Effects of fluoride content on structure and properties of steel slag glass-ceramics

Journal Pre-proof Effects of Fluoride Content on Structure and Properties of Steel Slag GlassCeramics

Zhihong Luo, Feng He, Wentao Zhang, Yongli Xiao, Junlin Xie, Ruijie Sun, Mengqin Xie PII:

S0254-0584(19)31341-0

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122531

Reference:

MAC 122531

To appear in:

Materials Chemistry and Physics

Received Date:

13 October 2019

Accepted Date:

06 December 2019

Please cite this article as: Zhihong Luo, Feng He, Wentao Zhang, Yongli Xiao, Junlin Xie, Ruijie Sun, Mengqin Xie, Effects of Fluoride Content on Structure and Properties of Steel Slag GlassCeramics, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys. 2019.122531

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Journal Pre-proof

Effects of Fluoride Content on Structure and Properties of Steel Slag Glass-Ceramics Zhihong Luoa, Feng Hea,b,∗ , Wentao Zhanga, Yongli Xiaoc,∗, Junlin Xiea,b, Ruijie Sunb, Mengqin Xiec a State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, China b School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China c Baoshan Iron & Steel Co., Lid., Shanghai, 201900, China

Abstract: The hot steel slag were utilized for preparing the glass-ceramics by melting method with different fluoride contents. The crystallization behaviors of the glass-ceramic were characterized by Differential Scanning Calorimeter (DSC), X-ray Diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR), Raman ,and the Field Emission Scanning Electron Microscope (FESEM).The results established that the transition from Nepheline to Cuspidine, and then to Kalsilite, and finally became Combeite in glass-ceramics occurred with the decrease of fluoride content Besides, a comprehensive analysis of FTIR and Raman revealed that the addition of fluoride contributed to the formation of the Si-O bridge oxygen in the glass structure, which improved the degree of polymerization of the glass structure and further affected the crystal phase of the glass-ceramics. In addition, when Nepheline and Cuspidine constituted as the major crystalline phases for the prepared glass-ceramics, a higher flexural strength and a smaller volume shrinkage were achieved. SEM depicted that the above mentioned results were ascribed to from the changes in the crystal type and the degree of crystallization, in which the size of the crystal phase had a greater influence on the performance of the glass-ceramic. It was worth noting that the highest flexural strength was 177.76 MPa with the lowest volume shrinkage of 0.06 %. Key words: Steel slag; Glass-ceramic; Fluoride; Glass structure; Mechanical property

1. Introduction Steel slag is a by-product of the steelmaking process which accounts for about 10–15 % in the steel

production, while the overall utilization rate of steel slag is only about 22 %[1]. This will cause enormous trouble for environmental treatment and corporate finance，so its comprehensive recycling plays a very

important role in the sustainable development of the steel industry. Many researchers had conducted numerous studies on the effective utilization of steel slag, such as agrochemical fertilizers

[2],

cement

production [3, 4], and the chemical extraction of rare elements[5]. Considering that the main components of steel slag include SiO2, Al2O3, CaO, and Fe2O3, it is feasible to be a kind of raw materials for

Journal Pre-proof preparing the high value-added glass-ceramics materials[6-13], which shows a number of outstanding characteristics such as high mechanical strength, stable dielectric constant, corrosion resistance, adjustable expansion coefficient and high temperature resistance. Meanwhile, the utilization of steel slag in preparing glass-ceramics can effectively fix harmful heavy metals such as Cr3+, V5+, and Ti4+ in steel slag, which can prevent environmental pollution[14]. Besides, one of the main reasons which limit the release of its wide application potential in cement industry is the f-CaO in steel slag

[4, 15],

while this restriction can be totally ignored during the

preparation of glass-ceramics by melting method for f-CaO will become a fixed part of glass-ceramics. Zhihong Yang's research[16] suggested that the increase of CaO/ SiO2 promoted the crystallization of glass-ceramics and facilitated the crystallization of Ca-containing crystals. In addition, Fe2O3 in the steel slag was not only beneficial to the melting of the glass, but also to the crystallization of glass-ceramics, and the effects of Fe2O3 content on steel slag glass-ceramics had been studied by sintering method[10], the result showed that the proper increase of Fe2O3 reduced the sintering temperature and improved the sintering performance. Furthermore, the comprehensive utilization of steel slag, fly ash, and waste glass to prepare glass-ceramics with flexural strength of 43.37 MPa was a strong evidence of effective utilization of other waste resources with steel slag [17]. At present, the refinement of the steel slag can be improved by modifying hot slag with the waste heat of the hot steel slag fully utilized simultaneously[18]. For preparing steel slag glass-ceramics with excellent performance, sintering method is one of the most popular method which conducts by adding modified materials to the hot steel slag and water-quenched the modified steel slag [19-21]. However, the glass-ceramics prepared by the sintering method is more susceptible to fluctuations in composition and sintering atmosphere, and the productions have a high shrinkage rate. Besides, the molten steel slag is difficult to be directly transformed into glass-ceramics unless it is modified by adding other chemical materials. Thus, the preparation of the glass-ceramics by sintering and the water quenching of the modified steel slag is not conducive to the continuity of industrial production, and also waste the existed heat inside the hot slag[22]. Fig. 1 shows two typical methods for the preparation of glass-ceramics using hot steel slag. To realize the direct production of glass-ceramics from modified steel slag, it is essential to find some reasonable components of modifying materials for determining suitable heat treatment system. In this experiment, the glass-ceramics were prepared by the melting method[23], and the glass-ceramics production with continuity are hoped to be directly prepared by the

Journal Pre-proof modified hot slag , and the energy consumption can be reduced. Therefore, the composition of the modifying materials is crucial for the direct preparation of steel slag glass-ceramics, and fluoride which is considered as a common nucleating agent will have a greater impact on the crystallization of glass[24, 25].

