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Immobilization of strontium-loaded zeolite A by metakaolin based-geopolymer
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Immobilization of strontium-loaded zeolite A by metakaolin basedgeopolymer ⁎
Zhonghui Xua, , Zao Jiangb, Dandan Wub, Xi Pengc, Yahong Xub, Na Lib, Yijin Qib, Ping Lib a Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China b Non-Coal Mine Safety Technology Key Laboratory of Sichuan Province Colleges and Universities, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China c Key Subject Laboratory of National Defense for Radioactive Waste and Environmental Security, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Immobilization Strontium Zeolite A Geopolymer Radioactive waste
Zeolites are the preferred inorganic ion exchange materials for purifying radioactive waste liquid. Radionuclideloaded zeolites, which are considered to be radioactive waste, are strictly required to be encapsulated within a solid matrix. In this paper, we investigate the feasibility of immobilizing exhausted zeolite A, loaded with 90Sr radionuclide, in metakaolin based-geopolymer. The geopolymer solidification blocks had better mechanical performance and leaching resistance in deionized water, sulfuric acid, magnesium sulfuric and acetic acid buffer solutions than the cemented blocks. While the compressive strength of the geopolymer solidification product was 37.62 MPa after curing for 28 days, the equivalent value for the cement block was only 11.32 MPa. The geopolymer solidification blocks also exhibited even lower compressive strength loss after high-temperature and freeze-thaw cycles tests. XRD and EDS analysis indicated that most of the strontium radionuclide in the geopolymer solidification blocks was incorporated in the zeolite structure as the charge balancing cation. The microscopic analysis revealed that geopolymer matrix appeared more compact and dense, and encapsulated the Sr-loaded zeolite A more tightly than did the cement. Therefore, it could be concluded that metakaolin basedgeopolymer are more compatible with exhausted zeolite A and present a remarkable advantage for radioactive waste immobilization.
1. Introduction Nuclear power is a potential and major solution to help address the global energy crisis. However, nuclear power plants may generate a large volume of low and intermediate level aqueous, nuclear wastes. These aqueous wastes need treatment to concentrate radionuclides and reduce storage volume. Zeolites are the preferred inorganic ion exchange materials for radionuclide concentrations from aqueous nuclear wastes because of their radiation stability, high selectivity, and cation exchange capacity [1–3]. Nevertheless, radionuclide-loaded zeolites are still considered to be radioactive waste. On account of the relatively long half-life of radionuclides, they require encapsulation in a solid matrix. Cement-based materials are widely proposed and investigated for use in the immobilization of low and intermediate level radioactive wastes [2,4]. However, hardened cement is a porous material with poor corrosion resistance and thermal stability [4,5]. As a result, cement is
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not the optimal material for nuclear waste immobilization. Recently, geopolymer has gradually presented an alternative, as they possess high mechanical strength [4], leaching resistance [6–8], and thermal stability [4,9]. Many scholars have reported that geopolymer matrix is very effective in immobilizing hazardous wastes, such as municipal solid waste incinerator (MSWI) fly ash [10,11], chromite ore processing residue [7], and radioactive wastes [3,4]. Geopolymers are formed by interactions between solid aluminosilicate minerals and aqueous alkali silicate or hydroxide solutions. The structure of geopolymer is likely confined to a closed cage cavity that can firmly adsorb radionuclides and other toxic substances. Many applications where geopolymer has been adopted to immobilize simulated water-soluble radionuclide ions have been reported. Scholars have evaluated the leaching behavior of radionuclides from geopolymer materials [4,12,13] and have explored the thermal stability of geopolymer solidification blocks [4]. Although the leaching of Cs+ from geopolymer waste forms increased as a result of the effect of
Corresponding author. E-mail address: [email protected] (Z. Xu).
http://dx.doi.org/10.1016/j.ceramint.2016.12.092 Received 7 October 2016; Received in revised form 17 December 2016; Accepted 17 December 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xu, Z., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.092
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a standard curing box at 25 ± 0.5 °C with humidity of 90 ± 1% and under ambient pressure for 24 h. After being removed from the molds, the samples were subjected to curing at room temperature without the polyethylene film for an additional 27 days.
gamma-ray irradiation on the hardened block, geopolymer was still proven to be effective for radionuclide immobilization [14]. Encapsulating spent radionuclide-loaded zeolites within geopolymer matrices may be an effective and alternative option. However, geopolymer has structures that are similar to that of zeolites at the nanoscale, and they may transform into each other under certain alkaline conditions [15–19]. Thus, reactions or decomposition of spent zeolite may occur under the high pH condition of the alkaline activator solution and lead to the release of radionuclides into the geopolymer gel [3]. Furthermore, the radionuclides could delay the geopolymer gel formation and influence the performance of the hardened block [20]. Therefore, the aim of this research was to examine the immobilization performance of exhausted zeolite A, loaded with 90Sr radionuclide, in a metakaolin-based geopolymer matrix and to compare the results with that of an ordinary Portland cement matrix. The leaching characteristics of strontium radionuclide from geopolymer and cement solidification blocks were investigated within the context of four different extraction solutions. Thermal stability by way of both freeze-thaw and high-temperature performance was also given full consideration.
