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Abstract

The preparation of ceramsite using lead–zinc tailings (LZT) as the primary raw material, fly ash as a sintering auxiliary material, and coal powder as a flux was investigated. The results indicate that the apparent density and water absorption of ceramsite decrease continuously with increasing sintering temperature, while the cylinder compressive strength shows the opposite trend. As the LZT content increases, the apparent density of ceramsite initially decreases and then slightly increases. Meanwhile, the water absorption rate shows an upward trend, and the cylinder compressive strength continuously declines. The primary crystalline phases of LZT-based ceramsite are anorthite, mullite and quartz, with mullite providing significant support to the strength of the ceramsite. Under the conditions of 50% LZT content and a sintering temperature of 1250°C, the ceramsite with an apparent density of 1165.8 kg/m³, a water absorption of 3.1%, and a cylinder compressive strength of 8.5 MPa was successfully produced.

Keywords

lead-zinc tailings-based ceramsite sintering temperature Al/Si ratio cylinder compressive strength apparent density

Introduction

Lead–zinc tailings (LZT) are the powdery or fine sandy waste residues discharged after crushing, grinding, and flotation of lead–zinc ores.¹ China is endowed with a wealth of lead–zinc mineral resources, and its lead and zinc production is the highest in the world.² However, due to the relatively low grade of the ore, the tailings yield from beneficiation plants typically exceeds 70%.³ The ongoing exploitation of lead–zinc mineral resources has resulted in a substantial increase in the quantity of tailings. The data indicates that by the conclusion of 2021, China's annual tailings production reached 16.49 billion tons. Nevertheless, the total utilisation amount was merely 312 million tons, resulting in an overall utilisation rate of less than 20%.⁴ The LZT that is not utilised is typically stored in tailings ponds. This practice not only contributes to the waste of land resources but also poses a risk of landslides, mudslides, and other geological disasters. These potential hazards have the further consequence of threatening human health and safety.⁵

Currently, the main utilisation approaches for LZT include: recovery of valuable components,^6–8 filling of underground mining zones,^9,10 and utilisation in building materials.^11,12 However, the content of valuable components remaining in LZT is relatively low, thereby rendering recovery a challenging endeavour. Despite the implementation of a re-selection processing technique, tailings are still generated, limiting the effectiveness of this method in reducing tailings. Although the filling of underground mining zones represents an effective method for the disposal of large quantities of LZT, with relatively low requirements for the physical and chemical properties and stability of tailings, its economic value is relatively limited. Ceramsite, as a kind of artificial lightweight aggregate produced by high-temperature sintering, has excellent properties such as light weight, high porosity, and good sound insulation and heat insulation effects. Depending on the different raw materials used in preparation, ceramsite can be classified into shale ceramsite,¹³ clay ceramsite,¹⁴ tailings ceramsite,¹⁵ fly ash ceramsite,¹⁶ sludge ceramsite,¹⁷ coal gangue ceramsite,¹⁸ etc., showing great potential in the field of resource utilisation. Therefore, based on the relevant experience in the utilisation of solid waste in the past, in order to achieve the comprehensive utilisation of LZT resources, if LZT is used as raw material to produce ceramsite and applied in fields such as building materials, horticulture, and water treatment,^19–21 it can realise its transformation from ‘waste’ to ‘resource’. It can maximise its economic benefits as well as the potential and value of green and sustainable development.^22–23

In recent years, the base materials for preparing ceramsite have gradually evolved from non-renewable clay and shale to industrial solid waste. The preparation of ceramsite using clay and shale not only causes environmental pollution but also puts pressure on the supply of raw materials.²⁴ Given that industrial solid waste (such as tailings, sludge, fly ash, coal gangue, etc.) has the advantages of wide sources and low cost, and its chemical composition is similar to that of clay and shale, numerous researchers have commenced investigations into the potential of utilising these wastes as raw materials for ceramsite.

