1 1. Introduction

Coal is a key energy source in developing countries. Coal gangue and slime are solid wastes produced during the coal mining and washing [1]. Their discharge and accumulation have caused serious environmental pollutions [2,3,4,5]. In order to quickly address this problem, coal gangue is used as backfill material and coal slime is used to generate electricity. However, the resources cannot be fully utilized and there are still potential environmental pollution problems, such as heavy metal pollution and fly ash [3, 6]. During the use of slime, too much ash is produced, thus resulting in serious damages to the equipment [7,8,9].

Mullite is a scarce ceramic material with various interesting properties in nature [10,11,12,13,14,15]. There have been reports on the preparation of mullite porous ceramics from coal gangue [16,17,18,19,20,21]. However, coal gangue only occupies 50–80 wt% of raw materials. Coal-based waste cannot be quickly consumed. Lü et al. used gangue to prepared porous mullite ceramics with high mechanical properties by in-situ synthesis method [19]. The enhanced mechanical properties result from the addition of bauxite and the high molding pressure. The use of corn starch as a pore former is not environmentally friendly. Li et al. used 80 wt% waste (coal gangue and quartz) to prepare porous ceramics with high porosity, but the mechanical properties were deteriorated [20]. In summary, the preparation of porous ceramics from waste is conducive to environmental protection and resource utilization. However, it is specifically a huge challenge to use solid wastes as the only raw materials to prepare porous ceramics without sacrificing mechanical properties.

In this work, coal gangue, coal slime, wood chips, waste glass powder and sodium carbonate were used as the raw materials to prepare mullite porous ceramics. The influence of the added amount of waste glass powder on structure and mechanical properties of the porous ceramics has been further studied. Not only the waste is effectively used, but also the purpose of preparing environmentally friendly materials at low cost is realized.

2 2. Experimental

2.1 2.1. Samples preparation

Contents and characteristics of the raw materials are listed in Table 1. Pore structure and porosity are regulated by the burning loss rate and particle size of the raw materials. Gangue (Huainan), slime (Huainan), sawdust, flat glass powder and Na2CO3 (Xuzhou reagent No. 2 Factory, AR) were mixed through ball milling (XGB-04, Nanjing Boyuntong Instrument Technology Co., Ltd). 5 % deionized water is added to the mixture, so the slime has viscosity and used as a binder. Cylindrical green samples (diameter 95mm and height 3mm) and bars green samples (4 ×  4 × 40mm3) were prepared at a pressure of 5 MPa for 1 min. Mechanical properties and calcining properties of the porous ceramics were studied as a function of the content of the glass powder. The samples with 0%, 2%, 4% and 6% glass powder were denoted as GP0, GP2, GP4 and GP6, respectively.

Table 1 Contents and characteristics of the raw materials
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2.2 2.2. Calcination

Figure 1 shows temperature profile to calcine the samples. Green samples were calcined with a box resistance furnace (SX2, Xiangtan Huafeng Instrument Manufacturing Co., Ltd, China) in oxygen atmosphere at different temperatures (1070 °C, 1100 °C, 1130 °C, 1150 °C and 1180 °C) for 3 h. Firstly, the green were samples heated from room temperature to 500 °C at a rate of 5 °C/min and kept at 500 °C for 1 h to decompose the organic phases. Then, the temperature was increased to 1000 °C at a heating rate of 3 °C/min. In this process, the glass powder melts gradually to form a liquid phase. Mechanical properties of the samples were improved due to the liquid phase can accelerate the reaction between the raw materials, while the samples were also densified. Finally, the temperature was increased to final temperatures at a heating rate of 1 °C/min and kept for 3 h. At this stage, mullite was formed, thus leading to further enhancement in mechanical properties.