In this paper, various content of fluoride in modifying materials with a constant amount of 50 wt% steel slag were utilized to prepare glass-ceramics. In order to make the modifying material easier to be melt, the alkali metal oxides K2O and Na2O were added into the modifying material. Moreover, due to the high CaO content in the steel slag[18], Na2SiF6 which was acted as the source of fluoride was used instead of the used CaF2. The effects of the fluoride content from 0 wt% to 6 wt% on the structure and properties of steel slag glass-ceramics were investigated.

Fig. 1. Two typical methods for the preparation of glass-ceramics using hot slag

2. Experimental methods 2.1 Raw materials The steel slag used in this experiment came from Pingxiang IRON & STEEL CO. LTD in Jiangxi province, China. The main chemical compositions of the steel slag were characterized by chemical analysis and shown in Table 1, and the steel slag were pretreated by grounding into particles less than 0.1 mm in diameter to eliminate the composition fluctuations in the raw materials. However, 50 % used in this experiment cannot satisfy the desired composition of the glass, so the modifying materials which was supplied by oxide and carbonate (analytical pure) reagents were added. Besides, the fluoride was all introduced by Na2SiF6 in the modifying materials, and the mass percentage of Na2O, SiO2 and Na2SiF6 stay constant in the studied chemical composition of glass-ceramics. Table 2 shows the investigated compositions of parent glass (including the contribution of steel slag).

Journal Pre-proof Table 1 Chemical composition of steel slag Composition wt%

CaO

SiO2

MgO

Al2O3

FeO

Fe2O3

f-CaO

Fe

46.94

19.65

6.42

7.00

7.56

8.80

3.11

0.54

Table 2 Chemical composition of the glass ceramics (wt%) CaO

SiO2

MgO

Al2O3

FeO

Fe2O3

K2O

Na2O

Na2SiF6

F

SF1

24.98

35.37

3.45

3.77

4.12

4.39

7.45

6.54

9.93

6.02

SF2

24.98

36.60

3.45

3.77

4.12

4.39

7.45

7.82

7.42

4.50

SF3

24.98

37.82

3.45

3.77

4.12

4.39

7.45

9.07

4.95

3.00

SF4

24.98

39.04

3.45

3.77

4.12

4.39

7.45

10.33

2.47

1.50

SF5

24.98

40.25

3.45

3.77

4.12

4.39

7.45

11.59

0.00

0.00

2.2 Preparation of glass-ceramics In this experiment, glass-ceramics were prepared by melting method. Firstly, 100 g steel slag was melted in alumina crucible at 1400 C for 1 h with electric furnace (GMT-24-17) in an air atmosphere at a heating rate of 2 C/min. Then the modifying materials weighed accurately and mixed by ball milling were added into the molten steel slag, and the mixed melt obtained in this way was stirred with a platinum rotator, and another holding time for 2 h at 1400 °C was applied on the mixed melts to guarantee the homogeneity. Subsequently, the melts were casted on a preheated steel mold to obtain the parent glass in size of about 80 mm×50 mm×15 mm. Then, the parent glass samples were annealed in a muffle furnace at 550 C for 1 h. Finally, glass-ceramics were prepared by the heat treatment of parent glass at the nucleation temperature (Tn) and crystallization temperature (Tc). Specifically, the heat treatment method was according to thermal behavior analysis of parent glass powders. 2.3 Characterization of samples Glass and glass-ceramic powders were obtained by grinding bulk samples to to pass through 200 mesh sieves. The heat treatment schedule was decided according to the DSC (NETZSCH STA 449 C Germany) results of parent glass powder ranging from room temperature to 1000 °C, at heating rate of 10 C/min. The crystalline phases were determined by X-ray diffractometer (D8 Advance, Bruker), with Cu-Ka radiation, at 40 kV and 25 mA, in the range of 10 °-70 ° with a scanning speed of 10°/min. Raman spectra of the samples were collected using Raman spectrometer (Renishaw RM-1000) with Ar excitation laser source at wavelength of 514.5 nm and 4 cm-1 resolution. The FTIR were performed using a Fourier transform infrared spectrometer (Nicolet 6700; Thermo Fisher America) . The samples were prepared using the tablet method in KBr and it were carried out over the wave number range of

Journal Pre-proof 400 cm−1-1600 cm−1 at room temperature. The absorption spectra were recorded in 64 scans and the resolution of 2 cm-1 In addition, for FESEM (ULTRA PLUS-43-13, Zeiss, Germany) analysis of the glass-ceramics, the studied glass-ceramics were polished and etched with an HF solution (5%) for 45 s, and immediately rinsed with deionized water in an ultrasonic cleaner, and the etched samples were coated with a thin layer of platinum before the FE-SEM analysis. The flexural strength of the glass-ceramics samples was investigated by three-point test method in the universal material tester (MTS810, MTS Co, USA) at a crosshead speed of 0.5 mm/min, a fulcrum span of 25 mm, a loading speed of 9.8 ± 0.1 N/S, and rectangular specimens with dimensions of 50 mm× 4 mm× 4 mm were used. The density of glass-ceramics were measured at room temperature using Archimedes's principle. Because the dimensional change of glass ceramics was difficult to identify with the naked eye, the volume shrinkage was obtained by testing the density and quality before and after crystallization of the same sample, which was calculated using the following equation:

