Preparation and Properties of CaO-Al 2 O 3-SiO 2 Glass-ceramics by Sintered Frits Particle from Mining Wastes

The paper reports on some experimental results obtained from the production of glass-ceramics containing gold tailings powder (GTP). Frits particle sintered technology was used to prepare glass ceramic products. SiO2, CaO, ZnO, BaO and B2O3 were selected to adjust the composition of the glass. Based on the results of differential thermal analysis (DTA), the nucleation and crystallization temperature of parent glass samples with different schedule were identified, respectively. X-ray diffraction (XRD) analysis of the produced glass-ceramics materials revealed that the main crystalline phase was β-wollastonite. With the increasing of CaO content, the intensity of crystal diffractive peaks also increases. The formation of β-wollastonite crystal could be accelerated by the increasing of CaO. The glassceramics with fine microstructure showed better physical, mechanical properties and chemical resistance. Overall results indicated that it was a feasible attempt to produce glassceramics for building and decorative materials from waste materials. The amount of GTP used in the glass batches was more than 65 wt% of the whole raw.


Introduction
Glass ceramics are a series of composite materials in which glass phase and crystalline phase coexist.They are famous for the numerous varieties, excellent properties and different preparation technologies.Glass-ceramics are attractive materials used in various applications such as building materials, cooking ceramics, machinable ceramics, bioceramics, optical materials, etc. [1,2].Among these glass-ceramics system, CaO-Al 2 O 3 -SiO 2 system is significant, due to its special durability and mechanical properties arise from precipitation of wollastonite phase [3], which can be produced by sintered of glass particles.Several advantages appear when using particles sintering technology to prepare glassceramics as decorative building materials [4].Compared with natural stones, sintering glassceramics decorated materials have the following advantages: dense structure, high strength and very nice corrosion resistance.Since free glass surfaces are preferred sites for devitrification, crystallization may occur without catalysts, especially for small grains [5,6].With the development of the modern society, the rational and safe use of various waste were paid more and more attention, so that many recycling technologies and materials which reused the waste materials were contrived continuously.Some researchers tried to reuse the wastes material by preparing glass-ceramics [7][8][9][10][11].Vitrification by melting was proposed as a convenient method to solidify raw materials, in which the glass product can immobilize and stabilize the heavy metals in the glass matrix [12].Gold tailings powder (GTP) is one of the mining wastes which are disaffirmed and imperiling entironment.Nevertheless, GTP was found that there were some usable elements in them.For instance, SiO 2 , Al 2 O 3 , CaO, the primary oxides in GTP, makes GTP very suiTab.for the raw material used to produce CaO-Al 2 O 3 -SiO 2 system glass-ceramics.GTP has been produced about 10 million tons a year in China since 2010 and also took up large numbers of land.These disaffirmed GTP flowed away in the rain or the wind easily, and the harm was spread in the entironment.Therefore the recycling of GTP in glass-ceramic production is an effective way for the volume reduction of wastes.
The paper reports on the possibility of glass-ceramic products obtained by the sintering of glass particles from Linghu GTP.The sintering and crystallization heat treatment were optimized in order to obtain glass ceramic products of high density and smooth surface, which also contained high volume percentage of crystalline phase.

Experimental 2.1 Glass preparation and heat treatment
GTP was provided by a mining company in Linghu, Henan province, China.The chemical composition of the GTP being used was listed in Tab.I.The amount of SiO 2 and Al 2 O 3 in the GTP was relatively higher in the compositions.If the GTP could also be used to melt into glasses and produce glass ceramics, the compositions should be adjusted.According to the relevant references [13][14][15][16][17] and many experiments, SiO 2 , CaO, ZnO, BaO and BB 2 O 3 were the useful components to adjust the base glasses in Tab.II.The compositions of them defined were shown in Tab.III.The amount of GTP used in the glass batches was more than 65 wt% of the whole raw.All of the raw materials used for adjusting the base glasses were reagent grade.The weighed batch materials, after mixing, were melted in alumina crucible in an electric furnace at 1520 ºC for 3h.The melts were then quenched in cold water to obtain frits which maintained glass phase structure and the size < 1 mm.These frits samples were prepared, with a pressure of 25MPa with polyvinyl alcohol (PVA) as binder, and were heated in an electric furnace, according to the schedule which was gained by differential thermal analysis.

