prepare

geopolymers was the following: 3.57< SiO2/Na2O < 5 and0.83< Al2O3/Na2O < 1.25; The SiO2/Al2O3 molar ratio of the kaolin

geopolymers of 3.28 was also fell in range of 3.5 < SiO2/

Al2O3 < 4.5. The H2O/Na2O molar ratio fell out the range proposed,

that was 15 < H2O/Na2O < 17.5. However, Davidovits and Sawyer

[43] has also proposed the range for H2O/Na2O of 1025.0. Based

on other researchers doing on metakaolin geopolymers, the

Na2O/SiO2 ratio of 0.25, SiO2/Al2O3 ratio of 3.30, H2O/Na2O ratio

of 10.0, SiO2/Al2O3 ratio of 3.0 and Na2O/SiO3 ratio of 0.25 were

the optimum chemical composition. Based on a study on metaka-

olin geopolymers, best mechanical performance were achieved

when the ratio of SiO2/Al2O3 is 3.0 and Na2O/SiO3 ratio is 0.25 [44].

3.5. SEM observation

Scanning Electron Microscope (SEM) depicted morphological

features of geopolymer of different degree of reaction at different

ratios of activation medium. The micrographs were taken with

the objective of analyzing their microstructural evolution. Dense

in appearance reflected the advances of geopolymerization reac-

tion of samples activated with alkaline solution.

Fig. 6 revealed the microstructure of pure kaolin. Morphological

features of kaolin have plate-like structure [45,46] and this plate-

like structure contributed smaller surface area for geopolymeriza-

tion process. In addition, as stated above in Section 3.2, kaolin com-

prised of one tetrahedral silica sheet and one octahedral alumina

sheet, joined together by sharing a common layer of oxygen and

hydroxyls. These layers were strongly held together by the hydro-

gen bonding, supplemented by dipoledipole and van der waalsinteractions. Thus, the layers only allowed very little, if any, substi-

tution of other elements and have layer charge close to zero and

thus contributed to low reactivity [47,48]. These were the reason

Table 3

Effect of SiO2/Al2O3, SiO2/Na2O, H2O/Na2O and Al2O3/Na2O molar ratios on compressive strength.

Mix no. Molar ratios Compressive strength (MPa)

SiO2/Al2O3 SiO2/Na2O H2O/Na2O Al2O3/Na2O 1 day 3 day 7 day 28 day 60 day 90 day 180 day

1 3.41 2.16 14.36 0.63 0.20 0.18 0.15 0.18 0.35 0.46 0.40

2 3.28 2.78 14.36 0.85 0.86 1.09 1.18 1.48 1.78 2.44 3.98

3 3.20 3.39 14.36 1.06 1.42 1.74 2.38 2.65 3.10 4.20 5.34

4 3.15 4.00 14.36 1.27 2.06 2.21 2.29 2.56 2.99 4.60 5.81

5 3.11 3.19 14.09 1.02 2.03 1.71 2.10 1.83 2.28 3.06 3.27

6 3.16 3.29 14.23 1.04 1.31 1.84 1.30 2.88 3.80 3.27 4.487 3.24 3.48 14.49 1.07 1.48 1.56 1.77 1.91 2.98 5.15 5.72

8 3.28 3.58 14.61 1.09 1.80 1.68 2.65 3.38 3.93 5.22 6.05

9 3.32 3.67 14.73 1.11 1.53 1.43 1.23 2.26 3.40 3.61 4.57



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Fig. 7. Kaolin geopolymers at days 1, 7 and 60 of ageing for (a) Mix 2, (b) Mix 3 and (c) Mix 4 (constant Na 2SiO3/NaOH ratio of 0.24).

Fig. 8. Kaolin geopolymers at days 1, 7 and 60 of ageing for (a) Mix 5 (b) Mix 6 and (c) Mix 7 (constant S/L ratio of 1.00).



9/12


for the very low compressive strength achieved. For fly ash-based

geopolymers, high compressive strength could be achieved [49]

mainly due to the large surface area of spherical fly ash particles

which allowed for optimum dissolution by alkaline solution and

geopolymerization reaction [50]. After activated by alkali activat-

ing solution, the edges of the kaolin particles were slightly altered

[51].

