Effects of different sintering temperatures on thermal, physical, and morphological of SiO2-Na2O-CaO-P2O5 based glass-ceramic system from vitreous and ceramic wastes

This research involved comprehensive studies on thermal, physical, and
 morphological properties of SiO2-Na2O-CaO-P2O5 (SNCP) glass-ceramic at
 various sintering temperatures. The study in SNCP glass-ceramic using
 soda-lime-silica (SLS) wastes glass and clam shell (CS) wastes as the main
 raw of materials via conventional melt-quenching technique and solid state
 sintering are interesting and challenging by considering the research using
 waste materials to fabricate novel SNCP glass-ceramic. The main peaks,
 Na3PO4 and Ca3Na6Si6O18 were assigned to high crystallization temperature
 (Tc) at 650-950?C. The density of samples increases at 550-750?C and
 decreases at 850-950?C due to the increase of sample thickness and higher
 specific volume at high sintering temperature. FESEM micrograph showed that
 existed porous increased at sintering temperature 850-950?C contributes
 effect to low densification of the sample.


Introduction
Glass-ceramics are polycrystalline materials of fine microstructure that are produced by the controlled nucleation and crystallization of a glass which has low-density properties at high sintering temperature [1,2]. The great varieties of compositions and microstructures with specific technological properties have allowed glass-ceramics to be used in a wide range of applications. The first bone bonding was discovered for certain compositional range containing chemical materials from SiO 2 , Na 2 O, CaO and P 2 O 5 in bioactive glass-ceramic [3]. In this research, SNCP glass-ceramic is composed of SLS waste glass (50 %), CS waste (25 %), NaCO 3 (20 %), and P 2 O 5 (5 %) in weight percentage. The materials of CaCO 3 and SiO 2 were produced from both waste materials (SLS and CS) [2]. Materials scientists discover waste materials properties to fabricate new products for future technologies to minimize the production cost and environmental problems associated with disposal. The galvanic sludge and glass frit wastes have been used to produce glass-ceramic products [4][5].
There is great interest in a glass-ceramics study that possesses appropriate physical and microstructure for medical applications. These materials should be biocompatible and in most cases bioactive to elicit a specific biological response at the interface of the material. Mostly, bioactive materials consist of bioglass, glass-ceramics, and calcium phosphate ceramics which have been applied to repair and reconstruct diseased, damaged bones or tissues [6]. The type of glass-ceramic used in this study is included in the group of bioactive materials. Certain glass compositions are appropriate precursors for glass-ceramics. Some glasses are too stable and difficult to crystallize, such as ordinary window glass, whereas others crystallize too readily in an uncontrollable manner resulting in undesirable microstructures. Glass-ceramic samples in this study are potential to be used in the damaged bone to do implantation [6][7]. Siqueira and Zanotto have reported on the production of the bioactive SLS containing phosphorous in a solid solution using solid-state reaction [8]. Mojtaba Abbasi and Babak Hashemi synthesized SNCP bioactive glass-ceramic from SLS glass by using solid-state reaction method, which is different from melt-quenching and subsequent heat treatment, which is the conventional method to obtain glass-ceramics [9].
The aim of this study is to investigate the effects of different sintering temperatures on phase composition, crystal system, structural bonding properties, density, and microstructures properties of glass-ceramic with low production cost by using waste materials such as clam shells and bottle glasses. Nevertheless, by reusing soda-lime-silica glass and clam shell wastes as source material for producing glass-ceramic is an interesting and feasible alternative option to the reuse of industrial wastes into useful products. The optimum physical and morphological properties of SNCP glass-ceramic has the potential to be used in orthopedic medical application.

