thermal treatment of kaolin_effect on the pozzolanic activity.pdf

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Procedia Materials Science 1 (2012) 343 350

2211-8128 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SAM/CONAMET 2011, Rosario, Argentina.

doi:10.1016/j.mspro.2012.06.046

11th

International Congress on Metallurgy & Materials SAM/CONAMET 2011.

Thermal treatment of kaolin: effect on the pozzolanic activity

A. Tironia*, M. A. Trezza

a, E. F. Irassar

a, A. N. Scian

b

aFacultad de Ingeniera Universidad Nacional del Centro de la Provincia de Buenos Aires,

Olavarra , Argentina.bCentro de Tecnologa de Recursos Minerales y Cermica CONICET La Plata - UNLP,

Gonnet 1900, Argentina.

Abstract

Thermally activated clays, especially kaolinitic clays, are today revaluated as a source of supplementary cementitious

materials to reduce the CO2 emissions and energy consumption originated in cement production. In this work, the

influence of different thermal treatments on the pozzolanic activity of raw kaolin with 98 % kaolinite and ordered

structure was studied. Results show that pozzolanic activity of calcined kaolin decays when using a thermal treatment at

high temperatures (800 C) and high periods of residence (30 minutes). Furthermore, low calcination temperature

(700 C) must be corresponding with a residence time that guarantees a high dehydroxylation percentage. Sample treated

during 10 minutes (94 %DH) was less reactive than the one treated during 30 minutes (96 %DH). Results contribute to the

industry purposes to reduce the energy consumption, the CO2 emission and to contribute with new alternatives of

sustainable development.

2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of 11 th

International Congress on Metallurgy & Materials SAM/CONAMET 2011.

Keywords: kaolinite, kaolin, metakaolin, pozzolanas

* Corresponding author. Telefax.: 54-2284-451055.

E-mail address:[email protected].

Available online at www.sciencedirect.com

2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SAM/

CONAMET 2011, Rosario, Argentina.


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344 A. Tironi et al. / Procedia Materials Science 1 (2012) 343 350

1.Introduction

Thermally activated clays, especially kaolinitic clays, are today revaluated as a source of supplementary

cementitious materials to reduce the CO2emissions and energy consumption originated in cement production

(Sabir et al., 2001; Samet et al., 2007). Clays containing high kaolinite percentage (Al 2O3.2SiO2.2H2O) are

commonly called kaolin (Mari, 1998). In the presence of water at ambient temperature, calcined kaolin reactswith the calcium hydroxide released by cement hydration to form compounds with cementing properties.

During the calcination, dehydroxylation of kaolinite produces an amorphous phase (metakaolinite) according

to the following reaction (Salvador, 1995):

Al2O3.2SiO2.2H2O (s) Al2O3.2SiO2(s) + 2H2O (g) (1)kaolinite metakaolinite water

Metakaolinite provides the reactive silica and alumina that react with Ca(OH)2, while its pozzolanic activity

depend on dehydroxylation degree and accommodation or available surface for reaction. The dehydroxylation

process must be coinciding with the amorphization, and this transition is affected by thermal treatment (time,

temperature, heating rate).

In this work, the pozzolanic activity of kaolin subjected to different thermal treatments was studied by theFrattini test and the electrical conductivity (Qijun et al., 1999). Results were compared to determine the

optimal heat treatment (temperature and the residence time) to obtain the better pozzolanic activity.

2.

Experimental

2.1.

Kaolin characterization

A kaolin from Patquia, La Rioja, Argentina, was used. Chemical analysis is shown in Table 1. The sample

has a high percentage of Al2O3and SiO2, close to pure composition of kaolinite.

X-ray diffraction (XRD), Fourier transformed infra-red spectroscopy (FTIR), and differential thermal

analysis combined with thermal gravimetric analysis (DTA-TG) were used to determine the mineralogical and

structural composition of kaolin. XRD was performed on Philips PW 3710 diffractometer operating with

CuK radiation at 40 kV and 20 mA. FTIR spectrum was obtained using a Nicolet Magna 500

spectrophotometer ranged from 4000 to 400 cm-1. DTA-TG was carried out using a NETZCH STA 409

thermobalance.

Table 1. Kaolin chemical analysis and loss on ignition (LOI).