Fig. 1
figure 1

Temperature profile to calcine the samples

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2.3 2.3. Characterization

Phase compositions of the samples were studied by using X-ray diffractometer (XRD, Smart Lab, Rigaku Corporation, Japan) [22, 23]. RIR-quantitative analysis was used to calculate the phase content according to the XRD data [24]. The microstructure of the samples was observed by scanning electron microscopy (FESEM, Sigma 300, Zeiss, Germany). Archimedes method was used to measure density and apparent porosity of the samples [25, 26]. Diameters of the samples before and after calcination were used to calculate the linear shrinkage (D) by using the following Eqs. [27, 28]:

$$D=\frac{{d}_{0}-d}{{d}_{0}}\times 100\%$$
(1)

where d0 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 five samples broken by using universal testing machine (WDW-50, Shenzhen Kaiqiangli Test Instrument Co., Ltd, China) [29, 30]. The flexural strengths were given according to the following Eqs. [31,32,33,34]:

$$F=\frac{3lf}{{2bh}^{2}}$$
(2)

where F is the flexural 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.

3 3. Results and discussion

3.1 3.1. Phase composition

Figure 2 shows XRD patterns of the raw materials. Kaolinite 1 A (Al2(Si2O5)(OH)4, PDF#80–0886), quartz (SiO2, PDF#89-1961) and kyanite (Al2SiO5, PDF#83-1569) are present as the main phases of gangue. The slime also contained kaolinite 1 A (Al2(Si2O5)(OH)4, PDF#80–0886) and quartz (SiO2, PDF#86-2237) as the major crystalline phases. Organic matter and minor unknown matters are observed in the slime, as demonstrated by the peaks at 20–30 ° and 29.5°. The effect of the organic and unknown matters was not studied because they were decomposed at high temperatures [14, 21, 35].

Fig. 2
figure 2

XRD patterns of the raw materials

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Figure 3 shows XRD patterns of the samples with different contents of glass powder calcined at different temperatures. The results show that the samples consist mainly of mullite (Al4.52Si1.48O9.74, PDF#79-1457) and quartz (SiO2, PDF#85–0930), phases are varied with the sintering temperature and the content of glass powder. Kyanite (Al2SiO5, PDF#83-1569) is found in samples calcined at 1070 °C, which come from coal gangue and cannot be decomposed. The kyanite crystals in GP0 calcined at 1130 °C grew up and then was decomposed to form mullite when the temperature was rised to 1180 °C. This is consistent with the observations reported by Zhang et al. [36], but the decomposition temperature in this study is lower because of the presence of impurities in the coal gangue [35, 37, 38]. The kyanite in GP2, GP4, and GP6 decomposed at 1130 °C. The liquid phase formed at high temperatures by glass powder promote the formation of mullite, because of the increased contact between the particles. Silicon oxide (SiO2, PDF#82-1554) in samples increases as glass powder, which confirmed that the glass powder was transformed into silicon oxide during the reaction.

Fig. 3
figure 3

XRD patterns of samples calcined at different temperatures a GP0, b GP2, c GP4 and d GP6

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Figure 4 shows XRD patterns and phase contents of the samples calcined at 1180 °C for 3 h. Mullite, quartz and silicon oxide are the main phases in the samples. As the amount of glass powder added increases, the content of silicon oxide increase, because the glass powder will transform into silicon oxide, as shown in Fig. 3. The content of mullite increase first and then decrease with increasing content of glass powder. The maximum is 64.2 % when the content of the glass powder is 4 wt%. The variation trend of quartz content is opposite to mullite. The sample with 4 wt% glass powder has a minimum content of quartz (30.3 %). Comparatively, GP4 has the largest mullite content and the smallest quartz content, which may be caused by calculation errors.

Fig. 4
figure 4

XRD patterns and phase contents of the samples with different contents of glass powder after calcining at 1180 °C for 3 h

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3.2 3.2. Densification

Figure 5 displays apparent porosity, density and diameter shrinkage of GP0 calcined between 1070 °C and 1180 °C[26]. The apparent porosity is more than 40.92 %, whereas the density and linear shrinkage are less than 1.50 g/cm3 and 9.27 %, respectively. However, these characteristics change irregularly with temperature. This may be related to the incomplete reaction due to the insufficient contact of raw materials after the organic matter was burned.

