English

• 0.   Introduction

  As the solid waste generated from limestone gypsum wet desulfurization technique, the flue gas desulfurization (FGD) gypsum, of which main content is calcium sulfate dihydrate (CSD), possesses the low application level and utilization rate. As a consequence, the bulk deposition of FGD gypsum have caused serious resource waste and environment pollution^[1]. α-Calcium sulfate hemihydrate (α-CSH) is a high value-added class of cementitious materials, which could be transformed from CSD via dehydration process^[2-4]. Owing to its superior mechanical strength, biocompatibility and biodegradability, α-CSH exhibits great potential for application in medicine such as dental impression, bone graft substitute and drug carrier^[5-9]. Therefore, the preparation of α-CSH from FGD gypsum is an effective way to improve FGD gypsum added value and utilization rate.

  There are mainly three methods to directly synthe-size α-CSH from FGD gypsum, including autoclave hydrothermal reaction, salt solution and alcohol-water solution methods. However, the synthesized α-CSH particles present excessively large particle size (40~100 µm) and aspect ratio (80~200), eventually impairing its mechanical strength and limiting its applications in medicine and other high-end fields. Currently, the crystal modifier has been commonly employed to manipulate the particle size and aspect ratio of α-CSH by adjusting nucleation and growth process, mainly including inorganic salt, organic acid, surfactant, macromolecule, etc^[10-13]. For instance, nonlattice univalent cations (Li⁺, Na⁺, NH⁴⁺, K⁺)and bivalent ions (Cu²⁺, Zn²⁺, Mn²⁺, Mg²⁺) increase the supersaturation of α-CSH by forming"[MSO₄]^(2-b)- ion pair", and α-CSH particles with reduced size (length of 40~60 µm, aspect ratio of 16~24) can be obtained^[11]. Compared with inorganic cations, organic acids, surfactants and macromolecules control the particle size and aspect ratio by establishing strong chemical adsorption on the crystal surface. Wang et al. studied the effects of EDTA-2Na and succinic acid on the morphology of α-CSH and found that the carboxyl group can effectively bind to the polar crystal face to produce elongated-prismatic or spherical α-CSH (length of 80~100 µm, aspect ratio of 3~5)^[12]. Shao et al. used sodium lauryl sulfate as surfactant to obtain short columnar α-CSH (length of 100 µm, aspect ratio of 2)^[13]. Li et al. controlled the crystal morphology of α-CSH (length of 48.99 µm, aspect ratio of 1.21) by complexation of L-aspartic acid and Ca²⁺ active sites^[10]. Obviously, the crystal modifier could significantly reduce the aspect ratio of α-CSH, whereas the particle size of α-CSH is still large, leading to an unsatisfactory injection performance and mechanical property. Moreover, the poor whiteness of α-CSH prepared from FGD gypsum has some restrictions for possible commercial applications. Therefore, it is desirable to synthesize small α-CSH with low aspect ratio and high whiteness to realize extensive and high-added value use.

  N, N′-methylenebisacrylamide (MBA) is a cross-linking agent, containing two double bonds, carbonyl groups and imino groups. Its double bonds can self-polymerize to form a three-dimensional network at high temperature^[14-15], and carbonyl oxygen atoms can coordinate with the Ca²⁺ active sites^[15-16]. Based on the properties of MBA, we proposed a novel two-step hydrothermal reaction to obtain small and quasi-spherical α-CSH particles with high whiteness. In the first hydrothermal process, MBA was adsorbed and self-polymerized on the surface of FGD gypsum to synthesize α-CSH intermediate with reduced aspect ratio. Then, this three-dimensional gel network formed by self-polymerization of MBA provided the confined space for the dissolution-recrystallization process of α-CSH intermediate in the second hydrothermal treatment. Combined with the regulation of succinic acid and dispersion of ethylene glycol (EG), small and quasi-spherical α-CSH particles with high whiteness were finally obtained. The effect of MBA addition on the crystal morphology and particle size of α-CSH is discussed. The regulation mechanism of MBA is illustrated by using FTIR and energy dispersire spectroscopy (EDS) measurements as well as EG concentration-dependent experiments. Besides, variation of the whiteness of the synthesized α-CSH will be investigated.

