# Influence of a Two-Stage Sintering Process on Characteristics of Porous Ceramics Produced with Sewage Sludge and Coal Ash as Low-Cost Raw Materials

Authors: Esam Al-Qadhi; Gaiye Li; Yaxing Ni
Journal: Advances in Materials Science and Engineering
 (2019)

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## Abstract

In this paper, the two-stage sintering process as a promising strategy has been adopted to prepare high-performance porous ceramics with high porosity. Sewage sludge and waste coal ash were used as raw materials and added were starch and CaCO3 as a mixed foaming agent. The chemical composition of the raw materials and mixed foaming agents were analyzed by standard techniques. All mixtures were prepared from fixed solid materials mixed at a ratio of 48 : 32 : 10 : 10 (wt.%) for coal ash, sludge, starch, and CaCO3, respectively, followed by granular molding, aging, sintering and property survey tests. The influence of heating rate, holding time, and heating temperature of the two stages on the characteristics of the as-obtained porous ceramics was deeply investigated. The properties investigated are apparent porosity, bulk density, compressive strength, and linear shrinkage. On the one hand, the results show that the major content of the raw materials is silica (SiO2). On the other hand, the results show that the first-stage sintering process had minimal influence on the properties, whereas the second-stage sintering process had a significant influence on the properties of the porous ceramics. With the optimized sintering parameters (temperature, heating rate, and holding time) of the first stage and second stage, we obtained a high-performance porous ceramic product with an apparent porosity up to 51.3%, bulk density up to 1.22 g/cm3, linear shrinkage up to 14.3%, and compressive strength up to 25.1 MPa. Also, the results of the phase composition and microstructure revolution showed sufficient glassy phases and feldspar (Ca, Na) (Si, Al)4O8, which are known to enhance bending strength of ceramics. A comparison of these properties and characteristics with those of standard porous ceramics revealed that the product developed in this study would compete favorably for real applications.


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## Body

## 1. Introduction

The contradiction between industry development and environmental protection has become more and more intense in recent years [1]. As a representative example, aluminum siliceous waste and resultant slime from urban sewage treatment had dependably been a troublesome ecological issue [2, 3]. Although many methods have been used to the dispose of wastes, such as landfill, agricultural compost, and incineration composting [4], secondary pollution to the earth and groundwater often happens due to the persistent organic pollutants and heavy metal infiltration [5]. Therefore, sintering wastes into porous ceramics has become one of the best ways to dispose of wastes in recent years [6], conforming to the concept of 3R system of reusing, reducing, and recycling.Porous ceramics are used in many applications, such as bioceramics, gas membranes, thermal insulators, radiant gas burners, catalyst supports, particle filters, and many other fields [7–10] that demand high resistance to thermal stress, wear, and corrosion that are unobtainable from metals and polymers [11, 12]. The use of recycled coal ash [13] and sewage sludge [14] to produce porous ceramics has enticed a lot of global interest due to the increased sewage sludge and coal ash production and the limitation of land available to produce ceramics [15, 16]. Thus, it is considered as a practical solution for problems such as cost expenditure on waste management and its impact on the environment [17]. At the same time, porous ceramics with high strength and high porosity are much anticipated in real applications, but the two key factors are usually competing with each other [18, 19]. Many alternatives have been investigated to improve characteristics of ceramics such as microstructure, compressive strength, and porosity of the ceramics by controlling the powder size, as well as changing their composition and treating methods [20, 21]. Among such efforts, the sintering parameters of ceramic materials by sintering curve control are much more preferred as a simple, effective, and relatively low-cost strategy [22].Earlier studies have tried to fabricate porous ceramics from sludge waste together with clay or coal ash by combining the preheating and sintering treatments as one process [23]. This is quite limited for real applications presently due to the strategic resource constraints of clay and the competing porosity and strength restraints [24–28]. As a promising method to obtain microstructure-tailored ceramics to enhance the mechanical, electrical, magnetic as well as piezoelectric properties [29], the two-step sintering strategy has been successfully applied elsewhere to consolidate structural ceramics, bioceramics, ferrites, piezoelectric ceramics, and electrolyte ceramics [30, 31]. For the consolidation of sewage sludge-containing wastes, such a two-step strategy should be much more useful by preheating treatment at low sintering temperature to decompose the organic compounds and sintering at a higher temperature for making pores [32]. The kinds of foam agent and parameters are not the same. Especially for porous preparation ceramic with waste sewage sludge and coal ash, the second-hand waste cannot be obtained. The extensive literature review has little information about the effect of a two-stage heating process (preheating, sintering temperature, heating rate, and holding time) on the properties of porous ceramics produced with sewage sludge as raw materials.Therefore, for this aim, we adopted a two-step strategy for the manufacture of high-performance porous ceramics using sewage sludge and coal ash as a low-cost raw material. Also, starch/CaCO3 was added to act as a mixed foaming agent.


## 2. Raw Materials and Methods

### 2.1. Raw Materials

Sewage sludge used in this study was taken from the Sewage Treatment Plant of Jiangning Development Zone in Nanjing, and coal ash from the Power Plant in Maanshan of China. Table1 shows the chemical components of coal ash and sewage sludge. As shown in Table 1, the chemical components of coal ash are mainly contained from SiO2, Al2O3 (accounting 83.7% of the total), and a small amount of Fe2O3, CaO, and K2O. The main chemical components of sewage sludge are SiO2, Al2O3, CaO, and Fe2O3, in which SiO2 and organic matter accounted for the majority.Table 1
The chemical composition of coal ash and sewage sludge.
Chemical composition
SiO2
Al2O3
CaO
Fe2O3
MgO
K2O
P2O5
Balance
Sewage sludge
29.33
19.28
25.58
10.95
3.74
1.54
4.37
5.21
Coal ash
50.69
32.99
4.64
5.33
0.60
1.42
0.31
4.20


### 2.2. Preparation of the Porous Ceramic Samples

All mixtures were prepared from fixed solid materials mixed at a ratio of 48 : 32 : 10 : 10 (wt.%) for coal ash, sludge, starch, and CaCO3, respectively. NaOH solution of 20 g/L was also added and mixed well for better consolidation. Figure 1 shows the preparatory process of the porous ceramic samples described as follows:(1)
Raw material processing: the sludge taken from the sewage treatment plant was block-shaped, directly crushed, and sealed for use.(2)
Ingredients: the coal ash, sewage sludge, pore-forming agent, and admixture were mixed according to the design formula, and stirred with a mixer.(3)
Granulation molding: freshly mixed material with adequate viscosity was transferred to a pelletizer disk for making the desired spherical pellet size. The uniformly mixed mixture was taken out, placed in a mortar, added with an appropriate amount of water, stirred again, pressed and formed by a custom mold, and pressed into a sample having a diameter of 20 mm and a height of 20 mm. The molding pressure was set at 0.03 MPa, and the holding time 5 seconds.(4)
Aging: the sample was allowed to stand for 6 to 10 hours so that the starch can be fully gelatinized to improve the bonding degree of the whole pattern.(5)
Sintering: the pressed sample is placed in a box-type horse boiling furnace and sintered according to a self-designed sintering temperature mounting system. The sintering process is divided into two stages, according to the pore-forming mechanism. The implementation steps of the two-stage sintering method are designed to study the effects of sintering temperature, heating rate, and holding time on the properties of the products.Figure 1
Preparatory flowchart for the porous ceramic samples.For the first-stage sintering process, the temperature was set at 300°C–700°C, the heating rate was set at 1, 3, 5, 7, and 9°C/min, and the holding time was set at 0, 5, 10, 15, and 20 min, respectively. The primary purpose of the first-stage sintering of the pellets was to eliminate the organic compounds in the sewage sludge and gradually study the effects of the sintering process.For the second-stage process, the pellets were sintered at different temperatures ranging from 950°C to 1150°C with a heating rate set at 1, 3, 5, 7, and 9°C/min, and held at 0, 5, 10, 15, and 20 min, respectively.