Chemical composition, %

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 LOI, %

45.9 37.0 0.77 0.08 0.12 0.06 0.40 0.99 13.3

2.2.

Thermal treatment

Kaolin sample was reduced to particle size smaller than 4 mm. The thermal treatment was carried out in a

programmable laboratory furnace Indef 272 using a fixed bed technique. The ground material was calcined at


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345A. Tironi et al. / Procedia Materials 1 (2012) 343 350

5 10 15 20 25 30 35 40 45 50 55 60 65 70

K

2 , deg

Intensity,

Sqr(counts)

K

KKK

KK

KKKK

K

KK

K

K

Q

KK

KK

K

K K

KAK

K

different temperatures (T = 700, 750 and 800 C) with different residence times (tres= 10, 20 and 30 min).

Finally, calcined samples were ground in a laboratory mill (Fritsch Pulverisette 2) at the same energy

quantity.

2.3.

Characterization and pozzolanic activity of calcined kaolin

Calcined samples were characterized by FTIR. Dehydroxylation percentage (% DH) was calculated with

the percentage in mass of water removed at the different thermal treatments mentioned. Blaine specific

surface (BSS) was determined according to ASTM C 204-04 procedure.

Pozzolanic activity was evaluated by Frattini test and electrical conductivity method (Qijun et al., 1999).

Frattini test was carried out according to the procedure described by EN 196:5 standard. The tested samples

were a blend of 70% of Portland cement (PC) and 30% by mass of ground calcined kaolin. This test implies

the determination of the amount of Ca2+ and OH- in the water of contact with the tested samples stored at

40 C during 2, 7 and 28 days. Then, comparing the amount of these ions with the solubility isotherm of

Ca(OH)2in an alkaline solution at the same temperature, the calcined sample is considered as active pozzolan

when the [Ca2+] and [OH-] determined in solution are located below the solubility isotherm.

The electrical conductivity test was done by mixing 20.00 ml of Ca(OH)2saturated solution at 40 C with2.00 g of calcined kaolin. At 2, 7 and 14 days the electrical conductivity was measured by a Jeway 4010

conductivity-meter. The electrical conductivity falls due to the drop of the [Ca2+

] and [OH-] attributable to the

ions consumption by the progress of pozzolanic reaction of calcined kaolin.

3.Results and discussion

3.1.Kaolin characterization

XRD pattern (Fig. 1) show that sample has very strong peaks of kaolinite (K), and a low intensity of the

peaks of quartz (Q) and anatase (A). Peaks assigned to kaolinite are intense and sharp showing an ordered

structure (Aparicio and Galan, 1999).

Fig. 1. XRD pattern of kaolin (K: kaolinite, Q: quartz, A: anatase).


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4000 3500 1500 1000 500

939Al-OH

3619-OH3

652-OH

3669-OH

912Al-OH

538Si-O-Al

754Si-O-Al

789Si-O-Al

696Mg/Al-O

H

Wave number, cm-1

Tran

smitance,

%

470Si-O

430Si-O

1009Si-O

1032Si-O

1115Si-O

3694-OH

In FTIR spectrum (Fig. 2), the characteristic absorption bands of kaolinite were identified. Kaolinite has

absorption bands between 3500 and 3750 cm-1corresponding to stretching frequencies of OH groups (Wilson,

1987; Madejov, 2003). When the four characteristic bands (3700, 3670, 3650 and 3620 cm-1) are well

defined, structure of kaolinite is ordered. When the band at 3670 cm -1 disappears, kaolinite structure is

disordered and easier to dehydrate (Bich, 2005). Kaolinite present in the analyzed sample has an ordered

structure, since the four bands are well defined. It is also calculated P 0 index as the ratio between bandintensity at 3620 and 3700 cm-1. In this case P0= 1,121>1, confirming that kaolinite has an ordered structure

(Bich, 2009).

Fig. 2. FTIR spectra of kaolin.

