Construction and Building Materials 53 (2014) 503–510
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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Effects SiO2/Na2O molar ratio on mechanical properties and the
microstructure of nano-SiO2 metakaolin-based geopolymers
Kang Gao a, Kae-Long Lin b,⇑, DeYing Wang a, Chao-Lung Hwang c, Hau-Shing Shiu b, Yu-Min Chang d,
Ta-Wui Cheng e
a
Department of Environmental and Material Engineering, Yan-Tai University, China
Department of Environmental Engineering, National Ilan University, Ilan 26047,Taiwan
Department of Construction Engineering, National Taiwan University of Science and Technology, Taiwan
d
Institute of Environmental Engineering and Management, National Taipei University of Technology, Taipei, Taiwan
e
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taiwan
b
c
h i g h l i g h t s
Applying the nano-SiO2 to a geopolymer enhances strength.
Geopolymerization filled pores, causing the microstructure to become dense.
SiO2/Na2O ratio of 1.5 exhibited more strength and less porosity.
Nanotechnology geopolymers have many potential applications.
a r t i c l e
i n f o
Article history:
Received 23 August 2013
Received in revised form 8 December 2013
Accepted 11 December 2013
Keywords:
Geopolymer
Metakaolin
Nano-SiO2
SiO2/Na2O molar ratio
Compressive strength
Microstructure
a b s t r a c t
An alkali-activating solution plays an important role in dissolving Si and Al atoms to form geopolymer
precursors and aluminosilicate material. This study explores the effects of the SiO2/Na2O molar ratio
(1.0–2.0) on the properties of nano-SiO2 metakaolin-based geopolymers. The setting time, compressive
strength, mercury intrusion porosimetry (MIP), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) are investigated to study the properties of the geopolymers. The results
show that the compressive strength increased, and that the products of geopolymerization filled pores
with curing time, causing the microstructure to become dense. Moreover, a nano-SiO2 metakaolin-based
geopolymer sample with a SiO2/Na2O ratio of 1.5 exhibited more strength, higher density, and less porosity. Applying the nano-SiO2 to a geopolymer enhances compactness, improves uniformity, and increases
strength.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Geopolymers are a novel type of high-performance cementitious material, first developed by Joseph Davidovits in the 1970s.
Geopolymerization involves a chemical reaction between aluminosilicate oxides and alkali-metal silicate solutions under highly
alkaline conditions, yielding amorphous-to-semicrystalline 3D
polymeric structures consisting of Si–O–Al bonds [1]. The main
properties of geopolymers are quick compressive strength
development, low permeability, resistance to acid attacks, good
resistance to freeze–thaw cycles, and a tendency to drastically
reduce the mobility of most heavy-metal ions contained within
the geopolymeric structure [2–4]. These characteristics enable
⇑ Corresponding author. Tel.: +886 3 9357400x7579; fax: +886 3 9364277.
E-mail address: [email protected] (K.-L. Lin).
0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.conbuildmat.2013.12.003
geopolymers to be promising building, high-strength, toxic-waste
immobilizing, sealing, and temperature-resistant materials [5,6].
Moreover, compared to Portland cement, geopolymers have significant advantages in the preparation process. The preparation does
not require high temperatures for calcining or sintering, meaning
that the polymerization reaction could be performed at room temperature. Almost no generation is created of NOx, SOx, and CO, and
the emission of CO2 is also significantly low [7].
Criado et al. suggested that the activation rate and the chemical
composition of the reaction product relied on the particle size and
chemical composition, and included a type of aluminosilicate
source and activator concentration [8]. In the activation of aluminosilicate materials such as metakaolin, the nature of the activator
solution plays a key role in determining the structural and
mechanical performance. An alkali-activating solution is vital for
dissolving Si and Al atoms to form geopolymer precursors, and
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K. Gao et al. / Construction and Building Materials 53 (2014) 503–510
finally, aluminosilicate material. The NaOH concentration in the
aqueous phase of the geopolymeric system acts on the dissolution
process and on the bonding of solid particles in the final structure
[9]. The most relevant characteristics related to the alkali activator
are the type of alkaline salt (usually silicate or hydroxide) [10], the
method of addition of the alkaline component (as a solution or in
the solid state) [11], and the dosage of the alkali component.
