Effect of Al2O3/SiO2 on the structure and properties of blast furnace slag glass-ceramics

Different Al 2 O 3 /SiO 2 glass-ceramics were prepared from blast furnace slag by traditional sintering method. The structure and properties of glasses or glass-ceramics were investigated by DSC, XRD, SEM, FTIR, 27 Al MAS NMR. The DSC results showed that with the increase of Al 2 O 3 /SiO 2 , the glass transition temperature (T g ) rst increased and then decreased, reached the minimum when Al 2 O 3 /SiO 2 was 0.34. The volume density, bending strength and microhardness of glass-ceramics also showed the same variation rule. The FTIR and 27 Al MAS NMR spectra results revealed this phenomenon. When Al 2 O 3 /SiO 2 was 0.19, a large amount of Si 4+ was added to the glass network to make the structure dense. With Al 2 O 3 /SiO 2 increased from 0.24 to 0.34, the amount of [AlO 6 ] in the glass increased while [AlO 4 ] decreased, and the degree of network polymerization of the glass decreased; as Al 2 O 3 /SiO 2 further increased from 0.34 to 0.39, [AlO 4 ] increased and [AlO 6 ] decreased in the glass, and the degree of network polymerization of the glass increased. The XRD results showed that the crystal phase of the glass-ceramics was composed of gehlenite, diopside and hyalophane. Moreover, with the increase of Al 2 O 3 /SiO 2 , the gehlenite content in the glass-ceramics increased, while the content of diopside and hyalophane decreased.

enter the glass network as [AlO 4 ], making the structure compact; on the other hand, Al 3+ can also exist in the glass as [AlO 6 ], causing the depolymerization of the glass network and making the structure loose [7].
In partial systems, Al 3+ can also act as the element of partial crystal phases when the content of Al 2 O 3 is high [8]. Wentao Zhang et al. studied the in uence of B 2 O 3 /Al 2 O 3 ratio and Al/Na ratio on BFS glassceramics, and indicated that Al 2 O 3 improved the thermal stability of glass, meanwhile, the crystallization activation energy and network structure were also affected with the change of Al 2 O 3 content [9,10].
Lishun Chen et al. also con rmed that, in the study of glass-ceramics with high calcium system, the crystallization activation energy rose and the main crystal phase changed from akermanite to gehlenite with the increase of Al 2 O 3 [11]. Jinshu Cheng et al. showed that the workability and kinetic fragility of glass were linked with the structure, with the increase of SiO 2 /Al 2 O 3 , the non-bridging oxygen in the glass network decreased, and the structure of the glass network became compact, resulting in the increase of the viscosity of the glass melt, meanwhile, the change of SiO 2 /Al 2 O 3 ratio were responsible for the increase of fragility [12]. The research of Janusz Partyka et al. also showed that the variation of SiO 2 /Al 2 O 3 ratio directly determined the type and content of crystal phase [13]. Therefore, it is important to investigate the in uence of SiO 2 /Al 2 O 3 on the structure and properties of BFS glass-ceramics.
In this paper, based on 60 wt.% BFS, the effects of different SiO 2 /Al 2 O 3 on the crystal phases, structure and properties of glass-ceramics were studied. In order to further reveal the in uence of the Al 2 O 3 /SiO 2 on the glass network, Fourier Transform Infrared Spectrometer and Solid State Nuclear Magnetic Resonance Spectrometer were used to deeply investigate the microstructure of glass and glass-ceramics.

Raw materials and experimental formula
The BFS used in this experiment was from China Baowu Iron & Steel Group. The chemical composition of BFS was measured by X-ray Fluorescence (XRF, Zetium, PANalytical B. V.). This experiment was based on 60 wt.% BFS and supplemented by pure chemical reagent to prepare glass-ceramics. The high content of alkaline earth metal oxide in BFS leads to the high alkalinity of base glass, which is not conducive to the sintering process due to the fast crystallization [14]. Therefore, the reagents were mainly acid oxides such as SiO 2 and Al 2 O 3 , and a small amount of oxides such as Na 2 O, K 2 O, BaO and B 2 O 3 were added to adjust the glass melting, clari cation, homogenization and sintering process. The oxides were introduced by Na 2 CO 3 , K 2 CO 3 , Ba 2 CO 3 and H 3 BO 3 respectively. The formula and the speci c oxide composition of the BFS were shown in Table 1. The SiO 2 /Al 2 O 3 ratios in the base glass were different, which were 0.19(A1), 0.24(A2), 0.29(A3), 0.34(A4) and 0.39(A5), respectively. Table 1 Oxide composition of glasses and BFS (wt.%).

