Construction and Building Materials 53 (2014) 503–510

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Construction and Building Materials

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Effects SiO2/Na2O molar ratio on mechanical properties and themicrostructure of nano-SiO2 metakaolin-based geopolymers

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.12.003

⇑ Corresponding author. Tel.: +886 3 9357400x7579; fax: +886 3 9364277.E-mail address: kllin@niu.edu.tw (K.-L. Lin).

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, Chinab Department of Environmental Engineering, National Ilan University, Ilan 26047,Taiwanc Department of Construction Engineering, National Taiwan University of Science and Technology, Taiwand Institute of Environmental Engineering and Management, National Taipei University of Technology, Taipei, Taiwane Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taiwan

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 2013Received in revised form 8 December 2013Accepted 11 December 2013

Keywords:GeopolymerMetakaolinNano-SiO2

SiO2/Na2O molar ratioCompressive strengthMicrostructure

a b s t r a c t

An alkali-activating solution plays an important role in dissolving Si and Al atoms to form geopolymerprecursors 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, compressivestrength, mercury intrusion porosimetry (MIP), Fourier-transform infrared spectroscopy (FTIR), and scan-ning electron microscopy (SEM) are investigated to study the properties of the geopolymers. The resultsshow that the compressive strength increased, and that the products of geopolymerization filled poreswith curing time, causing the microstructure to become dense. Moreover, a nano-SiO2 metakaolin-basedgeopolymer sample with a SiO2/Na2O ratio of 1.5 exhibited more strength, higher density, and less poros-ity. Applying the nano-SiO2 to a geopolymer enhances compactness, improves uniformity, and increasesstrength.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Geopolymers are a novel type of high-performance cementi-tious material, first developed by Joseph Davidovits in the 1970s.Geopolymerization involves a chemical reaction between alumino-silicate oxides and alkali-metal silicate solutions under highlyalkaline conditions, yielding amorphous-to-semicrystalline 3Dpolymeric structures consisting of Si–O–Al bonds [1]. The mainproperties of geopolymers are quick compressive strengthdevelopment, low permeability, resistance to acid attacks, goodresistance to freeze–thaw cycles, and a tendency to drasticallyreduce the mobility of most heavy-metal ions contained withinthe geopolymeric structure [2–4]. These characteristics enable

geopolymers to be promising building, high-strength, toxic-wasteimmobilizing, sealing, and temperature-resistant materials [5,6].Moreover, compared to Portland cement, geopolymers have signif-icant advantages in the preparation process. The preparation doesnot require high temperatures for calcining or sintering, meaningthat the polymerization reaction could be performed at room tem-perature. Almost no generation is created of NOx, SOx, and CO, andthe emission of CO2 is also significantly low [7].

Criado et al. suggested that the activation rate and the chemicalcomposition of the reaction product relied on the particle size andchemical composition, and included a type of aluminosilicatesource and activator concentration [8]. In the activation of alumi-nosilicate materials such as metakaolin, the nature of the activatorsolution plays a key role in determining the structural andmechanical performance. An alkali-activating solution is vital fordissolving Si and Al atoms to form geopolymer precursors, and

504 K. Gao et al. / Construction and Building Materials 53 (2014) 503–510

finally, aluminosilicate material. The NaOH concentration in theaqueous phase of the geopolymeric system acts on the dissolutionprocess and on the bonding of solid particles in the final structure[9]. The most relevant characteristics related to the alkali activatorare the type of alkaline salt (usually silicate or hydroxide) [10], themethod of addition of the alkaline component (as a solution or inthe solid state) [11], and the dosage of the alkali component.

Nanoparticles are particles between 1 and 100 nm that createvolume and surface effects. Nanoparticle preparation and applica-tion and their size and surface effects offer various benefits; theycan be used to develop high-performance polymers [12]. Nanosyn-thesis technology has been widely applied to develop novel mate-rials with better properties and to improve conventional materialproperties. Studies have been conducted on nanocoatings, high-strength structural materials, macromolecular-based nanocompos-ites, magnetic materials, optical materials, and bionic materials[13].