Analysis methods
The particle size measurements of the powdered glasses were carried out by a laser particle size analyzer (Mastersizer 2000).Non-isothermal DTA was performed in dry nitrogen by heating 30 mg samples in a Pt crucible with Al 2 O 3 as the reference material within the temperature range between 20 ºC and 1000 ˚C at a heating rate of 5 ºC /min (Netzech thermal analysis system).Glass powder (<100 micron) was analyzed by differential thermal analysis (DTA), so the temperature of crystallization peak on the DTA curve was a function of the glass particle size (<100 micron).As the glass particle size used to analyze was smaller than the size used to prepare the glass ceramic sample, crystallization temperatures of the parent glass samples should be higher than the temperature of crystallization peak on the DTA curve.
X-ray diffraction was utilized to analyze which crystalline phases occurred in the produced samples.X-ray diffraction spectra were acquired by a D/MAX-RB X-ray diffraction system (RIGAKU) operating at 40 kV and 30 mA utilizing Cu Kα radiation on powder samples passed through 300 grade mesh.The detector was scanned over a range from 10• to 80• for two thete angles, at a step size of 0.02• and a dwell time of 2 s per step.The resulting powder diffraction patterns were analyzed utilizing a software package program.
The glass-ceramics sample fracture surface which had been etched for 30 s in 5 % HF (v%) solution were prepared beforehand for scanning electron microscopy (SEM).After the etched step, the samples were rinsed with excess distilled water immediately, and then were cleaned in ethanol for 3 min.Next the samples were gold-coated and observed with a JSM 600 microscope operating at 25kV.The volume and shape of crystals occurred in the microstructure of the glass-ceramic samples were determined.An irregularly-shaped blocky area was used to provide detailed microchemistry information.Elemental analysis was carried out using Emispec energy dispersive spectroscopy (EDS) in scanning SEM mode.
A representative sample of glass-ceramic was analyzed for microstructural to investigate the scale and distribution of crystalline phases and their connectivity.Transmission electron microscopy (TEM) was on a JEOL JEM 2100F transmission electron microscope.The samples were prepared following traditional TEM sample preparation methods.The crystallized sample which was grinded in an agate mortar to a grain size < 65 μm was used for TEM.

Properties tests
The density of nucleated glass-ceramics samples were tested using the procedure outlined in GB/T 9966.3-2001.Three-point bending strength test was carried out with a bending test machine.The rectangular specimens of dimensions used for the test were 60 mm × 5 mm × 5 mm.For each sample, ten measurements were made.The chemical resistances of nucleated glass-ceramics samples were tested in 5% H 2 SO 4 and 5% NaOH solutions.In this study, the prepared samples were treated at 373K for 2 h in 100 ml solutions respectively, and the size of the test samples was 20 mm × 20 mm × 10 mm, then every small blocks of which were weighed carefully, and the original mass (w 0 /g) was obtained.After the dunked process, samples were washed, dried and weighed again to obtain the final mass (w 1 /g).The following formula was used to calculate the chemical resistances property of the samples, with units of mg / g.

Differential thermal analysis
With respect to the effect of glass particle size on its sinter ability, the frits were grinded to a mean particle size (Fig. 1).This was the accumulated mass distribution curve of glass powder, and the glass powder which size was > 500 micron accounted for more than 50% of the whole mass of the glass powder.Nevertheless, the specific surface area of the glass powder was very high.The differential thermal analysis (DTA, 5 ºC /min heating rate) curve of base glass samples were able to demonstrate the glass transition temperature and crystallization temperature, as illustrated by Fig. 2. Transition temperature occurred around at Tg 1 = 686 ºC, Tg 2 = 675 ºC and Tg 3 = 664 ºC, respectively.Crystallization temperatures of the parent glass samples were respectively identified as Tc 1 = 975 ºC, Tc 2 = 969 ºC and Tc 3 = 959 ºC.DTA results showed that the glass transition and crystallization temperatures of the samples reduced due to different CaO content.In the components of the glass samples, metal oxides, including Fe 2 O 3 , CaO, MgO, K 2 O, and Na 2 O, led to lower energy input in crystalline phase transition.Previous studies showed that the optimum nucleation temperature usually occurred in the range from 50 ºC to 100 ºC above the glass transition temperature [18].In our study, the nucleation temperatures of C 1 , C 2 and C 3 samples were identified respectively as Tn 1 = 766 ºC for 1 h, Tn 2 = 755 ºC for 1 h and Tn 3 = 744 ºC for 1 h.The crystallization temperatures of C 1 , C 2 and C 3 samples were identified as Tc 1 = 1025 ºC for 2 h, Tc 2 = 1019 ºC for 2 h and Tc 3 = 1009 ºC for 2 h, respectively.The crystallization temperatures used to prepare the glass ceramic sample were higher than the temperature of crystallization peak on the DTA curve because of the explanation in 2.2.All heating and cooling rates of the tests were set at 5 ºC /min.This was conducted to assess the simultaneous occurrence of densification and crystallization in these materials.

