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Thermal Treatment of Kaolin: Effect on the Pozzolanic Activity
Available online at www.sciencedirect.com
Procedia Materials Science 1 (2012) 343 – 350
11th International Congress on Metallurgy & Materials SAM/CONAMET 2011.
Thermal treatment of kaolin: effect on the pozzolanic activity A. Tironia*, M. A. Trezzaa, E. F. Irassara, A. N. Scianb a
Facultad de Ingeniería
Universidad Nacional del Centro de la Provincia de Buenos Aires, Olavarría , Argentina. b Centro de Tecnología de Recursos Minerales y Cerámica 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.
2012Published PublishedbybyElsevier ElsevierLtd. Ltd.Selection Selectionand/or and/orpeer-review peer-review under responsibility 11th ©©2012 under responsibility of of SAM/ International2011, Congress on Metallurgy CONAMET Rosario, Argentina. & Materials SAM/CONAMET 2011. Keywords: kaolinite, kaolin, metakaolin, pozzolanas
* Corresponding author. Telefax.: 54-2284-451055. E-mail address: [email protected].
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
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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 CO2 emissions 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 reacts with 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) kaolinite
Al2O3.2SiO2 (s) + 2H2O (g) metakaolinite water
(1)
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 the Frattini 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 Al2O3 and 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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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 ºC during 2, 7 and 28 days. Then, comparing the amount of these ions with the solubility isotherm of Ca(OH)2 in 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)2 saturated solution at 40 °C with 2.00 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).
Intensity, Sqr (counts)
K
K
K K K
K A K K Q
5
10
15
20
25
KK K K
30
K
35
K K
K
40
2 , deg
Fig. 1. XRD pattern of kaolin (K: kaolinite, Q: quartz, A: anatase).
K K KK K
45
50
K
55
K K
K K
60
65
70
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A. Tironi et al. / Procedia Materials Science 1 (2012) 343 – 350
4000
1500
Wave number, cm
1000
470 Si-O 430 Si-O
538 Si-O-Al
789 Si-O-Al 754 Si-O-Al 696 Mg/Al-OH
939 Al-OH 912 Al-OH
1115 Si-O 1032 Si-O
3500
1009 Si-O
3669 -OH 3652 -OH
3619 -OH
3694 -OH
Transmitance, %
In FTIR spectrum (Fig. 2), the characteristic absorption bands of kaolinite were identified. Kaolinite has absorption bands between 3500 and 3750 cm-1 corresponding to stretching frequencies of OH groups (Wilson, 1987; Madejová, 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 band intensity at 3620 and 3700 cm-1. In this case P0 = 1,121>1, confirming that kaolinite has an ordered structure (Bich, 2009).
500
-1
Fig. 2. FTIR spectra of kaolin.
The DTA curve for kaolinite shows an endothermic peak in the temperature range 500-600 ºC due to dehydroxylation 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 ºC, the centre of endothermic peak appears at 577 ºC, and the maximum temperature for kaolinite dehydroxylation occurs at 700 ºC. 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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DTA, V
Weight loss, %
985 °C Exo
13.49 %
69 °C Endo 577 °C Endo
0
300
600
900
1200
1500
0
200
400
600
800
1000
1200
1400
Temperature, °C
Temperature, ºC
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
700/10
700/30
750/20
800/10
800/30
%DH
94
96
99
99
100
BSS, m2/kg
969
997
776
927
784
T, ºC / tres, min
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-1 to 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 wave numbers. These FTIR spectrums (Fig. 4) confirm the transformation of kaolinite into reactive amorphous phase by thermal treatment. For calcination temperature of 800°C and a residence time of 30 minutes, bands corresponding to amorphous phases were obtained with lower area (Fig. 4).
