Preparation of geopolymer from fluidized bed combustion bottom ash

Preparation of geopolymer from fluidized bed combustion bottom ash

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Preparation of geopolymer from fluidized bed combustion bottom ash R. Slavik a,∗ , V. Bednarik a , M. Vondruska a , A. Nemec b a

Tomas Bata University in Zlin, Faculty of Technology, Department of Environment Protection Engineering, 762 72 Zlin, Czech Republic b Atel Energetika Zl´ın, Ltd., 76001 Zlin, Czech Republic

a r t i c l e

i n f o

a b s t r a c t

Article history:

This study has shown that it is possible to use fluidized bed combustion bottom ash (FBC-BA)

Received 15 March 2007

without any thermal activation as a partial- or full-replacement for kaolinitic raw material

Received in revised form

in geopolymerization. Test specimens prepared from FBC-BA have reached compressive

24 August 2007

strength of almost 50 MPa after 90 days of hardening. After 50 freeze-thaw cycles, the com-

Accepted 10 September 2007

pressive strength of test specimens has not decrease below 80% of compressive strength of unaffected reference specimens, which represents an acceptable freeze-thaw resistance. Testing of wet-dry resistance has shown that wet-dry cycles cause an increase of the com-

Keywords:

pressive strength values. Testing of acid resistance of prepared geopolymer specimens has

Fluidized bed combustion bottom

shown that replacement of usual kaolinitic raw material by FBC-BA results in slight increase

ash

of leachability.

Geopolymer

© 2007 Elsevier B.V. All rights reserved.

Compressive strength Freeze-thaw resistance Wet-dry resistance Acid resistance

1.

Introduction

At present, we are witnessing an intensive research in the field of geopolymer synthesis and preparation of new technically applicable materials from these geopolymers (Davidovits, 1991, 1994, 2005; Xu and Van Deventer, 2000). As it is known, the basic raw materials used for geopolymer synthesis are aluminosilicate minerals such as kaolinite, albite, stilbite (Xu and Van Deventer, 1999, 2003, 2002). These raw materials have to be transformed into reactive form by several hours heating at approximately 750 ◦ C before the geopolymer synthesis. This process, called activation of basic raw material, requires appreciable energy consumption.



Corresponding author. E-mail address: [email protected] (R. Slavik). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.09.008

In the presented paper, a replacement of classical aluminosilicate raw material by fluidized bed combustion bottom ash (FBC-BA) is tested. Fluidized bed combustion is considered as an advanced technology for energy utilization of coal from the viewpoint of both gas emission reduction and economical feasibility (Valk, 1995). FBC-BA is a by-product that has got a convenient chemical composition from the point of geopolymer synthesis (Bednarik et al., 2000) and, as well, it is a material produced at temperatures around 800 ◦ C with detention time of a few hours. The mentioned temperature and time represent almost ideal conditions for activation of kaolinitic raw materials needed for geopolymer synthesis. The amount of produced FBC-BA is relatively large at present and it will be

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probably increasing in the future (for example, in a town with 80,000 inhabitants, the heating plant produced 4500 tonnes of FBC-BA in the year 2006). The aim of the presented study is to test possibilities of FBC-BA application for industrial production of technically valuable materials by means of geopolymer synthesis. With regard to the fact that FBC-BA is predominantly disposed to landfills at present, a technology utilizing FBC-BA would offer significant economical and also environmental benefit.

2.

Materials and methods

2.1.

Fluidized bed combustion bottom ash (FBC-BA)

Table 1 – Chemical composition (wt.%) of FBC-BA sample and kaolinitic clay determined by XRF analysis Oxide

FBC-BA

Kaolinitic clay

Calibration error

Al2 O3 SiO2 K2 O CaO TiO2 Fe2 O3

20.5 38.2 1.1 17.9 1.3 5.5

23.9 67.2 0.24 0.12 0.37 0.35

± 0.200 ± 0.343 ± 0.013 ± 0.050 ± 0.004 ± 0.005

Analyzed using spectrometer ElvaX (Elvatech Ltd., http://www. elvatech.com).

Table 2 – Loss on ignition of FBC-BA sample A sample of FBC-BA was supplied by Atel Energetika Zlin, Ltd. (Czech Republic, Europe). A mixture of bituminous and brown coal was being used as the fuel in the heating plant in the time of the sample collection. The sample had the appearance of small grey stones with dimensions up to 1 cm. The sample was firstly pulverized in ball mill to decrease the particle size to 2 mm and the obtained material was subsequently milled in jet mill to final particle size bellow 10 ␮m, as it is shown in Fig. 1 (particle size distribution was measured by laser particle size analyzer CILAS 920, CILAS U.S., Inc.). It is obvious from the figure that the size of the most frequent particles is 5 ␮m. An approximate chemical composition of the FBC-BA sample was determined by X-ray fluorescence analysis (XRF); the results are shown in Table 1. If the FBC-BA composition is compared with composition of typical kaolin, which is classical raw material for geopolymer synthesis, it is obvious that FBC-BA contains markedly less SiO2 and Al2 O3 and relatively large amount of calcium. Considering the fact that the coal is combusted, stirred and mixed with limestone, the FBC-BA contains calcium in the form of CaO (product of limestone decomposition by combustion), CaCO3 (undecomposed