Microstructure and densification
The results of XRD spectra were shown in Fig. 3.It was clear that the major crystalline phase was β-CaSiO 3 (β-wollastonite), and the crystal diffractive peak was very clear, because the crystalline phase of β-wollastonite was complete.It could be noticed that with CaO content increased, the intensity of crystal diffractive peaks was also increasing in Fig. 3.The reason is that the increasing of CaO can efficiently hasten CaSiO 3 to be separated out from the glass phase.It was an ineviTab.fact that amount of β-wollastonite crystal increased with CaO content increasing.4 showed SEM images of the samples sintered under different conditions.SEM observations revealed that a large number of block-shaped crystals occurred in the glass ceramics materials.The glass-ceramic samples for SEM test were eroded in the HF solution and the samples were washed many times by ultrasonic cleaning instrument after erosion.So the glass phase of the glass-ceramic structure was eroded and dissolved, and the microcrystalline phase was retained.With the increasing of CaO, the amount and the sizes of crystal particles also grew up.The samples had been eroded by hydrofluoric acid, and the glass phases were dissolved, so the crystalline phases were outstanding.It could be seen from the micrographs that the microstructure of the sample was dense, moreover the glass phases and crystalline phases adhered to each other.The structure was beneficial to improve the integral intensity and wear-resistance of materials.When using frits particle sintered technology, the heat treatment at Tn had two very important functions.Firstly, when sintered at Tn, the density of the glass grains increased.Secondly, in the course of sintering, there were many crystallites produced.Due to the existing of nucleus, crystal growing became very easy.The major crystalline phase grew completely.Tab.IV listed the results of the density of samples sintered at their own conditions.The data in Tab.IV showed that with CaO content increased, the density of samples increased too.According to the results of XRD and SEM, with CaO content increased, β-wollastonite crystal of samples increased.In addition, the density of β-wollastonite crystal is about 2.85 g/cm 3 , which is higher than the glass phase.

Crystallization time and densification
In order to realize the influence of crystallization time, the heat treatment schedules were changed.According to the result in 3.2, the schedules were chosen to prepare sample C 2 .Tn 2 was 755 ºC, Tc 2 was 1019 ºC from 0.5 h to 4 h.Tab.V listed the results of the density of samples sintered at different conditions.The data in Tab.V showed that the density of C 2 increased when crystallization time extending.In other words, the longer crystallization time used, the higher the density of samples obtained.Previous study indicated that the density of glass-ceramics samples increased with the enhancement of crystallization degree [19].In our experiment, the process of glass particles were concerned with liquid phases, and the liquid phase benefited to particles moving.
Tab. V The results of the density of C 2 produced at different schedules.In Fig. 5, XRD results showed that as crystallization time increased, the intensity of crystal diffractive peaks increased.It could be seen that there were some weak peaks in the curves when the crystallization time was 0.5 h, and the curves presented the states of dissemination.It could be inferred that there were some crystallization in glass phase.When the crystallization time went up to 1 h, the intensity of diffraction peaks ascended in the curves.As the crystallization time rose to 2 h, obvious diffraction peaks were shown in the curves, which indicated that there was considerable β-wollastonite crystallized.When the crystallization time turned to 3 h and 4 h, the diffraction peaks were more evident and the peak intensity of β-wollastonite was much higher, indicated that the crystallization was more sufficient.This kind of glass ceramics could be used as decorated material, so the quality of the surface was one of the most important characters of the material.From the surface of the samples, it could be manifested that with the sintering temperature increasing, the surface of the samples become better and have beautiful luster.Fig. 6 showed SEM images of C 2 crystallized at different thermal conditions.C 21 was produced at Tn 2 = 755 ºC for 1 h, Tc 2 = 1019 ºC for 0.5 h, C 22 was produced at Tn 2 = 755 ºC for 1 h, Tc 2 = 1019 ºC for 1 h, C 23 was produced at Tn 2 = 755 ºC for 1 h, Tc 2 = 1019 ºC for 2 h, C 24 was produced at Tn 2 = 755 ºC for 1 h, Tc 2 = 1019 ºC for 3 h and C 25 was produced at Tn 2 = 755 ºC for 1 h, Tc 2 = 1019 ºC for 4 h.With the increase of crystallization time, the quantity of crystals in glass-ceramics increase.Meanwhile, the sphere-shaped crystals became equirotal and aswarm.It could be inferred that crystallization time had a very important effect on the crystallization.In general, the interaction between crystallization and densification could be altered by the heat treatment.Previous studies indicated that the quantity and the size of the crystallites in glassceramics increased with the increase of crystallization time [17,19].When doing further researches on the irregularly-shaped crystals in SEM, energy spectrum analysis was used, which could help finding that Ca weight accounted for 33.64%,Si weight accounted for 25.28%,and O weight accounted for 41.08%.The ratio matched the element content of wollastonite, so we believed that the irregularly-shaped phase was wollastonite microcrystalline phase.Macor-type composition as cast reveals phase separation with continuous in Fig. 7 (a).It is well known that phase separation occurs in these glasses at the nanometer level and those phase-separated regions serve as nucleation sites during the heat treatment.Fig. 7 (b) was a partial enlarged drawing of Fig. 7 (a), the circle marked 1 in the figure were mainly amorphous, the circle marked 2 presented a crystallization trend, and the circle marked 3 showed the monocrystal status.In Fig. 7(b), the contrast between different areas can be clearly seen.It could be declared that there were distinctly crystallization area, transition area and amorphous area in the structure.The glass phase and the crystalline phase were enchased each other, and the crystal lattice stripes could be found in Fig. 7 (b).It showed that the crystallites phase was distributed as isolated pockets between adjacent glass phase.