Recent studies have shown that nanoparticles have a high sur-face area and the potential for tremendous chemical reactivity. Alyet al. showed that adding 3 wt.% of nano-SiO2 accelerated pozzola-nic activity, improved workability, enhanced the strength anddurability of concrete [14]. Rodríguez et al. found that geopolymermotors produced using alternative nano-SiO2-based activatorsexhibited similar or better mechanical performance than thoseproduced using conventional activators [15]. To understand the ef-fects of nano-SiO2 on metakaolin-based geopolymers, this paperpresents a discussion of the effects of different SiO2/Na2O molar ra-tios (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 bymercury intrusion porosimetry (MIP), Fourier transform infraredspectroscopy (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 �Cfor 3 h. Before and after calcination, the proportions of kaolin and metakaolin were1.74 and 1.66, respectively (compared to kerosene). However, the pH values of kao-lin and metakaolin were 6.52 and 5.74, respectively. This is because dehydroxyla-tion during calcination makes metakaolin weakly acidic. The chemical

Table 1Chemical composition of raw materials.

Composition Kaolinite Metakaolin

SiO2 (%) 53.70 59.60Al2O3 (%) 37.88 38.00Fe2O3 (%) 0.88 1.30CaO (%) 0.20 0.24MgO (%) N.D. N.D.SO3 (%) N.D. 0.04Na2O (%) 0.04 0.04K2O (%) 0.34 0.32LOI (%) 6.96 0.46

N.D.: not detected.

Table 2Properties of nano-SiO2.

Properties Content of SiO2 (%) Phase Compaction

Content 99.9 Non-crystal white powder 0.14

Properties Impurity (%)

Cl Cu Al

Content 0.028 0.003 0.002

composition of raw materials was determined using X-ray fluorescence. The resultsshow that kaolin is mainly composed of SiO2 (53.7%) and Al2O3 (37.88%). Metakao-lin is composed of 59.6% SiO2 and 38% Al2O3. Table 1 shows the chemical composi-tion 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% pureNaOH was used. The nano-SiO2 used was a white powder. Table 2 shows that theSiO2 content was 99.9%, the average particle size was approximately 10 nm, andthe 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 tocool to room temperature. With constant stirring, nano-SiO2 was added to the alkaliactivator. 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 solu-tion was prepared 24 h before use to ensure that the activator component wasmixed uniformly. Table 3 shows the mixture proportions. A mechanical mixerwas 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). Sampleswere placed into an oven at 80 �C for 24 h. The compressive strength and the micro-structure 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, accordingto ASTM C109.

2. Setting time: The test methods determine the setting time of samples by meansof the Vicat needle according to the ASTM C191.

3. Mercury intrusion porosimetry (MIP): A Quantachrome Autoscan MercuryIntrusion Porosimeter was used, with intrusion pressures up to 60,000 psi. Byusing the Washburn equation, p ¼ � 2c cos h

r , the pore volume (V) and the corre-sponding radius (r) were synchronously plotted by an X–T plotter, under theassumption 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 performedusing an automated RIX 2000 spectrometer. Specimens were prepared forXRF analysis by mixing 0.4 g of sample and 4 g of 100 Spectroflux at a dilutionratio of 1:10. The homogenized mixtures were placed in Pt–Au crucibles beforebeing heated for 1 h at 1000 �C in an electrical furnace. The homogeneousmelted sample was recast into glass beads 2 mm thick and 32 mm in diameter.

5. FTIR: Fourier transformation infrared spectroscopy (FTIR) was carried out onsamples using a Elmer FTIR Spectrum L16000A Spectrometer and the KBr pellettechnique (1 mg powdered sample mixed with 150 mg KBr).

density (g/cm3) Average particle size (nm) Specific surface area (m2/g)

10 670

Ca Fe Mg Sn

0.002 0.001 0.001 0.001

10nm

Fig. 1. SEM of nano-SiO2.