Materials and Experimental Procedures
The samples were prepared from SLS waste glass powder, CS waste powder, sodium carbonate (Na 2 CO 3 , 99.99 %, Alfa Aesar), and phosphorus (V) oxide (P 2 O 5 , 99.99 %, Alfa Aesar). The XRF analysis of SLS waste glass and clam shell waste are tabulated in Table I. In this study, the batch formulation of SNCP glass-ceramic was prepared via conventional meltquenching technique and solid-state sintering. The waste materials that were used to fabricate SNCP glass-ceramic are derived from SLS glass and CS. The SLS and CS waste materials were destructed using plunger into coarse pieces and dry milled until formed fine powder and sieved both materials in 45 μm. SLS powder, CS powder, Na 2 CO 3 , and P 2 O 5 are mixed together and dry milled the powders until homogenous. The mixtures were heated and taken out from the furnace at 1400 °C when fully melting for about 2 hours soaking time and heating rate 10 °C/min. Then, the molten samples were rapidly quenched in water to form the glass-frits. The glass-frits were crushed into fine powder by using the milling technique (24 hours). Then, the fine SNCP glass powder powders were sieved and weighed one gram each. After that, the powders were pressed into pellet form at 3 t pressure. Finally, the samples were sintered at 550-950 °C for 3 hours.
TGA/DSC thermal analysis were investigated by Mettler Toledo TGA/DSC1 HT simultaneous analyzer 70 µl platinum pan with heating rate 10 °C/min through 50 ml/min N 2 gas flow rate. The phase transformation was investigated through XRD diffractometer (PANalytical X'Pert Pro PW3050/60) with Cu-Kα radiation (λ = 1.54060 Ǻ) in the range of 20°−80° by a 2θ scan mode and the XRD results were analyzed by using PANalytical X-Pert High ScorePlus Programme. The types of structural bonding in the compositions were detected via Fourier transform infrared spectroscopy (FTIR) were recorded using Bruker Vector 22 FTIR spectrometer in a frequency range of 280−4000 cm −1 at room temperature. Densities and volume of samples were measured by using an electronic densitometer that utilizes Archimedes' method with distilled water as the immersion liquid. Sample's micrographs that were coated with gold (Au) were observed by Nova Nano 230 Field Emission Scanning Electron Microscope (FESEM).
Tab. I Chemical composition of SLS glass waste and clam shell waste (wt.%).   1 illustrates the DSC/TGA analysis of SNCP glass powder was sintered from room temperature until 1000 °C. The phase crystallization of Na 2 CO 3 in the XRD results which have been reported by Arakcheeva and Chapuis [10] in the early stage of SNCP sintering indicated exothermic peak can be observed at 282 °C. There are two overlapped peaks, namely glass transition temperature (T g ) and glass crystallization temperatures (T c ) between 550 and 600 °C respectively with a decrease in weight corresponding to the TG curve. The weight loss in the TG curve shows the decomposition of Na 2 CO 3 and CaCO 3 in the SNCP system at temperature 550-600 °C which corresponding to the formation of sodium calcium silicate and calcium silicate phases as shown in Fig. 2. Arstila et al. divided the silicate-based glass-ceramic into two groups [11]. The glasses with the transition temperatures

Thermal analysis
of about 500 °C and the glass crystallization below 750 °C included in the first group. The second group referred to the glasses with T g between 550-600 °C and T c at about 900 °C. The first group glasses form the sodium calcium silicate phase, whereas the glasses related to the second group form Wollastonite. Therefore, the sintered SNCP samples belong to the first group and the temperature at 550 °C attributed to the onset temperature of crystallization, T c(on) . DSC result in Fig. 1 shows the transformation from the amorphous phase (SNCP powder at room temperature) into the crystalline phase occurs at 550-600 °C followed by glass crystallization temperature peak beginning at T c is 600 °C. The formation of silicate and phosphate-rich phase occurred after glass transition and the glassy phase separation have been investigated by Lefebvre