The DTA curve for kaolinite shows an endothermic peak in the temperature range 500-600 C due todehydroxylation of the mineral (Wilson, 1987) and it is associated with a weight loss of 13.76 % to pure

kaolinite (Shvarzman et al., 2003). The results obtained by DTA/TG for sample are presented in Fig. 3. For

TG, the calculated kaolinite content in the sample was 98 % (Fig. 3a) corresponding with the main phase

observed by XRD. According to DTA analysis (Fig. 3b), the initial temperature of kaolinite dehydroxylation

ranges is 450 C, the centre of endothermic peak appears at 577 C, and the maximum temperature for

kaolinite dehydroxylation occurs at 700 C. The slope ratio (SR) is the ratio between the slope of the

descending branch of the dehydroxylation peak in the DTA curve and the slope of the ascending branch of

this peak. The slope ratio characterizes the presence of surface defects. When SR=1, the peak is symmetric

and kaolinite does not present many surface defects. When SR=2, many surface defects are present (Bich,

2009). For this sample, SR is 1.76 having few superficial defects. The exothermic peak at 985 C is assigned

to the metakaoline (amorphous) transformation to spinel and amorphous silica.


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0 300 600 900 1200 1500

Weightloss,%

Temperature, C

13.49 %

0 200 400 600 800 1000 1200 1400

Endo

Endo

Exo

69 C

985 C

577 C

DTA

,V

Temperature, C

Fig. 3. TG (a) and DTA (b) curves.

3.2.Calcined kaolin characterization

After calcining, %DH from the different thermal treatments was determined (Table 2). This indicates the

degree of kaolinite transformation in metakaolinite according to eq. 1. For all thermal treatments used, the

%DH was higher than 94. Values of Blaine specific surface corresponding to the different calcined and

ground samples are presented in Table 2. All of them are within the same order.

Table 2. Dehydroxylation percentage (%DH), Blaine specific surface (BSS)

Samples

T, C / tres, min

700/10 700/30 750/20 800/10 800/30

%DH 94 96 99 99 100

BSS, m2/kg 969 997 776 927 784

In agreement with Chakchouk et al. (2009), FTIR spectrums of calcined clays (Fig. 4) present the

following changes: the absence of detectable -OH and Al-OH bands; the transformation Si-O characteristic

bands of kaolinite present in the raw clay at 1115, 1032 and 1009 cm-1

to a single absorption band at

1082 cm-1 which is characteristic of the amorphous silica; the transformation Al-O-Si bands at 789 and

754 cm-1to a single absorption band at 810 cm-1, characteristic of amorphous phase; the disappearance of the

band at 534 cm-1 relative to Al-O-Si; and the displacement of the Si-O band at 470 cm-1 to high wavenumbers. These FTIR spectrums (Fig. 4) confirm the transformation of kaolinite into reactive amorphous

phase by thermal treatment. For calcination temperature of 800C and a residence time of 30 minutes, bands

corresponding to amorphous phases were obtained with lower area (Fig. 4).


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4000 3500 1500 1000 500

4658121078

Transmitance,

%

Wave number, cm-1

700/10

700/30

800/10

800/30

750/20

3441

3439

1085 808 466

3435

1086

809 463

34231090

812 465

3436

1086814 465

Fig. 4. FTIR spectra of calcined kaolin at different temperatures and residence times.

3.3.Pozzolanic activity

Figs. 5, 6 and 7 present the results of Frattini test at 2, 7 and 28 days, corresponding to the samples with

different thermal treatments. This test is not sensitive to difference between the calcination temperatures and

the residence time. However, it was sensitive to evaluate the progress of reaction. The pozzolanic activity was

higher at 7 days than at 2 days (Figs. 5 and 6), while it presents a similar value between the 7 and 28 reaction

days (Figs. 6 and 7).

Results of electrical conductivity (EC) in function of the reaction age for the different thermal treatmentsare shown in Fig. 8. The EC decreases when the reaction age increases due to a consumption of Ca 2+and OH-.

Difference between 2 and 7 days is not abrupt, only slightly higher to the 7 to 14 days period. At 800 C and

30 minutes, the EC is higher and so the reactivity of the sample can be lower.

The EC in function to residence time for the different calcination temperatures (T) and reaction ages

(Fig. 9) show that:

For T = 700 C, at higher residence time, conductivity decreases, and pozzolanic activity

increases.

Inversely, T = 800 C, at higher residence time, conductivity increases, and pozzolanic activity

decreases.

At 750 C, EC of calcined clay behaves in an intermediate position.