Table 3Ratios of material in mixture.

SiO2/Na2O Addition of nano-SiO2 (%) Mixture proportion by weight (g)

Metakalinite Nano-SiO2 Na2SiO3 NaOH H2O

1.00 1 1000 10 575.1 158.3 350.81.25 1 1000 10 718.9 141.4 289.71.50 1 1000 10 862.7 124.5 228.51.75 1 1000 10 1006.5 107.6 167.32.00 1 1000 10 1150.3 90.7 106.2

K. Gao et al. / Construction and Building Materials 53 (2014) 503–510 505

6. SEM: the microstructure of the geopolymer was observed using an electronbeam from a Tescan Vega TS5136MM. For SEM analysis, samples were takenfrom specimens that had been fractured during compression testing andmounted in epoxy resin, polished and sputter coated with gold–palladiumalloy. The samples were vacuum-dried overnight prior to SEM. The beam wasapplied at 10 KeV.

3. Results and discussion

3.1. Compressive strength of nano-SiO2-metakaolin-basedgeopolymers with solid-to-liquid ratios

Fig. 2 shows the compressive strength of nano-SiO2-metakao-lin-based geopolymers at various solid-to-liquid ratios. This oc-curred because the solid component in the mixture was morethan a fluid medium with the increase in the ratio, which led toa reduced workability, exhibiting a decline in compressivestrength. Therefore, an optimal solid-to-liquid ratio exists for

Fig. 2. Compressive strength of nano-SiO2 metakaolin-ba

metakaolin geopolymers. When the solid-to-liquid ratio was rela-tively small, the dissolution of Si and Al ions was insufficient, itmade the generation of leading material too little, and then thepolycondensation level was relatively low. Panias et al. indicatedthat a relationship exists between polycondensation and the com-pressive strength of metakaolin geopolymers [16]. When a low so-lid-to-liquid ratio was used, there was more fluid medium thansolid content in the mixture, and contact between the activatingsolution and reacting materials was limited because of the largefluid volume. The dissolution rate of aluminosilicate was slow. Thisexplains the low compressive strength of geopolymers with a so-lid-to-liquid ratio of 0.97. By contrast, when a higher solid-to-li-quid ratio was used, the solid content increased. Contactbetween the activating solution and reacting materials improvedand increased the compressive strength [17]. The compressivestrength became lower as the solid-to-liquid ratio continued to in-crease. The main reason was solid component in mixture was more

sed geopolymers with various solid-to-liquid ratios.

506 K. Gao et al. / Construction and Building Materials 53 (2014) 503–510

than fluid medium with the increase of the ratio, which made theworkability reduced and paste soon harden, so that the compres-sive strength became weak. Therefore, an optimal solid-to-liquidratio exists for metakaolin geopolymers. Fig. 2 also shows thatthe compressive strength of the samples increased significantlyafter adding nano-SiO2. The best amount of nano-SiO2 to add is1% when the solid-to-liquid ratio is 1.03. Numerous unsaturatedbonds and different hydroxy bonding states may exist in thenano-SiO2 surface in an active and high free-energy state. This in-creases the speed and degree of polymerization. In addition, nano-particles could fill the paste pores, and thus, raise its strength. Thisexplains why adding nano-SiO2 to the paste improves the earlygeopolymer strength. When numerous nanoparticles were added,various unreacted nano-SiO2 particles reduced the compressivestrength because of their dispersion effect [18].