2.3. Static leaching tests
2. Materials and methods
Static leaching tests were conducted in four different extraction solutions, including deionized water, sulfuric acid solution (pH=1), magnesium sulfuric solution (5 wt% MgSO4) and acetic acid buffer solution (pH=3.6). Acid and brine solutions were adopted to detect the leaching behavior of Sr from hardened blocks of cement and metakaolin-based geopolymer under extreme conditions. Each prepared sample was immersed in 200 ml of extraction solution in sealed polyethylene bottles. All bottles were stored in the laboratory at room temperature. The leachate was sampled and the entire 200 ml solution was replaced with a fresh extraction solution after the 1st, 3rd, 7th, 10th, 14th, 21st, 28th, 35th and 42nd day. After this, the sampled leachate was analyzed using Atomic Absorption Spectroscopy (AA700, PE, US). The cumulative leach fraction (cm) was calculated according to the relevant literature [2].
2.1. Materials
2.4. Mechanical performance and thermal stability tests
The sodium silicate solution (molar mass ratio SiO2/Na2O=3.3, 26 wt% SiO2, 8.2 wt% Na2O) was obtained from Guangzhou Huixin Chemical Industry Co., Ltd (Guangzhou, China) and adjusted by a certain amount of sodium hydroxide (analytical reagent grade) to make a solution with a modulus (i.e. molar mass ratio SiO2/Na2O) of 0.8, which was used as the activator in the geopolymerization. Kaolinite was obtained from the Shanghai Fengxian Fengcheng Chemical Reagent Factory in China. The metakaolin was prepared by calcining kaolinite at 850 °C for 2 h. The cement was supplied by Beichuan Sixing Cement Co., Ltd (Sichuan, China). The chemical compositions of metakaolin and cement are presented in Table 1. The zeolite A was provided by Chengdu Kelong Chemical Reagent Factory (Sichuan, China). Its chemical composition is given in Table 1. Strontium was exchanged onto zeolite A by contacting 50 g of zeolite A with 1 L of 1.5 g/L Sr(NO3)2 solution (labeled with 90Sr) under stirring by magnetic stirring apparatus for 2 h. The suspensions were then filtered via a vacuum filter. The strontium-loaded zeolite A obtained from adsorption process was dried at 105 °C for 24 h.
2.4.1. Thermal stability After standard curing for 28 days, the solidification products were subjected to freeze-thaw cycles. During freezing the samples were kept at −20 °C for 3 h and immediately immersed in water at 15–20 °C for 4 h to thaw. Thus, a full freeze-thaw cycle was achieved within 7 h. The compressive strength of each matrix was measured after 15 freeze-thaw cycles. Hardened blocks of geopolymer and cement specimens were exposed to elevated temperatures at 600, 800, and 1000 °C under a gradual and incremental rate of approximately 5 °C/min from room temperature. After holding for 2 h, the specimens were naturally cooled in the furnace.
2.2. Sample preparation
2.5. Microstructural and mineralogical characterization
Geopolymer paste was prepared with metakaolin, zeolite, water and water glass at the mass ratio of 1:1:0.6:0.8. To compare geopolymeric and conventional immobilization methods, OPC was also used to prepare an additional solidification matrix according to a fixed mass ratio (cement:zeolite:water=1:1:0.5). Before curing, the slurries were cast into 20 mm cube molds and vibrated for 10 min to release bubbles. Subsequently, the molds were sealed with polyethylene film and kept in
The prepared samples included the original zeolite A, exhausted zeolite A and solidification matrices, which were characterized via XRD and FTIR. The microscopic analysis of the geopolymer and cement solidification blocks was conducted using a Zeiss Ultra 55 scanning electron microscope.
Table 1 Chemical compositions of zeolite A, cement and metakaolin.
3.1. Microstructural and mineralogical characterization
Component
Zeolite A
OPC
Metakaolin
SiO2 Al2O3 Na2O MgO Fe2O3 CaO K2O SO3 Others
54.6 27.8 11.67 2.77 1.51 0.54 0.38
29.89 8.15 0.59 2.2 4.04 49.82 1.28 2.78 1.25
54.25 43.92 0.14
0.73
2.4.2. Mechanical performance The compressive strengths of the 28-d cured geopolymer and cement blocks were measured with a CMT5504 compressive strength testing apparatus (Shenzhen, China). The specimens were also measured after thermal stability tests. Each result was obtained by taking an average of the results from three specimens.