Preparation of glass-ceramics
In this paper, glass-ceramics were prepared using conventional sintering method. The melting temperature of the base glass was 1450 ℃, the holding time was 1 h. The melted glass liquid was poured into the clear water to obtain the base glass slag. And the glass slag was ground with planetary grinding ball for 30 min and then passed through a 200 mesh sieve to obtain the parent glass powder which was pressed into 4 mm × 40 mm strip samples under a pressure of 50 MPa, and then the samples were sintered in a resistance furnace, the sintering temperature was obtained by thermal analysis, the heating rate was 10 ℃/min, the holding time was 1.5 h.

Characterizations
Thermal analysis of base glass powder was performed using Differential Scanning Calorimetry (DSC, STA449F3, NETZSCH), the temperature range was 0-1000 ℃ in the air and the heating rate was 10 ℃/min. The crystal phase was measured by X-ray Diffractometer (XRD, D8 Advance, BRUKER AXS),the scanning range was 10-70º. where F is the fracture load (N), L is the span (mm), b is the fracture width (mm), and h is the fracture thickness (mm). The volume density of glass-ceramics was measured by Archimedes-drainage method. The indentation method was used to measure the Vickers hardness of the glass-ceramics, the loading force was 0.98 N, the loading time was 10 s.  Table 2 showed the speci c characteristic temperature values for A1-A5, where T g was the glass transition temperature, T c1 was the rst crystallization temperature, and T c2 was the second crystallization temperature. In Fig. 1, there were two obvious exothermic peaks within the range of 780-900 ℃, among which the rst crystal peak shape of A1 parent glass was relatively at. Table 2 Characteristic temperature of glasses with different Al 2 O 3 /SiO 2 in DSC. The DSC curve of the base glasses showed that the peak of T c1 became sharp and moved towards low temperature gradually for A1-A5, while T c2 showed a trend of moving towards high temperature for A2-A5. While the charge of Al 3+ is less than that of Si 4+ , and the self-diffusion coe cient of Al 3+ is higher than that of Si 4+ [15]. Therefore, the higher Al 2 O 3 content in the glasses was more conducive to particle diffusion in the crystallization process, which reduced T c1 for A1-A5 [15]; then the crystal phase was precipitated on the surface of the glass particle, which increased the viscosity of the glasses, hindered the further diffusion of the particle, and led to the rise of T c2 [16]. The rst crystallization peak of the A1 glass was not obvious and the T c2 was high, which may be related to the high SiO 2 content in the A1 glass, as Si 4+ can gather the network, increase the viscosity of the glass, hinder the diffusion of particles inside the glass, and increase the crystallization temperature [17]. Figure 1 and Table 2 showed that T g decreased rst from A1 to A5 (reached the minimum in A4) and then increased, which indicated that the thermal stability of glasses deduced rst and then rose with the increase of Al 2 O 3 /SiO 2 . The thermal stability of glasses is closely related to the network structure of glass [18], which also indicated that the network polymerization degree of glasses showed a trend of rst decreasing and then increasing in series A glasses. In this experiment, according to the DSC curve ( Fig. 1), in order to ensure that the crystallization process of each sample was fully carried out, the sintering temperature of the sample was set at 890 ℃ and the holding time was 1.5 h.