  1.   Experimental

  1.1   Materials

  FGD gypsum with the whiteness of 39.22% was generated from the flue gas desulfurization system of an alkali plant. The main components and SEM image of FGD gypsum were shown in Table S1 and Fig. S1 (Supporting information), respectively. Sodium chloride (NaCl) was obtained from Tianjin Yingda Chemical Co., Ltd., China. Succinic acid was purchased from Tianjin First Chemical Co., Ltd., China. MBA was obtained from Tianjin Fuchen Chemical Co., Ltd., China. EG was purchased from Tianjin Jierzheng Chemical Trading Co., Ltd., China. The reagents used in the experiment are all analysis grade.

  1.2   Physical purification of FGD gypsum

  FGD gypsum was mixed with water in a beaker of 500 mL with mass ratio of 1∶3, where the magnet was fixed on the wall of the beaker to adsorb magnetic impurities. After stratification, the middle layer of the slurry was washed three times to obtain the preliminarily-purified FGD gypsum followed by drying at (45± 3) ℃ in the oven. The preliminarily-purified FGD gypsum was sifted through 120 mesh sieve to obtain the FGD gypsum with a size of less than 75 µm.

  1.3   Synthesis of small and quasi-spherical α-CSH particles

  The small and spherical α-CSH particles was synthesized by using two-step hydrothermal method. The detailed process was described as follows. In the first step, the preliminarily-purified FGD gypsum (4.5 g), distilled water (50 mL) and MBA (0.15 g) were mixed into Teflon-lined autoclave at 120 ℃ with the speed of 100 r·min^-1 for 140 min. The obtained product was immediately filtered and washed three times with boiling water followed by drying at 110 ℃ for 4 h. The above-mentioned product was added into 10 mL distilled water to form a slurry with a mass fraction of 18%. NaCl, succinic acid, and EG were also added to this slurry with contents of 3%, 0.02%, and 1.61%, respectively. The reaction was carried out in the auto-clave at 120 ℃ for 140 min. The synthesized product was filtrated immediately and washed three times with boiling water before drying at 110 ℃ for 4 h.

  1.4   Characterization

  The crystal morphology of products was observed using scanning electron microscopy (SEM, Nano SEM 450), the test voltage was 10 kV, and the magnification was 400~5 000. The chemical composition of products was tested by EDS. The crystal type of products was determined by X-ray diffraction (XRD, D8-Focus) using Cu Kα radiation (λ =0.154 18 nm) with a scanning rate of 12 (°)·min^-1, a scanning 2θ range of 5°~90°, a tube voltage of 40 kV and a tube current of 200 mA. The adsorption and self-polymerization of MBA on the crystal surface of α-CSH was demonstrated by Fourier transform infrared spectra (FTIR, TENSOR 27). Chemical composition of FGD gypsum and α-CSH products was measured by method of GB/T 5484-2000. The whiteness of α-CSH and FGD gypsum was determined by automatic colorimeter (SC-80). The amount of CSD was calculated according to JC/T 2074-2011 and GB/T 5484-2000. 1 g of FGD gypsum was dried in an oven at 45 ℃ to a constant weight, and the content of adhesive water (X₁) was analyzed by the following Eq.₁. Another 1 g of FGD gypsum was heated to a constant quantity at 230 ℃ in an oven, and the content of crystal water (X₂) was defined by the following Eq. 2. The amount of CSD (R) could be expressed by Eq.3.