Fig. 7 displayed the microstructures of kaolin geopolymers withvarious S/L ratios at constant Na2SiO3/NaOH ratio of 0.24 whereas

Fig. 8 showed the SEM micrographs of kaolin geopolymers with

various Na2SiO3/NaOH ratios at constant S/L ratio of 1.00. Both fig-

ures clearly showed that the kaolin has been activated by the acti-

vating solution with changes at the edges of the plate-like particles

and became more compact from day 1 to day 60. As shown in

Fig. 7, SEM micrograph of kaolin geopolymers with S/L ratio of

1.00 (Mix 3) has more geopolymeric gel than other samples. The

microstructure of kaolin geopolymers with S/L ratio of 1.20 (Mix

4) was nearly similar to that with S/L ratio of 1.00 (Mix 3), except

that it has larger precipitation. The solids ratio did not influence

the rate of alumino-silicates formation, but larger product precip-

itation was produced when the solids ratio increased. This was also

agreed by Chen et al. [52] where higher S/L ratio favored the forma-tion of precipitations. For Fig. 8, it could be observed that as the

Na2SiO3/NaOH ratio increased, the microstructure of the activated

kaolin become more compact, which further ascertained that the

presence of more waterglass liquid favored the geopolymer forma-

tion as stated above. Fig. 9 revealed the microstructure of kaolin

geopolymers at day 180. At longer age, micrograph revealed that

numerous kaolin particles being involved in the alkali activation,

and coexisting with partially reacted or unreacted particles. The

loosely grained structure contributed to the imperfect microstruc-

ture of kaolin geopolymers would be one of the main cause of poor

compressive strength obtained. Table 4 showed the mass percent-age of Si, Al and Na in the kaolin geopolymers as determined by

EDX analysis. The result of EDX suggested that the element of kao-

lin geopolymers were in good agreement with their respective

components. The kaolin geopolymers composed mainly of Al, Si

and Na. The kaolin geopolymer has Si/Al ratio of 1.50 (Fig. 9a)

and 1.60 (Fig. 9b).

In summary, the kaolin geopolymers does not undergo com-

plete geopolymerization and are not totally incorporated into the

polymeric structure. The low reactivity of kaolin due to the

plate-like and the layered structure, the dissolution of kaolin by

alkaline solution was extremely slow and caused slow develop-

ment of geopolymer gel and formation of strong structure. This

was in agreement with Xu and van Deventer [53] who stated that

the geopolymer structure using kaolinite source material aloneproduced weak structure. However, when the kaolinite was added

to the fly ash as minor portion, a strong and low cracking geopoly-

mer was formed. As kaolin needed longer time for the interaction

Fig. 10. XRD pattern of pure kaolin and kaolin geopolymers with different (a) x = S/L ratios and (b) y = Na2SiO3/NaOH ratios. D represents day of ageing (K = kaolinite;

A = Alunite; Q = Quartz and D = Dickite).

Fig. 9. Kaolin geopolymers at day 180 of ageing for (a) Mix 3 (at Na2SiO3/NaOH ratio of 0.32) and (b) Mix 7 (at S/L ratio of 1.0).



10/12


between the source materials, thus it was believed that the

strength will be increased if the unreacted part and fastened struc-

ture reacted to form denser structure.

3.6. XRD analysis

Fig. 10a showed the XRD pattern of geopolymer samples with

various S/L ratios while Fig. 10b revealed the XRD pattern of geo-

polymer samples with various Na2SiO3/NaOH ratios. Pure kaolin

comprised of kaolinite (K) as major mineral [51]. The characteristic

kaolinite peaks are at 2h values of 12.3, 19.8, 24.9, 45.4, 55.1

and 62.2 [12]. Besides this, kaolin also contained some dickite

(D) and quartz (Q). Alunite (A) could be found in trace amount.

These alunite mineral and especially quartz phase has been found

to be largely unreactive [13]. The reflection peak still remained in

the system and indicated that they did not take part in the geopo-

lymerization process, but their intensities were slightly lower due

to a dilution effect [54,55].

After geopolymerization process, a number of characteristic

kaolinite peaks was remained in diffractogram of geopolymer sam-

ples due to the lower activities of pure kaolin [33]. However, these

kaolinite peaks decreased in intensity in all geopolymer products

and the intensity continued to decrease with ageing days as shown

in Fig. 10 (spectra 1, 4, 6 and 7 in Fig. 10a and spectra 1, 5, 6 and 7

in Fig. 10b). This meant that the dissolution increased with ageing

days. The position of halo (2h values between 20 and 30) shifted

to higher angular values (Fig. 10) indicating the formation of alka-

line aluminosilicate gel [13]. But, the halo was very small in the

kaolin geopolymers. According to Cristobal et al. [51], the activa-

tion of kaolin with alkaline solution will form an amorphous alu-

minosilicate gel which acted as precursor for zeolite formation.