3. Results and discussion
3.1.1. X-ray diffraction Fig. 1 exhibits XRD patterns of zeolite A, Sr loaded-zeolite A and solidification blocks. According to the chemical composition of the zeolite A and the XRD analysis, the main crystalline phase of the four specimens is zeolite NaA. The secondary phases of the zeolite A were also mainly composed of zeolite A-X, which is a series of tecto-alumosilicate of K, Ag, Co and Pb, etc. This resulted from the impurities of the zeolite precursors, which are not marked in the figure. Owing to the ion exchange process that Sr replaced Na as the charge balancing cation in the zeolite structure, zeolite A (Sr) was detected in the XRD pattern of
0.39 0.13 0.41 0.76
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occurred during the geopolymer and cement encapsulation processes. In addition, the asymmetric stretching of CO32− at 1434 cm−1 and the out-of-plane bending at 876 cm−1 in the FTIR spectrum of the cement solidification matrix indicate carbonation during the curing process [24,25], which is consistent with the XRD result. 3.1.3. SEM/EDS analysis SEM images of both geopolymer and cement solidification blocks are shown in Figs. 3 and 4. The geopolymer block appears more compact and dense (Fig. 3), while the cement specimen tends to be more porous (Fig. 4). The figures also indicate that metakaolin basedgeopolymer encapsulates Sr loaded-zeolite A more tightly than does cement. These characteristics may be the direct reason for the better leaching resistance of geopolymer solidification blocks. Furthermore, the surface morphology of zeolite phase in cement shows more obvious and severe deterioration. EDS analysis results are shown in Table 2. The Si/Al ratio of zeolite phase in solidification blocks and Sr content in geopolymer and cement gel phase indicated that the cement encapsulation process caused more serious dealumination of exhausted zeolite A, and a lot more strontium radionuclide released into the cement gel. It is noteworthy that only a small proportion of Sr was detected within the geopolymer and cement gel, demonstrating that most of the strontium radionuclide was still incorporated into the zeolite structure.
Fig. 1. XRD patterns of zeolite specimens and solidification blocks.
Sr loaded-zeolite A (PDF 38-0243, (Sr, Na)Al2Si1.85O7.7·xH2O, 2θ position (°): 7.146, 10.136, 23.940, 25.020, 26.071, 27.064, 29.899, 30.774, 32.496, 34.128). Moreover, Calcite was found in the XRD pattern of the cement solidification matrix, probably resulting from the cement hydration reactions involving atmospheric CO2. Quartz was also observed in the XRD patterns, originating from the impurity of zeolite A and cement. Although X-ray diffraction peak intensities of zeolite phases in the XRD patterns of geopolymer and cement blocks decreased obviously, the crystal phase of zeolite A did not collapse. Meanwhile, there was no new crystalline phase found to be associated with the strontium radionuclide in the XRD patterns of geopolymer and cement solidification specimens. Most of the strontium radionuclide within the solidification blocks was incorporated into the zeolite structure as the charge balancing cation, or chemically bonded into the amorphous phase of the geopolymer and cement matrices.
3.2. Leaching The variations of the cumulative leach fractions of Sr2+ ions from geopolymer and cement blocks which were immersed in four different extraction solutions are depicted in Fig. 5. The leaching behavior can be described as a combination of two processes [2]: (1) surface wash-off mechanism; (2) diffusion stage mechanism. In the initial period, the surface wash-off mechanism mainly determined the leaching behavior and resulted in nearly a similar leaching rate of Sr2+ ions between geopolymer and cement solidification blocks within deionized water (Fig. 5a). When Sr2+ ions present in the surface of solidification blocks had been leached out into solution, ions from deeper within solidification matrices began to diffuse into the solution through longer pathways, thus decreasing the leaching rate in the later period. Meanwhile, as a result of the higher permeability of the cement specimen [4,9], the cumulative leach fractions of Sr2+ ions from the geopolymer solidification matrix in the later period were lower than those from the cement matrix in deionized water. The leaching rates of Sr2+ ions from geopolymer and cement solidification blocks in magnesium sulfate solutions (Fig. 5b) were much higher than those in deionized water and sulfuric acid solutions (Fig. 5a and c). On account of the high concentration of magnesium sulfate solutions, ion exchange reactions may have occurred between Mg2+ ions and Sr2+ ions in solidification blocks, resulting in much greater leaching rates. The leaching behavior of Sr2+ ions in the magnesium sulfate solutions once again proved that most of the strontium radionuclide within solidification blocks remained within the zeolite structure, acting as the charge balancing cation. In addition, leaching rates of Sr2+ ions from the cement blocks were significantly larger than those from geopolymer matrix in the later period because of the higher porosity of the cement matrix. In an acidic environment, the geopolymer solidification blocks remained structurally stable with slightly softer surface, while the cement solidification specimens exhibited severe deterioration and even complete decomposition within the acetic acid buffer solution (Fig. 5c and d). The cumulative leach fractions of Sr2+ ions from the cement solidification blocks in the sulfuric acid solution were also obviously larger than those from geopolymer solidification matrices (Fig. 5c). Many studies reported that the presence of Ca-rich phases such as calcium hydroxide, ettringite and calcium silicate hydrate gel in the hardened cement was the primary causal factor behind the dissolution and decalcification of the binder system in the acidic media
3.1.2. Fourier transform infrared spectroscopy Fig. 2 shows the FTIR spectra of zeolite A, Sr-loaded zeolite A and solidification blocks. As the figure depicts, there is no significant difference between the FTIR spectra of the four different samples, as a result of the zeolite-like structure of geopolymer [15,16]. The broad band at 3450–3425 cm−1 and the sharp band at 1650–1640 cm−1 are attributed to the water component of the OH stretching band and the chemically bonded water (H–O–H bending) respectively [8]. The bands are linked to the presence of weak H2O [19,21,22] and seem to be sensitive for ion exchange. The bands around 1070–950 cm−1 corresponding to Si(Al)–O stretching vibration of zeolite structure, NASH gel in geopolymer and CSH gel in cement also can be identified in the four FTIR spectra [23–25]. The shift of these bands towards the higher frequencies is the evidence for the breakdown of Al-O bond in AlO4 tetrahedrons and the removal of aluminium from the zeolite lattice [22]. Thus, we could conclude that dealumination of zeolite A
Fig. 2. FTIR spectra of zeolite specimens and solidification blocks.