Temperature (◦ C)

LOI (wt.%)

550 800

7.5 11.0

residuum of limestone) and CaSO4 (product of SO2 fixation). Loss on ignition (LOI) values of the FBC-BA sample at 550 and 800 ◦ C are shown in Table 2. The value of LOI at 550 ◦ C reflects complete combustion of coal residues and the value of LOI at 800 ◦ C includes moreover decomposition of carbonate. From the LOI value at 550 ◦ C it is thus evident that the FBC-BA sample contained 7.5% of uncombusted coal residues. From the difference of LOI values at 800 and 550 ◦ C, considering that the weight loss is caused by the loss of CO2 , the content of undecomposed limestone can be calculated according following equation: % CaCO3 = (LOI800 − LOI550 ) ×

MCaCO3 MCO2

where M are values of molar mass. The calculated content of CaCO3 amounted to 7.96%. The content of free CaO was

Fig. 1 – Particle size distribution of the milled FBC-BA sample.

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Fig. 2 – Conductometric titration curve of FBC-BA water suspension; FBC-BA weight 0.2505 g, water volume 200 mL.

determined by conductometric titration of FBC-BA water suspension by HCl solution and amounted to 5.63%. The titration curve is shown in Fig. 2. The content of CaCO3 determined by conductometric titration amounted to 7.75%, what it is in acceptable agreement with result obtained from difference of LOI values.

composition determined by XRF is presented in Table 1. Kaolinite content determined by LOI at 1000 ◦ C amounted to ca. 47%. Particle size distribution is shown in Fig. 3. It is obvious from the figure that the clay contains particles up to 100 ␮m and approximately 60% of the material are particles smaller than 20 ␮m. The kaolinitic clay was thermally activated at 750 ◦ C for 6 h before the geopolymerization.

2.2. Kaolinitic clay—by-product from glass sand mining

2.3.

The kaolinitic clay was used as a comparative raw material for geopolymer preparation. The sample was provided by Sklopisek Strelec, Inc. (Czech Republic, Europe). Chemical

Sodium water glass with composition 30.2% SiO2 and 10.8% Na2 O was used. The sample was provided by Vodni sklo, Ltd., Brno (Czech Republic, Europe).

Water glass

Fig. 3 – Particle size distribution of kaolinitic clay sample.

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Table 3 – Composition of reaction mixtures for preparation of geopolymer specimens FBC-BA (g) Kaolinitic clay (g) Water glass (g) 5 M-NaOH (mL)

2.4.

0

10

25

50

65

75

90

100 100 10

90 90 9

75 85 8

50 80 7.5

35 70 6.5

25 65 6

10 60 5.5

Preparation of geopolymer specimens

Weighted quantities of FBC-BA and thermally activated kaolinitic clay were mixed together in a dry state; the content of FBC-BA in the mixture varied from 0 to 100%—see Table 3. The mixture was mixed for 5 min to achieve a good homogeneity. Then the other reaction components were added: water glass and solution of NaOH. The mixture was mixed intensively in a laboratory kneader for 15 min and then was filled into cylindrical plastic moulds (inner diameter 27.6 mm, height 50 mm). For removing of air bubbles, the filling was carried out on a vibration plate. The mixtures were kept in closed moulds for 24 h to solidify. Then the moulds were opened and the prepared specimens were left to harden in the moulds under laboratory conditions (relative air humidity ca. 45%, temperature 22 ◦ C) until the time of their testing.

0 55 5

(2 h at −20 ◦ C)/thawing (2 h at +20 ◦ C in water) and then their compressive strengths were measured. Simultaneously, compressive strengths of unaffected reference specimens were measured for comparison. Similarly the testing of wet-dry resistance was carried out: specimens were exposed to 50 cycles of drying (2 h at +80 ◦ C in hot air)/soaking (2 h at +20 ◦ C in water) and then their compressive strengths were measured. Chemical resistance against acidic solutions was tested by using a 0.1 M-HCl solution: specimens were firstly dried to constant weight (m0 ) at 105 ◦ C and then leached in HCl solution for 24 h. The volume of HCl solution in millilitres equalled to ten times the amount of the dried specimen weight in grams. Then the specimens were dried at 105 ◦ C to constant weight (m). The resistance was expressed as relative loss of specimen weight: m(%) =

2.5.

