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U.P.B. Sci. Bull., Series B, Vol. 74, Iss. 1, 2012 ISSN 1454-2331 SYNTHESIS AND CHARACTERIZATION OF GEOPOLYMER BINDERS FROM FLY ASH Irina CĂTĂNESCU 1 , Maria GEORGESCU 2 , Alina MELINESCU 3 Valorificarea unor deşeuri rezultate din activităţi industriale sau de minerit, a devenit un domeniu important în cercetare. O categorie de materiale în care aceste deşeuri pot fi valorificate, sunt lianţii geopolimeri. Lucrarea prezentă aduce unele date privind sinteza şi caracterizarea unor mase liante geopolimere din cenuşă de termocentrală şi soluţie de NaOH. S-au folosit cenuşi cu caracteristici compoziţionale şi dispersionale diferite, dozaje diferite de NaOH, dar şi condiţii diferite de sinteză. Procesele de formare şi proprietăţile liante au fost studiate în corelare cu factori compoziţionali şi de procesare. Pentru investigarea acestora, s-au utilizat metode de caracterizare fizico-mecanică şi metode fizice de investigare, precum spectroscopia IR, difractia de raze X, microscopia electronică. Valorization of wastes from industrial or mining activities has become an important area on research. A category of materials in which these wastes can be recovered, are geopolymer binders. The present paper brings some data on the synthesis and characterization of geopolymer binder masses of fly ash and NaOH solution. Fly ashes with different compositional and dispersional characteristics and different dosages of NaOH were used as well as different conditions of synthesis. The formation processes and binder properties have been studied in correlation with compositional and processing factors. In order to investigate these, compressive strength tests and physical methods of investigation, as FTIR spectroscopy, X-ray diffraction were used. Keywords: Fly ash, Geopolymer binders, Compressive strength 1. Introduction In the last years, more several experimental researches have as their object efficient valorization, with low cost, of fly ashes, which result in large quantities, by burning coal in the power stations and storage which has negative implications on the environment. 1 PhD student, Faculty of Applied Chemistry and Material Science, University POLITEHNICA of Bucharest, e-mail: [email protected] 2 Prof., Faculty of Applied Chemistry and Material Science, University POLITEHNICA of Bucharest, Romania 3 Lecturer, Faculty of Applied Chemistry and Material Science, University POLITEHNICA of Bucharest, Romania

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Page 1: synthesis and characterization of geopolymer binders from fly ash

U.P.B. Sci. Bull., Series B, Vol. 74, Iss. 1, 2012 ISSN 1454-2331

SYNTHESIS AND CHARACTERIZATION OF GEOPOLYMER BINDERS FROM FLY ASH

Irina CĂTĂNESCU1, Maria GEORGESCU2, Alina MELINESCU3

Valorificarea unor deşeuri rezultate din activităţi industriale sau de minerit, a devenit un domeniu important în cercetare. O categorie de materiale în care aceste deşeuri pot fi valorificate, sunt lianţii geopolimeri.

Lucrarea prezentă aduce unele date privind sinteza şi caracterizarea unor mase liante geopolimere din cenuşă de termocentrală şi soluţie de NaOH. S-au folosit cenuşi cu caracteristici compoziţionale şi dispersionale diferite, dozaje diferite de NaOH, dar şi condiţii diferite de sinteză.

Procesele de formare şi proprietăţile liante au fost studiate în corelare cu factori compoziţionali şi de procesare. Pentru investigarea acestora, s-au utilizat metode de caracterizare fizico-mecanică şi metode fizice de investigare, precum spectroscopia IR, difractia de raze X, microscopia electronică.

Valorization of wastes from industrial or mining activities has become an

important area on research. A category of materials in which these wastes can be recovered, are geopolymer binders.

The present paper brings some data on the synthesis and characterization of geopolymer binder masses of fly ash and NaOH solution. Fly ashes with different compositional and dispersional characteristics and different dosages of NaOH were used as well as different conditions of synthesis.

The formation processes and binder properties have been studied in correlation with compositional and processing factors. In order to investigate these, compressive strength tests and physical methods of investigation, as FTIR spectroscopy, X-ray diffraction were used.