Volume shrinkage 

V0  Vgc V0



m0 / 0  mgc /  gc m0 / 0

100%

Where m0, V0 and ρ0 stood for the quality, volume, and density of sample before crystallization, respectively. And mgc, Vgc and ρgc referred to the quality, volume and density of the same sample after crystallization, respectively. 3. Results and discussion 3.1. DSC analysis and heat treatment The DSC curves of the five different specimens containing various amounts of fluoride (SF1, SF2, SF3, SF4, and SF5) at a heating temperature of 10 C/min have been illustrated in Fig. 2. According to the change of heat capacity of glass in DSC curve, the glass transition temperature Tg could be found. It was seen that the glass transition temperature Tg located in the range of 540 °C to 550°C, the temperature of the endothermic peak Tx was about 574 °C, and the exothermic peak Tc was in the range of 680 °C to 745 °C, which changed with the decrease of the fluoride content in parent glass. The appearance of the exothermic peak Tc was attributed to the rearrangement of the molecules, during the process of glass nucleation[25]. Fig. 2 clearly indicated that the area of the peak Tx valleys increased first and then decreased when fewer fluoride was added in it, and eventually disappeared. The nucleation temperature of fluoride was usually lower than the crystal growth temperature[26],thus the endothermic peak area could indicate the number of fluoride nuclei formed during the nucleation

Journal Pre-proof process. Moreover, the existence of Si-F group due to the different valence states of F- and O2-was beneficial to the break of the silicon-oxygen network structure, thereby the viscosity of the glass would be reduced and the diffusion and migration of ions in glass structure would be promoted during the crystallization process[24, 27, 28], which explained the nucleation mechanism of fluoride. The formation of the nucleus in this study, was caused by the appearance of fluoride, which was responsible for the increase of the nucleation endothermic peak Tx area and the decrease of the crystallization exothermic peak temperature Tc. The various fluoride content only affected the half-width of the endothermic peak, but made no differences on the position of the endothermic peak. Specifically, the endothermic peak area of SF2 sample indicated that the number of nuclei formed was the largest, which could be attributed to the influence of fluoride on the glass structure.

722.5°C

Exothermic SF1

574°C 683.9°C 541.4°C 712.3°C 539.5°C 738.6°C

SF2 745.8°C 547.2°C

SF3

548.6°C

SF4

549.2°C

SF5 200

400

600

800

1000

Temperature/°C Fig. 2. DSC curves of parent glass with different fluoride additions at 10 C/min

Thus, the increased in fluoride content promoted the formation of nucleation in the glass melt, which was beneficial to improve the crystallization of glass under certain conditions. The nucleation temperature was an important factor for nucleus formation. Previous studies[29,

30]

showed that the

optimum nucleation temperature usually occurred in the range from 50 to 100 C above the glass transition temperature Tg. In this work, the heat treatment of the glass-ceramics was determined to be nucleated at 600 °C for 0.5 h and crystallized at 720 °C for 1 h, which was on the basis of the position of the crystallization exothermic peak and the nucleation condition in the DSC curves. 3.2 Phase analysis by XRD Fig. 3 showed the XRD patterns of glass-ceramics with different fluoride addition. It can be seen

Journal Pre-proof from that after the heat treatment of the parent glass, the Nepheline (Na3K(Si0.56Al0.44 )8O16;JCPDS NO.01-078-1650) and Cuspidine (Ca4Si2O7F2;JCPDS NO.00-041-1474) were identified as major phases in SF1 and SF2 sample. As the fluoride content decreased, the intensity of Nepheline and Cuspidine diffraction peak became weaker simultaneously. Moreover, new crystalline phase Kalsilite (KAlSiO4; JCPDS NO.01-085-1413) and Combeite (Na4.24Ca3.8(Si6O18); JCPDS NO.01-078-1650) precipitated in SF3. When fluoride disappeared in SF5 glass sample, the main crystalline phase turned to be Kalsilite and Combeite. Table 3 the major crystalline phases in glass-ceramic Na3K(Si0.56Al0.44)8O16

Ca4Si2O7F2

KAlSiO4

Na5.27Ca3(Si6O18)

Nepheline

Cuspidine

Kalsilite

Combeite

SF1

√

√

SF2

√

crystalline phase

√

√

SF3

√

√

√

SF4

√

√

√

√

√

SF5

As shown in Fig. 3, with the decrease of fluoride content, the intensity of the Cuspidine diffraction peak gradually decreased and Cuspidine was absent in SF5, which indicated that the appearance of fluoride was the main reason for the formation of Cuspidine. Meanwhile, from SF1 to SF2, it can be found that with the decrease of Cuspidine, the diffraction peak of Kalsilite appeared but a small amount of Nepheline phase was still present in the SF2 sample. This suggested that the presence of F- was favorable for the precipitation of Nepheline phase. Furthermore, fluoride also suppressed the subsidiary crystallization of Kalsilite phase, and the similar results had been reported in the relevant literatures [31, 32].

Kalsilite is a hexagonal system, and its Si/O is 1:4 which belongs to an island structure. The

participation of F- was beneficial to the absorption of the Si-O structure in the residual glass phase[33] to form a Nepheline phase of the framework structure. During the formation of the crystalline phase, Fe in the glass matrix will enter into the Nepheline phase to replace Al by doping[34, 35]. Nepheline has good accommodation for Fe ion, so it is an excellent crystalline phase for preparing glass-ceramics with high iron content.

