Effect of Waste Glass Powder on Phase, Microstructure and Mechanical Properties of Mullite Based Porous Ceramics with Coal-series Solid Waste as Raw Materials

Mullite based porous ceramics were successfully prepared by using a solid-phase method with gangue, slime and sawdust as raw materials. The effects of the content of glass powder and calcining temperature on microstructure and mechanical properties of the samples were systematically studied. It is found that glass powder can prevent the formation of Kyanite and reduce the synthesis temperature of mullite. The samples with 4% glass powder have a large amount of mullite calcined at 1150°C for 3h. Meanwhile, alumina in the sample is completely converted into mullite at 1180°C for 3h. Mullite content reaches 64.2wt%. The minimum apparent porosity is 31.22%, while the maximum volume density and diameter shrinkage are 1.74g/cm 3 and 8.48%, respectively. The ratio of pores of <8μm to those of 8~20μm is varied from 7:2 to 2:7, due to the addition of the glass powder. With increasing content of mullite, the exural properties of the samples are increased from 7.7MPa to 28.36MPa. Therefore, with solid wastes as the resource, the porous ceramics have a bright prospect.


Introduction
Coal is a key energy source in developing countries. Coal gangue and slime are solid wastes produced in the process of coal mining and washing [1]. The main components of coal gangue are silica and alumina, which come from the extraction of roadway and the selection of coal preparation process [2][3][4]. Coal gangue is piled up, which causes environmental pollution and has potential safety hazards, due to possibly spontaneous combustion [5][6][7]. As llers or making build materials, gangue can be used in a value-adding way. However, mechanical dispersion of coal gangue as llers may cause environmental pollution through the leaching of heavy metal elements [4,5]. It is therefore desired to develop more effective technology for gangue utilization. The pollution of slime is higher than that of gangue due to its small particle size and chemical properties [8][9][10]. At the same time, the low calori c value and high ash content make the reuse of coal slime to be di cult. If they can be utilized effectively, we can solve the above pollution issue and promote the development of regional economy.
Mullite is a scarce ceramic material with various interesting properties in nature [11][12][13][14][15][16]. Mullite ber is often used as reinforcing material of ceramics [17][18][19][20][21]. Mullite porous ceramics not only have high stability at high temperatures, but also are used in various situations because of their abundant pore structures [17,18,[22][23][24][25]. Lü et al. prepared porous mullite ceramics with solid waste by using an in-situ synthesis method [25,26]. Gröttrup et al. used porous mullite ceramics for photocatalytic degradation of wastewater [24]. Wang et al. fabricated mullite foams from coal-series kaolin using freeze-cast [22]. Three dimensional mullite ber reinforced mullite reported by Wang et al with a new method [27]. In summary, fabrication of mullite porous ceramics prepared from solid wastes is of bright prospect in industrial application and environmental remediation. However, it is still a challenge, because organic burning may cause matrix damage.
In this study, mullite based porous ceramics were fabricated by calcining of coal gangue and slime, with sawdust as the pore agent. Meanwhile, the effect of glass powder on the properties of samples at different temperatures was studied.

Samples preparation
Tab. 1 lists contents and characteristics of the raw materials. Pore structure and porosity are regulated by burning rate and particle size of the raw materials. Gangue (Huainan), slime (Huainan), sawdust, at glass powder and Na 2 CO 3 (Xuzhou reagent No.2 Factory, AR) were mixed through ball milling (XGB-04, Nanjing Boyuntong Instrument Technology Co., Ltd). Slime was viscose and used as binder together with 5% deionized water. Cylindrical green samples (diameter 95mm and height 3mm) and bars green samples (4×4×40mm 3 ) were prepared at a pressure of 5MPa for 1min. Mechanical properties and calcining properties of the porous ceramics were studied as a function of amount of the glass powder.
Tab.1 Contents and characteristics of the raw materials.