Zeolite reflection peaks were found in all kaolin geopolymer pastes.

Peaks at 2h values of 14.0, 32.0, 35.0, 43.5, 50.5, 52.5, 59.0

and 61.0 most properly was corresponding to zeolite X (ICDD#

39-0218) [56] and will only appear in geopolymer samples. Ini-

tially, the zeolite X intensity was high but the peak lowered at

longer age [45].

For day 1, the XRD pattern for S/L ratio of 1.0 (Spectrum 2 in

Fig. 10a) was initially very similar to the raw material (Spectrum

1) due to early strength gain of geopolymer with only 1.42 MPa.

Similar observation was found in Na2SiO3/NaOH ratio of 0.32 for

day 1 (Spectrum 2 in Fig. 10b), where only 1.79 MPa has been re-

corded. At day 7, sample of Na2SiO3/NaOH ratio of 0.32 (Spectrum

5 in Fig. 10b) has overall lower intensities of characterized peak

compared to the other sample and obtained the highest strengthgain in the similar day, with 2.68 MPa. This situation was also ap-

peared in S/L ratio of 1.0 at day 7 (Spectrum 4 in Fig. 10a), with

2.38 MPa being recorded. Another similar observation obtained

from XRD pattern was that, all the geopolymer sample reduced

in intensities which becoming amorphous at longer age with high-

er strength gain around 56 MPa at day 180 for both cases.

In general, XRD pattern of geopolymer samples showed a large

part of unreacted materials remained. These unreacted parts con-

tributed to the low strength of geopolymer products. After alkaline

activation, the peak shifted slightly to the right which indicated the

occurrence of changes in the raw materials to geopolymers

products.

3.7. FT-IR analysis

Figs. 11 and 12 presented the IR spectra of kaolin geopolymers

with different S/L ratios and Na2SiO3/NaOH ratios, respectively.

Fig. 11. IR spectra of kaolin (1) and geopolymer products with S/L ratio of (2) Mix 3,

1 day; (3) Mix 2, 7 days; (4) Mix 3, 7 days; (5) Mix 4, 7 days; (6) Mix 3, 60 days and

(7) Mix 3, 180 days at constant Na2SiO3/NaOH ratio of 0.24.

Fig. 12. IR spectra of kaolin (1) and geopolymer products with Na2SiO3/NaOH ratio

of (2) Mix 8, 1 day; (3) Mix 6, 7 days; (4) Mix 3, 7 days; (5) Mix 8, 7 days; (6) Mix 8,

60 days and (7) Mix 8, 180 days at constant S/L ratio of 1.00.

Table 4

Mass percentage pure kaolin, and kaolin geopolymer using Energy Dispersive X-ray

analysis (EDX).

Mass percentage

and ratios

Pure kaolin

(Fig. 6)

Kaolin

geopolymers

(Fig. 9a)

Kaolin

geopolymers

(Fig. 9b)

Si 26.36 26.66 27.39

Al 18.33 17.79 17.12

Na 0.20 3.04 4.07



11/12


Transformation took place during the synthesis was indicated by

the different absorption frequencies of kaolin and the synthesized

geopolymers. In IR spectrum of kaolin, bands at 3688 cm1,

3617 cm1 and 1643 cm1 were corresponded to OH vibration.

The peak around 1113 cm1 was attributed to SiO symmetrical

stretching in tetrahedral, which vanished after geopolymerization

reaction. Also, a weak band of SiO symmetrically stretching vibra-

tion was observed at 640 cm1. Absorption at 790 cm1 and

749 cm

1

were assigned to SiOSi symmetrical stretching [57].Peakat 995 cm1 was assigned as alternating SiO and AlO bonds.

A shift of the asymmetric bending of the bonds OSiO and OAlO

to lower frequencies was confirmed by previous findings [34,58].

Band at 907 cm1 was the Al-OH bending mode. Another band at

544 cm1 was AlIVOSi, where Al3+ is in octahedral coordination

[57].