Keywords: Fly ash, Geopolymer binders, Compressive strength

1. Introduction

In the last years, more several experimental researches have as their object

efficient valorization, with low cost, of fly ashes, which result in large quantities, by burning coal in the power stations and storage which has negative implications on the environment. 1 PhD student, Faculty of Applied Chemistry and Material Science, University POLITEHNICA of

Bucharest, e-mail: [email protected] 2 Prof., Faculty of Applied Chemistry and Material Science, University POLITEHNICA of

Bucharest, Romania 3 Lecturer, Faculty of Applied Chemistry and Material Science, University POLITEHNICA of

Bucharest, Romania

Page 2: synthesis and characterization of geopolymer binders from fly ash

4 Irina Catanescu, Maria Georgescu, Alina Melinescu

Besides using fly ash as the admixture in the blended cements - limited in some countries (including our country too), it can be considered as material aluminosilicate that, by alkaline activation, leads to obtaining of the binders that are different from those considered classics, namely geopolymer type binders. Geopolymers are poorly crystalline materials (almost amorphous), similar from the point of view of composition to the zeolites, and are formed by the polymerization processes of silicate and aluminate groups favored, in general, by higher temperature [1-5].

Their synthesis from aluminosilicate materials, as fly ash or metakaolin, take place by the so-called geopolymerization process, which involves polycondensation phenomena of aluminate and silicate groups with formation of -Si-O-Al- type bonds.

Geopolymerization mechanism consists, by different authors [4-7], in the following main stages:

- dissolution of aluminosilicate material in alkaline medium, with the destruction of the surface links, in highly alkaline pH conditions, with movement in solution of hydrated silicate and aluminate ions;

- polycondensation of silicate and aluminates ions, with the participation of alkali ions and formation of a gelic product, type sialate;

- evolution of sialate groups into more organized structural formations, type of poly (sialate-siloxo) [4]; the gel formation is passing, in time at semi-crystalline compounds.

Page 3: synthesis and characterization of geopolymer binders from fly ash

Synthesis and characterization of geopolymer binders from fly ash 5

The alkalis compounds are necessary for the dissolution of silica and

alumina from initial solid component and favor the polycondensation process. Alkaline cations and Ca2+ cations is assumed to be embedded in the reaction products, so physical, in structural cavities, and chemical, bound by Al-O and Si-O bonds[4].

Silico-aluminate materials used in geopolymers performing can be: fly ash, granulated blast furnace slag, calcined clay (metakaolin), together eventually, with some admixtures as: cement kiln dust, lime etc. Commonly used alkaline activators are: NaOH, Na2CO3, K2CO3, K2SO4 [1-13]. The nature of these influences the geopolymerisation process and the properties of the obtained product. Usually, geopolymerisation processes involves initial thermal treatment of reactants mixture [1-13] that enhance these processes. Less used is the curing at room temperature, when the geopolymerisation processes are slower [14].

The present paper brings some data regarding synthesis and properties of some geopolymer type binders from Romanian fly ashes, having specific compositional and dispersional characteristics, activated with NaOH. For synthesis were considered the initial hydrothermal treatment at different temperatures (60 – 100ºC). Geopolymerisation processes and reaction products formation, in correlation with initial hydrothermal treatment and subsequent storage in air, were investigated by X-ray diffractions and FTIR spectroscopy.

The binding properties were appreciated by the mechanical strengths determination, for the same conditions.

2. Experimental

Two types of fly ash from Romanian power stations were considered. Their chemical composition is shown in the Table 1. Some compositional differences can be mentioned:

- amount (SiO2+ Al2O3 +Fe2O3): 80.66% - fly ash T and 84.11% –fly ash D

Page 4: synthesis and characterization of geopolymer binders from fly ash

6 Irina Catanescu, Maria Georgescu, Alina Melinescu

- the ratio SiO2 / Al2O3: 2.25 - fly ash T and 2.17 –fly ash D; - CaO content: 10.01% - fly ash T and 6.69% –fly ash D; - LOI: 1.10% - fly ash T and 2.69% - fly ash D.

Table 1

Chemical compositions of the fly ashes Fly ash type

Composition (%)

LOI SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O

T 1.10 48.93 21.72 10.01 10.70 2.61 1.21 __ __

D 2.69 51.5 23.68 8.93 6.69 1.99 1.10 0.50 2.18

The ashes were ground at corresponding specific surface areas of 5900cm2/g approximately. The alkaline activator was NaOH – as solutions 8M and 12M. The liquid/solid ratio of binding pastes was 0.39, for all compositions (table 2).