Nanoparticles are particles between 1 and 100 nm that create
volume and surface effects. Nanoparticle preparation and application and their size and surface effects offer various benefits; they
can be used to develop high-performance polymers [12]. Nanosynthesis technology has been widely applied to develop novel materials with better properties and to improve conventional material
properties. Studies have been conducted on nanocoatings, highstrength structural materials, macromolecular-based nanocomposites, magnetic materials, optical materials, and bionic materials
[13].
Recent studies have shown that nanoparticles have a high surface area and the potential for tremendous chemical reactivity. Aly
et al. showed that adding 3 wt.% of nano-SiO2 accelerated pozzolanic activity, improved workability, enhanced the strength and
durability of concrete [14]. Rodríguez et al. found that geopolymer
motors produced using alternative nano-SiO2-based activators
exhibited similar or better mechanical performance than those
produced using conventional activators [15]. To understand the effects of nano-SiO2 on metakaolin-based geopolymers, this paper
presents a discussion of the effects of different SiO2/Na2O molar ratios (1.00–2.00) on the nano-SiO2-metakaolin-based geopolymers.
The setting time and compressive strength were determined.
Moreover, the microstructure of all samples was determined by
mercury intrusion porosimetry (MIP), Fourier transform infrared
spectroscopy (FTIR), and scanning electron microscopy (SEM).
2. Experimental materials and methods
2.1. Materials
The metakaolin used in this study was obtained from kaolin calcined at 650 °C
for 3 h. Before and after calcination, the proportions of kaolin and metakaolin were
1.74 and 1.66, respectively (compared to kerosene). However, the pH values of kaolin and metakaolin were 6.52 and 5.74, respectively. This is because dehydroxylation during calcination makes metakaolin weakly acidic. The chemical
composition of raw materials was determined using X-ray fluorescence. The results
show that kaolin is mainly composed of SiO2 (53.7%) and Al2O3 (37.88%). Metakaolin is composed of 59.6% SiO2 and 38% Al2O3. Table 1 shows the chemical composition of the raw materials.
The Na2SiO3 solution was produced in Taiwan. It was composed of 29.5% SiO2,
9.1% Na2O, 61.4% H2O, and the Ms (SiO2/Na2O) was 3.4. Analytical-grade 99% pure
NaOH was used. The nano-SiO2 used was a white powder. Table 2 shows that the
SiO2 content was 99.9%, the average particle size was approximately 10 nm, and
the specific surface area was 670 m2/g. Fig. 1 shows an SEM image of the nano-SiO2.
2.2. Sample preparation
NaOH and distilled water were mixed with a Na2SiO3 solution and allowed to
cool to room temperature. With constant stirring, nano-SiO2 was added to the alkali
activator. The solid-to-liquid ratios were 1.03 and the addition of nanosilica was 1%.
The SiO2/Na2O ratios were 1.00, 1.25, 1.50, 1.75 and 2.00. The alkali activator solution was prepared 24 h before use to ensure that the activator component was
mixed uniformly. Table 3 shows the mixture proportions. A mechanical mixer
was used to mix the activator solution and 1000 g metakaolin for a few minutes.
The fresh paste was then rapidly poured into plastic molds (5 5 5 cm). Samples
were placed into an oven at 80 °C for 24 h. The compressive strength and the microstructure performance of specimens were tested after 1, 7, 14, 28 and 60 days.
2.3. Analytical methods
1. Compressive strength: Compressive strength tests were performed after 1, 7,
14, 28 and 60 days using a 50 mm 50 mm 50 mm cubic sample, according
to ASTM C109.
2. Setting time: The test methods determine the setting time of samples by means
of the Vicat needle according to the ASTM C191.
3. Mercury intrusion porosimetry (MIP): A Quantachrome Autoscan Mercury
Intrusion Porosimeter was used, with intrusion pressures up to 60,000 psi. By
h
using the Washburn equation, p ¼ 2c cos
, the pore volume (V) and the correr
sponding radius (r) were synchronously plotted by an X–T plotter, under the
assumption that mercury wetting angle is h = 140°. In this equation, p, c, r, h,
stand for the applied pressure, surface tension, pore radius and wetting angle,
respectively.
4. Chemical composition: The X-ray fluorescence (XRF) analysis was performed
using an automated RIX 2000 spectrometer. Specimens were prepared for
XRF analysis by mixing 0.4 g of sample and 4 g of 100 Spectroflux at a dilution
ratio of 1:10. The homogenized mixtures were placed in Pt–Au crucibles before
being heated for 1 h at 1000 °C in an electrical furnace. The homogeneous
melted sample was recast into glass beads 2 mm thick and 32 mm in diameter.