Crystal phase analysis
The XRD graph of glass-ceramics with different Al 2 O 3 /SiO 2 was illustrated in Fig. 2. It shows that the peaks of A1-A5 glass-ceramics were basically the same. The analysis by Jade 6.5 indicated that the crystal phase of A series glass-ceramics was composed of the main crystal phase gehlenite(Ca 2 (Al(AlSi)O 7 ,PDF#74-1607), the secondary crystal phase diopside(CaMgSi 2 O 6 PDF#74-1607) and hyalophane (K. 6 Ba. 4 Figure 2 also showed that the diffraction peak intensity of the main crystal phase increased while that of the secondary crystal phase decreased gradually for A1-A5. It means that the content of gehlenite in the glass-ceramics increased while diopside and hyalophane decreased with the increase of Al 2 O 3 /SiO 2 . The degree of polymerization of anionic groups is different between the primary phase (gehlenite) and the secondary phase (diopside and hyalophane). The reaction process is as follows: 6 Ba. 4  Si 4+ in the glass network, which was conducive to the formation of [AlSiO 7 ] and promoted the precipitation of gehlenite. On account of the large amount of precipitation of gehlenite, Si 4+ content in the glasses was decreased, which led to the reduction of the anion group of the secondary crystal phase and reduced the precipitation of diopside and hyalophane.
The SEM graph was showed in Fig. 3. It illustrated that there were a large number of strip or gridded crystals, and the XRD analysis results showed that the crystal phase was gehlenite; a few clusters of granular crystals were seen in A1-A3, which were speculated to be diopside based on the XRD results. In the A4 and A5 samples, no other crystal phases were found, which was mainly attributed to the high precipitation of gehlenite and the low precipitation of secondary phase. In the SEM graph of A1, there were more holes left by HF solution erosion. With the increase of Al 2 O 3 /SiO 2 , the holes decreased gradually, indicating that the increase of crystal evolution led to the decrease of glass phase for A1-A5. When the Al 2 O 3 /SiO 2 increased to 0.34 (A4), it was seen that the growth of crystal grains was united and arranged in a certain direction, and the defect area without crystal phase existed; when the Al 2 O 3 /SiO 2 ratio continued to increase to 0.39 (A5), the crystal grains continued to grow into long strips, and the long strips growing in different directions were interspersed and nested together.

FTIR analysis
FTIR is a common method used to test the internal structure of glass and glass-ceramics. Figure 4 was the FTIR of the base glasses with different Al 2 O 3 /SiO 2 at 400 cm − 1 -1400 cm − 1 . In Fig. 4, the shape of vibration absorption peak of A1-A5 parent glasses was substantially the same. There were mainly three wide vibration absorption bands, which were in the range of 400-600 cm − 1 , 600-800 cm − 1 and 800-1200 cm − 1 respectively. The irregular arrangement of ions (ion clusters) in the glasses and the existence of non-bridging oxygen bonds made the angle and length of Si-O bond change, which made the infrared vibration absorption peak shift to a certain extent and become gentle and broad [21,22].
In the infrared vibration spectra of glasses, the vibration absorption band in the range of 400-600 cm − [25][26][27]. The vibration absorption peak in the range of 800-1200 cm − 1 can be decomposed into stretching vibration of silicon-oxygen tetrahedron with different degree of polymerization by Gaussian function (Fig. 5), whose symbol is Q n , where n is the number of bridging oxygen (O b ) in the silicon-oxygen tetrahedron (n = 0,1,2,3,4,5) [19,28]. Figure 5 was the Gaussian deconvolution diagram of the infrared vibration absorption peak at the range of 800-1200 cm − 1 of the series A glasses, and table 3 was the speci c peak values of each Q n . The results of Fig. 5 and Table 3 showed that the vibration absorption peaks of A1-A5 glasses at 800-1200 cm − 1 were all decomposed into 5 vibration absorption peaks corresponding to Q 0 , Q 1 , Q 2 , Q 3 and Q 4 respectively. Moreover, the peak positions of Q n were relatively similar for each sample of series A glasses, indicating that the network structure of the base glass did not change signi cantly with the increase of Al 2 O 3 /SiO 2 . Figure 6 was the variation distribution chart of peak position of Q n for A1-A5, which indicated that the peak position of Q n was rstly shifted to direction of the low wavenumbers and then to the high wavenumbers, among which the peak position of Q n reached the lowest when the Al 2 O 3 /SiO 2 was 0.34 (A4). The vibration absorption peak of Q n is closely related to the glass network [29]. When the glass network tends to be close, the vibration peak is shifted to the direction of high wavenumbers; on the contrary, when the glass network structure tends to be loose, the vibration peak moves to the direction of low wavenumbers [29]. The curve in Fig. 6 illustrated that the glass network structure tended to be loose with the increase of Al 2 O 3 /SiO 2 from 0.19 to 0.34; subsequently, the glass network structure tended to be close with the Al 2 O 3 /SiO 2 continues to increase from 0.34 to 0.39. The infrared vibration absorption spectra of the glass-ceramics sintered at 890 ℃ was showed in Fig. 7.
It indicated that the peak range of the glass-ceramics sample was roughly the same as that of the base glasses (Fig. 4), which was still divided into three vibration absorption bands, however, there were some sharp absorption peaks which were not found in the infrared vibration absorption spectra of the base glasses. This was mainly attributed to the generation of crystal phase, which made the originally covered absorption peaks appear [21,23]. The position and shape of the infrared absorption peaks of A1-A5 were basically the same, indicating that the crystal phase of glass-ceramics with different Al 2 O 3/ SiO 2 were basically the same, and the XRD analysis also proves this conclusion.
In the range of 400-600cm − 1 , the vibration absorption peak at 470 cm − 1 was divided into two absorption peaks: 472 cm − 1 and 424 cm − 1 . The peak at 472 cm − 1 was attributed to the bending vibration of Si-O-Si in the residual glass phase and diopside phase (a small amount of hyalophane), the peak at 424 cm − 1 was attributed to the bending vibration of Si-O-Al bond in gehlenite [24,30]. The intensity of the vibration absorption peak at 424 cm − 1 gradually increased for A1-A5, which indicated that the content of gehlenite increased in glass ceramics with the increase of Al 2 O 3 /SiO 2 . There were two vibration absorption peaks at 544 cm − 1 and 579 cm − 1 , which were not found in the base glasses in Fig. 4. This was due to the coupling effect between the bending vibration of O-Si-O in the crystal phase and the stretching vibration of Ca-O [24,27]. In the range of 600-800 cm −