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0

2

46

8

10

12

14

16

18

30 40 50 60 70 80 90 100

[OH-], mmol/l

[CaO],mmol/

700/10

700/30

750/20

800/10

800/30

7 days

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

0 10 20 30 40

residence time, min

EC,mS 700 C / 2 days

700 C / 7 days

700 C / 14 days

750 C / 2 days

750 C / 7 days

750 C / 14 days

800 C / 2 days

800 C / 7 days

800 C / 14 days

Fig. 5. Result of Frattini test at 2 days. Fig. 6. Result of Frattini test at 7 days.

Fig. 7. Result of Frattini test at 28 days.

Fig. 8. Electrical Conductivity of samples in function of the

reaction age

Fig. 9. Electrical Conductivity samples in function of the residence

time

0

2

46

8

10

12

14

16

18

30 40 50 60 70 80 90 100

[OH-], mmol/l

[CaO],mmol/

700/10

700/30

750/20

800/10

800/30

2 days

0

2

4

6

8

10

12

14

16

18

30 40 50 60 70 80 90 100

[OH-], mmol/l

[CaO],mmol/

700/10

700/30

750/20

800/10

800/30

28 days

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

0 7 14 21Age, days

EC,mS

800/30

800/10

700/10

750/20

700/30


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4.Conclusions

For the production of metakaolin to be used as pozzolanic material starting from a high purity (98%) and

high ordered kaolin, the calcination temperature and the residence time are variables to be considered.

The Frattini test did not show sensitivity in order to differentiate the diverse thermal treatments used in this

work, while the test based in the electrical conductivity did it.The used kaolin treated at 700 C showed that its pozzolanic activity were directly proportional to the

residence time. On the other hand, when it was treated at 800 C the pozzolanic activity decreases when the

residence time increases.

The peak area of bands assigned to the amorphous phase in FTIR spectrum of kaolin calcined at 700 C

during 30 min is higher than that obtained at 800 C. From differential thermal diagram, the reduction of

amorphous phase is attributed to structural rearrangement to form later spinel phase. For 800 C and 750 C, a

low residence time (10 minutes) reduces the structural rearrangement and increases the pozzolanic activity.

References

Aparicio, P., Galan, E., 1999. Mineralogical interference on kaolinite crystallinity index measurements, Clays and Clay Minerals 47, p.12.

ASTM C 204-04. Standard Test Method for Fineness of Portland Cement by Air Permeability Apparatus.Bich, Ch., 2005. Contribution l'tude de l'activation thermique du kaolin: volution de la structure cristallographique et activit

pouzzolanique, Ph. D. Thesis, Institut National des Sciences Appliqus de Lyon, France.

Bich, Ch., Ambroise, J., Pra, J., 2009. Influence of degree of dehidroxylation on the pozzolanic activity of metakaolin, Applied Clay

Science 44, p. 194.

Chakchouk, A., Trifi, L., Samet, B., Bouaziz, S., 2009. Formulation of blended cement: Effect of process variables on clay pozzolanic

activity, Construction and Building Materials 23, p. 1365.EN 196-5 Standard: methods for testing cement. Part 5: pozzolanicity test for pozzolanic cements.

Madejov, J., 2003. FTIR techniques in clay mineral studies: review, Vibrational Spectroscopy 31, p. 1.

Mari, E.A., 1998. Los Materiales Cermicos, Editor. Librera y Editorial Alsina, Bs. As., Argentina.Qijun, Yu, Sawayama, K., Sugita, S., Shoya, M., Isojima, Y., 1999. The reaction between rice husk ash and Ca(OH) 2solution and the

nature of its product, Cement and Concrete Research 29, p. 37.Sabir, B.B., Wild, S., Bai, J., 2001. Metakaolin and calcined clays as pozzolans for concrete: a review, Cement and Concrete Composites

23, p. 441.

Salvador, S., 1995. Pozzolanic properties of flash-calcined kaolinite: a comparative study with soak-calcined products, Cement and

Concrete Research 25, p. 102.

Samet, B., Mnif, T., Chaabouni, M., 2007. Use of a kaolinitic clay as a pozzolanic material for cements: Formulation of blended cement,

Cement & Concrete Composites 29, p. 741.Shvarzman, A., Kovler, K.G., Grader, G.S., Shter, E., 2003. The effect of dehydroxylation/amorphization degree on pozzolanic activity

of kaolinite, Cement and Concrete Research 33, p. 405.

Wilson, M.J., 2003. A Handbook of determinative methods in clay mineralogy, Editor. Chapman and Hall Publ., USA.

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