Table 2

Geopolymer type binder′s indicatives and curing conditions

The cylindrical samples with d=h=20 mm were prepared by pressing, and

cured for geopolymerisation and hardening in the next conditions: 24 hours – hydrothermal treatment at 60°C, 80°C and 100°C (in the tight closed box, with relative humidity approximately 100%) + n days (till 28) in the air (T=20±2°C and RH= 55-60%).

On the hardened samples were determined compressive strengths, after thermal treatment at different periods of time, and respectively, after subsequent storage in the air.

Fly ash type

Sa (cm2/g)

NaOH concentra

tion

Curing conditions

Liquid/ solution

ratio

Indicative of cured samples in air, for:

24 hours 7 days 28 days T19

5952

12M

24 hours 60°C

0.39

T19.3 T19.4 T19.5 T20 24 hours 80°C T20.3 T20.4 T20.5 T21 24 hours 100°C T21.3 T21.4 T21.5 T22 atm T22.3 T22.4 T22.5 T23 8M 24 hours 60°C T23.3 T23.4 T23.5 T24 24 hours 80°C T24.3 T24.4 T24.5 D36

5908

12M

24 hours 60°C D36.3 D36.4 D36.5 D37 24 hours 80°C D37.3 D37.4 D37.5 D38 24 hours 100°C D38.3 D38.4 D38.5 D39 atm D39.3 D39.4 D39.5 D40 8M 24 hours 60°C D40.3 D40.4 D40.5 D41 24 hours 80°C D41.3 D41.4 D40.5

Page 5: synthesis and characterization of geopolymer binders from fly ash

Synthesis and characterization of geopolymer binders from fly ash 7

Information on the geopolymerisation process and reaction products formation was obtained by X-ray diffraction analysis and FTIR spectroscopy on the geopolymer samples, after different curing periods of time.

3. Experimental results

Mechanical strengths of the geopolymer type binders, synthesized in different conditions, are presented in Figs. 1 and 2.

T19 T20 T21 T22T23

T24

2 ore

5 ore

24 ore7 zile

28 zile

0

5

10

15

20

25

30

35

40

45

Cs

[MP

a]

D36 D37 D38 D39 D40 D41

2 ore

5 ore24 ore

7 zile28 zile

0

5

10

15

20

25

30

35

40

45

Cs

[MP

a]

Fig. 1. Compressive strengths of the geopolymer binders resulted from fly ash T and 12M / 8M

NaOH solution: Sa = 5952 cm2/g;

Fig. 2. Compressive strengths of the geopolymer binders resulted from fly ash D and 12M, 8M NaOH solution: Sa = 5908

cm2/g;

Higher compressive strengths are achived for geopolymer compositions made from fly ash T, compared with those of type D masses.

The main compositional characteristics of T fly ash, which contribute to the better binding properties of the derived geopolymers, are:

- a greater CaO content (10.7% value is very close to the limit of the fly ashes type C and respectively of type F;

- a smaller loss on ignition, suggesting a lower carbon content, a high carbon content which could negatively influences the fly ash reactivity

The initial thermal treatment, of precursor mixtures, at temperatures up to 100ºC, favors the development of compressive strengths, even after short periods of time (the first hours), better under conditions of high temperatures (100ºC), in good correlation with literature date [1, 4]. Otherwise, for both categories of the masses, the best mechanical strengths were developed by the compositions T21 and D38, initial treated at 100°C (see Figs.1 and 2). The continuous curing up to 7 days (28 days), in air, leads to further increases of strengths, for all compositions,

Page 6: synthesis and characterization of geopolymer binders from fly ash

8 Irina Catanescu, Maria Georgescu, Alina Melinescu

explained by the structuring processes of reaction products, which continues in these conditions.

Curing the samples at normal temperature (20±2°C) determines a slow development of compressive strengths. Theirs values, determined for 7 and 28 days, are closely to those of the samples initial treated, at 100ºC. This also confirms the accelerator effect of thermal treatment of these mixtures, on compressive strengths development, in correlation with the geopolymerisation process.

Concentration of NaOH solution influences the mechanical strength development. A higher NaOH concentration (12 M) favors the development of better mechanical strength, as can see in Fig. 3.