Bending strength of glass-ceramic
Bending strength is an important parameter for evaluating the mechanical properties of glass-ceramic.As shown in Fig. 8, there was a relation between bending strength and the crystallization time of C 2 produced by the schedule.The bending strength values of C 2 samples were in the range of 58-120 Mpa.Previous studies indicated that crystal was very effective to increase the bending strength of glass-ceramics.Therefore, the C 2 sample produced under the heat treatment of Tn 2 = 755 ºC, Tc 2 = 1019 ºC for 3.0 h had the highest bending strength value because of the highest crystallization degree in glass ceramic samples.High crystallization degree and density values could increase the bending strength of the samples directly and obviously.Crystallization time( h) C 2

Chemical resistance of glass-ceramics materials
The chemical resistance is one of the base properties of building decorative glass ceramics.The chemical resistance of glass-ceramics samples was tested by immersion in H 2 SO 4 and NaOH solutions.It can be seen that the sequence which samples showed better chemical stability was C 3 > C 2 > C 1 (Tab.VI), or with crystallization time increasing, chemical stability of the samples improved.Through the analysis of XRD and SEM, the increase of the content of CaO in raw material or crystallization time caused more amorphous phase occurring in glass-ceramics.The durabilities of glass-ceramics samples correlated well with the crystallization degree of the produced samples [20].Additionally, the durability of glassceramic samples also correlated with the morphological character of crystalline phase.The glass-ceramics with fine microstructure showed better chemical resistance.

Conclusion
Mixtures of GTP and chemical reagent can be melted at 1520 °C, from which glass powder can be prepared by quenching the melt in water.
The main phases of the glass-ceramics were wollastonite.The volume of crystalline phase increased with the increase of CaO in raw materials.It was also observed that the crystal size became much larger from 1 micron to 15 micron with the raise of crystallization time.
The chemical durability of the GTP glass-ceramics is excellent in alkali but relatively poor in acid.
It was a feasible attempt to produce glass-ceramics for building and decorative materials from GTP frits and the amount of GTP used in the glass batches was more than 65 wt% of the whole raw.

Fig.
Fig.4showed SEM images of the samples sintered under different conditions.SEM observations revealed that a large number of block-shaped crystals occurred in the glass ceramics materials.The glass-ceramic samples for SEM test were eroded in the HF solution and the samples were washed many times by ultrasonic cleaning instrument after erosion.So the glass phase of the glass-ceramic structure was eroded and dissolved, and the microcrystalline phase was retained.With the increasing of CaO, the amount and the sizes of crystal particles also grew up.The samples had been eroded by hydrofluoric acid, and the glass phases were dissolved, so the crystalline phases were outstanding.It could be seen from the micrographs that the microstructure of the sample was dense, moreover the glass phases and crystalline phases adhered to each other.The structure was beneficial to improve the integral intensity and wear-resistance of materials.

Fig. 8 .
Fig. 8. Relation between bending strength and the crystallization time.
The results of the density of samples.
Chemical resistances of the glass-ceramics samples.