### 2.3. Characterization of the Raw Materials and Products

The phase composition of the raw materials and the as-sintered products were characterized by X-ray diffraction (X-ray diffraction, BrukerAXS-D8) at a rate of 10°C/min from 5° to 90° at a voltage of 40 kV, a current of 40 mA, and a test accuracy ≤0.02°. Then, phase composition, according to the XRD patterns, was analyzed with MDI-JADE software. The microstructural morphology was observed by scanning electron microscopy (SEM, Hitachi SU8020, Japan) equipped with energy dispersive spectroscopy (EDS). The weight and the heat change of mixed powders were recorded by a thermogravimetric-differential thermal scanning synchronous thermal analyzer (TG/DSC, US Discovery SDT 650 type). The heating was conducted in the air from room temperature to 1200°C at 10°C/minute.Porosity and bulk density of the porous ceramics were tested by the boiling drainage method GB/T 1966–1996 [33]. Shrinkage was determined by measuring the change in the diameter of the prepared sample before and after sintering.The compressive strength is determined by using the universal material testing instrument REGER-3010 with the national standard of GB/T 1964–1996 [33]. The pattern was placed at the center of the universal testing machine, and the load was applied at a rate of 0.2 mm/min until the pattern was broken. The pore size distribution was examined by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics Instrument Corporation).


## 2.1. Raw Materials

Sewage sludge used in this study was taken from the Sewage Treatment Plant of Jiangning Development Zone in Nanjing, and coal ash from the Power Plant in Maanshan of China. Table1 shows the chemical components of coal ash and sewage sludge. As shown in Table 1, the chemical components of coal ash are mainly contained from SiO2, Al2O3 (accounting 83.7% of the total), and a small amount of Fe2O3, CaO, and K2O. The main chemical components of sewage sludge are SiO2, Al2O3, CaO, and Fe2O3, in which SiO2 and organic matter accounted for the majority.Table 1
The chemical composition of coal ash and sewage sludge.
Chemical composition
SiO2
Al2O3
CaO
Fe2O3
MgO
K2O
P2O5
Balance
Sewage sludge
29.33
19.28
25.58
10.95
3.74
1.54
4.37
5.21
Coal ash
50.69
32.99
4.64
5.33
0.60
1.42
0.31
4.20


## 2.2. Preparation of the Porous Ceramic Samples

All mixtures were prepared from fixed solid materials mixed at a ratio of 48 : 32 : 10 : 10 (wt.%) for coal ash, sludge, starch, and CaCO3, respectively. NaOH solution of 20 g/L was also added and mixed well for better consolidation. Figure 1 shows the preparatory process of the porous ceramic samples described as follows:(1)
Raw material processing: the sludge taken from the sewage treatment plant was block-shaped, directly crushed, and sealed for use.(2)
Ingredients: the coal ash, sewage sludge, pore-forming agent, and admixture were mixed according to the design formula, and stirred with a mixer.(3)
Granulation molding: freshly mixed material with adequate viscosity was transferred to a pelletizer disk for making the desired spherical pellet size. The uniformly mixed mixture was taken out, placed in a mortar, added with an appropriate amount of water, stirred again, pressed and formed by a custom mold, and pressed into a sample having a diameter of 20 mm and a height of 20 mm. The molding pressure was set at 0.03 MPa, and the holding time 5 seconds.(4)
Aging: the sample was allowed to stand for 6 to 10 hours so that the starch can be fully gelatinized to improve the bonding degree of the whole pattern.(5)
Sintering: the pressed sample is placed in a box-type horse boiling furnace and sintered according to a self-designed sintering temperature mounting system. The sintering process is divided into two stages, according to the pore-forming mechanism. The implementation steps of the two-stage sintering method are designed to study the effects of sintering temperature, heating rate, and holding time on the properties of the products.Figure 1
Preparatory flowchart for the porous ceramic samples.For the first-stage sintering process, the temperature was set at 300°C–700°C, the heating rate was set at 1, 3, 5, 7, and 9°C/min, and the holding time was set at 0, 5, 10, 15, and 20 min, respectively. The primary purpose of the first-stage sintering of the pellets was to eliminate the organic compounds in the sewage sludge and gradually study the effects of the sintering process.For the second-stage process, the pellets were sintered at different temperatures ranging from 950°C to 1150°C with a heating rate set at 1, 3, 5, 7, and 9°C/min, and held at 0, 5, 10, 15, and 20 min, respectively.


## 2.3. Characterization of the Raw Materials and Products

The phase composition of the raw materials and the as-sintered products were characterized by X-ray diffraction (X-ray diffraction, BrukerAXS-D8) at a rate of 10°C/min from 5° to 90° at a voltage of 40 kV, a current of 40 mA, and a test accuracy ≤0.02°. Then, phase composition, according to the XRD patterns, was analyzed with MDI-JADE software. The microstructural morphology was observed by scanning electron microscopy (SEM, Hitachi SU8020, Japan) equipped with energy dispersive spectroscopy (EDS). The weight and the heat change of mixed powders were recorded by a thermogravimetric-differential thermal scanning synchronous thermal analyzer (TG/DSC, US Discovery SDT 650 type). The heating was conducted in the air from room temperature to 1200°C at 10°C/minute.Porosity and bulk density of the porous ceramics were tested by the boiling drainage method GB/T 1966–1996 [33]. Shrinkage was determined by measuring the change in the diameter of the prepared sample before and after sintering.The compressive strength is determined by using the universal material testing instrument REGER-3010 with the national standard of GB/T 1964–1996 [33]. The pattern was placed at the center of the universal testing machine, and the load was applied at a rate of 0.2 mm/min until the pattern was broken. The pore size distribution was examined by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics Instrument Corporation).


## 3. Results and Discussion

### 3.1. Raw Materials Analysis

Figure2 shows the results of X-ray diffraction analysis of the mineral composition of coal ash and sludge. As indicated in Figure 2, according to the analysis of related material phase, the mineral coal ash is mainly composed of SiO2 and mullite (Al6Si2O13) formed by the combination of silica (SiO2) and alumina (Al2O3). The presence of Ca element from sludge, Al from coal ash, and Si from both was also proved. Also, in Figure 2, it can be noticed that a large number of silicate glass phases exist in coal ash. The existence of CaCO3 in the sludge was due to the artificial addition of lime to improve the dewatering rate and deodorization of the sludge, whereas the sludge mainly contains SiO2, feldspar, and CaCO3.Figure 2
X-ray diffraction patterns of (a) sewage sludge and (b) fly ash.Figure3 shows the SEM morphology of sewage sludge and coal ash. As shown in SEM images of Figures 3(a) and 3(b), the morphology of the coal ash looks like a standard spherical structure. On the other hand, as shown in Figures 3(c) and 3(d), the morphology of the sewage sludge looks lose with irregularity in shapes and sizes. Also, the particle volume of the sewage sludge is much larger than that of coal ash. These morphologies are consistent with those found elsewhere [34].Figure 3
SEM images of (a, b) coal ash and (c, d) sewage sludge.Figure4 shows the thermogravimetric analysis of the mixture of sludge and coal ash. As shown in Figure 4, first, due to an increase in the temperature to 200°C, the weight loss was –7.9%, mainly due to the evaporation of free water. Second, due to an increase in the temperature from 200°C to 600°C, the weight loss was –12.69%, mainly due to the combustion of organic matters. Simultaneously, during the sintering process in this temperature range, the mixture releases most of its energy. Therefore, it is necessary to reduce the heating rate during this temperature range. Primarily, when the pore foaming agent is used as well, in this setup, the liquid-phase reaction resulting from the addition of CaCO3 can prevent the existence of the cracks. Next, due to an increase in the sintering temperature from 600°C to 900°C, CaCO3 decomposes to form CO2 gas. In this case, the weight loss was negligible. At an increase in the sintering temperature 600°C to 900°C, the weight loss of the sample was paltry –1.43%. Then, due to an increase in the sintering temperature higher than 900°C, the solid-phase reaction accelerates and, at the same time, the bonding phase of ceramics was believed to form while overheating. Therefore, the optimization of the sintering temperature should be of vital importance to compromise the competing relationship of strength and porosity of the sintered porous ceramics.Figure 4
TG/DSC analysis of the mixture of sludge and fly ash.


### 3.2. The Influence of the Stage Sintering Process in the Characteristics of the Porous Ceramics

In this study, the stage sintering process mainly lies on the sintering temperature, heating rate, and holding time. The influences of different preheating, sintering temperatures, heating rate, and holding times were determined with regard to the porosity, density, shrinkage, and compressive strength of the porous ceramics understudied. Sections3.2.1 and 3.2.2 contain the details of the influence of a one-stage and two-stage sintering process on the characteristics of the porous ceramics understudied.