Journal Pre-proof  







 ▼ 



▼

Intensity (a.u.)

■

▼

■



■



■ ■





 ■ ▼

▼

▼ ▼

■

▼

■

■ 

SF1



SF2

■

■

SF3

■

SF4

■

SF5 ■ 01-078-1650 Na Ca (Si O ) 5.27 3 6 18

▼

01-085-1413-KAlSiO4

 01-076-2467-Na3K (Si0.56Al0.44)8O16

 00-041-1474-Ca4Si2O7F2 20

25

30

35

40

45

50

55

60

65

70

75

80

2 / degree Fig. 3. XRD of glass-ceramics samples with different fluoride addition and Crystal phase standard

With the decrease of fluoride, the crystal of Kalsilite in the glass-ceramics raised, and the increase of the main diffraction peak of Kalsilite at 28.6 ° can be confirmed. The XRD diffraction peak about 35 ° is a characteristic diffraction peak of Combeite, which became more intensive with the decrease of fluoride. Furthermore, the position of the diffraction peak had moved and overlapped, which might be caused by incomplete crystallization or different substitutions of Na+ and Ca2+ in Combeite. With the same heat treatment, fluoride will participate in the formation of crystalline phase and contribute to different type and quantity of crystal phase in glass-ceramics. Thus, it indicated that F- ions will not remain in the dissolved state in the residual glass phase, which is different from the method of introducing F- by using CaF2 to replace CaO[36, 37]. 3.3 Raman analysis results Raman spectroscopy is a very important method to obtain the vibrational spectroscopic properties of glass[38-40], which has many advantages to analyze silicate structure and acquire information as an analytical technique . Fig. 4 showed Raman spectra of parent glass samples with different fluoride addition, the broad band in the range about 700 cm-1 belonged to the characteristic vibration band of [AlO6] octahedron[41, 42]. While, the band in the range 800–1200 cm-1 ascribed to Si‒O symmetric bonds associated with network Si–O–Si bridges in Raman spectra. The state of oxygen atoms was classified

Journal Pre-proof by the cations group: the bridging oxygen (BO) connects two tetrahedrons and the non-bridging oxygen (NBO) connects a tetrahedron and a modifier. The Raman shifts for all Qn were listed in Table 4[40, 43, 44]. However, Q4 had a too low intensity in the studied glasses to be found at 800–1200 cm-1 in Fig. 4. Table 4 Raman shift for Qn corresponding structural units.

Structural unit

NBO/（Qn）

Wavenumber （cm-1）

[SiO4]4-, monomer [Si2O7]6-, dimmer [Si2O6]4-, chain [Si2O5]2-, plate SiO2, network

4（Q0） 3（Q1） 2（Q2） 1（Q3） 0（Q4）

850-880 900-920 950-980 1050-1100 1190

1

2

0 Q Q Q

3

Q

[AlO6]

Intensity

SF5 SF4 SF3 SF2 SF1

600

700

800

900

1000 1100 1200 1300 1400 1500 -1

Raman shift/cm

Fig. 4. Raman spectra of parent glass samples with different fluoride addition

Fig. 5. Typical deconvolution of the Raman spectra of the parent glass samples

According to the location of Qn in Table 4, the typical deconvolution results based on Gaussian function for the parent glasses samples at 800-1200 cm-1 was shown in Fig. 5. Frantz and Mysen[45, 46]

Journal Pre-proof had stated the content of the Qn structure could be calculated from the deconvoluted Raman bands’ peak area ratio. For calculating the Qn structure relative content, the following equation (1), equation (2) and precise Raman scattering coefficients were necessary:

X n   n An (n  0,1, 2,3)

(1)

An A 3 /  n 0 n Sn Sn

(2)

Xn 

Where Xn, θn, An represent the mole fraction of Qn structure, the Raman scattering coefficient, and the peak area ratio of Qn structure, respectively. The precise values of θn is defined as Sn ,which have been acquired by Wu et al

[47].

In this study, S0 , S1, S2, and S3 are accordingly calculated as 1, 0.514,

0.242, and 0.09 in alkaline earth silicate glasses as shown in similar reports[38]. Table 5 showed the Xn obtained from formula (1) (2) with Gaussian deconvolution of the Raman spectra, R represented the coefficient of determination, R2> 0.9943 indicated that the deconvolution results were valid. Table 5 the Xn obtained from Gaussian deconvolution of the Raman spectra samples

F wt%

X0(Q0)

X1(Q1)

X2(Q2)

X3(Q3)

R2

SF1

6.02

0.0038

0.0345

0.2810

0.6808

0.9977

SF2

4.50

0.0049

0.0242

0.2243

0.7466

0.9949

SF3

3.00

0.0081

0.0359

0.3514

0.6046

0.9943

SF4

1.50

0.0100

0.0387

0.4074

0.5438

0.9943

SF5

0.00

0.0138

0.0362

0.4077

0.5423

0.9960

0.8

Mole fractions of Q

n

0.7 0

Q 1 Q 2 Q 3 Q

0.6 0.5 0.4 0.3 0.2

2+

1

-

4Ca +2Q +2F Ca4Si2O7F2 (3) 2

3

1

2

Q +Q Q

4

1

(4)

1

(5)