5. FTIR: Fourier transformation infrared spectroscopy (FTIR) was carried out on
samples using a Elmer FTIR Spectrum L16000A Spectrometer and the KBr pellet
technique (1 mg powdered sample mixed with 150 mg KBr).
Table 1
Chemical composition of raw materials.
10nm
Composition
Kaolinite
Metakaolin
SiO2 (%)
Al2O3 (%)
Fe2O3 (%)
CaO (%)
MgO (%)
SO3 (%)
Na2O (%)
K2O (%)
LOI (%)
53.70
37.88
0.88
0.20
N.D.
N.D.
0.04
0.34
6.96
59.60
38.00
1.30
0.24
N.D.
0.04
0.04
0.32
0.46
N.D.: not detected.
Fig. 1. SEM of nano-SiO2.
Table 2
Properties of nano-SiO2.
Properties
Content of SiO2 (%)
Phase
Compaction density (g/cm3)
Average particle size (nm)
Specific surface area (m2/g)
Content
99.9
Non-crystal white powder
0.14
10
670
Properties
Content
Impurity (%)
Cl
Cu
Al
Ca
Fe
Mg
Sn
0.028
0.003
0.002
0.002
0.001
0.001
0.001
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K. Gao et al. / Construction and Building Materials 53 (2014) 503–510
Table 3
Ratios of material in mixture.
SiO2/Na2O
Addition of nano-SiO2 (%)
1.00
1.25
1.50
1.75
2.00
1
1
1
1
1
Mixture proportion by weight (g)
Metakalinite
Nano-SiO2
Na2SiO3
NaOH
H2O
1000
1000
1000
1000
1000
10
10
10
10
10
575.1
718.9
862.7
1006.5
1150.3
158.3
141.4
124.5
107.6
90.7
350.8
289.7
228.5
167.3
106.2
6. SEM: the microstructure of the geopolymer was observed using an electron
beam from a Tescan Vega TS5136MM. For SEM analysis, samples were taken
from specimens that had been fractured during compression testing and
mounted in epoxy resin, polished and sputter coated with gold–palladium
alloy. The samples were vacuum-dried overnight prior to SEM. The beam was
applied at 10 KeV.
3. Results and discussion
3.1. Compressive strength of nano-SiO2-metakaolin-based
geopolymers with solid-to-liquid ratios
Fig. 2 shows the compressive strength of nano-SiO2-metakaolin-based geopolymers at various solid-to-liquid ratios. This occurred because the solid component in the mixture was more
than a fluid medium with the increase in the ratio, which led to
a reduced workability, exhibiting a decline in compressive
strength. Therefore, an optimal solid-to-liquid ratio exists for
metakaolin geopolymers. When the solid-to-liquid ratio was relatively small, the dissolution of Si and Al ions was insufficient, it
made the generation of leading material too little, and then the
polycondensation level was relatively low. Panias et al. indicated
that a relationship exists between polycondensation and the compressive strength of metakaolin geopolymers [16]. When a low solid-to-liquid ratio was used, there was more fluid medium than
solid content in the mixture, and contact between the activating
solution and reacting materials was limited because of the large
fluid volume. The dissolution rate of aluminosilicate was slow. This
explains the low compressive strength of geopolymers with a solid-to-liquid ratio of 0.97. By contrast, when a higher solid-to-liquid ratio was used, the solid content increased. Contact
between the activating solution and reacting materials improved
and increased the compressive strength [17]. The compressive
strength became lower as the solid-to-liquid ratio continued to increase. The main reason was solid component in mixture was more
Fig. 2. Compressive strength of nano-SiO2 metakaolin-based geopolymers with various solid-to-liquid ratios.
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K. Gao et al. / Construction and Building Materials 53 (2014) 503–510
than fluid medium with the increase of the ratio, which made the
workability reduced and paste soon harden, so that the compressive strength became weak. Therefore, an optimal solid-to-liquid
ratio exists for metakaolin geopolymers. Fig. 2 also shows that
the compressive strength of the samples increased significantly
after adding nano-SiO2. The best amount of nano-SiO2 to add is
1% when the solid-to-liquid ratio is 1.03. Numerous unsaturated
bonds and different hydroxy bonding states may exist in the
nano-SiO2 surface in an active and high free-energy state. This increases the speed and degree of polymerization. In addition, nanoparticles could fill the paste pores, and thus, raise its strength. This
explains why adding nano-SiO2 to the paste improves the early
geopolymer strength. When numerous nanoparticles were added,
various unreacted nano-SiO2 particles reduced the compressive
strength because of their dispersion effect [18].