Characterization
Phase compositions of the samples were studied by using X-ray diffractometer (XRD, SmartLab, Rigaku Corporation, Japan). RIR-quantitative analysis was used to calculate the phase content with the XRD data [28]. Microstructure of the samples was observed by using scanning electron microscopy (FESEM, Sigma 300, Zeiss, Germany). Archimedes method was used to measure density and apparent porosity of the samples. Diameters of the samples before and after calcination were used to calculate the diameter shrinkage (D) by using the following equation (1) [29,30]: where d 0 and d are the sample diameters (mm) before and after calcination, respectively.
Pore size distribution of the samples was measured by suing a pore size analysis instrument. Flexural strength of the samples was tested by using the three-point bending method with GB/T 1965-1996. The breaking force of each group is the average value of ve samples broken by using universal testing machine (WDW-50, Shenzhen Kaiqiangli Test Instrument Co., Ltd, China). The exural strengths were given according to the following equation (2) [18,31,32]: where F is the exural strength (MPa), l is the span (mm) between the sample and the mold, f is the maximum stress (N) of the samples, b and h are the width (mm) and height (mm) of the sample, respectively. ,PDF#80-0886) and quartz (SiO 2 ,PDF#86-2237) as the major crystalline phases [21,24].

Phase analysis
Organic matter and minor unknown matters are also observed in the slime, as demonstrated by the peaks at 20°~30° and 29.5°. Organic and unknown matter were not studied because they were decomposed at high temperatures [15,26,33  The content of mullite is increased gradually, while that of quartz is decreased, as the content of glass powder is increased to 4%. Above 4%, the content of quartz is slightly increased. Correspondingly, the content of mullite is decreased. In other words, the highest mullite content is 64.2% in the sample with 4% glass powder. In other words, 4% glass powder is optimal for mullite formation. for 3h, the sample is not densi ed, with small and uniform pores. The pore size is increased with increasing calcination temperature, because mullite is formed due to the reaction of the raw materials. Obvious crystallization is observed after calcining at 1180°C. The surface of GP4 is smoother than that of GP0 after calcining at 1070°C. A large number of rod-like crystals are present in GP4 calcined 1130°Cand the matrix is denser at 1180°C. It is believed that the liquid phase derived from the glass powder promoted the formation and growth of mullite. Fig. 6 shows cross-sectional SEM images of the samples with different contents of glass calcined at 1180°C for 3h. There are grains on the surface of GP0, while interlaced rod crystals and broken crystals are observed on the fracture surface and inside the holes of GP2, GP4 and GP6. The interlaced mullite in the matrix would improve exural properties of the samples [31]. Large holes and small pits are present on the fracture surface of GP6, as shown in Fig. 6(d). It means that the sample contains closed pores, which are formed because of the interaction between the melting glass and the raw materials. Fig. 7 displays apparent porosity, density and diameter shrinkage of GP0 calcined between 1070°C and 1180°C. The apparent porosity is more than 40.92 %, whereas the density and linear shrinkage are less than 1.50 g/cm 3 and 9.27%, respectively. This suggests that the sample without glass powder has not reacted completely at 1180°C. This observation is consistent with the results of XRD and SEM. Fig. 8 shows the apparent porosity, density and linear shrinkage of GP2, GP4 and GP6 with calcination temperature. The apparent porosity is decreased, while both the density and diameter shrinkage are increased, with increasing calcination temperature. Therefore, the content of glass powder affects all the apparent porosity, bulk density and diameter shrinkage of the samples. At a given calcination temperature, the apparent porosity is decreased, while the density and diameter shrinkage are increased with increasing content of glass powder. For example, after calcining at 1180°C, the apparent porosities are 42.59%, 34.62%, 33.15% and 31.22%, for GP0, GP2, GP4 and GP6, respectively. The densities are 1.50g/cm 3 , 1.68g/cm 3 , 1.71g/cm 3 and 1.75g/cm 3 , respectively. The corresponding diameter shrinkages are 9.27%, 6.93%, 7.65% and 8.48%. It is thus concluded that the apparent porosity, bulk density and linear shrinkage of the samples can be effectively controlled.