Phase analysis
Nucleation and crystallization mechanisms used in the development of glass-ceramics are controlled by taking advantage of various solid-state reactions. The quantification of the different crystalline phases is indeed important for optimizing the properties of the glassceramics. In order to understand and explain the details of the crystallization sequence in the most promising glass samples, after subjecting them to controlled heat treatment conditions, they were analyzed by XRD. In this study, the evolution of crystalline and phase formation was investigated as a function of sintering temperature at the angle 20°−80° (2θ) as shown in Fig. 2. XRD gives information of crystalline region only and contribution from amorphous grain surface does not contribute. The crystallization, or devitrification, of glass to form a glass-ceramic is a heterogeneous transformation. The process consists of two stages, namely a nucleation stage and a growth stage. In the nucleation stage, small, stable volumes of the product (crystalline) phase are formed, usually at preferred sites in the parent glass. The preferred sites are interfaces within the parent glass or the free surface.
The silicate and phosphate that existing in the glass system would easily cause the occurrence of crystallization in the glass-ceramic [16]. Before sintering, the SNCP sample showed amorphous phase at room temperature. After sintering, at 550 °C, the main phase obtained from XRD analysis is sodium calcium silicate, Na 2 CaSiO 4 which is cubic crystal system with space group Fm-3m system is revealed by reference International Centre for Diffraction Data (ICCD) code 98-001-6165. The intensity phase of the diffraction peak is (022) and at the angle is 34.24° as shown in Fig. 2. It is has been established that materials with a cubic crystal structure are prone to grow into a spherical shape to minimize surface tension [17]. This result was in agreement with Omer Kygili finding which showed the same main composition [18].
There was peak separation formed at the main peak which both peak are the same phase and belong to trisodium phosphate, Na 3 PO 4 at 650 °C with reference code 98-005-8525. Phillips and McMillan [19] have revealed that the addition of P 2 O 5 increased the extent of two-phase separation and widens the range of phase-separable compositions. The crystal system of Na 3 PO 4 composition is cubic with space group Fm-3m have the same crystal system, diffraction peak, and space group with the sample at sintering temperature 550 °C.
Chemical compound P 2 O 5 reacted as a nucleating agent in the sample which can promote phase separation and volume nucleation [20]. The combeite phase begins to crystallize at temperatures higher than 650 °C as stated by Filho et al. [21]. When sintering temperature increased at 750-950 °C, the main phase formed is combeite high, Ca 3 Na 6 Si 6 O 18 at diffraction peak (024) with hexagonal crystal system with space group R-3m which is assigned to high crystallization temperature (T c ) is detected by ICCD reference code 98-002-1442. From the intensity of XRD peaks, it is observed that the intensity of the diffraction pattern of the phases and peak increases as the sintering temperature increases.

Microstructure analysis
The FESEM microstructure varies with the samples at different sintering temperatures as shown in Fig. 3 by using magnification 2000×. Generally, microstructural evolution caused with increasing sintering temperatures. The microstructure observed at 550-750 °C showed less porosity. Samples at 850 and 950 °C showed that the grains started to combine each other to form agglomerate, coalescence and porosity is increased. Addition of a few percentages of P 2 O 5 to silicate glass compositions promotes the volume nucleation and glass-ceramic formation [21]. This reaction affected the grain size increases, the porosity of the sample increased at 850 and 950 °C. The weight percentage of P 2 O 5 used in this study is 5 wt% which showed a highly crystalline interlocking microstructure is revealed also by Holand et al. when the final product of glass-ceramic used was 3.2 wt% P 2 O 5 [22].
When sintering temperature increases at 850 and 950 °C, grain growth slowdown densification by increasing the length of the diffusion path, but it is most damaging when the grain grows around pores, leaving a large pore volume that is not intercrossed by grain boundaries [23]. Decrease in density happened when sintering temperature increases, which might be related to the formation of more phase crystallization induced increasing of porosity at a higher temperature. One or more crystalline phases formed during sintering as shown in XRD results and as their composition is normally different from the precursor (parent) glass, it follows that the composition of the residual glass is also different to the parent glass have been explained by Rawlings et al. [24]. The sample's surface at 850 °C and 950 °C as depicted in Fig. 3(d) and 3(e) was loose and grains were agglomerated due to incomplete crystallization at some angle of diffraction pattern [25].
Prolonged sintering of SNCP glass-ceramic, P 2 O 5 as the nucleation agent, results in a solid state transformation in which small spinal shape formed at the expense of the silica content in the grains which from the Ca 3 Na 6 Si 6 O 18 phase have been observed in Fig. 3(d) at sintering temperature 850 °C. Such solid-state transformations are often encountered in systems used for glass-ceramic applications in which a metastable phase first crystallizes or in which initial crystallization at a low temperature is followed by a higher temperature. Carbonates decompose over a temperature range from 400 to 1000 °C, also depending on the particular composition [26]. When samples at 850 and 950 °C, the thickness of samples are expanded because of CO 2 gases trapped in the samples. This reaction occurred because of the chemical reaction of CaCO 3 compound during the sintering process [26]. When the sample is sintered at high temperature, the gaseous product pressure exceeded which affected damage on grain formation and lead towards the sealing of pores before complete dissociation as shown in Fig. 3(e).