27 Al NMR analysis
To further study the coordination of Al 3+ in glass, the 27 Al MAS NMR spectra were used to investigate the glass structure, as shown in Fig. 8, there was an obvious wide peak in the range of -25-100 ppm. ]. Therefore [31,32], 27 Al NMR spectra were deconvoluted by Gaussian function, and the results were shown in Fig. 9 and Fig. 10.

Physical and mechanical properties analysis
The physical and mechanical properties of glass-ceramics with different Al 2 O 3 /SiO 2 were showed in The results of Fig. 8 showed that the volume density of the glass-ceramics was between 2.735-2.770 g/cm 3 , the bending strength was between 95-120 MPa and the microhardness was between 540-610 Hv. Meanwhile, with the increase of Al 2 O 3 /SiO 2 , the volume density, bending strength and microhardness of glass-ceramics all decreased rst (reached the minimum in A4, the Al 2 O 3 /SiO 2 was 0.34) and then increased. The physical and mechanical properties of glass-ceramics are closely connected to the crystal phase, the glass phase and the combination among them [33,34]. According to the previous analysis, with the Al 2 O 3 /SiO 2 gradually raising from 0.19 to 0.34, the precipitation of main crystal phase gehlenite (Ca 2 (Al(AlSi)O 7 )) increased, while the precipitation of secondary crystal phase decreased, and the remaining glass phase in the glass-ceramics also decreased. With the precipitation and growth of crystal phase, the internal stress of the glass-ceramics rose, and the decrease of liquid phase content was harmful to the sintering process, which led to the insu cient connection between crystal phase and led to more holes and defects in the glass-ceramics. At the same time, more Al 3+ destroyed the glass network structure with [AlO 6 ], making the microstructure of the glasses tend to be loose. These results combined to reduce the physical properties (volume density, bending strength, microhardness) of the glass-ceramics with the increase of which was bene cial to strengthen the glass network and make the structure tend to be dense. Meanwhile, the crystal phase in the glass-ceramics was further precipitated and grown, and the long strip crystal phase was interspersed with each other, which made the bonding between the crystal phase more complex and compact, thus increasing the density, bending strength and microhardness.

Conclusion
The effects of different Al 2 O 3 /SiO 2 on the structure and properties of BFS glass-ceramics were studied in this paper. With the increase of Al 2 O 3 /SiO 2 , the gehlenite content in the glass-ceramics increased, while the diopside and hyalophane contents decreased. As Al 2 O 3 /SiO 2 increased from 0.24 to 0.34, the content of [AlO 6 ] in the glass network increased and [AlO 4 ] decreased gradually, the glass network structure became loose and the temperature of T g decreased, simultaneously, the volume density, bending strength and microhardness of the glass-ceramics also decreased. With further increased Al 2 O 3 /SiO 2 from 0.34 to 0.39, the content of [AlO 6 ] in the glass decreased and [AlO 4 ] increased, the glass network structure became densi ed and the temperature of T g increased, meanwhile, the volume density, bending strength and microhardness of the glass-ceramics increased. When Al 2 O 3 /SiO 2 was 0.19, the network structure of glass was relatively compact as high SiO 2 content, and the performance of glass-ceramics was the best, the volume density was 2.77 g/cm 3 , the bending strength was 120.50 MPa, the microhardness was 604.21 Hv.