T19 D36 T20 D37

2 ore 5 ore

24 ore 7 zile

28 zile

0

5

10

15

20

25

30

35

40

Rc

[MPa

]

2 ore

5 ore

24 ore

7 zile

28 zile

T23 D40 T24 D41

2 ore 5 ore

24 ore 7 zile

28 zile

0

5

10

15

20

25

30

Rc

[MPa

] 2 ore

5 ore

24 ore

7 zile

28 zile

a b Fig. 3. Compressive strengths of some geopolymer binders from T fly ash and D fly ash, with: a-

NaOH 12 M; b-NaOH 8M; curing conditions: 24h 27°C + 24h 60°C + n days in air, and 24h 27°C + 24h 80°C + n days in air (for binders indicatives, see Table 2)

This influence is in correlated with a higher solubility of alumino-silicates

compounds in concentrated alkaline solutions [4], the geopolymerisation process being also, favored. Information on the geopolymerisation processes evolution and reaction products formation that ensure the mechanical strengths development, are obtained of FTIR spectroscopy and X-ray diffraction analysis.

Page 7: synthesis and characterization of geopolymer binders from fly ash

Synthesis and characterization of geopolymer binders from fly ash 9

Fig. 4. FTIR spectra of fly ashes T and D and geopolymer binder prepared from these ashes and

NaOH 12M, after thermal treatment at 60°C for 24 hours, and subsequent storage in air for 6 days respectively 27 days: a- fly ash T; b- fly ash D

FTIR spectra of the fly ashes show the following absorption bands (Fig. 4): - 1075 cm-1 - attributed to the asymmetric vibrations of bonds Si-O and Al-O [1]; - 465 cm-1 - attributed to the deformation vibrations of Si-O bonds; - 797 cm-1 - attributed to Al-O bonds vibration.

500 1000 1500 2000 2500 3000 3500 4000

1.0

1.5

2.0

2.5

2327

23242370

2371

2848

28551651

16591418

1507

1418

2920

2927

782695

675

783462

458

3858

37473852

3747

3437

344815141003

1009

Abs

(%)

1/λ (cm-1)

38763748

34092922

28591647

1520

1075

797

691

465

460

1008

29202855 3448

37473864

1650

1522

1431712

2374

2333

500 1000 1500 2000 2500 3000 3500 40001.0

1.5

2.0

DDDD

Abs

(%)

1/λ (cm-1)

3866

37552927

28541647

1519

1433800

557

465

3404

1076460

576

796

1012

14591528

16432369

2344 28602921 3490

37463862

471

10123455 3759

387629242860

2370

16501457788574

460

5741016

793 16581457

2369

2343

2342

29292856

34633761 3853

a b Fig. 5. FTIR spectra of fly ashes and geopolymer binders prepared from these, by activation with NaOH 12M, after thermal treatment at 80°C for 24 hours, and subsequent storage in air for 6 days

respectively 27 days: a- fly ash T, b- fly ash D

500 1000 1500 2000 2500 3000 3500 4000

1.0

1.5

2.0

2.52929

28512374

344816541428

T T T T

2342

23342375

2374

2851

28591654

165414181507

1428

2921

2929

782690

683

783458

4583858

3750 3852

3750

3437

3448

15061004

1004

Abs

(%)

1/λ (cm-1)

3876

3748

34092922

285916471520

1075

797

691

465

458

10127063758

38521506

2338

500 1000 1500 2000 2500 3000 3500 40001.0

1.5

2.0

D D 3 D 3 D 3

3853

3853

3752

3744

2930

2928

2930

2848

2848

2369

2327

2323

2368

3853

3744

577

560

575

28482327

2360

803

795

802

15111655

3654

15181645

16551508

459

465

458

1014

1013

1002

3573

3573

1/λ (cm-1)

3755

3866

3404

2927

28541647

15191433

1076

800

557

451

Abs

(%)

3654

3573

a b

Page 8: synthesis and characterization of geopolymer binders from fly ash

10 Irina Catanescu, Maria Georgescu, Alina Melinescu

500 1000 1500 2000 2500 3000 3500 4000

1.0

1.5

2.0

2.5

TTTT

2339

2332

2324 2370

2371

2377

2852

2844

28511647

1654

163914511519

1452

1459

2904

2919

2927789691

782684

623

692465

458

461

3868

3748

3869

3747

3862

3762

3439

3431

3416

1512

1008

1000

998

Abs

(%)

1/λ (cm-1)

38763748

3409

29222859

1647

1520

1075

797

691

465

500 1000 1500 2000 2500 3000 3500 40001.0

1.5

2.0

D D D D

Abs

(%)

1/λ (cm-1)