3Q Q +2Q

(6)

2

3

2Q Q +Q

0.1 0.0 -0.1

6.02

4.5

3

1.5

0

F/ wt% Fig. 6. Mole fractions of Qn in parent glass with different fluoride content

Fig. 6 showed the Qn molar fraction and its variation trend with different fluoride content. Usually, the reason why fluoride is used as nucleating agent is that in the process of annealing and cooling of

Journal Pre-proof glass, fluoride will accumulate and cause phase-separation in the melt, thus fluoride played the role of nucleation. According to XRD analysis of glass-ceramics, Cuspidine precipitated in glass-ceramics with the increase of fluoride content. Therefore, it can be inferred that fluoride nuclei with the similar structure to Cuspidine were formed during the annealing process of glass melt. In the case of high calcium content, the formation of Cuspidine in glass melt needed to consume Q1 and combined with F-, so with the increase of fluoride content, the equation (3) was promoted and more fluoride nuclei formed. Meanwhile, the consumption of Q1 also promoted the equation (4) and (5). Therefore, with the increase of fluoride from 0-4.5 wt%, the molar fraction of Q3 increased and the molar fraction of Q2 decreased respectively. With the continuous increase of fluoride content, the available Ca2+ in glass melt was depleted, the equation (3) stopped, and the molar fraction of Q1 increased. According to the principle of equation equilibrium, the equation (6) will be promoted. Finally, when the fluoride content increased from 4.5 to 6.02 wt%, the molar fraction of Q3 decreased and the molar fraction of Q2 increased.

X4(DOP)

2.4 2

2

(X3) /X2

4

3

Q +Q 2Q

2.0 1.6 1.2 0.8

SF1

SF2

SF3

SF4

SF5

Fig. 7. The (X3)2/X2 ratio of parent glass samples

Fig. 7 showed the (X3)2/X2 ratio of parent glass samples, they derived from the following formulas: 2Q3↔Q2+Q4

K

X4  K 

X2X4 ( X 3 )2

( X 3 )2  DOP X2

(7)

(8)

(9)

Equation (7) is a common conversion in silicate glass structures, where K in equation (8) stands for the equilibrium constant decided by temperature. The molar fraction of Q4 stands for the highest polymerization, which is proportional to (X3)2/X2, at a certain temperature. Hence, the ratio of (X3)2/X2 can be considered as the degree of polymerization (DOP) of silicate structure network[48, 49] .

Journal Pre-proof Fig. 7 showed (X3)2/X2 of different fluoride content in parent glasses. As the fluoride content increased, the DOP increased first and then decreased. In particular, these results were in accordance with the conclusions deduced from DSC analysis. From the analysis of Fig. 6, it can be known that the increase of the area of endothermic peak contributed to the formation of fluoride in the annealing process of parent glass. The amount of fluoride was a key factor affecting the number of crystallization nuclei generated during the nucleation stage. The equation (3) showed that the formation of fluoride required the consumption of Ca2+ and F-, which increased the molar fraction of bridging oxygen structure in glass structure, so that the DOP of glass increased accordingly. However, as the fluoride continued to increase, available Ca2+ was consumed, excess F- and [Si2O7]6-(2Q1) would restrain the formation of fluoride which would combine with silica-oxygen structures such as Q4 and Q3 to form non-bridge oxygen bonds. This was the reason why the DOP of glass network decreased. 3.4 FTIR analysis results The FTIR spectroscopy is a common method for determining the structure of glass, and it appeared to be a sensitive tool for detecting the crystallization of glass induced by heat treatment[50, 51]. Fig. 8 showed that the infrared spectrum of the parent glass with different fluoride content was similar. The infrared absorption bands of the parent glass were mainly located in three parts of 1150-850 cm-1, 750-650 cm-1 and 450-550 cm-1. Since it showed a large half-height width in wavenumber of 1150-850 cm-1, which corresponds to an infrared absorption band characteristic of the glass network structure, it can be inferred that the parent glass was mainly composed of an amorphous phase[52].

SF1 SF2 SF3 SF4 SF5

1600

1400

1200

1000

800

600

400

-1

Wave number/cm

Fig. 8. FTIR spectra of parent glass

Fig. 9 showed the FTIR spectrum of glass-ceramics after heat treatment. In comparison with Fig. 8, the shift of the absorption bands and the presence of new absorption bands highlighted the deformation of the silica network and the formation of new crystalline after the heat treatment, which were obvious

Journal Pre-proof evidences of the crystallization for the parent glass. The sharper infrared absorption bond of the glass-ceramic compared with the parent glass meant that crystalline phases were precipitated in the glass matrix. Besides, obvious differences were observed among the FTIR spectra of the as-prepared glass-ceramic with different fluoride content. The change in the position and shape of the infrared absorption bonds of the glass ceramics indicated that the types of crystalline phases formed after the heat treatment were different, which was consistent with the results of XRD.

SF1

691

SF2

689

855

SF3

982

447

857

688

982 856 986928

SF4

462

619 453

689 619 526

SF5

524

1038 980 919 1600

1400

1200

1000

453

688 617

1032 984 922

800

600

451 400

-1

Wave number/cm

Fig. 9. FTIR spectra of glass ceramic

The infrared absorption band of the glass-ceramic is an average reflection of the glass matrix and the crystalline phase. Differences at wavenumber of 980-986 cm-1, 688-691 cm-1, and 447-462 cm-1 for the infrared absorption bands were also found in various fluoride content glass-ceramics, which was the main absorption band of the glass matrix portion. Combined with the analysis of XRD, it was supposed that the infrared absorption bonds at 855 cm-1 belonged to the characteristic absorption band of the Cuspidine. Moreover, the new infrared absorption bands at 619 cm-1 and 524 cm-1 were contributed to the appearance of Kalsilite and Combeite.