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Fig. 3. SEM micrographs of geopolymer solidification blocks.
Fig. 4. SEM micrographs of cement solidification blocks.
product was equal to 37.62 MPa, while the cement block exhibited a strength of only 11.32 MPa. The major difference indicated better compatibility between geopolymer and the exhausted zeolite A. All of the cement specimens cracked after calcination at elevated temperatures, while the compressive strength of geopolymer blocks showed a slight downward trend, with 26.74 MPa after 2 h of calcination at 1000 °C. After 15 freeze-thaw cycles, the geopolymer block exhibited better mechanical performance than did the cement block, with only 9.4% compressive strength loss. Although the differing mechanical performance of the geopolymer and cement solidification blocks may be an important reason for their different thermal stabilities, the role played by different pore structures could not be ignored. Initially, the total porosity of the geopolymer block was much less than that of cement materials [4,9]. Then, thermal expansion and contraction of air in the more developed pore structure of the cement blocks may have caused more serious internal micro cracking and obvious compressive strength loss [4,9,26]. In addition, the larger proportion of connected pores in the geopolymer blocks may have led to lower internal pore pressure and less serious destruction at elevated temperatures [27]. From the results obtained above, we conclude that the geopolymer matrix has an obvious advantage in terms of excellent thermal stability.
Table 2 EDS analysis results of geopolymer and cement solidification blocks. Areas
A B C D
Atom (%) Si
Al
Na
13.53 21.47 10.67 13.05
11.17 11.69 1.94 5.52
11.02 8.76 4.02
Ca
Sr
4.63
0.36 0.04 1.38 0.29
[4,8,23]. Although acid attack also caused dealumination and depolymerization of the geopolymer structure [8,23], the geopolymer solidification matrix exhibited better performance, which was attributed to its stable cross-linked aluminosilicate polymer structure. The metakaolin based-geopolymer seems to be more suitable for immobilization of the exhausted zeolite A.
3.3. Thermal stability Solidification blocks may be buried in some areas, where the temperature varies greatly between day and night. Meanwhile, radioactive disintegration gradually releases decay heat and can lead to a temperature rise of solidification blocks throughout the long half-life of radioactive waste. Thus, solidification blocks were subjected to testing their thermal stability. The changes in compressive strength of the solidification blocks both before and after those thermal stability tests are presented in Table 3. Before the thermal stability tests were conducted, the compressive strength of geopolymer solidification
4. Conclusions This paper investigated the comparative performance of the encapsulation of strontium-loaded zeolite A by metakaolin-based geopolymer and ordinary Portland cement. Based on the results presented above, the following conclusions can be drawn: 4
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Fig. 5. CFLC of Sr2+ ions in four different solutions: (a) deionized water, (b) magnesium sulfate solution, (c) sulfuric acid solution, (d) acetic acid buffer solution.
Talent Foundation of Sichuan Province (No. 20132065).
Table 3 Compressive strength of geopolymer/OPC solidification blocks before and after thermal stability tests/MPa. Before
Geopolymer Cement
37.62 11.32
After calcination
References
After freeze-thaw cycles
600 °C
800 °C
1000 °C
15 cycles
Loss (%)
33.53 cracked
28.44 cracked
26.74 cracked
34.08 8.95
9.4 21
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1. Metakaolin based-geopolymer solidification blocks exhibited better mechanical performance. While the compressive strength of geopolymer solidification product was 37.62 MPa after curing for 28 days, the cement solidification product measured only 11.32 MPa. Meanwhile, the microstructure of geopolymer specimens appeared both more compact and more dense. 2. The geopolymer solidification blocks had better leaching resistance than did the cement specimens in deionized water, sulfuric acid solution (pH=1), magnesium sulfuric solution (5 wt% MgSO4) and acetic acid buffer solution (pH=3.6). Through XRD and EDS analysis, it also could be concluded that most of the strontium radionuclide within solidification blocks remained within the zeolite structure, acting as the charge balancing cation. 3. The geopolymer solidification block had better thermal stability than did the cemented form. The compressive strength of the geopolymer solidification matrix measured up to 26.74 MPa after 2 h of calcination at 1000 °C and up to 34.08 MPa after 15 freeze-thaw cycles. In contrast, the cement solidification products cracked after 2 h of calcination at elevated temperatures and lost 21% of the initial associated compressive strength after 15 freeze-thaw cycles.