In geopolymer products, major bands were broad band at 3000

3500 cm1 and 16501655 cm1, which were the OH stretching

and OH bending, respectively. These peaks indicated the presence

of weak H2O bond absorbed in the surface or caught in the cavities

of structure [51,55]. From Fig. 11(2)(4) and Fig. 12(2)(5), it was

clear that the peaks at 907 cm1 were shifted to higher frequency

which was around 937 cm1, due to the alkalination from Al-OH

into Al-(OH)(). The geopolymer bonding was lesser; hence, the

strength was lower. In Fig. 11(5)(7) and Fig. 12(6)(7), the fre-

quency change to around 967 cm1 which was vibration of asym-

metric tension of SiOAl and AlOSi groups [51]. Table 5

represented the peak height in percentage transmittance of these

absorption peaks (SiOAl and AlOSi groups) over age. The per-

centage transmittance was measured directly from the FTIR curves.

The lower the percentage transmittance meant the greater the per-

centage absorption and hence the greater intensities of the peak.

From Table 4, it was obvious that the percentage transmittance

of the SiOT peak decreased over age for both S/L and Na2SiO3/

NaOH ratios. Thus, this peak increased in intensities over age

[59] and contributed to higher strength gain (Figs. 4 and 5). Besides

this, peak around 619662 cm1 which corresponded to zeolite

[51]. Chandrasekhar and Pramada [45] also reported that the

absorption peak near 670 cm1 was a typical band of zeolite X.

These peaks showed decrease in intensities at latter age. This

was clearly proved by XRD pattern (Fig. 10). It was believed that

crystalline phase transformed into amorphous phase over ageing

time. In general, large part of unreacted kaolin remained in the sys-

tem which was shown by peaks at 1400 cm1 and 530 cm1 corre-

sponded to asymmetric stretching vibration of SiO/AlO bonds

and AIIVOSi.

4. Conclusion

In this paper, the effect of S/L and Na2SiO3/NaOH ratios on the

properties of kaolin geopolymers was investigated. Results con-

cluded that kaolin geopolymers showed good volume stabilitywith no crack and disintegration in water. This proved the existing

of geopolymer bonding which hold the geopolymers from collapse.

Both S/L and Na2SiO3/NaOH ratios affected the workability of the

kaolin geopolymers. The workability of the kaolin geopolymers

mixes decreased with increasing in the ratios. The kaolin geopoly-

mers have relatively low bulk densities (12501500 kg/m3). The

bulk densities were consistent with the compressive strength mea-

sured. The bulk densities decreased from 7 to 28 days mainly due

to the loss of unbound water to the atmosphere during curing pro-cess. The S/L and Na2SiO3/NaOH ratios affected compressive

strength of kaolin geopolymers. However, the compressive

strengths achieved by the kaolin-based geopolymers were quite

low. The reason for the low compressive strength achieved was

the limitation due to the structure of kaolin with kaolinite stacks

and plate of low surface area. In addition, the strongly held alu-

mina and silica sheets avoid the cations exchange and the substitu-

tion of elements. In spite of this, the strength of kaolin

geopolymers increased with the ageing time but very slowly. From

the experimental results, the S/L ratio of 1.00 and Na2SiO3/NaOH

ratio of 0.32 were the optimum ratios for kaolin geopolymers. In

term of oxide molar ratios, the compressive strength was the best

when the Al2O3/Na2O and SiO2/Na2O molar ratios were 1.09 and

3.58, respectively. These values fell in the range proposed by manyresearchers. SEM analysis of kaolin geopolymers revealed the coex-

isting of both partially reacted or unreacted particles while EDX re-

sults of the reaction products confirmed the Si, Al and Na were the

main elements. XRD pattern showed that kaolinite peaks de-

creased in intensity in all geopolymer products and the intensity

continued to decrease with ageing days. The presences of crystal-

line peaks in the kaolin-based geopolymers were Zeolite X. FTIR

spectra emphasized the increased in the intensity of the absorption

peaks (SiOSi/SiOAl) and further ascertained the continuous

dissolution and polycondensation of the reacting materials over

age.

Geopolymers possessed the ability to resist chemical attack and

weathering and has long term durability. Kaolin geopolymers, like

other geopolymers, produced from the activation of kaolinite with

alkaline solution. The reaction products consisted SiOSi and Si

OT geopolymer bondings. Thus, it was believed that the kaolin

geopolymers should also durable. This was one of the characteris-

tic of geopolymer products. However, the actual durability of the

kaolin geopolymers should be tested in future work to improve

its applications in various fields.

Acknowledgment

The authors of the present work wish to acknowledge the

KACST for the support provided in this study through collaboration

between KACST UniMAP.

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