100

m0 − m × 100 m0

Assessment of prepared geopolymer specimens

The prepared specimens were assessed by compressive strength measurement, testing of freeze-thaw resistance, wetdry resistance and chemical resistance against leaching by acidic solution. Compressive strength was measured after 7, 28 and 90 days of specimens hardening. Details of the measurement are described in European norm EN 14617-15:2005, that is designed for testing of artificial stone. All compressive strength measurements were carried out with three simultaneously prepared specimens. Freeze-thaw test according to European norm EN 146175:2005 was carried out with specimens after 28 days of hardening: specimens were exposed to 50 cycles of freezing

3.

Results and discussion

Effect of FBC-BA content in the raw material on a quality of the prepared geopolymer is shown in Fig. 4. This figure presents results of the compressive strength measurements of the prepared specimens after 7, 28 and 90 days of hardening. It is evident that increasing of FBC-BA content to about 50% causes decreasing of the compressive strength (for example, 90 days’ specimens demonstrate a decrease from 57 to 49 MPa). The compressive strength of specimens prepared from the raw material with FBC-BA content higher than 50% shows no obvious trend (for example, values of 90 days’ specimens vary around 50 MPa).

Fig. 4 – Dependence of specimen compressive strength on FBC-BA content in raw material after 7, 28 and 90 days of hardening at temperature ca. 23 ◦ C. Presented values represent average compressive strengths of three simultaneously prepared and tested specimens.

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Fig. 5 – Results of freeze-thaw resistance and wet-dry resistance testing of specimens after 50 cycles in dependence on FBC-BA content in raw material. The percentage values at the column graphs represent relative comparison with reference specimens. The values of compressive strengths represent average values of three simultaneously prepared and tested specimens.

The fact that, for example, the geopolymer prepared from 100% FBC-BA shows the compressive strength after 90 days of hardening higher than 42.5 MPa (value required for ordinary concrete), is significant. This favourable result indicates exceptional potential of FBC-BA for technical applications. However, for practical applications, besides the compressive strength of non-affected specimens, a good resistance against impacts, such as temperature changes, moisture and effect of aqueous acidic solution, is also important. The compressive strengths of specimens exposed to 50 freeze-thaw or 50 wet-dry cycles are shown in Fig. 5. Again, it is obvious that only a small decrease of compressive strength occurs with increasing FBC-BA content. It is noteworthy that compressive strengths of all speci-

mens (including the one with 100% FBC-BA content) have not decreased after 50 freeze-thaw cycles below 80% of reference specimens’ compressive strengths. The frostresistance parameter (presented in Fig. 5) is higher than 75% for all tested specimens, so it can be concluded, that all tested specimens are frost-resistant for number of 50 cycles. This result meets the requirements for frostresistance of commonly used concrete (class T50). Thus, the freeze-thaw resistance tests confirmed that geopolymer prepared from FBC-BA has a potential for technical applications. Specimens exposed to wet-dry cycles show higher compressive strength than reference specimens, which proves the fact that higher temperatures accelerate the geopolymerization reaction, and thereby the specimen hardening. The study of the resistance against acidic solution, whose results are shown in Fig. 6, has provided seemingly inconsistent results. It is evident, that all specimens containing FBC-BA have higher leachability than the specimens without FBC-BA. The leachability increases up to FBC-BA content of 75%; however specimens with 90 and 100% of FBC-BA show a reverse trend—decrease of leachability. This can be explained through a high content of free CaO and CaCO3 in the FBC-BA, which causes neutralisation of HCl solution. Therefore, the specimens containing 90 and 100% of FBC-BA soften the acidity of HCl solution and further acid leaching cannot occur. Generally, it is possible to state, that the mass loss by leaching in 0.1 MHCl corresponds to the values of ordinary concrete (Bayoux et al., 1990).

4. Fig. 6 – Relative loss of specimen weight upon leaching in 0.1 M-HCl solution in dependence on FBC-BA content in raw material. The volume of 0.1 M-HCl was 1000 mL per 100 g of dried specimen.

Conclusions

The presented study has shown that the FBC-BA without any thermal activation can be used not only as a partial-, but also as the full-replacement for classical kaolinitic raw material.

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The results of testing of physical and chemical properties can be summarized as follows:

references

• Specimens prepared from the FBC-BA alone have reached the 90 days’ compressive strength nearly 50 MPa. • Testing of freeze-thaw resistance has shown a decrease of compressive strength after 50 cycles to approximately 80% in comparison with reference specimens, which represents acceptable freeze-thaw resistance. • Testing of wet-dry resistance has shown an increase of specimens’ compressive strengths. The increased temperature accelerates the geopolymerization reaction and consequently the specimen hardening. • Testing of resistance against leaching by 0.1 M solution of HCl has shown that the replacement of kaolinitic material by FBC-BA has slightly increased the geopolymer specimen leachability. It means that technical material prepared from FBC-BA without any modifying admixtures or appropriate surface treatment is not suitable for applications requiring direct resistance against acid environment.

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Acknowledgement Financial support of this study was provided by the Ministry of Education of the Czech Republic—project VZ MSM 7088352101.