3866

37552927

28541647

15191433

800

557

465

3404

1076464

571

797

10131456

1528

1648

2855

29243463

37563866

471

10073479

374838672926

285616501456

788573

460

573

1005

780

1457

1515

2343

15162927

28633487

37483867

1647

a b Fig. 6. FTIR spectra of fly ash and geopolymer binder prepared from fly ash and NaOH 12M, after thermal treatment at 100°C for 24 hours, and then kept in air for 6 days respectively 27 days; a- fly

ash T, b- fly ash D The main changes that are recorded on the geopolymer IR spectra, after

hydrothermal treatment and subsequent storage in air (Fig. 4-6 T21.1-T21.3), consist in: - the shifting of the main band (1075cm-1) to smaller wave numbers (1003-998cm-

1), in parallel with the decrease of the its amplitude, which suggests the silica solubilisation and polycondensation processes, with partial substitution of SiO4

4- with AlO4

5- groups, in the newly formed network [3]; the valences compensation is achieved by Na+ ions; the shift is significantly for the thermal treated samples and practically does not change, for the storage in air; - the disappearance of the band at 797 cm-1, what point out the consumption of tetrahedral coordinated Al [3]; - the appearance of a two new bands, at about 3450 and 1650cm-1, which belonging to the vibration bonds H-O and H-OH from aluminate-silicates hydrates, formed as reaction products; - a well outlined band, at about 1459-1500 cm-1, is attributed to C-O bond from alkaline carbonates formed by atmospheric CO2 action.

X-ray diffraction analysis, performed on a geopolymer binders from fly ashes T and D, hydrothermal synthesized at 60ºC, 80ºC and 100ºC - 24 hours and then kept in the laboratory atmosphere (air), up to 7 and 28 days respectively, are presented in Figs. 7 and 8.

Page 9: synthesis and characterization of geopolymer binders from fly ash

Synthesis and characterization of geopolymer binders from fly ash 11

5 10 15 20 25 30 35 40 45 50 55 60

200

400

600

800

1000

1200

1400

1600

1800

I(CP

S)

2θ(deg)

- quartz - mullite - hematite - anortite - NaAlSi2O6*3H2O - NaHCO3 - Na2CO3*H2O

TT T 2T 2

5 10 15 20 25 30 35 40 45 50 55 600

200

400

600

800

1000

1200

1400

1600

I(CP

S)

2θ(deg)

- quartz - mullite - hematite - anortite - NaAlSi2O6*3H2O - NaHCO3 - Na2CO3*H2O

T T 1 T 2 T 2

a b

5 10 15 20 25 30 35 40 45 50 55 60

200

400

600

800

1000

1200

1400

1600

1800

I(CPS

)

2θ(deg)

- quartz - mullite - hematite - anortite - NaAlSi2O6*3H2O - NaHCO3 - Na2CO3*H2O

TT T T

Fig.7. X-ray spectra of fly ash T and geopolymer binder prepared from this ash and

NaOH 12M, after: a- 24 hours thermal treatment at 60º, 80º and 100°C; b - 24 hours thermal treatment and then kept in air for 6

days; c -24 hours thermal treatment and then kept in air for 27 days

c

On the X-ray spectra of the geopolymer a small halo is present, in the

range of 2�=20º - 40º, which suggest the presence of poorly crystalline phases (nano-sized) - geopolymeric gel. As crystalline compounds are found the fly ash main components – quartz, mullite, hematite, and some reaction products, these compounds, of zeolites type (hershelit (NaAlSi2O6·3H2O) [11], phylipsit {KCa(Si5Al3)O16·6H2)} have a certain crystallization degree. Theirs formation is favored by hydrothermal treatment [13]. Beside these products, the X-ray spectra suggest the presence of alkali carbonates too, in good correlation with IR spectra [16].