3.2. Compressive strength of nano-SiO2-metakaolin-basedgeopolymers with various SiO2/Na2O ratios

Fig. 3 shows the compressive strength of nano-SiO2-metakao-lin-based geopolymers at various SiO2/Na2O ratios. The resultsshow that the strength of geopolymers increased with the curingtime. The development of the early strength was quick and re-mained steady. Significant differences were noted in the compres-sive strength development of the geopolymers with various SiO2/Na2O ratios. The figure shows that the maximum compressivestrength was obtained when the SiO2/Na2O ratio was 1.50. Whenthe SiO2/Na2O ratio increased from 1.00 to 1.50, the compressivestrength of the metakaolin-based geopolymers gradually im-proved. A soluble silicate is an essential factor of the geopolymer-ization process. It provides the aqueous phase of the geopolymericsystem with the soluble-silicate species that are necessary for theinitiation of oligomer (e.g., monomers, dimmers, trimmers, and tet-ramers) formation, and consequently, for polycondensation [19].The increase of the SiO2/Na2O ratio in the geopolymeric systemcaused the dissolution of considerable aluminosilicates and thesubsequent gradual shift of the chemical system from the monosi-licate chains and cyclic trimers to species with larger rings andcomplex structures and polymers, resulting in the 3D polymericframework and increasing the mechanical properties of the result-ing geopolymeric materials.

Moreover, the increase of the NaOH concentration acceleratesthe dissolution rate of the Si and Al phases of metakaolin, improv-ing the effectiveness of the geopolymerization process. Increased Si

Fig. 3. Compressive strength of nano-SiO2-metakaolin-based geopolymers withvarious SiO2/Na2O ratios.

and Al contents in the aqueous phase of the geopolymeric systemare essential for the initiation of the formation of oligomeric pre-cursors and polycondensation, which is critical to strength devel-opment in the geopolymeric materials. Although the formation ofthe oligomeric precursors is enhanced by the increased contentsof Si and Al in the aqueous phase, which is caused by the increaseddissolution rates, it is inhibited under extremely high NaOH con-centrations (and a low SiO2/Na2O ratio) [20]. An excess of the OH�

concentration results in the early precipitation of aluminosilicategel, causing a lower compressive strength [21]. When the SiO2/Na2O ratio continued to increase from 1.50 to 2.00, the metakao-lin-based geopolymer strength decreased. When the SiO2/Na2Oratio increased, the high content of the sodium-silicate solutioncaused the geopolymer paste to become sticky because of theviscous nature of the sodium-silicate solution. The high amountof the sodium-silicate solution may inhibit the geopolymerizationprocess [17].

3.3. Setting time of nano-SiO2-metakaolin-based geopolymers

Fig. 4 and Table 4 show the initial and final setting times of thenano-SiO2 metakaolin-based geopolymers with various SiO2/Na2Oratios. The initial and final setting times increased significantlywhen the SiO2/Na2O ratio increased. The initial setting times ofthe 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 increasedwith the SiO2/Na2O ratio. Because of its viscous property, the pastebecame sticky, workability decreased, and the setting time grew.Previous literature indicated that the setting time could be con-trolled by the temperature and M2O (M = K or Na) content in thesystem [22]. In addition, the alkaline condition accelerated thereaction process of activation because the presence of OH�

Fig. 4. Initial and final setting time of nano-SiO2-metakaolin-based geopolymerswith various SiO2/Na2O ratios.

Table 4Setting time of nano-SiO2-metakaolin-based geopolymers with various SiO2/Na2Oratios.

SiO2/Na2O

Addition of Nano-SiO2

(%)Initial setting(min)

Final setting(min)

1.00 1 185 2251.25 1 225 2701.50 1 255 3001.75 1 300 3452.00 1 343 390

K. Gao et al. / Construction and Building Materials 53 (2014) 503–510 507

enhanced the dissolution of raw material and increased the solu-bility of the silica and alumina [23]. The small SiO2/Na2O ratio(i.e., high OH� concentration) contributed to the dissolution ofthe raw material and shortened the setting time; these resultswere consistent with those observed by Bernal. Moreover, addingthe nano-SiO2 reduced the initial and final setting times becauseof the surface effect caused by the small size and high surface en-ergy of the nanosilica and numerous atoms located at the surface.These surface atoms accelerated the reaction rate of the system be-cause of their high activity and instability [24]. In addition, thenanomaterial with a high specific surface area is combined with

Fig. 5. Cumulative pore volume and percentage of pore volume of nano-

Fig. 6. FTIR spectra of nano-SiO2-metakaolin-base

water first, so that the addition of the nanomaterial increases thepaste viscosity.