  $ X_{1}=\frac{m_{1}-m_{2}}{m_{1}} \times 100 \% $

  (1)

  $ X_{2}=\frac{m_{3}-m_{4}}{m_{3}} \times 100 \%-X_{1} $

  (2)

  $ R=\frac{X_{2}}{20.9275} \times 100 \% $

  (3)

  where m₁ and m₂ represent the mass of FGD gypsum before and after drying at 45 ℃ (g), m₃ and m₄ stand for the mass of FGD gypsum before and after heating at 230 ℃ (g).

  2.   Results and discussion

  2.1   Synthesis of α-CSH intermediate with reduced size and low aspect ratio

  Fig. 1a~1e show SEM images and crystal size distributions of α-CSH intermediate synthesized with different amounts of MBA. In the absence of MBA, the synthesized α-CSH intermediate possesses morphology of whiskers with a length of 177 µm and an average aspect ratio of about 177 (Fig. 1a). With the MBA amount of 0.15 g, uniform α-CSH rhombus blocks (RBs) were synthesized (Fig. 1b), the length of which was decreased to about 60 µm, while the diameter was increased to 25.4 µm. The corresponding average aspect ratio was dramatically reduced to 2.36. When the MBA was increased to 1 g or more, the aspect ratio of the synthesized α-CSH intermediate displayed an augmented tendency, and the corresponding morphology changed from uniformly rhombus to non-uniformly short rod-like shape (Fig. 1c and 1d). The XRD patterns of the synthesized α-CSH intermediate are shown in Fig. 1f. All samples only exhibited the diffraction peaks ascribed to α-CSH, revealing that the amount of MBA in the synthesis solution has no influence on the crystal type. However, with the MBA amount of 1 and 2 g, the peak intensities of α-CSH intermediate were gradually decreased, suggesting that excessive MBA adsorbed on the crystal surface leads to a reduction in the crystallinity of the synthesized α-CSH intermediate.

  Figure 1

  

  Figure 1.  SEM images of α-CSH intermediate synthesized with MBA amounts of 0 g (a), 0.15 g (b), 1 g (c) and 2 g (d), and their corresponding lengths, diameters and aspect ratios (e), XRD patterns (f)

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  To illustrate the regulation mechanism of MBA on the crystal morphology, EDS analysis was performed on the synthesized α-CSH RBs (Table 1). In addition to the constituent elements of α-CSH RBs (Ca, S and O atoms), both C and N atoms were detected, suggesting the adsorption of MBA on the crystal surface of synthesized α-CSH RBs. Furthermore, Fig. 2 gives the FTIR spectra of α-CSH synthesized with the amount of MBA of 0 and 0.15 g. The band at 1 625 cm^-1 is attributed to the —OH of crystal water. The peaks at 1 008, 660 and 606 cm^-1 correspond to the stretching modes of SO₄^2-. Compared with FTIR spectrum of α-CSH synthesized with the amount of MBA of 0 g (Fig. 2B), it should be noted that the synthesized α-CSH RBs exhibited two new bands at 2 923 and 2 850 cm^-1, which are attributed to the stretching vibration of —CH₂— group in alkane (Fig. 2C), while MBA molecules showed two peaks at 3 080 and 2 990 cm^-1 ascribed to the stretching vibration of —CH₂— group in olefin (Fig. 2A). These results give strong evidence that MBA is successfully absorbed and self-polymerized on the surface of α-CSH RBs.

  Table 1

  

  Table 1.  EDS results of α-CSH RBs

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  Chemical element	C	N	O	S	Ca
  Atomic fraction/%	17.70	5.31	64.25	6.41	6.32

  Figure 2

  

  Figure 2.  FTIR spectra of MBA (A), and α-CSH intermediate synthesized with MBA amount of 0 g (B) and 0.15 g (C)

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  2.2   Preparation of small and quasi-spherical α-CSH particles