#### 3.2.1. Influence of One-Stage Sintering Process on the Characteristics of the Porous Ceramics

Figures5–7 show the influence of the one-stage sintering process on the characteristics of the porous ceramics. On the one hand, as shown in Figure 5(a), it can be observed that increasing the sintering temperature from 200 to 300°C decreases the bulk density, whereas increasing the sintering temperature from 300 to 400°C increases the bulk density of the porous ceramics. Also, in Figure 5(a), it can be observed that the increase in the sintering temperature from 200 to 300°C increases the apparent porosity, whereas increasing the sintering temperature from 300 to 400°C decreases the apparent porosity of the porous ceramics. Still, in Figure 5(a), it can be observed that for the one-stage sintering process, at a specific sintering temperature of 300°C, the bulk density is the lowest, whereas the apparent porosity is the highest. The reason for this trend is that there is a carbonization process in the organic matter during the heating process, and some of the organic matter (starch is the main factor) reacts with the oxygen in the air to form pores. Another part of the organic matter that cannot touch the air will carbonize. Thus, it could continue to make uniform pores in the subsequent sintering process with the oxygen generated at the high temperature of Fe2O3 to form pores, and this is more uniform than the pores formed by sintering of organic matter [6]. As indicated in Figure 5(a), it could be inferred that the influence of one-stage sintering process on the bulk density was less significant because it fell in the range of 0.89–0.95 g/cm3.Figure 5
Influence of one-stage sintering process on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of the porous ceramics.
(a)
(b)Figure 6
The influence of the heating rate on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for one-stage sintering process.
(a)
(b)Figure 7
The influence of the holding time on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for one-stage sintering process.
(a)
(b)On the other hand, as shown in Figure5(b), the compressive strength and linear shrinkage decreases with the increase of the sintering temperature from 200 to 300°C, whereas it increased with the increase in the sintering temperature from 300 to 400°C. Still, in Figure 5(b), it can be observed that at a sintering temperature of 400°C, the porous ceramics exhibited the highest compressive strength and linear shrinkage of about 4.3 MPa and 8.9%, respectively. As indicated in Figure 5(b), further increase in the sintering temperature higher than 400°C gradually decreases the compressive strength and linear shrinkage of the porous ceramics. The authors attributed the gradual decreasing of the compressive strength and linear shrinkage of the samples to the higher amount of gas released under the higher sintering temperature involved in this stage. Also, the authors believe some cracks could have occurred in the samples, resulting in deterioration of compressive strength. Overall, as shown in Figures 5(a) and 5(b), it can be observed that the linear shrinkage rate is 7∼8.9%, bulk density is 0.89∼0.95 g/cm3, apparent porosity is 61.4∼65.5%, and the compressive strength is 3∼4.3 MPa.In the current study, a preheating temperature of 300°C is considered the most suitable for the carbonization of organic matter, because of the indicated lowest bulk density and the highest apparent porosity at this temperature. Similar results for one-stage sintering process in similar products were reported elsewhere [27, 31].Figure6 shows the influence of the heating rate on the characteristics of the porous ceramics. As shown in Figure 6(a), it could be observed that the porous ceramics exhibited the highest and lowest apparent porosity and bulk density, respectively, at a heating rate of 5°C. It implies that at the heating rate of 5°C/min, the amount of carbonization of organic matter was the highest, contributing to the lowest value of bulk density and highest value of the porosity. Quantitatively, as indicated in Figure 6(a), the apparent porosity is 62∼63%, whereas the bulk density is 0.98∼1.05 g/cm3. As shown in Figure 6(b), it can be seen that the compressive strength decreased sharply and continuously at a heating rate from 1 to 5°C/min, whereas it increased gradually at a heating rate from 5 to 7°C/min and sharply from 7 to 9°C/min. Still, in Figure 6(a), it could be observed that the range of the compressive strength is 7.9∼9 MPa for the range of the heating rate understudied. However, it is expected that the compressive strength of porous ceramics can be mostly controlled by subsequent second-stage sintering temperature. Furthermore, as shown in Figure 6(b), it is evident that the linear shrinkage decreased sharply and continuously at a heating rate from 1–7°C/min, and then it increased sharply at a heating rate from 7 to 9°C/min. As shown in Figure 6(b), it can be seen that the range of the linear shrinkage is 10∼11%. According to the results shown in Figure 6, it could be inferred that the variation of the heating rate in one-stage sintering process has minimal influence on the characteristics of the porous ceramics. The authors believed that the variation of the heating rate in the two-stage sintering process would give a more significant influence on the characteristics of the porous ceramics.Moreover, Figure7 shows the influence of the heating holding time of the one-stage sintering process on the characteristics of the porous ceramics. As shown in Figure 7(a), it can be seen that the apparent porosity increases with the holding time, whereas the bulk density decreases with the holding time, both upto 15 min. Still, in Figure 7(a), it can be observed that the range of the apparent porosity is 58∼61%, whereas that of the bulk density is 1.07∼1.1 g/cm3. According to results indicated in Figure 7(a), a holding time of 15 min at 300°C would give the porous ceramics with the lowest bulk density and highest apparent porosity. The authors attributed these to the carbonization of the organic matters, and the generated carbon will continue to be a pore-forming agent in the subsequent sintering with the oxygen generated at the high temperature. Thus, the most conducive pores form, causing the lowest bulk density and the highest porosity.Furthermore, as shown in Figure7(b), the linear shrinkage and compressive strength of the sintered ceramics decrease with the increase in the holding time from 0 to 5 minutes, and then a small increase was observed at the holding time of 10 minutes. Still, in Figure 7(b), the range of the compressive strength is ∼8.2–9.9 MPa, whereas the range of the linear shrinkage rate is ∼10.5–12%. An opposite happened for the porosity of the prepared ceramics. However, it should be noted that the influence of the holding time on compressive strength and porosity observed in this study is minimal compared with those obtained by previous studies [25, 30].