3.2. Compressive strength of nano-SiO2-metakaolin-based
geopolymers with various SiO2/Na2O ratios
and Al contents in the aqueous phase of the geopolymeric system
are essential for the initiation of the formation of oligomeric precursors and polycondensation, which is critical to strength development in the geopolymeric materials. Although the formation of
the oligomeric precursors is enhanced by the increased contents
of Si and Al in the aqueous phase, which is caused by the increased
dissolution rates, it is inhibited under extremely high NaOH concentrations (and a low SiO2/Na2O ratio) [20]. An excess of the OH
concentration results in the early precipitation of aluminosilicate
gel, causing a lower compressive strength [21]. When the SiO2/
Na2O ratio continued to increase from 1.50 to 2.00, the metakaolin-based geopolymer strength decreased. When the SiO2/Na2O
ratio increased, the high content of the sodium-silicate solution
caused the geopolymer paste to become sticky because of the
viscous nature of the sodium-silicate solution. The high amount
of the sodium-silicate solution may inhibit the geopolymerization
process [17].
3.3. Setting time of nano-SiO2-metakaolin-based geopolymers
Fig. 3 shows the compressive strength of nano-SiO2-metakaolin-based geopolymers at various SiO2/Na2O ratios. The results
show that the strength of geopolymers increased with the curing
time. The development of the early strength was quick and remained steady. Significant differences were noted in the compressive strength development of the geopolymers with various SiO2/
Na2O ratios. The figure shows that the maximum compressive
strength was obtained when the SiO2/Na2O ratio was 1.50. When
the SiO2/Na2O ratio increased from 1.00 to 1.50, the compressive
strength of the metakaolin-based geopolymers gradually improved. A soluble silicate is an essential factor of the geopolymerization process. It provides the aqueous phase of the geopolymeric
system with the soluble-silicate species that are necessary for the
initiation of oligomer (e.g., monomers, dimmers, trimmers, and tetramers) formation, and consequently, for polycondensation [19].
The increase of the SiO2/Na2O ratio in the geopolymeric system
caused the dissolution of considerable aluminosilicates and the
subsequent gradual shift of the chemical system from the monosilicate chains and cyclic trimers to species with larger rings and
complex structures and polymers, resulting in the 3D polymeric
framework and increasing the mechanical properties of the resulting geopolymeric materials.
Moreover, the increase of the NaOH concentration accelerates
the dissolution rate of the Si and Al phases of metakaolin, improving the effectiveness of the geopolymerization process. Increased Si
Fig. 4 and Table 4 show the initial and final setting times of the
nano-SiO2 metakaolin-based geopolymers with various SiO2/Na2O
ratios. The initial and final setting times increased significantly
when the SiO2/Na2O ratio increased. The initial setting times of
the nano-SiO2 samples at SiO2/Na2O ratios of 1.00, 1.25, 1.50,
1.75, and 2.00 were 185, 225, 255, 300, and 343 min, respectively,
and their final setting times were 225, 270, 300, 345, and 390 min,
respectively. The content of the sodium silicate solution increased
with the SiO2/Na2O ratio. Because of its viscous property, the paste
became sticky, workability decreased, and the setting time grew.
Previous literature indicated that the setting time could be controlled by the temperature and M2O (M = K or Na) content in the
system [22]. In addition, the alkaline condition accelerated the
reaction process of activation because the presence of OH
Fig. 4. Initial and final setting time of nano-SiO2-metakaolin-based geopolymers
with various SiO2/Na2O ratios.
Table 4
Setting time of nano-SiO2-metakaolin-based geopolymers with various SiO2/Na2O
ratios.
Fig. 3. Compressive strength of nano-SiO2-metakaolin-based geopolymers with
various SiO2/Na2O ratios.