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Fig. 10. The deconvolution of the FTIR spectra obtained for SF1 and SF5 samples

The detailed interpretations of FTIR spectra of amorphous and glass-ceramic materials are very difficult due to the mentioned high full width at half maximum (overlapping bands) and the presence of characteristic bands of individual crystalline phase[52]. To facilitate the interpretation of the spectra of such materials, the deconvolution of FTIR spectra which included parent glass and glass-ceramic for two selected samples was performed: SF1 with a higher fluoride content and SF5 without fluoride addition. Fig. 10 (a) showed an absorption band of the parent glass of SF1. Since the studied material was a kind of typical alumino-silicate glass, the interpretation of the parent glass spectrum deconvolution was the most unambiguous one due to the lack of crystalline phases. Therefore, the band at 1079 cm-1 corresponded to the stretching vibrations of Si-O-Si (Si, Al), the band at 956 cm-1 and 834cm-1 could be assigned to the stretching vibrations of Si-O-(NBO) and Si-O-(2NBO), and the band at 898 cn-1 could be attributed to the stretching vibrations of Si-O-[53-55], respectively. In the range of 400-800 cm -1, the band at 720cm-1 is assigned to the bending vibrations of Si-O-Si or Si-O-Al, the band at 679 cm-1 is assigned to Si-O-Si symmetric stretching bridging oxygen between tetrahedral, the band at 468 cm -1 is assigned to the bending vibrations of O-Si-O or O-Al-O[54, 56, 57]. Compared with Fig. 10 (c), the location of absorption bands in the range of 400-800 cm -1 for SF1 were similar with SF5. However, the absorption bond at 1300-800 cm-1 was broader and a shift in wavenumber had occurred in SF5. As shown in Fig. 10 (a), the absorption bond area at 1079 cm-1 and

Journal Pre-proof 956 cm-1 was larger, indicating that the stretching vibration of Si-O-Si (Al) and Si-O-(NBO) was more dominant. However, as shown in Fig. 10 (c), obvious difference of the area between the absorption bonds at 1098 cm-1, 998 cm-1, 906 cm-1, and 823 cm-1 can not be found, indicating that the absence of fluoride in glass strengthened the stretching vibration of Si-O-(2NBO) non-bridge oxygen and Si-O- , and increased the content of non-bridge oxygen in the glass structure. It was supposed that the addition of Na2SiF6 led to the generation of fluoride, resulting in the difference of the number of non-bridged oxygen in the glass structure, which was similar to the results of Raman analysis. The difference between the parent glass and the glass-ceramic of SF1 was compared, a sharper absorption bond was found in the glass-ceramics. This was contributed to the fact that the angle and the strength of the Si-O bond were relatively fixed after the crystalline phases were precipitated, leading to the relative concentration of the distribution of vibration bond[53, 58]. Similar to Fig. 10 (a), the band at 1062 cm-1,964 cm-1,898 cm-1, and 814 cm-1shuould be assigned to the stretching vibration of Si-O-Si (Al) and Si-O- as mentioned before. Since Nepheline belong to a framework structure, each Si atom is linked by three bridge oxygen bonds at least. Therefore, the absorption bond area of the Si-O- and Si-O-(2NBO) non-bridge oxygen stretching vibration was relatively smaller than that of the parent glass. A new band located at 853 cm-1 was found in Fig. 10 (b), which was supposed the the feature band of Cuspidine

[59].

The band at 694 cm-1 should be assigned to the Si-O-Si symmetric stretching

bridging oxygen between tetrahedral[54,

55].

The band at 540 cm-1 was associated with double

six-membered rings vibration[55, 60], whereas the band at 469 cm-1 was attributed to the bending vibrations of O-Si-O or O-Al-O[54, 55, 58]. Some studies had pointed out that the band at 540 cm-1 was ascribed to the

presence of six-membered rings vibration related to Nepheline. Further more, the band at 694 cm-1 became sharper, which might be contributed to the silicon tetrahedron present in Nepheline.

Fig. 10 (d) showed the FTIR spectra of SF5 glass-ceramic. The band at 1134 cm-1 should be assigned to stretching vibrations of Si=O[53], and the three bands at 1028 cm-1, 923 cm-1, and 843 cm-1 were contributed to the stretching vibration of Si-O-Si(Al), the stretching vibrations of Si-O-(NBO), and Si-O-(2NBO), respectively[53-55]. Since the crystalline phases in SF5 were Kalsilite and Combeite, and more non-bridge oxygen bonds were existed than Nepheline. The intensity of bands of Si-O-(NBO) and Si-O-(2NBO) non-bridged oxygen stretching vibrations increased with more Ca2+, Na+, and K+ participated from the parent glass. The band located at 1134 cm-1 was ascribed to stretching vibrations of Si=O which was mainly originated from the structure in the remaining glassy matrix. Similarly to

Journal Pre-proof the glass-ceramic of SF1 shown in Fig. 10 (b), the band located at 688 cm-1 should be assigned to Si-O-Si symmetric stretching bridging oxygens between tetrahedral[54, 55, 61]. Besides, bands located at 617 cm-1, 529 cm-1, and 441 cm-1 were related to Combeite [58, 62, 63]. Band at 617 cm-1 was contributed to the six-membered rings of Si-O-Si, while bands at 529 cm-1 and 441 cm-1 were referred to the bending vibration of -O-Si-O- and the bending vibration of Si-O-Si. Thus, the abovementioned three bands confirmed the precipitation of Combeite in glass-ceramics[62].