#### 3.2.2. Influence of Two-Stage Sintering Process on the Characteristics of the Porous Ceramics

Figures8–10 show the influence of the two-stage sintering process on the characteristics of the sintered ceramics. As shown in Figure 8(a), it can be observed that the apparent porosity decreases continuously with the increase in the heating rate, whereas the bulk density increases continuously with the heating rate for the rate investigated. Also, in Figure 8(a), it can be seen that the heating rate has a significant influence on the apparent porosity and bulk density in the two-stage sintering process. As indicated in Figure 8(a), the range of the apparent porosity is ∼52–64.5%, whereas the range of the bulk density is ∼0.94–1.27 g/cm3. Similarly, as shown in Figure 8(b), the range of the compressive strength is ∼3.9–14.4 MPa, whereas the range of the shrinkage rate is ∼8.4–15%. The authors believe the reason for this trend is that after the first stage, a large number of pore structures have been formed in the sintered ceramics due to combustion and carbonization of organic matter. Therefore, if the heating rate in the second stage is too fast, the decomposition of CaCO3 and the reaction between Cao and Fe2O3 are relatively high [35]. Thus, it will destroy the pore structure in the sintered ceramics. At the same time, the continuous reaction will increase the volume shrinkage of the sample. This will result in an irregular volume shrinkage, and the contact area between the particles is enlarged. The particle aggregation and the center distance are reduced, and a grain boundary is gradually formed as the temperature increases. Also, the volume continues to shrink and finally becomes a less porous polycrystalline.Figure 8
The influence of the heating rate on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Figure 9
The influence of the holding time on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Figure 10
The influence of the sintering temperature on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Furthermore, Figure9 shows the influence of the holding time on the characteristics of the sintered ceramics for the second-stage sintering process. As shown in Figures 9(a) and 9(b), it can be seen that the increase in holding time had a significant influence on the characteristics of the sintered ceramics. As indicated in Figure 9(a), the range of the apparent porosity is ∼52.4–58.5%, whereas that of the bulk density is ∼1.03–1.17 g/cm3. Also, as indicated in Figure 9(b), the range of the compressive strength is ∼8–11 MPa, whereas that of the linear shrinkage is ∼10.0–12.2%. Also, it can be observed in Figures 9(a) and 9(b) that at the holding time higher than 15 min, the performance of the sintered ceramics was relatively stable. The authors attributed this behavior of the sintered ceramics to the prolongated holding time, which allows the liquid phase in the sample to further encapsulate small grains, growth of the grains or secondary recrystallization, and closer bonding between the particles. As the reaction almost completed after holding for 15 min, fewer changes were seen in the performance of the product.Figure10 shows the influence of sintering temperature on the characteristics of the sintered ceramics. Similar to those of the heating rate and holding time, the increase in the sintering temperature had a significant influence on the characteristics of the sintered ceramics. As shown in Figure 10(a), the range of the apparent porosity is ∼33.4–71.1%, whereas that of the bulk density is ∼0.72–1.66 g/cm3. Also, as shown in Figure 10(b), the range of the compressive strength is ∼1.71–51 MPa, whereas that of the linear shrinkage is ∼3–22%. These results are consistent with other studies carried out elsewhere [36–38]. It is worthy of note that the increase of the sintering temperature in the second-stage sintering process causes the increase of the linear shrinkage rate and compressive strength at the expense of the apparent porosity. As shown in Figure 10(a), at the sintering temperature range of 1000°C–1150°C, there was a continuous decrease of the apparent porosity.On the other hand, the compressive strength and linear shrinkage of the sintered ceramic showed a significant increase at the sintering temperature of 950°C–1150°C. The range of the compressive strength at this range of sintering temperature was 28–52 MPa, whereas those of the linear shrinkage was 1.27–1.66 g/cm3 as indicated in Figure 10(b). The authors attributed these trends to the intensification of the solid-phase reaction in the sintered ceramics. In the beginning, part of SiO2 dissolved in the liquid phase fuses into the various crystal grains, thereby promoting the fusion of the grain boundaries and accelerating the whole densification process. After the temperature continues to rise, various substances begin to undergo chemical changes and elements such as calcium, sodium, silicon, and aluminum begin to dissolve together and form a feldspar phase [28].At the same time, SiO2 will dissolve and embed into the skeleton of the porous ceramic, and thus the compressive strength, bulk density, and shrinkage increase but at the expense of the apparent porosity of the sintered ceramics. According to the results of the second-stage sintering process, it could be inferred that the characteristics of the porous ceramics understudied can be actively controlled by the two-stage sintering process.


### 3.3. The Phase Composition and Microstructural Revolution

Figure11 shows the XRD patterns of the porous ceramics sintered at a temperature range from 950°C to 1150°C. It can be observed in Figure 11(b) XRD pattern that the crystalline phase at a sintering temperature of 950°C only contains a small amount of feldspar (Ca, Na) (Si, Al)4O8, except SiO2(silica), Al6Si2O13, Fe2O3, and CaCO3. With the increase in the sintering temperature to 1000°C (as indicated in Figure 11(c) XRD pattern), SiO2 (silica) gradually reacts with mullite and CaCO3 to form albite (Ca, Na) (Si, Al)4O8, and thus the peak intensity of feldspar keeps rising. Simultaneously, the peaks of CaCO3 gradually decrease until its disappearance at 1100°C, indicating complete decomposition and transformation of CaO to feldspar (as indicated in Figure 11(d) XRD pattern). The XRD patterns of coal ash and sewage sludge in Figure 2 have proved the existence of SiO2 phase, some of which is believed to be involved in the reaction of feldspar formation. The rest of SiO2 (silica) continues to undergo liquid-phase transformation and fills into the pores of the sintered ceramics, which has been verified to cause the decrease of the apparent porosity and compressive strength in the porous ceramics understudied.Figure 11
XRD patterns of (a) the mixed powder and the sintered samples at (b) 950°C, (c) 1000°C, (d) 1050°C, (e) 1100°C, and (f) 1150°C.Figure12 shows SEM images of the as-obtained porous ceramics sintered at various temperatures ranging from 950°C to 1150°C. At 1000°C (Figure 12(a)), the particles form a preliminary contact under the action of surface diffusion, and the sintering phenomenon is not much observable due to a loose accumulation state, indicating the solid-phase reaction is at the just beginning, and no liquid phase can be identified. Just for this reason, the compressive strength is only about 2 MPa, as indicated earlier in Figure 10. With the further increase in the sintering temperature to 1050°C, the liquid phase begins to appear as indicated in Figure 12(b), thereby forming bubble-like structures as clearly observable in the sintered skeleton. As the molten liquid phase begins to connect other solid-phase particles, the contact frequency among crystals is enhanced, promoting grain boundary dissolution and densification. At 1100°C (Figure 12(c)), together with the central open pores, some close pores also appear due to the blocking effect of the CO2 gas from the decomposition of CaCO3. As a result, the mechanical properties of the sintered ceramic would increase more rapidly. Such temperature of 1100°C has also been used as the optimum temperature for the preparation of porous ceramics by other researchers [39–41]. At 1150°C (Figure 12(d)), the open pores decreased significantly, and the matrix compactness is further enhanced to form the sludge coal ash into a block. At this sintering temperature, although the compressive strength is as high as 50 MPa, the apparent porosity decreased to about 30%.Figure 12
Fracture surface morphology of the sintered samples at (a) 1000°C, (b) 1050°C, (c) 1100°C, and (d) 1150°C.
(a)
(b)
(c)
(d)


### 3.4. Influence of the Sintering Temperature on the Pore Size and Distribution

Figure13 shows the influence of the sintering temperature on the pore size and distribution in the porous ceramics. As shown in Figure 13, it can be observed that the change in the sintering temperature causes slight variations in both compositional and structural. Still, in Figure 13, it can be observed that the porous ceramics exhibited maximum cumulative volume in a pore size 3–4 μm. It is believed that at the maximum pore size in the ceramics, feldspar phase begins to grow, the liquid phase has not been formed in large quantities, and the structure does not seem to be combined. Thus, the apparent porosity is substantial, the density is minimal, the compressive strength is small, and many of the pores could be formed. This phenomenon happens due to the sintering temperature being comparatively low for the production of porous ceramic less than 1100°C, which shows that there are a large number of SiO2 on the surface of ceramic, less albite (Al6Si12O13), and where an incomplete-glassy phase is entrenched. The skeleton structure produced by feldspar and liquid-phase formation can be seen when the temperature reaches 1100°C. At a temperature of 1100°C, the fine bonding pore between grains begins to be fused, resulting in coarser and larger pores, some of which could exceed 15 μm. The porous ceramics sintered at this temperature have visible porosity and high mechanical strength, while the sample sintered at temperatures exceeding 1100°C has fewer pores and rougher surfaces. When the temperature increases to 1150°C, the liquid phase increases further, and a large number of apparent pores are blocked. At this time, the combination method still produces an excellent pore-forming effect, leaving more pores in the ceramic body. The diameter of the pores is much larger than that formed at a lower temperature, and a large number of closed pores are formed due to the liquid-phase encapsulation.Figure 13
Influence of the sintering temperature on the pore size and distribution in the porous ceramics.


### 3.5. Pore-Formation Mechanism and Performance Comparison

Figure14 shows the schedule for preparing sewage sludge-coal ash porous ceramics. It can be divided into two stages. The first stage is from room temperature to temperature (300°C, 400°C, 500°C, 600°C, and 700°C). In this stage, a large number of ceramic bodies are shaped into pores. The pore-forming mechanism at this stage can be expressed as the starch, and organic matter burns out to leave pores directly in the ceramic body. However, the first stage is also the most dangerous because it involves an extensive exothermic reaction, which could damage the organic matter due to the formation of cracks. Therefore, the heating rate should be appropriately optimized for full combustion of the organic matter in contact with air or carbonization inside the compact for the pore formation in the second stage. The pore formation in the second stage depends on the decomposition of CaCO3 to produce CO2 in the ceramic matrix to form pores, and the oxygen in the air will continue to react with the carbon produced in the first stage to form pores.Figure 14
The schedule for preparing porous ceramics from fly ash/municipal sludge.Based on the above discussion, it could be inferred that in order to obtain some robust porous ceramics of this kind, first, it would require the first-stage sintering process at the temperature to 300°C at a heating rate of 5°C/min held for 15 min. Second, it would require the second-stage sintering process at the sintering temperature of 1100°C at a heating rate of 3°C/min held for 10 min. With these processes, the authors obtained the high-quality samples with an apparent porosity up to 51.3%, density up to 1.22 g/cm3, linear shrinkage up to 14.3%, and comparable high strength of 25.1 MPa. These results were compared with the sintering parameters and performance of some porous ceramics prepared from clay and other wastes [3, 27, 39–43], ceramics prepared from wastes without natural resource [26, 44–47], and ceramics prepared from a natural resource without using any wastes [48], as listed in Table 2 of Appendix 1 (Supplementary Materials). The result shows that the porous ceramics prepared in this study are highly promising for real applications in consideration of the cost of manufacture as well as the performance (high porosity and high strength are obtained both as the two key factors).