Chen et al.²⁵ prepared porous ceramsite with a bulk density of 785 kg/m³, a water absorption rate of 3.5%, and a cylinder compressive strength of 5.9 MPa using coal gangue as the main raw material. Qin et al.²⁶ used municipal sludge as the main raw material and produced high-strength ceramsite with a compressive strength of 4.89 MPa and a density of 1320 kg/m³ according to the optimal ratio. Qi et al.²⁷ used coal gangue as the main raw material and added urban sludge with a mass fraction of 20% to 50% to produce ceramsite with a density of 1030 to 1200 kg/m³. Sun et al.²⁸ used red mud strong magnetic tailings as the main raw material and optimised the process conditions to produce lightweight ceramsite with uniform pores. Wang et al.²⁹ used low-silicon iron tailings as the main raw material and optimised the preparation process of ceramsite to produce lightweight porous ceramsite with a porosity of 43.46%, which improved the utilisation efficiency of tailings. Li et al.³⁰ produced low-density, low-water-absorption lightweight ceramsite by controlling the content of waste glass. Xiao et al.³¹ used sludge as the main raw material, silica fume as a conditioning agent, and phosphorus tailings as a pore-forming agent to produce lightweight ceramsite with an expansion rate of up to 133.7%. Long et al.³² discovered that by modifying the proportion of fly ash (FA) to clay, it was feasible to produce the ceramsite with a compressive strength of 32.90 MPa at a FA doping level of 40 wt.%. Zhao et al.³³ used iron tailings as the main raw material, studied the raw material ratio, and produced super-strong ceramsite with a strength of 56.5 MPa. The above research fully demonstrates the feasibility of preparing ceramsite from various industrial solid wastes as raw materials. Meanwhile, by rationally adjusting the raw material combination, the expansion performance and mechanical properties of ceramsite can be effectively regulated, providing inspiration for the study of using LZT to prepare ceramsite in this article. Shao et al.³⁴ investigated the effects of sintering processes on ceramsite properties using FA and sludge as main raw materials, revealing that the sintering temperature is a crucial factor determining ceramsite performance. Liu et al.³⁵ demonstrated that ceramsite sintering necessitates a transition from a powder state to a polycrystalline state. They also established that the transition proceeds more smoothly and the resulting ceramsite is of superior quality when the sintering temperature approaches the material's melting point. Additionally, extensive research has indicated that the phase composition and pore structure of ceramsite are intrinsic factors affecting its performance.^36–38 To sum up, the relevant research on the preparation of ceramsite from industrial solid waste has been quite extensive and mature, but at present, there are few studies on the preparation of ceramsite from LZT tailings. The SiO₂ content in LZT is relatively high, while the Al₂O₃ content is very low. A high SiO₂/Al₂O₃ ratio is not conducive to the sintering of ceramsite. The SiO₂ content of FA is similar to that of LZT, but the Al₂O₃ content is much higher than that of LZT. Adding FA can effectively supplement the aluminum content of LZT, thereby promoting the sintering of ceramsite. This study aims to achieve the resource utilisation and harmless treatment of LZT. It uses LZT and FA as raw material to prepare ceramsite through high-temperature baking, thereby realising the comprehensive utilisation of LZT resources and providing a new technical route for the harmless and sustainable solidification of LZT heavy metals.

It can be reasonably deduced that high-temperature sintering of ceramsite represents an effective approach to the utilisation of LZT resources. By optimising the material composition and sintering process, the properties of LZT-based ceramsite can be effectively regulated. The present study will employ LZT as the primary raw material, FA as the sintering auxiliary material, and a small amount of coal powder as the fluxing agent, to produce ceramsite. The research will investigate the impact of sintering temperature and LZT content on the properties of ceramsite, including apparent density, water absorption and cylinder compressive strength. Furthermore, a comprehensive examination of the intrinsic factors and mechanisms influencing performance of the prepared ceramsite will be conducted. X-ray diffraction (XRD) and scanning electron microscope (SEM)-energy dispersive spectroscopy (EDS) will be employed to analyse the phase composition and microstructure of the ceramsite in order to reveal their constitutive relationship with the properties. The heavy metal curing effect of the prepared ceramsite was evaluated. This study is expected to achieve large-scale, high value-added utilisation of lead–zinc tailings, the heavy metals in LZT are effectively solidified, alleviating the problem of insufficient raw materials for traditional ceramsite production and providing a green and high-performance material.