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3423 1090 3435
Transmitance, %
1086
812 465 809 463
1086
814 465
700/30
3439 1085 3441 1078 3500
800/10
750/20
3436
4000
800/30
1500
808 466
700/10 812 465
1000
500
-1
Wave number, cm
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 treatments are shown in Fig. 8. The EC decreases when the reaction age increases due to a consumption of Ca2+ 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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A. Tironi et al. / Procedia Materials 1 (2012) 343 – 350 18
18
2 days
16
14
700/30
14
12
750/20 800/10
[CaO], mmol/l
10
[CaO], mmol/l
700/10
16
800/30
8 6
700/10 700/30
7 days
750/20 800/10
12 10
800/30
8 6
4
4
2
2 0
0 30
40
50
60 70 [OH ], mmol/l
80
90
30
100
[CaO], mmol/l
Fig. 5. Result of Frattini test at 2 days.
40
50
60 70 [OH ], mmol/l
80
90
Fig. 6. Result of Frattini test at 7 days.
18 16 14 12 10 8 6 4 2 0
700/10
28 days
700/30 750/20 800/10 800/30
30
40
50
60 70 [OH ], mmol/l
80
90
100
Fig. 7. Result of Frattini test at 28 days.
2,0
2,0
1,8
1,8
1,6
1,6
750 °C / 2 days
1,4
1,4
800 °C / 7 days
1,2
1,2
EC, mS
EC, mS
800 °C / 2 days
1,0 0,8 0,6 0,4 0,2
800/30
0,8
700/10 800/10 750/20
0,6
14 Age, days
700 °C / 7 days 750 °C / 14 days
0,2
700 °C / 14 days
0,0 7
800 °C / 14 days
0,4
700/30 0
700 °C / 2 days 750 °C / 7 days
1,0
21
0,0 0
10
20
30
40
residence time, min
Fig. 8. Electrical Conductivity of samples in function of the reaction age
Fig. 9. Electrical Conductivity samples in function of the residence time
100
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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 ºC during 30 min is higher than that obtained at 800 ºC. 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 à l'étude de l'activation thermique du kaolin: évolution de la structure cristallographique et activité pouzzolanique, Ph. D. Thesis, Institut National des Sciences Appliqués de Lyon, France. Bich, Ch., Ambroise, J., Péra, 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. Madejová, J., 2003. FTIR techniques in clay mineral studies: review, Vibrational Spectroscopy 31, p. 1. Mari, E.A., 1998. Los Materiales Cerámicos, Editor. Librería 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) 2 solution 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.
Procedia Materials Science 1 (2012) 343 – 350
11th International Congress on Metallurgy & Materials SAM/CONAMET 2011.
Thermal treatment of kaolin: effect on the pozzolanic activity A. Tironia*, M. A. Trezzaa, E. F. Irassara, A. N. Scianb a
Facultad de Ingeniería
Universidad Nacional del Centro de la Provincia de Buenos Aires, Olavarría , Argentina. b Centro de Tecnología de Recursos Minerales y Cerámica 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.
2012Published PublishedbybyElsevier ElsevierLtd. Ltd.Selection Selectionand/or and/orpeer-review peer-review under responsibility 11th ©©2012 under responsibility of of SAM/ International2011, Congress on Metallurgy CONAMET Rosario, Argentina. & Materials SAM/CONAMET 2011. Keywords: kaolinite, kaolin, metakaolin, pozzolanas
* Corresponding author. Telefax.: 54-2284-451055. E-mail address: [email protected].
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
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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 CO2 emissions 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 reacts with 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) kaolinite
Al2O3.2SiO2 (s) + 2H2O (g) metakaolinite water
(1)
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 the Frattini 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 Al2O3 and 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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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 ºC during 2, 7 and 28 days. Then, comparing the amount of these ions with the solubility isotherm of Ca(OH)2 in 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)2 saturated solution at 40 °C with 2.00 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).
Intensity, Sqr (counts)
K
K
K K K
K A K K Q
5
10
15
20
25
KK K K
30
K
35
K K
K
40
2 , deg
Fig. 1. XRD pattern of kaolin (K: kaolinite, Q: quartz, A: anatase).