## 3.1. Raw Materials Analysis

Figure2 shows the results of X-ray diffraction analysis of the mineral composition of coal ash and sludge. As indicated in Figure 2, according to the analysis of related material phase, the mineral coal ash is mainly composed of SiO2 and mullite (Al6Si2O13) formed by the combination of silica (SiO2) and alumina (Al2O3). The presence of Ca element from sludge, Al from coal ash, and Si from both was also proved. Also, in Figure 2, it can be noticed that a large number of silicate glass phases exist in coal ash. The existence of CaCO3 in the sludge was due to the artificial addition of lime to improve the dewatering rate and deodorization of the sludge, whereas the sludge mainly contains SiO2, feldspar, and CaCO3.Figure 2
X-ray diffraction patterns of (a) sewage sludge and (b) fly ash.Figure3 shows the SEM morphology of sewage sludge and coal ash. As shown in SEM images of Figures 3(a) and 3(b), the morphology of the coal ash looks like a standard spherical structure. On the other hand, as shown in Figures 3(c) and 3(d), the morphology of the sewage sludge looks lose with irregularity in shapes and sizes. Also, the particle volume of the sewage sludge is much larger than that of coal ash. These morphologies are consistent with those found elsewhere [34].Figure 3
SEM images of (a, b) coal ash and (c, d) sewage sludge.Figure4 shows the thermogravimetric analysis of the mixture of sludge and coal ash. As shown in Figure 4, first, due to an increase in the temperature to 200°C, the weight loss was –7.9%, mainly due to the evaporation of free water. Second, due to an increase in the temperature from 200°C to 600°C, the weight loss was –12.69%, mainly due to the combustion of organic matters. Simultaneously, during the sintering process in this temperature range, the mixture releases most of its energy. Therefore, it is necessary to reduce the heating rate during this temperature range. Primarily, when the pore foaming agent is used as well, in this setup, the liquid-phase reaction resulting from the addition of CaCO3 can prevent the existence of the cracks. Next, due to an increase in the sintering temperature from 600°C to 900°C, CaCO3 decomposes to form CO2 gas. In this case, the weight loss was negligible. At an increase in the sintering temperature 600°C to 900°C, the weight loss of the sample was paltry –1.43%. Then, due to an increase in the sintering temperature higher than 900°C, the solid-phase reaction accelerates and, at the same time, the bonding phase of ceramics was believed to form while overheating. Therefore, the optimization of the sintering temperature should be of vital importance to compromise the competing relationship of strength and porosity of the sintered porous ceramics.Figure 4
TG/DSC analysis of the mixture of sludge and fly ash.


## 3.2. The Influence of the Stage Sintering Process in the Characteristics of the Porous Ceramics

In this study, the stage sintering process mainly lies on the sintering temperature, heating rate, and holding time. The influences of different preheating, sintering temperatures, heating rate, and holding times were determined with regard to the porosity, density, shrinkage, and compressive strength of the porous ceramics understudied. Sections3.2.1 and 3.2.2 contain the details of the influence of a one-stage and two-stage sintering process on the characteristics of the porous ceramics understudied.


### 3.2.1. Influence of One-Stage Sintering Process on the Characteristics of the Porous Ceramics

Figures5–7 show the influence of the one-stage sintering process on the characteristics of the porous ceramics. On the one hand, as shown in Figure 5(a), it can be observed that increasing the sintering temperature from 200 to 300°C decreases the bulk density, whereas increasing the sintering temperature from 300 to 400°C increases the bulk density of the porous ceramics. Also, in Figure 5(a), it can be observed that the increase in the sintering temperature from 200 to 300°C increases the apparent porosity, whereas increasing the sintering temperature from 300 to 400°C decreases the apparent porosity of the porous ceramics. Still, in Figure 5(a), it can be observed that for the one-stage sintering process, at a specific sintering temperature of 300°C, the bulk density is the lowest, whereas the apparent porosity is the highest. The reason for this trend is that there is a carbonization process in the organic matter during the heating process, and some of the organic matter (starch is the main factor) reacts with the oxygen in the air to form pores. Another part of the organic matter that cannot touch the air will carbonize. Thus, it could continue to make uniform pores in the subsequent sintering process with the oxygen generated at the high temperature of Fe2O3 to form pores, and this is more uniform than the pores formed by sintering of organic matter [6]. As indicated in Figure 5(a), it could be inferred that the influence of one-stage sintering process on the bulk density was less significant because it fell in the range of 0.89–0.95 g/cm3.Figure 5
Influence of one-stage sintering process on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of the porous ceramics.
(a)
(b)Figure 6
The influence of the heating rate on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for one-stage sintering process.
(a)
(b)Figure 7
The influence of the holding time on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for one-stage sintering process.
(a)
(b)On the other hand, as shown in Figure5(b), the compressive strength and linear shrinkage decreases with the increase of the sintering temperature from 200 to 300°C, whereas it increased with the increase in the sintering temperature from 300 to 400°C. Still, in Figure 5(b), it can be observed that at a sintering temperature of 400°C, the porous ceramics exhibited the highest compressive strength and linear shrinkage of about 4.3 MPa and 8.9%, respectively. As indicated in Figure 5(b), further increase in the sintering temperature higher than 400°C gradually decreases the compressive strength and linear shrinkage of the porous ceramics. The authors attributed the gradual decreasing of the compressive strength and linear shrinkage of the samples to the higher amount of gas released under the higher sintering temperature involved in this stage. Also, the authors believe some cracks could have occurred in the samples, resulting in deterioration of compressive strength. Overall, as shown in Figures 5(a) and 5(b), it can be observed that the linear shrinkage rate is 7∼8.9%, bulk density is 0.89∼0.95 g/cm3, apparent porosity is 61.4∼65.5%, and the compressive strength is 3∼4.3 MPa.In the current study, a preheating temperature of 300°C is considered the most suitable for the carbonization of organic matter, because of the indicated lowest bulk density and the highest apparent porosity at this temperature. Similar results for one-stage sintering process in similar products were reported elsewhere [27, 31].Figure6 shows the influence of the heating rate on the characteristics of the porous ceramics. As shown in Figure 6(a), it could be observed that the porous ceramics exhibited the highest and lowest apparent porosity and bulk density, respectively, at a heating rate of 5°C. It implies that at the heating rate of 5°C/min, the amount of carbonization of organic matter was the highest, contributing to the lowest value of bulk density and highest value of the porosity. Quantitatively, as indicated in Figure 6(a), the apparent porosity is 62∼63%, whereas the bulk density is 0.98∼1.05 g/cm3. As shown in Figure 6(b), it can be seen that the compressive strength decreased sharply and continuously at a heating rate from 1 to 5°C/min, whereas it increased gradually at a heating rate from 5 to 7°C/min and sharply from 7 to 9°C/min. Still, in Figure 6(a), it could be observed that the range of the compressive strength is 7.9∼9 MPa for the range of the heating rate understudied. However, it is expected that the compressive strength of porous ceramics can be mostly controlled by subsequent second-stage sintering temperature. Furthermore, as shown in Figure 6(b), it is evident that the linear shrinkage decreased sharply and continuously at a heating rate from 1–7°C/min, and then it increased sharply at a heating rate from 7 to 9°C/min. As shown in Figure 6(b), it can be seen that the range of the linear shrinkage is 10∼11%. According to the results shown in Figure 6, it could be inferred that the variation of the heating rate in one-stage sintering process has minimal influence on the characteristics of the porous ceramics. The authors believed that the variation of the heating rate in the two-stage sintering process would give a more significant influence on the characteristics of the porous ceramics.Moreover, Figure7 shows the influence of the heating holding time of the one-stage sintering process on the characteristics of the porous ceramics. As shown in Figure 7(a), it can be seen that the apparent porosity increases with the holding time, whereas the bulk density decreases with the holding time, both upto 15 min. Still, in Figure 7(a), it can be observed that the range of the apparent porosity is 58∼61%, whereas that of the bulk density is 1.07∼1.1 g/cm3. According to results indicated in Figure 7(a), a holding time of 15 min at 300°C would give the porous ceramics with the lowest bulk density and highest apparent porosity. The authors attributed these to the carbonization of the organic matters, and the generated carbon will continue to be a pore-forming agent in the subsequent sintering with the oxygen generated at the high temperature. Thus, the most conducive pores form, causing the lowest bulk density and the highest porosity.Furthermore, as shown in Figure7(b), the linear shrinkage and compressive strength of the sintered ceramics decrease with the increase in the holding time from 0 to 5 minutes, and then a small increase was observed at the holding time of 10 minutes. Still, in Figure 7(b), the range of the compressive strength is ∼8.2–9.9 MPa, whereas the range of the linear shrinkage rate is ∼10.5–12%. An opposite happened for the porosity of the prepared ceramics. However, it should be noted that the influence of the holding time on compressive strength and porosity observed in this study is minimal compared with those obtained by previous studies [25, 30].