3.4. MIP results of nano-SiO2-metakaolin-based geopolymers

Polymerization generates the hydration products of the geo-polymers. These products increase with the reaction degree, fillingholes, and concentration of the structure. According to the IUPACpore radius classification, pores are classified as micropores,mecropores, macropores, and air voids or cracks. The aperture rangeof the micropores is <1 nm; mecropores: 1–25 nm; macropores:

SiO2-metakaolin-based geopolymers with various SiO2/Na2O ratios.

d geopolymers with various SiO2/Na2O ratios.

508 K. Gao et al. / Construction and Building Materials 53 (2014) 503–510

25–5000 nm; and voids/cracks: 5000–50,000 nm [25]. The MIPtechnique provides a better understanding of the effects of differ-ent SiO2/Na2O ratios and nano-SiO2 additions on the connectivityand capacity of the pore structure in the assessed geopolymers.Fig. 5 shows the cumulative pore volume and percentage of thepore volume of the nano-SiO2-metakaolin-based geopolymers atvarious SiO2/Na2O ratios. As shown in Fig. 5(a), the cumulativepore volume was low when the SiO2/Na2O ratio was 1.50. Thecumulative pore volume was smaller, and the structure of thepaste tended toward densification. The cumulative pore volumeof 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 werepresent into the aluminosilicate gel network. Macropores maytransform to mesopores with the polycondensation of the hydratedgels and the effect of the nano-SiO2 because of the filling of the lar-ger pores with the new reaction products. The literature indicatedthat pores larger than 200 nm in the geopolymer paste were likelyto be associated with the interfacial spaces between partially re-acted or unreacted raw material and the geopolymer gel [15].Fig. 5(b) shows that the proportion of the macropores decreasedwhen the SiO2/Na2O ratio of metakaolin-based geopolymers in-creased from 1.00 to 1.50. The percentage of mesopores was a lar-ger proportion of approximately 95% when the SiO2/Na2O ratio was1.50. However, the macropore volume increased so that the meso-pore volume declined when the SiO2/Na2O ratio continued to grow,

(a) SiO2/Na2O=1.00

(c) SiO2/Na2O=1.50

(e) SiO2/Na2O=2.00

50 µm

50 µm

50 µm

Fig. 7. SEM photographs of nano-SiO2-metakaolin-ba

which was consistent with the results obtained from the compres-sive strength.

3.5. FTIR analysis of nano-SiO2-metakaolin-based geopolymers

Fig. 6 presents the FTIR spectra of the nano-SiO2 metakaolin-based geopolymers with various SiO2/Na2O ratios at different cur-ing times. In the FTIR spectrum of the geopolymer product, bandsbetween 1650 and 1655 cm�1 are associated with OH vibrations,which are characteristic of weak H2O molecules that have beeneither surface absorbed or caught in the structure cavities [17].The apparent bond at approximately 470 cm�1 is attributed toSi–O–Si bending. The FTIR spectrum of the geopolymers shows adistinct intensity band between 1300 and 900 cm�1 associatedwith the Si–O–T asymmetric vibration. This bond is often used todetermine the degree of polymerization because this peak is moreobvious than the Si–O–Si bonding peak [26]. Adsorption at700 cm�1 is assigned to Si–O–Al bending, indicating that the maingeopolymer structure generated after the reaction between the sil-icon aluminates and the highly alkali solution was a bending Si–O–Al. A weak Al–O–T bending band was observed between 540 and555 cm�1. The band at approximately 1460 cm�1 is related to car-bonate formation because of alkali-metal hydroxide reacting withthe atmospheric CO2 [27]. Previous researchers have used differentSiO2/Na2O ratios (1.0–1.6) to synthesize metakaolin-based geo-polymers, and found that the wavenumber of the Si–O–T bonding

(b) SiO2/Na2O=1.25

(d) SiO2/Na2O=1.75

50 µm

50 µm

sed geopolymers with various SiO2/Na2O ratios.