K K KK K
45
50
K
55
K K
K K
60
65
70
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A. Tironi et al. / Procedia Materials Science 1 (2012) 343 – 350
4000
1500
Wave number, cm
1000
470 Si-O 430 Si-O
538 Si-O-Al
789 Si-O-Al 754 Si-O-Al 696 Mg/Al-OH
939 Al-OH 912 Al-OH
1115 Si-O 1032 Si-O
3500
1009 Si-O
3669 -OH 3652 -OH
3619 -OH
3694 -OH
Transmitance, %
In FTIR spectrum (Fig. 2), the characteristic absorption bands of kaolinite were identified. Kaolinite has absorption bands between 3500 and 3750 cm-1 corresponding to stretching frequencies of OH groups (Wilson, 1987; Madejová, 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 band intensity at 3620 and 3700 cm-1. In this case P0 = 1,121>1, confirming that kaolinite has an ordered structure (Bich, 2009).
500
-1
Fig. 2. FTIR spectra of kaolin.
The DTA curve for kaolinite shows an endothermic peak in the temperature range 500-600 ºC due to dehydroxylation 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 ºC, the centre of endothermic peak appears at 577 ºC, and the maximum temperature for kaolinite dehydroxylation occurs at 700 ºC. 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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DTA, V
Weight loss, %
985 °C Exo
13.49 %
69 °C Endo 577 °C Endo
0
300
600
900
1200
1500
0
200
400
600
800
1000
1200
1400
Temperature, °C
Temperature, ºC
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
700/10
700/30
750/20
800/10
800/30
%DH
94
96
99
99
100
BSS, m2/kg
969
997
776
927
784
T, ºC / tres, min
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-1 to 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 wave numbers. These FTIR spectrums (Fig. 4) confirm the transformation of kaolinite into reactive amorphous phase by thermal treatment. For calcination temperature of 800°C and a residence time of 30 minutes, bands corresponding to amorphous phases were obtained with lower area (Fig. 4).
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3423 1090 3435
Transmitance, %
1086
812 465 809 463
1086
814 465
700/30
3439 1085 3441 1078 3500
800/10
750/20
3436
4000
800/30
1500
808 466
700/10 812 465
1000
500
-1
Wave number, cm
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 treatments are shown in Fig. 8. The EC decreases when the reaction age increases due to a consumption of Ca2+ 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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A. Tironi et al. / Procedia Materials 1 (2012) 343 – 350 18
18
2 days
16
14
700/30
14
12
750/20 800/10
[CaO], mmol/l
10
[CaO], mmol/l
700/10
16
800/30
8 6
700/10 700/30
7 days
750/20 800/10
12 10
800/30
8 6
4
4
2
2 0
0 30
40
50
60 70 [OH ], mmol/l
80
90
30
100
[CaO], mmol/l
Fig. 5. Result of Frattini test at 2 days.
40
50
60 70 [OH ], mmol/l
80
90
Fig. 6. Result of Frattini test at 7 days.
18 16 14 12 10 8 6 4 2 0
700/10
28 days
700/30 750/20 800/10 800/30
30
40
50
60 70 [OH ], mmol/l
80
90
100
Fig. 7. Result of Frattini test at 28 days.
2,0
2,0
1,8
1,8
1,6
1,6
750 °C / 2 days
1,4
1,4
800 °C / 7 days
1,2
1,2
EC, mS
EC, mS
800 °C / 2 days
1,0 0,8 0,6 0,4 0,2
800/30
0,8
700/10 800/10 750/20
0,6
14 Age, days
700 °C / 7 days 750 °C / 14 days
0,2
700 °C / 14 days
0,0 7
800 °C / 14 days
0,4
700/30 0
700 °C / 2 days 750 °C / 7 days
1,0
21
0,0 0
10
20
30
40
residence time, min
Fig. 8. Electrical Conductivity of samples in function of the reaction age
Fig. 9. Electrical Conductivity samples in function of the residence time
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A. Tironi et al. / Procedia Materials Science 1 (2012) 343 – 350
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 ºC during 30 min is higher than that obtained at 800 ºC. 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 à l'étude de l'activation thermique du kaolin: évolution de la structure cristallographique et activité pouzzolanique, Ph. D. Thesis, Institut National des Sciences Appliqués de Lyon, France. Bich, Ch., Ambroise, J., Péra, 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. Madejová, J., 2003. FTIR techniques in clay mineral studies: review, Vibrational Spectroscopy 31, p. 1. Mari, E.A., 1998. Los Materiales Cerámicos, Editor. Librería 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) 2 solution 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.