Fig. 5
figure 5

Apparent porosity, density and diameter shrinkage of GP0 with calcination temperature

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Figure 6 shows apparent porosity, density and linear shrinkage of GP2, GP4 and GP6 at different calcination temperatures. Compared with those of GP0, the apparent porosity, density and diameter shrinkage all have regular trend with temperature. The apparent porosity is decreased, while both the density and diameter shrinkage are increased, with increasing calcination temperature. This characteristic trend is stable up to 1180 °C. At the same temperature, the influence of glass powder on these characteristics also shows a certain variation profile. With increasing content of the glass powder, the apparent porosity decreases, while the density and diameter shrinkage increase. After calcining at 1180 °C, the apparent porosities are 34.62 %, 33.15 % and 31.22 %, for GP2, GP4 and GP6, respectively. The densities are 1.68 g/cm3, 1.71 g/cm3 and 1.75 g/cm3, respectively. The corresponding diameter shrinkages are 6.93 %, 7.65 % and 8.48 %. The liquid phase formed due to the glass powder at high temperatures promoted the formation of mullite by increasing the contact between the particles of the raw materials. However, the linear shrinkage rate and bulk density of the samples will increase, while the apparent porosity will decrease. The content of liquid phase in GP2 is less than those in GP4 and GP6, resulting in smaller linear shrinkage and bulk density, but the apparent porosity is higher than those of GP4 and GP6.

Fig. 6
figure 6

Apparent porosity (a ) density (b) and linear shrinkage (c) of GP2, GP4, GP6 with calcination temperature

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3.3 3.3. Microstructure

Figure 7 shows SEM images of GP0 and GP4 calcined at different temperatures. There are numerous small pores in GP0, which were formed by the accumulation of raw materials after calcining at 1070 °C. Increasing the temperature promotes the densification of GP0, but the pore size distribution is not uniform. Comparatively, GP4 calcined at 1070 °C is much compact, while the pore size and number of pores are evenly distributed. As the temperature increased, the density of GP4 is further increased. After sintering at 1180 °C, the pores can be observed clearly. This indicates that the liquid phase formed by the glass powder promotes the densification of the matrix, which is beneficial to the mechanical properties of porous ceramics.

Fig. 7
figure 7

SEM of representative samples a GP0 1070 °C, b GP0 1130 °C, c GP0 1180 °C, d GP4 1070 °C, e GP4 1130 °C and f GP4 1180 °C

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Figure 8 shows cross-sectional SEM images of the samples with different contents of glass calcined at 1180 °C for 3 h. The GP2 sample exhibits a promising pore structure, although the distribution is not uniform, as shown in Fig. 8a. With the content increases of glass powder, the pore distribution becomes to be more uniform and the connectivity between the pores is improved, as shown in Fig. 8c. The pores are clearly observed in the enlarged image. Staggered arrangement of crystals can be seen in the pores. In addition, the unevenness of the fracture surface increases with the increase of glass powder, which is caused by crystal growth. This verified that the liquid phase derived from the glass powder promoted densification and crystal growth of the samples.

Fig. 8
figure 8

Cross-sectional SEM images of the samples calcined at 1180 °C for 3 h (a, d) GP2, (b, e) GP4 and (c, f) GP6

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3.4 3.4. Pore structure

Figure 9 shows pore structure characteristics of GP4 calcined at different temperatures. As presented in Fig. 9a, the samples contain pores with diameters of ~ 4 μm and ~ 10 μm, together with a few nanosized pores, simply because of the slime and sawdust powder have different particle sizes. However, the pore size distribution profiles of GP4 calcined at different temperatures are different. With increasing calcination temperature, the numbers of nanopore and ~ 4 μm pore are decreased, while that of ~ 10 μm pore is increased. At high temperatures, the liquid phase from the glass powder promoted densification and increased the size of the pores. As seen in Fig. 9b, ~ 90 % of the pores are less than 20 μm. The samples calcined at 1070 °C, 1100 °C and 1130 °C contain mainly pores of < 8 μm and a small number of 8 ~ 20 μm pores, with a ratio of 2:7. However, for the samples calcined at 1150 °C, the ratio is 7:2. All these indicate that the pore structure is changed after the addition of the glass powder. The temperature at which the pore structure changes is 1150 °C[39].