(a) Curing Time=1 day (b) Curing Time= 7 days

(c) Curing Time=28 days (d) Curing Time=60 days

50 µm 50 µm

50 µm 50 µm

Fig. 8. SEM photographs of nano-SiO2-metakaolin-based geopolymers at different curing times.

K. Gao et al. / Construction and Building Materials 53 (2014) 503–510 509

increased with the SiO2/Na2O [28]. With the increase in curingtime, this bonding shifted to a high frequency, which indicates thatthe polymerization degree strengthened [23]. Bands near 800 cm�1

on the spectra for the initial metakaolin (corresponding to theAl(IV)–O vibration band) were also observed [29]. At a higherSiO2/Na2O ratio, this band peak is more obvious, which indicatedthat the geopolymer structure included substantial unreactedmetakaolin. The sequence of the spectra also shows that the bandcentered at approximately 460 cm�1 (Si–O bending) underwent aslight shift toward higher wavenumbers, consequent to the incor-poration of alumina in the geopolymer. This band shifts consider-ably toward higher wavenumbers for geopolymers composedonly 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 geopoly-mers with various SiO2/Na2O ratios (1.00–2.00) at 60 days. Themicrographs reveal that the specimens were homogeneous, andthat certain large particles and pores were embedded in the matrix.Considerable unreacted metakaolin particles were clearly observedin the samples with a high SiO2/Na2O ratio. With a SiO2/Na2O ratioincrease, the microstructure of the activated metakaolin-basedgeopolymer became compact, and the densification degree of thecementing material gradually increased. The structure of the spec-imen with a SiO2/Na2O ratio of 1.50 was more compact with fewerunreacted particles. This is consistent with the results of the previ-ous experiment. The metakaolin-based geopolymer with a SiO2/Na2O ratio of 1.50 exhibited the most strength, highest bulk den-sity, and lowest porosity.

Fig. 8 shows the microstructure of the nano-SiO2-metakaolin-based geopolymers with the same SiO2/Na2O ratio of 1.50 after dif-ferent curing times. The structure gradually became dense with theincrease in curing time. Heah indicated that with a growth withage, the microstructure showed that numerous kaolin particleswere activated by the alkali solution, and coexisted with the unre-acted particles [17]. This was consistent with the experimental re-sults. The unreacted metakaolin particles gradually became fewer,

the product was refined, and the structure became compact anduniform with the increase of curing time. A loosely grained struc-ture contributing to the imperfect microstructure of the metakao-lin geopolymers is one of the main causes of poor compressivestrength that was obtained.

4. Conclusions

This study demonstrates the effects of the SiO2/Na2O ratio onnano-SiO2-metakaolin-based geopolymers. Based on the resultsof the setting experiment, the setting time of the nano-SiO2-metakaolin-based geopolymer increased with the SiO2/Na2O ratiobecause of the viscous property of the sodium silicate. When theSiO2/Na2O ratio of metakaolin-based geopolymers was 1.50, thesample exhibited a smaller cumulative pore volume; consequently,its compressive strength was greater. The compressive strength ofthe geopolymers increased with the curing time. Early strengthdeveloped rapidly and steadily. The FTIR spectrum of the geopoly-mer product contained a distinct intensity band at 1300–900 cm�1

associated with the Si–O–T asymmetric vibration. This bond is of-ten used to determine the degree of polymerization. SEM photo-graphs show that the microstructure of the geopolymer is morecompact with fewer unreacted particles when the SiO2/Na2O ratiois 1.50. These benefit structural development. The results alsoshowed that nanotechnology geopolymers have many potentialapplications.

Acknowledgement

The authors would like to thank the National Science Council ofthe Republic of China, Taiwan, for financially supporting this re-search under Contract Nos. NSC 99-2622-E-197-003-CC and 101-2923-I-011-001-MY4.

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