Acknowledgements This work was supported by the Natural Science Foundation of China (No. 51404200), and the Science and Technology Innovation 5
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Immobilization of strontium-loaded zeolite A by metakaolin basedgeopolymer ⁎
Zhonghui Xua, , Zao Jiangb, Dandan Wub, Xi Pengc, Yahong Xub, Na Lib, Yijin Qib, Ping Lib a Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China b Non-Coal Mine Safety Technology Key Laboratory of Sichuan Province Colleges and Universities, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China c Key Subject Laboratory of National Defense for Radioactive Waste and Environmental Security, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Immobilization Strontium Zeolite A Geopolymer Radioactive waste
Zeolites are the preferred inorganic ion exchange materials for purifying radioactive waste liquid. Radionuclideloaded zeolites, which are considered to be radioactive waste, are strictly required to be encapsulated within a solid matrix. In this paper, we investigate the feasibility of immobilizing exhausted zeolite A, loaded with 90Sr radionuclide, in metakaolin based-geopolymer. The geopolymer solidification blocks had better mechanical performance and leaching resistance in deionized water, sulfuric acid, magnesium sulfuric and acetic acid buffer solutions than the cemented blocks. While the compressive strength of the geopolymer solidification product was 37.62 MPa after curing for 28 days, the equivalent value for the cement block was only 11.32 MPa. The geopolymer solidification blocks also exhibited even lower compressive strength loss after high-temperature and freeze-thaw cycles tests. XRD and EDS analysis indicated that most of the strontium radionuclide in the geopolymer solidification blocks was incorporated in the zeolite structure as the charge balancing cation. The microscopic analysis revealed that geopolymer matrix appeared more compact and dense, and encapsulated the Sr-loaded zeolite A more tightly than did the cement. Therefore, it could be concluded that metakaolin basedgeopolymer are more compatible with exhausted zeolite A and present a remarkable advantage for radioactive waste immobilization.
1. Introduction Nuclear power is a potential and major solution to help address the global energy crisis. However, nuclear power plants may generate a large volume of low and intermediate level aqueous, nuclear wastes. These aqueous wastes need treatment to concentrate radionuclides and reduce storage volume. Zeolites are the preferred inorganic ion exchange materials for radionuclide concentrations from aqueous nuclear wastes because of their radiation stability, high selectivity, and cation exchange capacity [1–3]. Nevertheless, radionuclide-loaded zeolites are still considered to be radioactive waste. On account of the relatively long half-life of radionuclides, they require encapsulation in a solid matrix. Cement-based materials are widely proposed and investigated for use in the immobilization of low and intermediate level radioactive wastes [2,4]. However, hardened cement is a porous material with poor corrosion resistance and thermal stability [4,5]. As a result, cement is
⁎
not the optimal material for nuclear waste immobilization. Recently, geopolymer has gradually presented an alternative, as they possess high mechanical strength [4], leaching resistance [6–8], and thermal stability [4,9]. Many scholars have reported that geopolymer matrix is very effective in immobilizing hazardous wastes, such as municipal solid waste incinerator (MSWI) fly ash [10,11], chromite ore processing residue [7], and radioactive wastes [3,4]. Geopolymers are formed by interactions between solid aluminosilicate minerals and aqueous alkali silicate or hydroxide solutions. The structure of geopolymer is likely confined to a closed cage cavity that can firmly adsorb radionuclides and other toxic substances. Many applications where geopolymer has been adopted to immobilize simulated water-soluble radionuclide ions have been reported. Scholars have evaluated the leaching behavior of radionuclides from geopolymer materials [4,12,13] and have explored the thermal stability of geopolymer solidification blocks [4]. Although the leaching of Cs+ from geopolymer waste forms increased as a result of the effect of
Corresponding author. E-mail address: [email protected] (Z. Xu).
http://dx.doi.org/10.1016/j.ceramint.2016.12.092 Received 7 October 2016; Received in revised form 17 December 2016; Accepted 17 December 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xu, Z., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.092
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a standard curing box at 25 ± 0.5 °C with humidity of 90 ± 1% and under ambient pressure for 24 h. After being removed from the molds, the samples were subjected to curing at room temperature without the polyethylene film for an additional 27 days.
gamma-ray irradiation on the hardened block, geopolymer was still proven to be effective for radionuclide immobilization [14]. Encapsulating spent radionuclide-loaded zeolites within geopolymer matrices may be an effective and alternative option. However, geopolymer has structures that are similar to that of zeolites at the nanoscale, and they may transform into each other under certain alkaline conditions [15–19]. Thus, reactions or decomposition of spent zeolite may occur under the high pH condition of the alkaline activator solution and lead to the release of radionuclides into the geopolymer gel [3]. Furthermore, the radionuclides could delay the geopolymer gel formation and influence the performance of the hardened block [20]. Therefore, the aim of this research was to examine the immobilization performance of exhausted zeolite A, loaded with 90Sr radionuclide, in a metakaolin-based geopolymer matrix and to compare the results with that of an ordinary Portland cement matrix. The leaching characteristics of strontium radionuclide from geopolymer and cement solidification blocks were investigated within the context of four different extraction solutions. Thermal stability by way of both freeze-thaw and high-temperature performance was also given full consideration.