Materials and methods

Materials

The LZT was obtained from the tailings pond of the Hunan Shuikoushan Nonferrous Metals Group Co., Ltd, while the FA was sourced from the Henan Gongyi Yuanheng Water Purification Material Factory. The coal powder used was a commercially available standard coal powder with a fixed carbon content of 57.7%, the chemical composition is shown in Tables 1 and 2. The chemical compositions of LZT and FA were determined by X-ray fluorescence (XRF), and the results are presented in Table 1. The LZT exhibits a relatively high SiO₂ content (63.50%), while the Al₂O₃ content is only 5.86 wt.%. The substantial SiO₂/Al₂O₃ ratio is an impediment to the sintering of ceramsite.³⁹ The SiO₂ content of FA is comparable to that of LZT, whereas its Al₂O₃ content is markedly higher (32.79%), effectively supplementing the aluminum content present in LZT. Heavy metal leaching toxicity tests were conducted on LZT, and the leaching concentrations and standard limits are shown in Table 3. It can be seen that the leaching concentration of Pb in the LZT adopted in this experiment exceeds the standard. The mineral compositions of LZT and FA were determined by XRD, and the results are displayed in Figure 1. The principal mineral constituents of LZT are quartz (SiO₂) and calcite (CaCO₃), with a minor occurrence of pyrite (FeS₂). The XRD spectra of FA demonstrates the presence of diffraction peaks associated with quartz (SiO₂) and mullite (3Al₂O₃-2SiO₂), in conjunction with a diffuse peak originating from the glassy phase.

Figure 1.

XRD spectra: (a) lead–zinc tailings and (b) fly ash. XRD: X-ray diffraction.

Table 1.

Chemical compositions of lead–zinc tailings and fly ash (wt.%).

Composition	SiO₂	CaO	Al₂O₃	Fe₂O₃	K₂O	SO₃	Other	LOI
Lead–zinc tailings	63.50	12.86	5.86	3.09	1.19	4.56	8.94	12.56
Fly ash	55.71	2.66	32.79	4.43	1.54	0.65	2.22	2.75

LOI: loss on ignition.

Table 2.

Chemical compositions of coal powder (wt.%).

Composition	SiO₂	CaO	Al₂O₃	Fe₂O₃	MgO	SO₃	Other
Coal powder	51.34	7.61	28.93	6.23	1.65	0.87	3.37

Table 3.

Leaching concentrations of raw LZT and ceramsite and their critical limits.

Heavy metal	Pb (mg/L)	Zn (mg/L)	Mn (mg/L)
Standards of the United States Environmental Protection Agency（US EPA 1311）	5.0	–	–
Identification standards for hazardous wastes （GB5085.3-2007）	5.0	100	–
The leaching concentration of LZT	22.78	52.64	36.5
The leaching concentration of ceramsite	2.14	10.46	2.21

LZT: lead–zinc tailings.

Preparation process of ceramsite

In this study, the mass ratio of LZT to FA was set at 7:3, 6.5:3.5, 6:4, 5.5:4.5 and 5:5 to ensure that the usage rate of LZT was above 50%. A dosage of 3 wt.% of coal powder was employed as the fluxing agent. The sintering temperatures were controlled at 1050°C, 1100°C, 1150°C, 1200°C and 1250°C. The specific preparation parameters for the ceramsite are outlined in Table 4.

Table 4.

Preparation parameters of ceramsite.