  The second hydrothermal reaction was the dissolution and recrystallization of α-CSH intermediate, where succinic acid was also added to further control the morphology of products. Fig. 3a~3e show SEM images and crystal size distributions of the recrystallized α-CSH. The recrystallized α-CSH particles prepared from α-CSH intermediate with the whisker morphology displayed non-uniform and short column with a length of 20~60 µm and aspect ratio of 3.1 (Fig. 3a), which could be attributed to the compression effect of succinic acid on (111) plane of the recrystallized α-CSH. It should be noted that the α-CSH clusters aggregated by quasispherical α-CSH particles with the crystal size of 2~5 µm were synthesized from α-CSH RBs (Fig. 3b). When α-CSH intermediates synthesized with the MBA amount of 1 and 2 g were used as raw materials, the recrystallized products were still crystal clusters formed by agglomeration of many α-CSH grains, where-as α-CSH grains exhibited columar morphology with increased aspect ratio in comparison to that synthe-sized from α-CSH RBs (Fig. 3c and 3d). This clustered morphology could be attributed to the fact that the dissolution and recrystallization processes of α-CSH particles were performed in the three-dimensional gel network formed by MBA self-polymerization on the surface of α-CSH intermediate. In addition, the crystal morphology and particle size of recrystallized products will be affected by that of the raw materials. In contrast to the α-CSH RBs synthesized with MBA amount of 0.15 g, the synthesized α-CSH intermediates synthesized with MBA amount of 1 and 2 g possessed short rod-like morphology with increased aspect ratios. Thus, the corresponding recrystallization products also exhibited increased aspect ratios. As shown in Fig. 3f, all products only presented the diffraction peaks attributed to α-CSH. The peak intensity of the recrystallized product synthesized from α-CSH RBs was the minimum, indicating that the recrystallized product synthesized from α-CSH RBs possesses the smallest crystal size.

  Figure 3

  

  Figure 3.  SEM images of recrystallized α-CSH products when the raw materials was α-CSH intermediate synthesized with MBA amount of 0 g (a), 0.15 g (b), 1 g (c) and 2 g (d), with their corresponding lengths, diameters and aspect ratios (e), XRD patterns (f)

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  EG was added as the dispersant during the second hydrothermal reaction for further understanding of the function of gel network formed by MBA self-polymerization. As shown in Fig. 4a~4e, without EG addition and with EG amount (mass fraction) of 0.96%, the recrystallized α-CSH were still clustered (Fig. 4a and 4b). When EG addition was 1.61%, the recrystallized α-CSH exhibited monodispersed and quasispherical morphology with a size of 6~10 µm and an aspect ratio of 1 (Fig. 4c), which could be attributed to the fact that EG exerted the destruction effect on this gel network, and the recrystallized α-CSH grains were disorganized. With EG addition of 50%, the recrystallized α-CSH showed the whisker morphology with a length of 65 µm and an average aspect ratio of 10.8 (Fig. 4d). At this time, the dispersion effect of EG is dominant, while the confinement effect of the gel network is tremendously weakened by the dispersion of EG, leading to an increase in the size and aspect ratio of the synthesized α-CSH particles. The XRD patterns of the recrystallized α-CSH particles synthesized with different amounts of EG were in good agreement with SEM observations (Fig. 4f). All characteristic diffraction peaks of products are ascribed to α-CSH. As the addition of EG augments, the intensities of diffraction peaks were gradually increased, indicating that the crystal size of the recrystallized α-CSH particles shows an increased tendency.