2.3. Static leaching tests
2. Materials and methods
Static leaching tests were conducted in four different extraction solutions, including deionized water, sulfuric acid solution (pH=1), magnesium sulfuric solution (5 wt% MgSO4) and acetic acid buffer solution (pH=3.6). Acid and brine solutions were adopted to detect the leaching behavior of Sr from hardened blocks of cement and metakaolin-based geopolymer under extreme conditions. Each prepared sample was immersed in 200 ml of extraction solution in sealed polyethylene bottles. All bottles were stored in the laboratory at room temperature. The leachate was sampled and the entire 200 ml solution was replaced with a fresh extraction solution after the 1st, 3rd, 7th, 10th, 14th, 21st, 28th, 35th and 42nd day. After this, the sampled leachate was analyzed using Atomic Absorption Spectroscopy (AA700, PE, US). The cumulative leach fraction (cm) was calculated according to the relevant literature [2].
2.1. Materials
2.4. Mechanical performance and thermal stability tests
The sodium silicate solution (molar mass ratio SiO2/Na2O=3.3, 26 wt% SiO2, 8.2 wt% Na2O) was obtained from Guangzhou Huixin Chemical Industry Co., Ltd (Guangzhou, China) and adjusted by a certain amount of sodium hydroxide (analytical reagent grade) to make a solution with a modulus (i.e. molar mass ratio SiO2/Na2O) of 0.8, which was used as the activator in the geopolymerization. Kaolinite was obtained from the Shanghai Fengxian Fengcheng Chemical Reagent Factory in China. The metakaolin was prepared by calcining kaolinite at 850 °C for 2 h. The cement was supplied by Beichuan Sixing Cement Co., Ltd (Sichuan, China). The chemical compositions of metakaolin and cement are presented in Table 1. The zeolite A was provided by Chengdu Kelong Chemical Reagent Factory (Sichuan, China). Its chemical composition is given in Table 1. Strontium was exchanged onto zeolite A by contacting 50 g of zeolite A with 1 L of 1.5 g/L Sr(NO3)2 solution (labeled with 90Sr) under stirring by magnetic stirring apparatus for 2 h. The suspensions were then filtered via a vacuum filter. The strontium-loaded zeolite A obtained from adsorption process was dried at 105 °C for 24 h.
2.4.1. Thermal stability After standard curing for 28 days, the solidification products were subjected to freeze-thaw cycles. During freezing the samples were kept at −20 °C for 3 h and immediately immersed in water at 15–20 °C for 4 h to thaw. Thus, a full freeze-thaw cycle was achieved within 7 h. The compressive strength of each matrix was measured after 15 freeze-thaw cycles. Hardened blocks of geopolymer and cement specimens were exposed to elevated temperatures at 600, 800, and 1000 °C under a gradual and incremental rate of approximately 5 °C/min from room temperature. After holding for 2 h, the specimens were naturally cooled in the furnace.
2.2. Sample preparation
2.5. Microstructural and mineralogical characterization
Geopolymer paste was prepared with metakaolin, zeolite, water and water glass at the mass ratio of 1:1:0.6:0.8. To compare geopolymeric and conventional immobilization methods, OPC was also used to prepare an additional solidification matrix according to a fixed mass ratio (cement:zeolite:water=1:1:0.5). Before curing, the slurries were cast into 20 mm cube molds and vibrated for 10 min to release bubbles. Subsequently, the molds were sealed with polyethylene film and kept in
The prepared samples included the original zeolite A, exhausted zeolite A and solidification matrices, which were characterized via XRD and FTIR. The microscopic analysis of the geopolymer and cement solidification blocks was conducted using a Zeiss Ultra 55 scanning electron microscope.
Table 1 Chemical compositions of zeolite A, cement and metakaolin.
3.1. Microstructural and mineralogical characterization
Component
Zeolite A
OPC
Metakaolin
SiO2 Al2O3 Na2O MgO Fe2O3 CaO K2O SO3 Others
54.6 27.8 11.67 2.77 1.51 0.54 0.38
29.89 8.15 0.59 2.2 4.04 49.82 1.28 2.78 1.25
54.25 43.92 0.14
0.73
2.4.2. Mechanical performance The compressive strengths of the 28-d cured geopolymer and cement blocks were measured with a CMT5504 compressive strength testing apparatus (Shenzhen, China). The specimens were also measured after thermal stability tests. Each result was obtained by taking an average of the results from three specimens.