### 3.2.2. Influence of Two-Stage Sintering Process on the Characteristics of the Porous Ceramics

Figures8–10 show the influence of the two-stage sintering process on the characteristics of the sintered ceramics. As shown in Figure 8(a), it can be observed that the apparent porosity decreases continuously with the increase in the heating rate, whereas the bulk density increases continuously with the heating rate for the rate investigated. Also, in Figure 8(a), it can be seen that the heating rate has a significant influence on the apparent porosity and bulk density in the two-stage sintering process. As indicated in Figure 8(a), the range of the apparent porosity is ∼52–64.5%, whereas the range of the bulk density is ∼0.94–1.27 g/cm3. Similarly, as shown in Figure 8(b), the range of the compressive strength is ∼3.9–14.4 MPa, whereas the range of the shrinkage rate is ∼8.4–15%. The authors believe the reason for this trend is that after the first stage, a large number of pore structures have been formed in the sintered ceramics due to combustion and carbonization of organic matter. Therefore, if the heating rate in the second stage is too fast, the decomposition of CaCO3 and the reaction between Cao and Fe2O3 are relatively high [35]. Thus, it will destroy the pore structure in the sintered ceramics. At the same time, the continuous reaction will increase the volume shrinkage of the sample. This will result in an irregular volume shrinkage, and the contact area between the particles is enlarged. The particle aggregation and the center distance are reduced, and a grain boundary is gradually formed as the temperature increases. Also, the volume continues to shrink and finally becomes a less porous polycrystalline.Figure 8
The influence of the heating rate on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Figure 9
The influence of the holding time on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Figure 10
The influence of the sintering temperature on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Furthermore, Figure9 shows the influence of the holding time on the characteristics of the sintered ceramics for the second-stage sintering process. As shown in Figures 9(a) and 9(b), it can be seen that the increase in holding time had a significant influence on the characteristics of the sintered ceramics. As indicated in Figure 9(a), the range of the apparent porosity is ∼52.4–58.5%, whereas that of the bulk density is ∼1.03–1.17 g/cm3. Also, as indicated in Figure 9(b), the range of the compressive strength is ∼8–11 MPa, whereas that of the linear shrinkage is ∼10.0–12.2%. Also, it can be observed in Figures 9(a) and 9(b) that at the holding time higher than 15 min, the performance of the sintered ceramics was relatively stable. The authors attributed this behavior of the sintered ceramics to the prolongated holding time, which allows the liquid phase in the sample to further encapsulate small grains, growth of the grains or secondary recrystallization, and closer bonding between the particles. As the reaction almost completed after holding for 15 min, fewer changes were seen in the performance of the product.Figure10 shows the influence of sintering temperature on the characteristics of the sintered ceramics. Similar to those of the heating rate and holding time, the increase in the sintering temperature had a significant influence on the characteristics of the sintered ceramics. As shown in Figure 10(a), the range of the apparent porosity is ∼33.4–71.1%, whereas that of the bulk density is ∼0.72–1.66 g/cm3. Also, as shown in Figure 10(b), the range of the compressive strength is ∼1.71–51 MPa, whereas that of the linear shrinkage is ∼3–22%. These results are consistent with other studies carried out elsewhere [36–38]. It is worthy of note that the increase of the sintering temperature in the second-stage sintering process causes the increase of the linear shrinkage rate and compressive strength at the expense of the apparent porosity. As shown in Figure 10(a), at the sintering temperature range of 1000°C–1150°C, there was a continuous decrease of the apparent porosity.On the other hand, the compressive strength and linear shrinkage of the sintered ceramic showed a significant increase at the sintering temperature of 950°C–1150°C. The range of the compressive strength at this range of sintering temperature was 28–52 MPa, whereas those of the linear shrinkage was 1.27–1.66 g/cm3 as indicated in Figure 10(b). The authors attributed these trends to the intensification of the solid-phase reaction in the sintered ceramics. In the beginning, part of SiO2 dissolved in the liquid phase fuses into the various crystal grains, thereby promoting the fusion of the grain boundaries and accelerating the whole densification process. After the temperature continues to rise, various substances begin to undergo chemical changes and elements such as calcium, sodium, silicon, and aluminum begin to dissolve together and form a feldspar phase [28].At the same time, SiO2 will dissolve and embed into the skeleton of the porous ceramic, and thus the compressive strength, bulk density, and shrinkage increase but at the expense of the apparent porosity of the sintered ceramics. According to the results of the second-stage sintering process, it could be inferred that the characteristics of the porous ceramics understudied can be actively controlled by the two-stage sintering process.