  Figure 4

  

  Figure 4.  SEM images of α-CSH particles synthesized with EG addition (mass fraction) of 0 (a), 0.96% (b), 1.61% (c) and 50% (d), and their corresponding lengths, diameters and aspect ratios (e), XRD patterns (f)

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  2.3   Whiteness of synthesized α-CSH particles

  Raw FGD gypsum is composed of CSD with the amount of 94.55%, as well as a small quantity of Fe₂O₃, Al₂O₃, SiO₂ and other impurities, which results in an unsatisfactory whiteness of the synthesized α-CSH products. Hence, the three-step decolorization process of FGD gypsum was proposed to obtain α-CSH particles with high whiteness, including physical decolorization and two-step chemical decolorization via hydrothermal crystallization process (Fig. 5). Fig. 6 gives variation of whiteness of FGD gypsum with washing times. The analysis of chemical compositions of FGD gypsum and α-CSH products is in Table 2. After washing three times, the content (mass fraction) of CSD in FGD gypsum was increased from 94.55% to 96.88%, while the content of Fe₂O₃ was decreased from 1.98% to 0.73% after magnetic adsorption, which is the primary factor affecting the whiteness of FGD gypsum. As a result, the corresponding whiteness of FGD gypsum was enhanced to 69.73% from 39.22%. In addition, most of SiO₂ and a small amount of MgO and Al₂O₃ were also removed in the physical decolorization, which contents were decreased to 0.81%, 0.89%, and 0.32%, respectively. The two-step chemical decoloarization process was carried out to further improve the whiteness of α-CSH products. In the first hydrothermal treatment, FGD gypsum was gradually dissolved and the α-CSH RBs product with the augmented whiteness of 89.96% was obtained in the mother liquor, where the concentrations of Ca²⁺ and SO₄^2- ions reached the supersaturation required for precipitation of α-CSH RBs. In contrast, most of the metal oxides were dissolved and remained in the mother liquor due to the fact that the contents of impurities were below their supersaturation. The contents of SiO₂, MgO, Fe₂O₃ and Al₂O₃ impurities were decreased to 0.17%, 0.26%, 0.32% and 0.19%, respectively. Furthermore, the addition of succinic acid in the second hydrothermal treatment can contribute to removal of MgO, Fe₂O₃ and Al₂O₃ in α-CSH products, and finally small and quasi-spherical α-CSH particles with the whiteness of 92.06% were obtained. At this time, the contents of SiO₂, MgO, Fe₂O₃, and Al₂O₃ impurities were only 0.12%, 0.08%, 0.04 % and 0.07%, respectively.

  Figure 5

  

  Figure 5.  Flow diagram of decolorization of FGD gypsum and the products

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  Figure 6

  

  Figure 6.  Dependence of whiteness and CSD content of FGD gypsum on washing times

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  Table 2

  

  Table 2.  Chemical compositions of FGD gypsum and α-CSH products %

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  Product	CaO	SO3	H2O	SiO2	MgO	Fe2O3	Al2O3	Others
  FGD gypsum	31.86	42.95	19.74	1.09	1.23	1.98	0.63	0.52
  Preliminarily-purified FGD gypsum	33.19	43.70	19.99	0.81	0.89	0.73	0.32	0.37
  α-CSH RBs	38.14	54.75	5.96	0.17	0.26	0.32	0.19	0.21
  Quasi-spherical α-CSH particles	38.55	55.02	6.01	0.12	0.08	0.04	0.07	0.11

  2.4   Role of MBA in synthesis of α-CSH particles

  Role of MBA in the synthesis of small and quasispherical α-CSH particles by using the two-step method is illustrated in Fig. 7. In the first step, the lone-pair electrons of the nitrogen atom in MBA molecules could be attracted to the oxygen atom of the carboxyl group, and the electron density of the oxygen atom is increased, leading to the reinforced coordination of carboxyl oxygen atom with Ca²⁺ active sites^[15-16]. According to the arrangement of Ca²⁺ and SO₄^2- ions on different crystal planes of α-CSH intermediate^[17], MBA molecules are preferentially absorbed on the (111) plane, restricting the growth of α-CSH intermediate along c axis. At the same time, MBA molecules could be self-polymerized at high temperature to form a three-dimensional gel network on the surface of α-CSH intermediate, resulting in the formation of α-CSH RBs with the reduced size synthesized in this limited space. During the second hydrothermal treatment, the three-dimensional gel network formed on the surface of α-CSH RBs provides the confined space for the dissolution and recrystallization of α-CSH RBs. This confinement effect impedes the free movement of dissolved Ca²⁺ and SO₄^2- ions, resulting in an increase in supersaturation of α-CSH. In addition, the space for α-CSH growth is sharply reduced. Combined with crystal manipulation of succinic acid and the dispersion of EG, the small and quasi-spherical α-CSH particles are obtained.