Fig. 11. Crystal structure diagram of glass-ceramics

In order to defining the difference between Nepheline and Combeite, Fig.11 illuminated the crystal cell structure of standard minerals coming from American Mineralogist Crystal Structure Database. The structure of double six-membered rings related to Nepheline and six-membered rings related to Combeite could be observed, in which the distribution of Si-O bonds in the crystalline phases

were consistent with the deconvolution of the FTIR spectra. The formation of Nepheline structure required more Si-O-Si bridging oxygen bond than Combeite. Combined with the previous analysis, the occurrence of Cuspidine in glass melt could controlled the number of Si-O-Si bridging oxygen, and then affected the types of precipitated crystals, which were related to the difference in the fluoride content. 3.4 Microstructure analysis results Fig. 12 showed SEM of glass-ceramics with different fluoride content under the same heat treatment conditions. It was obvious that the morphology and size of the crystalline phases were

Journal Pre-proof changed with various fluoride content in glass-ceramics, which was in accordance with the analysis of crystalline phase types. Crystalline phases with similar morphology and size were precipitated in SF1 and SF2 samples. SEM and EDS analysis confirmed that the crystalline phase presented a size of 1 μm and a hexagonal plate shape for Nepheline. The EDS shown in Fig. 13 indicated that a high content of Ca and F existed in Point B, proving that such randomly distributed particles in the glass matrix with small-sized were Cuspidine. With the content of fluoride reduced, the hexagonal plate-shaped Nepheline in SF3 disappeared. Furthermore, the shape of the crystalline phases in SF3 was uncertain, and the boundary between the crystalline phases and the glass matrix was not clear, which was ascribed to the incomplete crystallization of crystalline phase. This was explained by the fact that the surface and non-uniform crystallization still occurred in the glass which would result in the coarse and heterogeneous microstructures of glass-ceramics though fluorides were introduced [37, 64]. In addition, SEM images of SF4 and SF5 with low content of fluoride in Fig. 12 exhibited that the crystalline phase became fine granular crystals with a size about 30-80 nm, which were clustered and distributed in the glass phase. A higher magnification SEM image of SF4 and SF5 were shown in Fig. 14, it could be seen that the crystal characteristics in SF4 were more complete than those in SF5. Besides, EDS of point C displayed that the granular crystalline phase in SF4 was Kalsilite. Different from SF4 sample, it was obvious that there was a region in SF5 sample where the crystalline phases were not uniform and the glass matrix and the crystal phase were difficult to distinguish, which was caused by the difference in the degree of crystallization.

Fig. 12. SEM images of the glass-ceramics form SF1 to SF5

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Fig. 13. EDS spectra of points A and B labeled in SF1

When the content of fluoride was high in glass, it was advantageous to the precipitation of crystalline phases with larger size. At the same time, the skeleton structure would be built so that the crystalline phases and the glassy part were interlocked with each other, and this structure was more compact than the granular structure. However, when the content of fluoride was low, small-sized granular crystalline phases were precipitated from the glass matrix. Fluoride could act as nucleating agents and facilitate the crystallization of glass-ceramics. Thus, different types of crystalline phases would precipitated in glass-ceramics with various content of fluoride.

Fig. 14. EDS spectra of points C and D labeled in SF4 and SF5

3.5 Physical properties of glass-ceramics The flexural strength was one of the important factors for evaluating the performance of glass-ceramics. Fig. 15 showed the flexural strength of glass-ceramics, it was obvious that there was significant difference in the flexural strength with different fluoride content. The maximum flexural

Journal Pre-proof strength reached 177.76 MPa, while the minimum flexural strength was 32.88 MPa. According to the XRD analysis, it was beneficial to the improvement of the flexural strength of the glass-ceramic when the main crystal phase was Nepheline and Cuspidine. At the same time, it can be found in SEM images that the flexural strength was also related to the integrity of the precipitated crystalline phases. Firstly, the flexural strength of glass-ceramics was mainly affected by the type of crystalline phases. As shown in the SEM, the Nepheline interleaved in the glass matrix was larger than Kalsilite and Combeite, which was beneficial to resist the expansion of cracks when the samples were going to be broken. However, when the major crystalline phases in glass-ceramics were Kalsilite and Combeite, the skeleton structure could not be built with the glass matrix effectively for the crystalline phases were granular with small size, which was responsible for the decrease in the flexural strength. Moreover, the degree of crystallization also affected the flexural strength properties of glass-ceramics, which was the reason that the number of intact crystals was gradually reduced from SF1 to SF3 glass-ceramics as shown in SEM. Nevertheless, the flexural strength of the glass-ceramics slightly increased with the absence of the fluoride in glass-ceramic from SF4 to SF5, which was ascribed to the fact that a higher content and more complete crystalline phase in SF4 than that in SF5 shown in Fig. 14. Thus, fluoride controlled the crystalline type and the crystallization state of the glass-ceramics which made a large influence on the flexural strength of the glass-ceramics.