## 3.2.1. Influence of One-Stage Sintering Process on the Characteristics of the Porous Ceramics

Figures5–7 show the influence of the one-stage sintering process on the characteristics of the porous ceramics. On the one hand, as shown in Figure 5(a), it can be observed that increasing the sintering temperature from 200 to 300°C decreases the bulk density, whereas increasing the sintering temperature from 300 to 400°C increases the bulk density of the porous ceramics. Also, in Figure 5(a), it can be observed that the increase in the sintering temperature from 200 to 300°C increases the apparent porosity, whereas increasing the sintering temperature from 300 to 400°C decreases the apparent porosity of the porous ceramics. Still, in Figure 5(a), it can be observed that for the one-stage sintering process, at a specific sintering temperature of 300°C, the bulk density is the lowest, whereas the apparent porosity is the highest. The reason for this trend is that there is a carbonization process in the organic matter during the heating process, and some of the organic matter (starch is the main factor) reacts with the oxygen in the air to form pores. Another part of the organic matter that cannot touch the air will carbonize. Thus, it could continue to make uniform pores in the subsequent sintering process with the oxygen generated at the high temperature of Fe2O3 to form pores, and this is more uniform than the pores formed by sintering of organic matter [6]. As indicated in Figure 5(a), it could be inferred that the influence of one-stage sintering process on the bulk density was less significant because it fell in the range of 0.89–0.95 g/cm3.Figure 5
Influence of one-stage sintering process on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of the porous ceramics.
(a)
(b)Figure 6
The influence of the heating rate on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for one-stage sintering process.
(a)
(b)Figure 7
The influence of the holding time on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for one-stage sintering process.
(a)
(b)On the other hand, as shown in Figure5(b), the compressive strength and linear shrinkage decreases with the increase of the sintering temperature from 200 to 300°C, whereas it increased with the increase in the sintering temperature from 300 to 400°C. Still, in Figure 5(b), it can be observed that at a sintering temperature of 400°C, the porous ceramics exhibited the highest compressive strength and linear shrinkage of about 4.3 MPa and 8.9%, respectively. As indicated in Figure 5(b), further increase in the sintering temperature higher than 400°C gradually decreases the compressive strength and linear shrinkage of the porous ceramics. The authors attributed the gradual decreasing of the compressive strength and linear shrinkage of the samples to the higher amount of gas released under the higher sintering temperature involved in this stage. Also, the authors believe some cracks could have occurred in the samples, resulting in deterioration of compressive strength. Overall, as shown in Figures 5(a) and 5(b), it can be observed that the linear shrinkage rate is 7∼8.9%, bulk density is 0.89∼0.95 g/cm3, apparent porosity is 61.4∼65.5%, and the compressive strength is 3∼4.3 MPa.In the current study, a preheating temperature of 300°C is considered the most suitable for the carbonization of organic matter, because of the indicated lowest bulk density and the highest apparent porosity at this temperature. Similar results for one-stage sintering process in similar products were reported elsewhere [27, 31].Figure6 shows the influence of the heating rate on the characteristics of the porous ceramics. As shown in Figure 6(a), it could be observed that the porous ceramics exhibited the highest and lowest apparent porosity and bulk density, respectively, at a heating rate of 5°C. It implies that at the heating rate of 5°C/min, the amount of carbonization of organic matter was the highest, contributing to the lowest value of bulk density and highest value of the porosity. Quantitatively, as indicated in Figure 6(a), the apparent porosity is 62∼63%, whereas the bulk density is 0.98∼1.05 g/cm3. As shown in Figure 6(b), it can be seen that the compressive strength decreased sharply and continuously at a heating rate from 1 to 5°C/min, whereas it increased gradually at a heating rate from 5 to 7°C/min and sharply from 7 to 9°C/min. Still, in Figure 6(a), it could be observed that the range of the compressive strength is 7.9∼9 MPa for the range of the heating rate understudied. However, it is expected that the compressive strength of porous ceramics can be mostly controlled by subsequent second-stage sintering temperature. Furthermore, as shown in Figure 6(b), it is evident that the linear shrinkage decreased sharply and continuously at a heating rate from 1–7°C/min, and then it increased sharply at a heating rate from 7 to 9°C/min. As shown in Figure 6(b), it can be seen that the range of the linear shrinkage is 10∼11%. According to the results shown in Figure 6, it could be inferred that the variation of the heating rate in one-stage sintering process has minimal influence on the characteristics of the porous ceramics. The authors believed that the variation of the heating rate in the two-stage sintering process would give a more significant influence on the characteristics of the porous ceramics.Moreover, Figure7 shows the influence of the heating holding time of the one-stage sintering process on the characteristics of the porous ceramics. As shown in Figure 7(a), it can be seen that the apparent porosity increases with the holding time, whereas the bulk density decreases with the holding time, both upto 15 min. Still, in Figure 7(a), it can be observed that the range of the apparent porosity is 58∼61%, whereas that of the bulk density is 1.07∼1.1 g/cm3. According to results indicated in Figure 7(a), a holding time of 15 min at 300°C would give the porous ceramics with the lowest bulk density and highest apparent porosity. The authors attributed these to the carbonization of the organic matters, and the generated carbon will continue to be a pore-forming agent in the subsequent sintering with the oxygen generated at the high temperature. Thus, the most conducive pores form, causing the lowest bulk density and the highest porosity.Furthermore, as shown in Figure7(b), the linear shrinkage and compressive strength of the sintered ceramics decrease with the increase in the holding time from 0 to 5 minutes, and then a small increase was observed at the holding time of 10 minutes. Still, in Figure 7(b), the range of the compressive strength is ∼8.2–9.9 MPa, whereas the range of the linear shrinkage rate is ∼10.5–12%. An opposite happened for the porosity of the prepared ceramics. However, it should be noted that the influence of the holding time on compressive strength and porosity observed in this study is minimal compared with those obtained by previous studies [25, 30].


## 3.2.2. Influence of Two-Stage Sintering Process on the Characteristics of the Porous Ceramics

Figures8–10 show the influence of the two-stage sintering process on the characteristics of the sintered ceramics. As shown in Figure 8(a), it can be observed that the apparent porosity decreases continuously with the increase in the heating rate, whereas the bulk density increases continuously with the heating rate for the rate investigated. Also, in Figure 8(a), it can be seen that the heating rate has a significant influence on the apparent porosity and bulk density in the two-stage sintering process. As indicated in Figure 8(a), the range of the apparent porosity is ∼52–64.5%, whereas the range of the bulk density is ∼0.94–1.27 g/cm3. Similarly, as shown in Figure 8(b), the range of the compressive strength is ∼3.9–14.4 MPa, whereas the range of the shrinkage rate is ∼8.4–15%. The authors believe the reason for this trend is that after the first stage, a large number of pore structures have been formed in the sintered ceramics due to combustion and carbonization of organic matter. Therefore, if the heating rate in the second stage is too fast, the decomposition of CaCO3 and the reaction between Cao and Fe2O3 are relatively high [35]. Thus, it will destroy the pore structure in the sintered ceramics. At the same time, the continuous reaction will increase the volume shrinkage of the sample. This will result in an irregular volume shrinkage, and the contact area between the particles is enlarged. The particle aggregation and the center distance are reduced, and a grain boundary is gradually formed as the temperature increases. Also, the volume continues to shrink and finally becomes a less porous polycrystalline.Figure 8
The influence of the heating rate on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Figure 9
The influence of the holding time on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Figure 10
The influence of the sintering temperature on (a) the apparent porosity and bulk density and (b) compressive strength and linear sintering shrinkage of as-sintered ceramics for the second-stage sintering process.
(a)
(b)Furthermore, Figure9 shows the influence of the holding time on the characteristics of the sintered ceramics for the second-stage sintering process. As shown in Figures 9(a) and 9(b), it can be seen that the increase in holding time had a significant influence on the characteristics of the sintered ceramics. As indicated in Figure 9(a), the range of the apparent porosity is ∼52.4–58.5%, whereas that of the bulk density is ∼1.03–1.17 g/cm3. Also, as indicated in Figure 9(b), the range of the compressive strength is ∼8–11 MPa, whereas that of the linear shrinkage is ∼10.0–12.2%. Also, it can be observed in Figures 9(a) and 9(b) that at the holding time higher than 15 min, the performance of the sintered ceramics was relatively stable. The authors attributed this behavior of the sintered ceramics to the prolongated holding time, which allows the liquid phase in the sample to further encapsulate small grains, growth of the grains or secondary recrystallization, and closer bonding between the particles. As the reaction almost completed after holding for 15 min, fewer changes were seen in the performance of the product.Figure10 shows the influence of sintering temperature on the characteristics of the sintered ceramics. Similar to those of the heating rate and holding time, the increase in the sintering temperature had a significant influence on the characteristics of the sintered ceramics. As shown in Figure 10(a), the range of the apparent porosity is ∼33.4–71.1%, whereas that of the bulk density is ∼0.72–1.66 g/cm3. Also, as shown in Figure 10(b), the range of the compressive strength is ∼1.71–51 MPa, whereas that of the linear shrinkage is ∼3–22%. These results are consistent with other studies carried out elsewhere [36–38]. It is worthy of note that the increase of the sintering temperature in the second-stage sintering process causes the increase of the linear shrinkage rate and compressive strength at the expense of the apparent porosity. As shown in Figure 10(a), at the sintering temperature range of 1000°C–1150°C, there was a continuous decrease of the apparent porosity.On the other hand, the compressive strength and linear shrinkage of the sintered ceramic showed a significant increase at the sintering temperature of 950°C–1150°C. The range of the compressive strength at this range of sintering temperature was 28–52 MPa, whereas those of the linear shrinkage was 1.27–1.66 g/cm3 as indicated in Figure 10(b). The authors attributed these trends to the intensification of the solid-phase reaction in the sintered ceramics. In the beginning, part of SiO2 dissolved in the liquid phase fuses into the various crystal grains, thereby promoting the fusion of the grain boundaries and accelerating the whole densification process. After the temperature continues to rise, various substances begin to undergo chemical changes and elements such as calcium, sodium, silicon, and aluminum begin to dissolve together and form a feldspar phase [28].At the same time, SiO2 will dissolve and embed into the skeleton of the porous ceramic, and thus the compressive strength, bulk density, and shrinkage increase but at the expense of the apparent porosity of the sintered ceramics. According to the results of the second-stage sintering process, it could be inferred that the characteristics of the porous ceramics understudied can be actively controlled by the two-stage sintering process.