Sample	LZT to FA ratio	Sintering temperature	Preheating temperature	Coal powder	Preheating time	Sintering time
T1	5:5	1050°C	450°C	3%	30min	60min
T2		1100°C
T3		1150°C
T4		1200°C
T5		1250°C
C1	7:3	1250°C
C2	6.5:3.5
C3	6:4
C4	5.5:4.5
C5	5:5

LZT: lead–zinc tailings.

The preparation process of the ceramsite is as follows: firstly, the FA and LZT samples were subjected to a drying process until a constant weight was achieved. Subsequently, they were pulverised to a particle size of less than 0.074 mm. A base of 2 kg of the dried mixed material was established, after which the LZT and FA were proportioned according to the preset ratios. Additionally, 3 wt.% coal powder was added to the mixture. The mixture was blended thoroughly using a powder mixing machine. Subsequently, water was selected as the binder, and spherical pellets were formed in a pot-type pelletizer with a diameter of 40 cm, a tilt angle of 30°, and a rotation speed of 25 r/min. Following pelletisation, the pellets were removed and sieved to obtain qualified pellets of 10 to 15 mm. These qualified pellets were then dried to constant weight in an oven at 105 ± 5°C. Eventually, the dried pellets were placed in a crucible and subjected to sintering in a muffle furnace. The sintering regime was as follows: initially, the temperature was increased from room temperature to 450°C at a rate of 10°C/min and maintained for 30 minutes; then, the heating rate was reduced to 5°C/min, and the temperature was increased to the sintering temperature and maintained for 60 minutes; finally, the ceramsite samples were allowed to cool naturally to room temperature with the furnace. The preparation of ceramsite is illustrated in Figure 2.

Figure 2.

Diagram of preparation process of ceramsite.

Methods

Physical properties of ceramsite

The properties testing of ceramsite is primarily conducted in accordance with the guidelines established in GB/T 17431.2—2010, entitled ‘Lightweight Aggregates and Their Testing Methods: Part 2: Testing Methods for Lightweight Aggregates’. This study is chiefly concerned with the apparent density, 1-hour water absorption and cylinder compressive strength of ceramsite.

The following steps are undertaken to measure the apparent density and water absorption: 300 g of ceramsite are weighed and immersed in a beaker filled with clean water for 1 hour. After immersion, the ceramsite is removed and placed on a 2.36 mm sieve to drain for 1 minute. Thereafter, the ceramsite is rolled back and forth on a wrung-out wet towel 10 times in order to remove surface moisture, whereupon its mass is duly recorded. The ceramsite is then added to a 1000 mL graduated cylinder containing 500 mL of clean water. The water level in the cylinder is subsequently recorded. The formula for calculating the apparent density is given by: $\begin{matrix} ρ = 1000 m / (v_{2} - v_{1}) \end{matrix}$ (1)where $ρ$ is the apparent density of the ceramsite (kg/m³); m the mass of the dried ceramsite (g); and $v_{1}$ and $v_{2}$ the water volumes in the graduated cylinder before and after adding the ceramsite (mL), respectively.

The formula for calculating the water absorption is: $\begin{matrix} ω = 100 (m_{1} - m) / m \end{matrix}$ (2)where $ω$ is the water absorption (%) and $m_{1}$ and m the masses of the ceramsite after immersion and when dry, respectively (g).

The cylinder compressive strength is assessed through the following procedure: ceramsite is placed into a standard compression cylinder until it exceeds the cylinder's rim, vibrated to compact, and then levelled off (or topped up) to the rim. Subsequently, a guiding cylinder and a pressing die are installed, ensuring that the scale line on the pressing die is aligned with the upper edge of the guiding cylinder. The compression cylinder is positioned at the centre of the lower platen of a compression machine, and the load is applied at a uniform rate of 300 N/s. Upon the pressing die reaching a depth of 20 mm, the pressure value is recorded. The formula for calculating the cylinder compressive strength is as follows: $\begin{matrix} f_{} = (p_{1} + p_{2}) / s \end{matrix}$ (3)where $f_{}$ is the cylinder compressive strength (MPa); $p_{1}$ and $p_{2}$ the pressure recorded by the compression machine and the mass of the pressing die (N), respectively; and S the pressurised surface area (mm²).