Page 10: synthesis and characterization of geopolymer binders from fly ash

12 Irina Catanescu, Maria Georgescu, Alina Melinescu

5 10 15 20 25 30 35 40 45 50 55 600

500

1000

1500

2000

2500

DD 3D 3D 3

- quartz - mullite - hematite - NaHCO

3

- Na2O*Al

2O

3*SiO

2*H

- Na6Al

6Si

10O

32*12H

2O

I (C

PS)

2θ (deg)

a

5 10 15 20 25 30 35 40 45 50 55 600

500

1000

1500

2000

2500

D D D D

- quartz - mullite - hematite - NaHCO3

- Na2O*Al2O3*SiO2*H2O- Na6Al6Si10O32*12H2

I (C

PS)

2θ (deg)

b

5 10 15 20 25 30 35 40 45 50 55 600

500

1000

1500

2000

2500

DDDD

- quartz - mullite - hematite - NaHCO3

- Na2O*Al2O3*SiO2*H2O- Na6Al6Si10O32*12H2

I (C

PS)

2θ (deg)

c

Fig. 8. X-ray spectra of fly ash D and geopolymer binders prepared from this ash and NaOH 12M, after: 24 hours of thermal

treatment at 60º, 80º and 100°C; b - 24 hours thermal treatment and then keeping in air for

6 days; c -24 hours thermal treatment and then keeping in air for 27 days

4. Conclusions - The geopolymer type binders prepared by alkaline activation of the fly

ashes with NaOH solution can develop good mechanical strengths, that are influenced by the compositional features and the processing conditions too.

- A great CaO and vitreous phase content, as well as small quartz content in fly ash T, exert a favourable influence on geopolymerisation and mechanical strengths development.

- A higher concentration of NaOH solution favours the geopolymerisation process and higher mechanical strengths development.

Page 11: synthesis and characterization of geopolymer binders from fly ash

Synthesis and characterization of geopolymer binders from fly ash 13

- The initial thermal treatment of the fly ash + NaOH solution mixtures, at temperatures up to 100°C, favours the geopolymerisation process and the development of mechanical strengths, even after short periods of curing (2 hours).

- The curing at normal temperature (20±2°C) determine a slow development of mechanical strengths, as consequence of a very slow geopolymerisation process.

Acknowledgements

This study has been made within the PhD Programme founded by European Social Fund (ESP) (Contract no. POSDRU/6/1.5/S/19).

R E F E R E N C E S

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2. M. Criado, A. Palomo, A. Fernandez-Jimenez – Alkali activated of fly ashes. Part 1: Effect of curing conditions on the carbonation of the reaction products – Fuel 84 (2005), 2048-2054

3. M. Stevenson, K. Sagoe-Crentsil – Relationships between composition, structure and strength of inorganic polymers. Part2 Fly ash-derived inorganic polymers – J. Mater. Sci. 40 (2005) 4247-4259

4. D. Khale and R. Chaudhary – Mechanism of geopolymerization and factors influencing its development: a review, J. Mater. Sci 42 (2007), 720-740

5. J. Davidovits – Geopolymer. Chemistry and applications, Institute Geopolymere, 2008 6. S. Kumar, R. Kumar, T.C. Alex, A. Bandopadhyay and S.P. Mehrotra – Influence of reactivity

of fly ash on geopolymerisation – Advances in Applied Ceramics (2007), v. 106, n.3 7. F. Pacheco-Torgal, J. Castro-Gomes, S. Jalali – Alkali activated binders: A review. Part 2.

About materials and binders manufacture - Construction and Building Materials xxx (2007) 8. Sang-Sook Park and Hwa-Young Kang – Strength and microscopic characteristics of alkali-

activated fly ash-cement – Korean J. Chem. Eng. 23 (3), 367-373 (2006) 9. K. Dombrowski, A. Buchwald, M. Weil – The influence of calcium content on the structure and

thermal performance of fly ash based geopolymers – J. Mater. Sci. 42 (2007), 3033-3043 10. Fr. Skvara, L. Kopecky, J. Nemecek, Z. Bittnar – Microstructure of geopolymer materials

based on fly ash – Ceramics – Silikaty 40(2006), 208-215 11. F. Pacheco-Torgal, J. Castro-Gomes, S. Jalali – Alkali activated binders: A review Part 1.

Historical background, terminology, reaction mechanism and hydration products – Construction and Building Materials xxx (2007)

12. S. Kumar, R. Kumar, S.P. Mehrotra – Influence of granulated blast furnace slag on the reaction, structure and properties of fly ash based geopolymers – J. Mater. Sci., published online, 15 oct. 2009

13. D. Ravikumar, S. Peethamparan, N. Neithalath – Structure and strength of NaOH activated concretes containing fly ash or GGBFS – Cem. Concr. Comp. xxx (2010)

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15. M. Georgescu, I. Cătănescu, G. Voicu – Fly ash based geopolymer binders. Influence of some compositional characteristics, Romanian Chemistry Journal 62 (2011), 872-877.