3. Results and discussion
3.1.1. X-ray diffraction Fig. 1 exhibits XRD patterns of zeolite A, Sr loaded-zeolite A and solidification blocks. According to the chemical composition of the zeolite A and the XRD analysis, the main crystalline phase of the four specimens is zeolite NaA. The secondary phases of the zeolite A were also mainly composed of zeolite A-X, which is a series of tecto-alumosilicate of K, Ag, Co and Pb, etc. This resulted from the impurities of the zeolite precursors, which are not marked in the figure. Owing to the ion exchange process that Sr replaced Na as the charge balancing cation in the zeolite structure, zeolite A (Sr) was detected in the XRD pattern of
0.39 0.13 0.41 0.76
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occurred during the geopolymer and cement encapsulation processes. In addition, the asymmetric stretching of CO32− at 1434 cm−1 and the out-of-plane bending at 876 cm−1 in the FTIR spectrum of the cement solidification matrix indicate carbonation during the curing process [24,25], which is consistent with the XRD result. 3.1.3. SEM/EDS analysis SEM images of both geopolymer and cement solidification blocks are shown in Figs. 3 and 4. The geopolymer block appears more compact and dense (Fig. 3), while the cement specimen tends to be more porous (Fig. 4). The figures also indicate that metakaolin basedgeopolymer encapsulates Sr loaded-zeolite A more tightly than does cement. These characteristics may be the direct reason for the better leaching resistance of geopolymer solidification blocks. Furthermore, the surface morphology of zeolite phase in cement shows more obvious and severe deterioration. EDS analysis results are shown in Table 2. The Si/Al ratio of zeolite phase in solidification blocks and Sr content in geopolymer and cement gel phase indicated that the cement encapsulation process caused more serious dealumination of exhausted zeolite A, and a lot more strontium radionuclide released into the cement gel. It is noteworthy that only a small proportion of Sr was detected within the geopolymer and cement gel, demonstrating that most of the strontium radionuclide was still incorporated into the zeolite structure.
Fig. 1. XRD patterns of zeolite specimens and solidification blocks.
Sr loaded-zeolite A (PDF 38-0243, (Sr, Na)Al2Si1.85O7.7·xH2O, 2θ position (°): 7.146, 10.136, 23.940, 25.020, 26.071, 27.064, 29.899, 30.774, 32.496, 34.128). Moreover, Calcite was found in the XRD pattern of the cement solidification matrix, probably resulting from the cement hydration reactions involving atmospheric CO2. Quartz was also observed in the XRD patterns, originating from the impurity of zeolite A and cement. Although X-ray diffraction peak intensities of zeolite phases in the XRD patterns of geopolymer and cement blocks decreased obviously, the crystal phase of zeolite A did not collapse. Meanwhile, there was no new crystalline phase found to be associated with the strontium radionuclide in the XRD patterns of geopolymer and cement solidification specimens. Most of the strontium radionuclide within the solidification blocks was incorporated into the zeolite structure as the charge balancing cation, or chemically bonded into the amorphous phase of the geopolymer and cement matrices.
3.2. Leaching The variations of the cumulative leach fractions of Sr2+ ions from geopolymer and cement blocks which were immersed in four different extraction solutions are depicted in Fig. 5. The leaching behavior can be described as a combination of two processes [2]: (1) surface wash-off mechanism; (2) diffusion stage mechanism. In the initial period, the surface wash-off mechanism mainly determined the leaching behavior and resulted in nearly a similar leaching rate of Sr2+ ions between geopolymer and cement solidification blocks within deionized water (Fig. 5a). When Sr2+ ions present in the surface of solidification blocks had been leached out into solution, ions from deeper within solidification matrices began to diffuse into the solution through longer pathways, thus decreasing the leaching rate in the later period. Meanwhile, as a result of the higher permeability of the cement specimen [4,9], the cumulative leach fractions of Sr2+ ions from the geopolymer solidification matrix in the later period were lower than those from the cement matrix in deionized water. The leaching rates of Sr2+ ions from geopolymer and cement solidification blocks in magnesium sulfate solutions (Fig. 5b) were much higher than those in deionized water and sulfuric acid solutions (Fig. 5a and c). On account of the high concentration of magnesium sulfate solutions, ion exchange reactions may have occurred between Mg2+ ions and Sr2+ ions in solidification blocks, resulting in much greater leaching rates. The leaching behavior of Sr2+ ions in the magnesium sulfate solutions once again proved that most of the strontium radionuclide within solidification blocks remained within the zeolite structure, acting as the charge balancing cation. In addition, leaching rates of Sr2+ ions from the cement blocks were significantly larger than those from geopolymer matrix in the later period because of the higher porosity of the cement matrix. In an acidic environment, the geopolymer solidification blocks remained structurally stable with slightly softer surface, while the cement solidification specimens exhibited severe deterioration and even complete decomposition within the acetic acid buffer solution (Fig. 5c and d). The cumulative leach fractions of Sr2+ ions from the cement solidification blocks in the sulfuric acid solution were also obviously larger than those from geopolymer solidification matrices (Fig. 5c). Many studies reported that the presence of Ca-rich phases such as calcium hydroxide, ettringite and calcium silicate hydrate gel in the hardened cement was the primary causal factor behind the dissolution and decalcification of the binder system in the acidic media
3.1.2. Fourier transform infrared spectroscopy Fig. 2 shows the FTIR spectra of zeolite A, Sr-loaded zeolite A and solidification blocks. As the figure depicts, there is no significant difference between the FTIR spectra of the four different samples, as a result of the zeolite-like structure of geopolymer [15,16]. The broad band at 3450–3425 cm−1 and the sharp band at 1650–1640 cm−1 are attributed to the water component of the OH stretching band and the chemically bonded water (H–O–H bending) respectively [8]. The bands are linked to the presence of weak H2O [19,21,22] and seem to be sensitive for ion exchange. The bands around 1070–950 cm−1 corresponding to Si(Al)–O stretching vibration of zeolite structure, NASH gel in geopolymer and CSH gel in cement also can be identified in the four FTIR spectra [23–25]. The shift of these bands towards the higher frequencies is the evidence for the breakdown of Al-O bond in AlO4 tetrahedrons and the removal of aluminium from the zeolite lattice [22]. Thus, we could conclude that dealumination of zeolite A
Fig. 2. FTIR spectra of zeolite specimens and solidification blocks.