180

Flexural stength/MPa

160 140 120 100 80 60 40 20

SF1

SF2

SF3

SF4

SF5

Fig. 15. Flexural strength of glass ceramics with different fluoride content

The density and volume shrinkage of glass-ceramics with different fluoride were shown in Fig. 16. The density of the glass-ceramics was consistent with the change of the volume shrinkage, indicating that the change of the glass-ceramics in this respect was mainly affected by the crystallization after heat treatment. It could be noticed that the volume shrinkage ratio was higher than 0, which meant that the

Journal Pre-proof density of the crystalline phase was higher than that of the glassy phase, thus the volume shrinkage of the glass-ceramic occurred. The prepared glass-ceramics could be divided into two parts depending on the type of crystalline phases. The major precipitations of SF1 and SF2 were Nepheline and Cuspidine, while those were Kalsilite and Combeite for SF3, SF4, and SF5. It was obvious that the density of the glass-ceramics was greater when the fluoride content was lower. 4

2.94 Density

2.92

-3

Density/(g•cm )

3 2.90

2

2.88 2.86

1

Volume shrinkage/%

Volume shrinkage

2.84 2.82

0 SF1

SF2

SF3

SF4

SF5

Fig. 16. Density and volume shrinkage of glass-ceramics with different fluoride content

It was reported[65, 66] that the difference in density between the glass matrix and the crystal phase revealed the structural difference in the crystal phase and the glass matrix. If the crystalline phase precipitated from the glass matrix differed greatly from the structure of the glass matrix, this might cause a significant change in the density of glass-ceramic, which related to uniform nucleation in the crystallization process. Combining with Raman analysis, the higher the DOP of the parent glass was, the smaller the volume shrinkage was, which indicated that the glass-ceramics with similar structure of the parent glass can be obtained with a higher DOP. On the other hand, due to the presence of the residual glassy phase, the degree of crystallization will also have influence on the density and volume shrinkage. When the volume shrinkage rate was high, local stress and micro cracks were easily generated, which was not conducive to the cutting and machining of the glass ceramic. It could be found from Fig. 12 that the granular crystals were more likely to cause volume shrinkage than sheet-like crystal phases. Besides, SF5 sample with the largest volume shrinkage presented more cracks on the microscopic morphology, which was unfavorable for the improvement of the flexural strength of the glass-ceramics. 4. Conclusion In this paper, the hot steel slag was used by melting method, the effect of fluoride content on the structure and properties of glass-ceramics was investigated. The addition of fluoride was beneficial to

Journal Pre-proof the improvement of the performance of the steel slag glass-ceramic. Moreover, a method for preparing glass-ceramics directly from steel slag was provided. 1. With the content of fluoride decreased, the crystalline phase of the glass-ceramic has changed under the same heat treatment. When the content of fluoride was high, Nepheline with size about 1 μm and Cuspidine were precipitated. While with the decrease of fluoride, the Nepheline was absent and the Kalsilite and Combeite were precipitated. 2. Fluoride affected the structure and properties of glass-ceramics by controlling the type of crystalline phase and crystallization process. The steel slag glass-ceramics with Nepheline and Cuspidine as the main crystalline phase had higher flexural strength than glass-ceramics mainly composed of Kalsilite and Combeite. When the major crystalline phases were Kalsilite and Combeite, the steel slag glass-ceramic showed a higher volume shrinkage. 3. The crystalline type of the glass-ceramic was related to the structure of the glass matrix. Fluoride had an great influence on the crystallization process by affecting the glass structure. The introduction of fluoride by Na2SiF6 increased the content of Si-O bridging oxygen bond in the glass structure, resulting in the formation of Nepheline with the frame structure. Moreover, the presence of fluoride contributed to the formation of nuclei and further promoted the precipitation of crystalline phases from the glass matrix. Acknowledgement This work was supported by the “National Key Technologies R&D Program” (program number: 2016YFB0601304). The authors wish to express their sincere gratitude to the “Center for Material research and analysis” of Wuhan University of Technology. References [1] Guzel G, Deveci H. Properties of polymer composites based on bisphenol a epoxy resins with original/modified steel slag[J]. Polymer Composites. 2018, 39(2): 513-521. [2] Ning D, Liang Y, Liu Z, et al. Impacts of steel-slag-based silicate fertilizer on soil acidity and silicon availability and metals-immobilization in a paddy soil [J]. PLOS ONE. 2016, 11(12): e168163. [3] Tsakiridis P E, Papadimitriou G D, Tsivilis S, et al. Utilization of steel slag for Portland cement clinker production[J]. Journal of Hazardous Materials. 2008, 152(2): 805-811. [4] Jiang L, Bao Y, Yang Q, et al. Formation of Spinel phases in oxidized BOF slag under different cooling conditions[J]. Steel research international. 2017, 88(11): 1700066. [5] Qiu H, Zhang H, Zhao B, et al. Dynamics study on vanadium extraction technology from chloride leaching steel slag[J]. Rare Metal Materials and Engineering. 2013, 42(4): 696-699. [6] Yang J, Liu B, Zhang S, et al. Glass-ceramics one-step crystallization accomplished by building Ca2+ and Mg2+ fast diffusion layer around diopside crystal[J]. Journal of Alloys and Compounds. 2016, 688:

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Journal Pre-proof Declaration of interests：

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Highlights：    

Fluoride affected the type of crystal phase and crystallization process. Fluoride contributed to the formation of the Si-O bridge oxygen bonds. Fluoride had abnormal effect in high alkalinity glass. The glass-ceramics with flexural strength of 177.76 MPa can be obtained.