## 3.3. The Phase Composition and Microstructural Revolution

Figure11 shows the XRD patterns of the porous ceramics sintered at a temperature range from 950°C to 1150°C. It can be observed in Figure 11(b) XRD pattern that the crystalline phase at a sintering temperature of 950°C only contains a small amount of feldspar (Ca, Na) (Si, Al)4O8, except SiO2(silica), Al6Si2O13, Fe2O3, and CaCO3. With the increase in the sintering temperature to 1000°C (as indicated in Figure 11(c) XRD pattern), SiO2 (silica) gradually reacts with mullite and CaCO3 to form albite (Ca, Na) (Si, Al)4O8, and thus the peak intensity of feldspar keeps rising. Simultaneously, the peaks of CaCO3 gradually decrease until its disappearance at 1100°C, indicating complete decomposition and transformation of CaO to feldspar (as indicated in Figure 11(d) XRD pattern). The XRD patterns of coal ash and sewage sludge in Figure 2 have proved the existence of SiO2 phase, some of which is believed to be involved in the reaction of feldspar formation. The rest of SiO2 (silica) continues to undergo liquid-phase transformation and fills into the pores of the sintered ceramics, which has been verified to cause the decrease of the apparent porosity and compressive strength in the porous ceramics understudied.Figure 11
XRD patterns of (a) the mixed powder and the sintered samples at (b) 950°C, (c) 1000°C, (d) 1050°C, (e) 1100°C, and (f) 1150°C.Figure12 shows SEM images of the as-obtained porous ceramics sintered at various temperatures ranging from 950°C to 1150°C. At 1000°C (Figure 12(a)), the particles form a preliminary contact under the action of surface diffusion, and the sintering phenomenon is not much observable due to a loose accumulation state, indicating the solid-phase reaction is at the just beginning, and no liquid phase can be identified. Just for this reason, the compressive strength is only about 2 MPa, as indicated earlier in Figure 10. With the further increase in the sintering temperature to 1050°C, the liquid phase begins to appear as indicated in Figure 12(b), thereby forming bubble-like structures as clearly observable in the sintered skeleton. As the molten liquid phase begins to connect other solid-phase particles, the contact frequency among crystals is enhanced, promoting grain boundary dissolution and densification. At 1100°C (Figure 12(c)), together with the central open pores, some close pores also appear due to the blocking effect of the CO2 gas from the decomposition of CaCO3. As a result, the mechanical properties of the sintered ceramic would increase more rapidly. Such temperature of 1100°C has also been used as the optimum temperature for the preparation of porous ceramics by other researchers [39–41]. At 1150°C (Figure 12(d)), the open pores decreased significantly, and the matrix compactness is further enhanced to form the sludge coal ash into a block. At this sintering temperature, although the compressive strength is as high as 50 MPa, the apparent porosity decreased to about 30%.Figure 12
Fracture surface morphology of the sintered samples at (a) 1000°C, (b) 1050°C, (c) 1100°C, and (d) 1150°C.
(a)
(b)
(c)
(d)


## 3.4. Influence of the Sintering Temperature on the Pore Size and Distribution

Figure13 shows the influence of the sintering temperature on the pore size and distribution in the porous ceramics. As shown in Figure 13, it can be observed that the change in the sintering temperature causes slight variations in both compositional and structural. Still, in Figure 13, it can be observed that the porous ceramics exhibited maximum cumulative volume in a pore size 3–4 μm. It is believed that at the maximum pore size in the ceramics, feldspar phase begins to grow, the liquid phase has not been formed in large quantities, and the structure does not seem to be combined. Thus, the apparent porosity is substantial, the density is minimal, the compressive strength is small, and many of the pores could be formed. This phenomenon happens due to the sintering temperature being comparatively low for the production of porous ceramic less than 1100°C, which shows that there are a large number of SiO2 on the surface of ceramic, less albite (Al6Si12O13), and where an incomplete-glassy phase is entrenched. The skeleton structure produced by feldspar and liquid-phase formation can be seen when the temperature reaches 1100°C. At a temperature of 1100°C, the fine bonding pore between grains begins to be fused, resulting in coarser and larger pores, some of which could exceed 15 μm. The porous ceramics sintered at this temperature have visible porosity and high mechanical strength, while the sample sintered at temperatures exceeding 1100°C has fewer pores and rougher surfaces. When the temperature increases to 1150°C, the liquid phase increases further, and a large number of apparent pores are blocked. At this time, the combination method still produces an excellent pore-forming effect, leaving more pores in the ceramic body. The diameter of the pores is much larger than that formed at a lower temperature, and a large number of closed pores are formed due to the liquid-phase encapsulation.Figure 13
Influence of the sintering temperature on the pore size and distribution in the porous ceramics.


## 3.5. Pore-Formation Mechanism and Performance Comparison

Figure14 shows the schedule for preparing sewage sludge-coal ash porous ceramics. It can be divided into two stages. The first stage is from room temperature to temperature (300°C, 400°C, 500°C, 600°C, and 700°C). In this stage, a large number of ceramic bodies are shaped into pores. The pore-forming mechanism at this stage can be expressed as the starch, and organic matter burns out to leave pores directly in the ceramic body. However, the first stage is also the most dangerous because it involves an extensive exothermic reaction, which could damage the organic matter due to the formation of cracks. Therefore, the heating rate should be appropriately optimized for full combustion of the organic matter in contact with air or carbonization inside the compact for the pore formation in the second stage. The pore formation in the second stage depends on the decomposition of CaCO3 to produce CO2 in the ceramic matrix to form pores, and the oxygen in the air will continue to react with the carbon produced in the first stage to form pores.Figure 14
The schedule for preparing porous ceramics from fly ash/municipal sludge.Based on the above discussion, it could be inferred that in order to obtain some robust porous ceramics of this kind, first, it would require the first-stage sintering process at the temperature to 300°C at a heating rate of 5°C/min held for 15 min. Second, it would require the second-stage sintering process at the sintering temperature of 1100°C at a heating rate of 3°C/min held for 10 min. With these processes, the authors obtained the high-quality samples with an apparent porosity up to 51.3%, density up to 1.22 g/cm3, linear shrinkage up to 14.3%, and comparable high strength of 25.1 MPa. These results were compared with the sintering parameters and performance of some porous ceramics prepared from clay and other wastes [3, 27, 39–43], ceramics prepared from wastes without natural resource [26, 44–47], and ceramics prepared from a natural resource without using any wastes [48], as listed in Table 2 of Appendix 1 (Supplementary Materials). The result shows that the porous ceramics prepared in this study are highly promising for real applications in consideration of the cost of manufacture as well as the performance (high porosity and high strength are obtained both as the two key factors).


## 4. Conclusions

In this study, according to the pore-formation mechanism, a two-step sintering strategy was proposed using sewage sludge and coal ash as raw materials, and starch/CaCO3 as a mixed foaming agent. The effect of sintering parameters on the properties of the sample was studied, and the related mechanism was discussed.(1)
The pore-forming process is divided into two stages: the first stage is the formation of pores by organic matter combustion and the second stage is the appearance of CO2 from the decomposition of CaCO3, which will be helpful for the further formation of pores in the as-consolidated ceramic body.(2)
The optimal sintering process is selected: the first-stage heating rate is 5°C/min, the temperature is increased to 300°C, and the holding time is 15 min; the second-stage heating rate is 3°C/min, the maximum temperature is designed according to the actual production needs, and the holding time is 10 min.(3)
XRD and SEM analyses indicated that the pores are staggered on the green body and connected. From the 1050°C grain growth to the 1100°C liquid phase began to block the pores, the optimal reaction temperature for making porous ceramics is between this; when the temperature reaches 1150°C, the ceramic cross-section is almost invisible. Moreover, there is no longer a spherical structure unique to coal ash, indicating the complete reaction to form feldspar here.(4)
After sintering at the maximum temperature of 1100°C under other optimized parameters, a high-performance porous ceramic material was obtained, with an apparent porosity up to 51.3%, density to 1.22 g/cm3, line shrinkage at 14.3%, and compressive strength of 25.1 MPa, which is quite competitive for real applications than most of those reported in literatures.



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*Source: 3710692-2019-11-12.xml*