  Figure 7

  

  Figure 7.  Schematic diagram of regulation effect of MBA on synthesis of α-CSH particles

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  3.   Conclusions

  We designed a facile and ingenious two-step hydrothermal method to synthesize small and quasispherical α-CSH particles with high whiteness from FGD gypsum under the regulation of MBA. In the first step, MBA could chelate Ca²⁺ ion to control aspect ratio of α-CSH intermediate. Simultaneously, MBA could be self-polymerized to form a three-dimensional gel network on the surface of α-CSH intermediate, leading to a reduction in the particle size of α-CSH intermediate. As a result, α-CSH RBs with a length of 60 µm and aspect ratio of 2.36 were synthesized with the MBA addition of 0.15 g. During the second hydrothermal treatment, the three-dimensional gel network formed on the surface of α-CSH RBs confines the movement of dissolved Ca²⁺ and SO₄^2- to generate an increased supersaturation for α-CSH nucleation, and provides a limited space for the growth of α-CSH particles. Combined with crystal regulation of succinic acid and the dispersion of EG, the small and quasi-spherical α-CSH particles with the length of 6~10 µm and aspect ratio of 1 were obtained. After undergoing washing and two-step hydrothermal treatment, the small and quasispherical α-CSH particles exhibited a satisfactory whiteness with the value of 92.06%. This study provides a simple and effective route to manipulate the morphology and whiteness of α-CSH particles synthesized from FGD gypsum, giving rise to the broad prospects in applications.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Figure 1  SEM images of α-CSH intermediate synthesized with MBA amounts of 0 g (a), 0.15 g (b), 1 g (c) and 2 g (d), and their corresponding lengths, diameters and aspect ratios (e), XRD patterns (f)

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  Figure 2  FTIR spectra of MBA (A), and α-CSH intermediate synthesized with MBA amount of 0 g (B) and 0.15 g (C)

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  Figure 3  SEM images of recrystallized α-CSH products when the raw materials was α-CSH intermediate synthesized with MBA amount of 0 g (a), 0.15 g (b), 1 g (c) and 2 g (d), with their corresponding lengths, diameters and aspect ratios (e), XRD patterns (f)

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  Figure 4  SEM images of α-CSH particles synthesized with EG addition (mass fraction) of 0 (a), 0.96% (b), 1.61% (c) and 50% (d), and their corresponding lengths, diameters and aspect ratios (e), XRD patterns (f)

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  Figure 5  Flow diagram of decolorization of FGD gypsum and the products

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  Figure 6  Dependence of whiteness and CSD content of FGD gypsum on washing times

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  Figure 7  Schematic diagram of regulation effect of MBA on synthesis of α-CSH particles

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  Table 1.  EDS results of α-CSH RBs

  Chemical element	C	N	O	S	Ca
  Atomic fraction/%	17.70	5.31	64.25	6.41	6.32

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  Table 2.  Chemical compositions of FGD gypsum and α-CSH products %

  Product	CaO	SO3	H2O	SiO2	MgO	Fe2O3	Al2O3	Others
  FGD gypsum	31.86	42.95	19.74	1.09	1.23	1.98	0.63	0.52
  Preliminarily-purified FGD gypsum	33.19	43.70	19.99	0.81	0.89	0.73	0.32	0.37
  α-CSH RBs	38.14	54.75	5.96	0.17	0.26	0.32	0.19	0.21
  Quasi-spherical α-CSH particles	38.55	55.02	6.01	0.12	0.08	0.04	0.07	0.11

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