Density
Density measurement revealed the degree of sample crystallization. The densities (ρ) of the samples are directly measured using densitometer by the standard Archimedes method and using distilled water (dw) as the immersion liquid. The samples are weighed in air, air and then in the immersion of distilled water, dw , with a density of water, dw is 1 g cm −3 . The densities of the samples are then calculated using the following equation: (1) Structures with close-packed oxygen ions have a high density of atomic packing and high values of atoms per cubic centimeter. The density is usually measured (grams per cubic centimeter), depends on this value and on the atomic weight of the constituents. Structures based on close packing of oxygen ions, many silicates, such as those have low densities of atomic packing [26]. The densities of SNCP samples in this study showed increment from 550-750 °C which attributed to high densities of atomic packing. The density decreased when sintering temperature increases at 850 to 950 °C due to the high content of silicates composition and existed pores which attributed to low densities as shown in Fig. 4. The decrement densities of samples at 850-950 °C also due to increases volume of samples and a high level of porosity exist in the microstructure as shown in Fig. 3. Samples at sintering temperature 850 and 950 °C have higher specific volume (lower density) than the low-temperature samples. This corresponds to an increase in sample volume at the transformation temperature.  As the temperature increases, the decomposition pressure increases and forms large pores, blistering, increasing thickness of the sample, and bloating as occurred with samples at 850 and 950 °C. This kind of defect is particularly common when high heating rates are used. This temperature gradient and the time required for oxidation of constituents or impurities are the two most important reasons for limiting the rate of heating during firing [26]. Decomposition also occurs to form new solid phases. This reaction proceeds with an increase in volume since silica glass has a lower density [26]. The specific volume of any given crystal increases with temperature, and the crystal tends to become more symmetrical. The general increase in volume with temperature is mainly determined by the increased amplitude of atomic vibrations about a mean position. Consequently, as the lattice energy and amplitude of vibration between the same energy positions of atoms leads to a higher value for the atomic separation corresponding to a lattice expansion when sintering temperature increased which are correlated with the results as shown in Table II [26].