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Fig. 3. SEM micrographs of geopolymer solidification blocks.
Fig. 4. SEM micrographs of cement solidification blocks.
product was equal to 37.62 MPa, while the cement block exhibited a strength of only 11.32 MPa. The major difference indicated better compatibility between geopolymer and the exhausted zeolite A. All of the cement specimens cracked after calcination at elevated temperatures, while the compressive strength of geopolymer blocks showed a slight downward trend, with 26.74 MPa after 2 h of calcination at 1000 °C. After 15 freeze-thaw cycles, the geopolymer block exhibited better mechanical performance than did the cement block, with only 9.4% compressive strength loss. Although the differing mechanical performance of the geopolymer and cement solidification blocks may be an important reason for their different thermal stabilities, the role played by different pore structures could not be ignored. Initially, the total porosity of the geopolymer block was much less than that of cement materials [4,9]. Then, thermal expansion and contraction of air in the more developed pore structure of the cement blocks may have caused more serious internal micro cracking and obvious compressive strength loss [4,9,26]. In addition, the larger proportion of connected pores in the geopolymer blocks may have led to lower internal pore pressure and less serious destruction at elevated temperatures [27]. From the results obtained above, we conclude that the geopolymer matrix has an obvious advantage in terms of excellent thermal stability.
Table 2 EDS analysis results of geopolymer and cement solidification blocks. Areas
A B C D
Atom (%) Si
Al
Na
13.53 21.47 10.67 13.05
11.17 11.69 1.94 5.52
11.02 8.76 4.02
Ca
Sr
4.63
0.36 0.04 1.38 0.29
[4,8,23]. Although acid attack also caused dealumination and depolymerization of the geopolymer structure [8,23], the geopolymer solidification matrix exhibited better performance, which was attributed to its stable cross-linked aluminosilicate polymer structure. The metakaolin based-geopolymer seems to be more suitable for immobilization of the exhausted zeolite A.
3.3. Thermal stability Solidification blocks may be buried in some areas, where the temperature varies greatly between day and night. Meanwhile, radioactive disintegration gradually releases decay heat and can lead to a temperature rise of solidification blocks throughout the long half-life of radioactive waste. Thus, solidification blocks were subjected to testing their thermal stability. The changes in compressive strength of the solidification blocks both before and after those thermal stability tests are presented in Table 3. Before the thermal stability tests were conducted, the compressive strength of geopolymer solidification
4. Conclusions This paper investigated the comparative performance of the encapsulation of strontium-loaded zeolite A by metakaolin-based geopolymer and ordinary Portland cement. Based on the results presented above, the following conclusions can be drawn: 4
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Fig. 5. CFLC of Sr2+ ions in four different solutions: (a) deionized water, (b) magnesium sulfate solution, (c) sulfuric acid solution, (d) acetic acid buffer solution.
Talent Foundation of Sichuan Province (No. 20132065).
Table 3 Compressive strength of geopolymer/OPC solidification blocks before and after thermal stability tests/MPa. Before
Geopolymer Cement
37.62 11.32
After calcination
References
After freeze-thaw cycles
600 °C
800 °C
1000 °C
15 cycles
Loss (%)
33.53 cracked
28.44 cracked
26.74 cracked
34.08 8.95
9.4 21
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1. Metakaolin based-geopolymer solidification blocks exhibited better mechanical performance. While the compressive strength of geopolymer solidification product was 37.62 MPa after curing for 28 days, the cement solidification product measured only 11.32 MPa. Meanwhile, the microstructure of geopolymer specimens appeared both more compact and more dense. 2. The geopolymer solidification blocks had better leaching resistance than did the cement specimens in deionized water, sulfuric acid solution (pH=1), magnesium sulfuric solution (5 wt% MgSO4) and acetic acid buffer solution (pH=3.6). Through XRD and EDS analysis, it also could be concluded that most of the strontium radionuclide within solidification blocks remained within the zeolite structure, acting as the charge balancing cation. 3. The geopolymer solidification block had better thermal stability than did the cemented form. The compressive strength of the geopolymer solidification matrix measured up to 26.74 MPa after 2 h of calcination at 1000 °C and up to 34.08 MPa after 15 freeze-thaw cycles. In contrast, the cement solidification products cracked after 2 h of calcination at elevated temperatures and lost 21% of the initial associated compressive strength after 15 freeze-thaw cycles.
Acknowledgements This work was supported by the Natural Science Foundation of China (No. 51404200), and the Science and Technology Innovation 5
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