SiO2/
Na2O
Addition of Nano-SiO2
(%)
Initial setting
(min)
Final setting
(min)
1.00
1.25
1.50
1.75
2.00
1
1
1
1
1
185
225
255
300
343
225
270
300
345
390
K. Gao et al. / Construction and Building Materials 53 (2014) 503–510
enhanced the dissolution of raw material and increased the solubility of the silica and alumina [23]. The small SiO2/Na2O ratio
(i.e., high OH concentration) contributed to the dissolution of
the raw material and shortened the setting time; these results
were consistent with those observed by Bernal. Moreover, adding
the nano-SiO2 reduced the initial and final setting times because
of the surface effect caused by the small size and high surface energy of the nanosilica and numerous atoms located at the surface.
These surface atoms accelerated the reaction rate of the system because of their high activity and instability [24]. In addition, the
nanomaterial with a high specific surface area is combined with
507
water first, so that the addition of the nanomaterial increases the
paste viscosity.
3.4. MIP results of nano-SiO2-metakaolin-based geopolymers
Polymerization generates the hydration products of the geopolymers. These products increase with the reaction degree, filling
holes, and concentration of the structure. According to the IUPAC
pore radius classification, pores are classified as micropores,
mecropores, macropores, and air voids or cracks. The aperture range
of the micropores is <1 nm; mecropores: 1–25 nm; macropores:
Fig. 5. Cumulative pore volume and percentage of pore volume of nano-SiO2-metakaolin-based geopolymers with various SiO2/Na2O ratios.
Fig. 6. FTIR spectra of nano-SiO2-metakaolin-based geopolymers with various SiO2/Na2O ratios.
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K. Gao et al. / Construction and Building Materials 53 (2014) 503–510
25–5000 nm; and voids/cracks: 5000–50,000 nm [25]. The MIP
technique provides a better understanding of the effects of different SiO2/Na2O ratios and nano-SiO2 additions on the connectivity
and capacity of the pore structure in the assessed geopolymers.
Fig. 5 shows the cumulative pore volume and percentage of the
pore volume of the nano-SiO2-metakaolin-based geopolymers at
various SiO2/Na2O ratios. As shown in Fig. 5(a), the cumulative
pore volume was low when the SiO2/Na2O ratio was 1.50. The
cumulative pore volume was smaller, and the structure of the
paste tended toward densification. The cumulative pore volume
of the specimen at other proportions was larger when the SiO2/
Na2O ratio was 1.50 and had a wider range of pore distribution,
so that the compressive strength was weaker. Mesopores were
present into the aluminosilicate gel network. Macropores may
transform to mesopores with the polycondensation of the hydrated
gels and the effect of the nano-SiO2 because of the filling of the larger pores with the new reaction products. The literature indicated
that pores larger than 200 nm in the geopolymer paste were likely
to be associated with the interfacial spaces between partially reacted or unreacted raw material and the geopolymer gel [15].
Fig. 5(b) shows that the proportion of the macropores decreased
when the SiO2/Na2O ratio of metakaolin-based geopolymers increased from 1.00 to 1.50. The percentage of mesopores was a larger proportion of approximately 95% when the SiO2/Na2O ratio was
1.50. However, the macropore volume increased so that the mesopore volume declined when the SiO2/Na2O ratio continued to grow,
which was consistent with the results obtained from the compressive strength.
3.5. FTIR analysis of nano-SiO2-metakaolin-based geopolymers
Fig. 6 presents the FTIR spectra of the nano-SiO2 metakaolinbased geopolymers with various SiO2/Na2O ratios at different curing times. In the FTIR spectrum of the geopolymer product, bands
between 1650 and 1655 cm1 are associated with OH vibrations,
which are characteristic of weak H2O molecules that have been
either surface absorbed or caught in the structure cavities [17].
The apparent bond at approximately 470 cm1 is attributed to
Si–O–Si bending. The FTIR spectrum of the geopolymers shows a
distinct intensity band between 1300 and 900 cm1 associated
with the Si–O–T asymmetric vibration. This bond is often used to
determine the degree of polymerization because this peak is more
obvious than the Si–O–Si bonding peak [26]. Adsorption at
700 cm1 is assigned to Si–O–Al bending, indicating that the main
geopolymer structure generated after the reaction between the silicon aluminates and the highly alkali solution was a bending Si–O–
Al. A weak Al–O–T bending band was observed between 540 and
555 cm1. The band at approximately 1460 cm1 is related to carbonate formation because of alkali-metal hydroxide reacting with
the atmospheric CO2 [27]. Previous researchers have used different
SiO2/Na2O ratios (1.0–1.6) to synthesize metakaolin-based geopolymers, and found that the wavenumber of the Si–O–T bonding
50 µm
50 µm
(b) SiO2/Na2O=1.25
(a) SiO2/Na2O=1.00
50 µm
50 µm
(d) SiO2/Na2O=1.75
(c) SiO2/Na2O=1.50
50 µm
(e) SiO2/Na2O=2.00
Fig. 7. SEM photographs of nano-SiO2-metakaolin-based geopolymers with various SiO2/Na2O ratios.