Archimedes method and linear shrinkage
Tab. 2 shows the data of apparent porosity, density and diameter shrinkage of different samples. Fig. 8 and Tab. 2 show a special phenomenon that the diameter shrinkage of Gp0 calcined at 1070 °C for 3 h is similar to that of GP2, GP4 and GP6, but the apparent porosity of GP2, GP4 and GP6 is less than that of Gp0, and the bulk density of GP2, GP4 and GP6 is greater than that of Gp0.And this phenomenon also exists at 1180°C. This is because the liquid phase formed by glass powder leads to the formation of closed pores, which leads to the deviation of test data. This con rmed the conclusion that SEM closed pore existed, and showed that glass powder had in uence on pore structure. These changes may also affect the mechanical properties. 3.4. Pore structure Fig. 9 shows pore structure characteristics of GP4 calcined at different temperatures. As presented in Fig.  9a, the samples contain pores with diameters of ~4μm and ~10μm, together with a few nanopores, which can be attributed to the fact that the slime and sawdust powder have different particle sizes. However, pore size distribution pro les of GP4 calcined at different temperatures are different. With increasing calcination temperature, the numbers of nanopore and ~4μm pore are decreased, while that of ~10um pore is increased. The diameter of the pores is increased as the calcination temperature is increased, due to the continuous densi cation of the samples.
As seen in Fig. 9b, ~90% of the pores are less than 20μm. The samples calcined at 1070°C,1100°Cand 1130°C contain mainly pores of <8μm and a small number of 8~20μm pores, with a ratio of 2:7. However, for the samples calcined at 1150°C, the ratio is 7:2. Nanopores with sizes of up to 100nm are found in the samples, with volume fraction of <2%. The difference in pore size and distribution would result in difference in mechanical properties. Closed pores may have positive effect on mechanical properties in some cases. Fig. 10 shows the pore structure characteristics of samples calcined at 1180°C with different contents of glass powder. It can be seen the size and number of pores are similar with GP4 samples calcined at 1500°C . Fig. 10 shows pore structure characteristics of the samples calcined at 1180°C with different contents of glass powder. The size and number of pores are similar to those of GP4 calcined at 1500°C. This means that the pore structure is not varied signi cantly if the calcination temperature is above 1150°C. According to Fig. 5 (h, I), mullite phase is continuously formed and the samples are densi ed as the calcination temperature is increased from 1130°C to 1180°C. In the temperature range of 1150-1180°C, the apparent porosity, bulk density and diameter shrinkage of the samples have almost unchanged, as illustrated in Fig. 8. Based on these results, it is further con rmed that the reaction has been complete at 1180°C. Fig. 11 shows exural strength of the samples calcined at different temperatures. The exural strength of GP0 is increased from 2.70 MPa to 7.47 MPa, as the calcination temperature is increased from 1070°C to 1180°C [18]. The fracture strengths of GP2, GP4 and GP6 calcined at 1180°C for 3h are 21.23MPa, 25.27MPa and 28.36MPa, respectively. This suggests that the reaction and densify are di cult at low temperatures due to the decomposition of organic matter. However, this problem can be readily solved by the addition of glass powder, owing to the formation of liquid phase to accelerate the reaction and densi cation.

Mechanical properties
Combining with SEM and pore structure analysis results, the exural strength curves of GP2, GP4 and GP6 can be divided into two sections,1070-1130°C and 1130-1180°C. Over 1070-1130°C, the exural strength increases rapidly because of the promoting effect of the liquid phase derived from the glass powder. Over 1130-1180°C, the increase in exural strength is slow down. This observation is closely related to the formation of mullite and the variation in pore structure. In the high temperature regime, the number of ~10μm pores is increased, while the number of ~4μm pores is decreased. Fig. 12 shows overall effects of calcination temperature and the content of glass powder on mechanical properties of the samples. It is found that the calcination temperature and glass powder have synergistic effect on the mechanical properties. Fig. 12 can be divided into three regions according to the effects of temperature and glass powder. In region I, the content of glass powder is less than 2%. The mechanical properties are increased with increasing content of glass powder. This phenomenon is more pronounced at high temperatures. The calcination temperature has relatively weak effect on exural strength. In region II, the content of glass powder is >2%, while the calcination temperature is in the range of 1070-1150°C. In this case, the exural strength is dependent on both the calcination temperature and the content of glass powder. In region III, the effects of the content of glass powder and the calcination temperature are weakened.

Conclusions
Porous ceramics were successfully prepared by using solid state reaction method. The exural strength of the samples was improved due to the addition of glass powder, due to the promoted formation of mullite and inhibited formation of kyanite.4% glass powder is the optimal content, leading to the highest mullite content of 75.80 % after calcining at 1180°C, corresponding to a exural strength of 28.36MPa. Higher content of glass powder results in more quartz. Speci cally, the sample calcined at 1180°C exhibits an apparent porosity of more than 31.22 %, while the diameter shrinkage is less than 8.48 %, which is suitable for production. This achievement has potential economic bene ts and practical values in terms of the utilization of solid wastes.         Pore structure characteristics of GP4 calcined at different temperatures.

Figure 10
Pore structure characteristics of the samples calcined at 1180°C with different contents of glass powder.

Figure 11
Flexural strength of the samples with calcination temperature.

Figure 12
Effects of calcination temperature and content of glass powder on exural strength of the samples.