FTIR
FTIR is an effective analytical instrument for detecting functional groups and characterizing covalent bonding information by producing an infrared transmission spectrum. The spectra produce a profile of the sample. FTIR spectrum of SNCP glass-ceramic was collected in the range 280-4000 cm -1 for samples before sinter, and after sinter at 550-950 °C as shown in Fig. 5. The spectrum of SNCP glass-ceramic consists of two main bands, one in the lower wavenumber and other in the higher wavenumber. Both bands contain a number of small peaks and shoulders with the main peak and the high-frequency band is relatively broader than a low-frequency band. Peaks at wavenumber at 441-448 cm -1 are assigned with bending modes of Si-O-Si bands indicated a vibration of angular deformation of Si-O-Si band rocking vibrational modes between SiO 4 tetrahedral as reported by Pereira et al. [27]. Si-O-Si bands became weak when the intensity of transmittance is decreased at 550-850 °C. FTIR spectra of the sintering sample near wavenumber 452-530 cm -1 attributed to wollastonite, calcium phosphate, and apatite-like phase [9,28]. In this study, wavenumber at 517 cm -1 was assigned to Ca-Si-O bands at sintering temperature 650-950 °C. The P 2 O 5 reaction as the sintering temperature increases showed increases of transmittance intensities and three distinct shoulders appear in the region 616 to 692 cm -1 at sintering temperature 650-950 °C. The peaks with three distinct shoulders can be attributed to P-O-P stretching vibration, referred to the presence of crystalline phosphate in the glasses [29][30][31]. Generally, the bands in the region 916-1016 cm -1 observed in amorphous silica glasses [32,33]. Bands at 916-924 cm -1 attributed to the formation of non-bridging oxygen (NBO) ions and the splitting between 916-924 cm -1 and 1016 cm -1 are assigned to the vibration of SiO 4 tetrahedral that contain non-bridging oxygen facing sodium [9,27,[34][35][36]. In the 1005-1115 cm −1 region the asymmetric vibration Si-O-Si is very strong due to the overlap of the stretching PO 4 vibrational peaks at sintering temperature 550-950 °C [9,27,13,37]. McMillan thought that the double oxygen/phosphorus (P=O) bond is favorable to phosphate phase formation in a silica network and thus increases the tendency towards a crystallization [38]. The presence of a broad peak at wavenumber 1424-1733 cm -1 referred to the symmetric vibrational mode of the CO 3 group [39]. As the temperature increased, this peak seems to diminish slowly with the decomposition of CaCO 3 and Na 2 CO 3 in the glass system. The wavenumber from 1424-1522 cm -1 attributed to CaCO 3 formed by clam shells used in this study. The clam shells used in this study contribute a large amount of solid waste. Clam shells are primarily composed of CaCO 3 which is mostly in the form of calcite [40]. When sufficient heat is applied to calcite, it will decompose to form calcium oxide (CaO) and release carbon dioxide (CO 2 ) gas. The IR spectrum of ark clam shell detected calcite band by  is positioned at 1429-1492 cm -1 with three intramolecular, three translator lattice, and two rotator lattice vibrations [39][40]. Besides, three weak IR bands were found in the spectrum at wavenumber 3342-3840 cm −1 at room temperature and 950 °C. They were corresponding to bond vibration of H-OH of water and Ca-OH 2 of calcite. H 2 O from atmospheric air was adsorbed by CaCO 3 via chemisorption and physisorption [39]. When at temperatures higher than 1000 °C, the beginning of the melting process can be observed in glass-ceramic samples [15].

Conclusion
The investigations discussed in this paper demonstrate the potential of turning silicate wastes into useful glass-ceramic products. The general process involves the vitrification of a silicate waste, or a mixture of wastes, followed by crystallization to form a glass-ceramic. The formation of SNCP glass-ceramic in this study occurred via two steps. The amorphous phase was obtained after the melt-quenching process (at room temperature) as the first step then, followed by crystallization of the minor nucleant-rich phases (silicate and phosphate), which serves as a precursor for the formation of the major crystalline phases as the sintering temperature increases. From the results obtained from XRD and DSC, the samples started crystallized to form sodium calcium silicate (Na 2 CaSiO 4 ) at 550 °C as the main phase. Twopeak separation occurred at the main phase at 650 °C which belongs to trisodium phosphate, Na 3 PO 4 and crystallization at 750-950 °C to form combeite high, Na 6 Ca 3 (Si 6 O 18 ). From the observation of microstructure, grain size and porosity increased as the sintering temperature of samples increases. The density of samples increased at 550-750 °C, while the density of sample decreases at 850 and 950 °C due to the decomposition of carbonate and oxides pressure increases from samples. The effects on SNCP sample at high sintering temperature have formed large pores and bloating which affected increases of samples thickness and volume.