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K. Gao et al. / Construction and Building Materials 53 (2014) 503–510
50 µm
50 µm
(b) Curing Time= 7 days
(a) Curing Time=1 day
50 µm
(c) Curing Time=28 days
50 µm
(d) Curing Time=60 days
Fig. 8. SEM photographs of nano-SiO2-metakaolin-based geopolymers at different curing times.
increased with the SiO2/Na2O [28]. With the increase in curing
time, this bonding shifted to a high frequency, which indicates that
the polymerization degree strengthened [23]. Bands near 800 cm1
on the spectra for the initial metakaolin (corresponding to the
Al(IV)–O vibration band) were also observed [29]. At a higher
SiO2/Na2O ratio, this band peak is more obvious, which indicated
that the geopolymer structure included substantial unreacted
metakaolin. The sequence of the spectra also shows that the band
centered at approximately 460 cm1 (Si–O bending) underwent a
slight shift toward higher wavenumbers, consequent to the incorporation of alumina in the geopolymer. This band shifts considerably toward higher wavenumbers for geopolymers composed
only of SiO4 tetrahedra [27].
3.6. SEM observation of the nano-SiO2 metakaolin-based geopolymers
Fig. 7 shows the SEM images of the metakaolin-based geopolymers with various SiO2/Na2O ratios (1.00–2.00) at 60 days. The
micrographs reveal that the specimens were homogeneous, and
that certain large particles and pores were embedded in the matrix.
Considerable unreacted metakaolin particles were clearly observed
in the samples with a high SiO2/Na2O ratio. With a SiO2/Na2O ratio
increase, the microstructure of the activated metakaolin-based
geopolymer became compact, and the densification degree of the
cementing material gradually increased. The structure of the specimen with a SiO2/Na2O ratio of 1.50 was more compact with fewer
unreacted particles. This is consistent with the results of the previous experiment. The metakaolin-based geopolymer with a SiO2/
Na2O ratio of 1.50 exhibited the most strength, highest bulk density, and lowest porosity.
Fig. 8 shows the microstructure of the nano-SiO2-metakaolinbased geopolymers with the same SiO2/Na2O ratio of 1.50 after different curing times. The structure gradually became dense with the
increase in curing time. Heah indicated that with a growth with
age, the microstructure showed that numerous kaolin particles
were activated by the alkali solution, and coexisted with the unreacted particles [17]. This was consistent with the experimental results. The unreacted metakaolin particles gradually became fewer,
the product was refined, and the structure became compact and
uniform with the increase of curing time. A loosely grained structure contributing to the imperfect microstructure of the metakaolin geopolymers is one of the main causes of poor compressive
strength that was obtained.
4. Conclusions
This study demonstrates the effects of the SiO2/Na2O ratio on
nano-SiO2-metakaolin-based geopolymers. Based on the results
of the setting experiment, the setting time of the nano-SiO2metakaolin-based geopolymer increased with the SiO2/Na2O ratio
because of the viscous property of the sodium silicate. When the
SiO2/Na2O ratio of metakaolin-based geopolymers was 1.50, the
sample exhibited a smaller cumulative pore volume; consequently,
its compressive strength was greater. The compressive strength of
the geopolymers increased with the curing time. Early strength
developed rapidly and steadily. The FTIR spectrum of the geopolymer product contained a distinct intensity band at 1300–900 cm1
associated with the Si–O–T asymmetric vibration. This bond is often used to determine the degree of polymerization. SEM photographs show that the microstructure of the geopolymer is more
compact with fewer unreacted particles when the SiO2/Na2O ratio
is 1.50. These benefit structural development. The results also
showed that nanotechnology geopolymers have many potential
applications.
Acknowledgement
The authors would like to thank the National Science Council of
the Republic of China, Taiwan, for financially supporting this research under Contract Nos. NSC 99-2622-E-197-003-CC and 1012923-I-011-001-MY4.
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