Fig. 9
figure 9

Pore structure characteristics of GP4 calcined at different temperatures

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Figure 10 shows pore structure characteristics of GP2 and GP6 calcined at 1180 °C. The size and number of pores are similar with GP2 and GP6 calcined at 1180 °C. And the pore structure is similar to the GP4 sample calcined at 1150 °C. This indicates that the small difference in log differential pore volume and cumulative distribution of sample is mainly affected by the glass powder content when samples are calcined at 1180 °C. When the calcining temperature is higher than 1150 ℃, the temperature has an effect on the density (Fig. 6), but it has little effect on the pore structure.

Fig. 10
figure 10

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

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3.5 3.5. Mechanical properties

Figure 11 shows flexural strength of the samples calcined at different temperatures. The flexural strength of GP0 is increased from 2.70 MPa to 7.47 MPa, as the sintering temperature is increased from 1070 °C to 1180 °C[34]. The fracture strengths of GP2, GP4 and GP6 calcined at 1180 °C for 3 h are 21.23 MPa, 25.27 MPa and 28.36 MPa, respectively. This suggests that the reaction and densification are insufficient at low temperatures, due to the decomposition of the organic matter. However, the presence of the liquid phase from the glass powder at high temperatures effectively enhanced the densification of the samples, thereby improving the mechanical properties.

Combining with SEM and pore structure analysis results, the flexural strength curves of GP2, GP4 and GP6 can be divided into two sections, 1070 °C ‒ 1150 °C and 1150‒1180 °C. At 1070‒1150 °C, the addition of glass powder and the increase in temperature increased the density of the samples and the pore structure was adjusted. Therefore, the flexural strength increases rapidly. Above 1150 °C, the increase in flexural strength depends on the enhancement in densification, while the pore structure no longer changes.

Fig. 11
figure 11

Flexural strength of the samples with calcination temperature

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Table 2 compares the results of this work with those reported in the open literature. Although various wastes have been used as a raw material to prepare porous ceramics, the information is not systematic. In this work, the content of waste is as high as 98.20 %, approaching an all-waste raw material utilization. Li and Ji synthesized ceramics at low temperatures with a high content of waste [20, 40]. The porous ceramic had an apparent porosity of 73 %, but the flexural strength was very low, due to the large apparent porosity [20]. Ji used fly ash to synthesize high-density ceramics, but the flexural strength is only 51.28 MPa [40]. Hua and Guo employed high-temperature sintering to enhance the mechanical properties of high-porosity ceramics, but the effectiveness was very limited [37, 41]. Therefore, it is difficult to achieve high porosity and high flexural performance at the same time for porous ceramics prepared entirely from waste.

Table 2 Comparison of this work with other ceramics
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4 4. Conclusions

Low-cost environmentally friendly porous ceramics have been prepared with wastes as the only raw materials. The liquid phase formed due to the addition of glass powder has a positive effect on densification after sintering at 1180 °C. The number ratio of pores of < 8 μm to those of 8 –20 μm was adjusted from 7:2 to 2:7. The flexural strength is increased from 7.47 MPa to 28.36 MPa. The sample calcined at 1180 °C exhibits an apparent porosity of 31.22 %, while the diameter shrinkage is less than 8.48 %, which is suitable for large-scale production. This achievement has potential economic benefits and practical values in terms of the utilization of solid wastes.