The calculation formula for the inflation index is as follows: $k = \frac{v_{2} - v_{1}}{v_{1}} \times 100 %$ (4)where k is the expansion index; $v_{1}$ the volume of ceramsite before expansion(cm³) and $v_{2}$ the volume of expanded ceramsite (cm³).

Thermogtavimetric-differential thermal analysis (TG-DTA)

One gram of sample was weighed and ground in a mortar for 7 minutes; and 10 ± 1 mg of ground sample was taken to a crucible and the thermal behaviour of the sample was tested by increasing the temperature from 35°C to 1300°C using a comprehensive thermal analyser in an air atmosphere at a heating rate of 10°C/min.

X-ray diffraction (XRD)

The ceramsite is pulverised and ground in a mortar for approximately 20 minutes until a powdery texture is achieved. The sample is detected by an XRD, with a scan range of 5° to 70°, scan rate of 2°/min, Cu target radiation source, current of 40 mA and voltage of 40 kV. The results are analysed using HighScore software.

Scanning electron microscope-energy dispersive spectroscopy (SEM-EDS)

The ceramsite were broken into small pieces of about 1 cm, and a gold film was sprayed on the surface of these small pieces. The microscopic morphology and elemental distribution of these fragments were observed by SEM-EDS.

Heavy metal leaching toxicity

The leaching toxicity test was conducted using the Toxicity Characteristic Leaching Procedure (TCLP) method, referring to the standard of the United States Environmental Protection Agency (US EPA 1311). Ceramsite particles with a particle size no larger than 9.5 mm were selected. Then the extraction agent was weighted and mixed with the crushed samples at a liquid–solid ratio of 20:1. After that, the crushed samples and the extraction agent were placed in a rotary stirring device and rotated at 25°C for 18 hours. Finally, the extraction agent was filtered through a 0.45 µm pore size filter membrane to determine the concentrations of Pb, Zn and Mn in the clarified liquid.

Results and discussion

Thermal behaviour

The thermal behaviour of the samples (LZT, FA and C1–C5) without added coal powder at elevated temperatures was characterised using TG-DTA technology, with the results presented in Figure 3. The TG curves indicate that the rate of weight loss for each sample increases with rising temperature. The weight loss rate of FA is only 2.75% between 100°C and 1190°C, with the majority of the loss occurring around 750°C, which is chiefly attributed to the decomposition of a small amount of carbonate in FA. The TG curve and the reaction peaks in the DTA demonstrate that between 100°C and 1210°C, the total weight loss of LZT is 12.84% and this weight loss process can be divided into four stages: The weight loss of the first stage (approximately 100°C to 460°C) was mainly caused by the loss of adsorbed and crystalline water, coupled with the combustion of trace organic matter. The weight loss in the second stage (approximately 460°C–580°C) is attributed to the decomposition of pyrite (FeS₂), generating FeS and S vapour. The third stage (approximately 580°C–880°C) exhibits a substantial weight loss of 9.69%. This predominantly attributable to the decomposition of calcite producing a considerable quantity of CO₂ gas. This process favours the volume expansion of the ceramsite and the formation of pore structures. Additionally, the FeS generated in the previous stage reacts with O₂ in the air to form FeO and SO₂ gas, contributing to further mass loss.⁴⁰ The fourth stage (approximately 1100°C–1210°C) is associated with weight loss caused by the reduction of FeO.⁴¹ The weight loss trends in the mixed samples (C1–C5) are comparable to those of the LZT sample. As the LZT content increases from 50% to 70%, the weight loss rate of the mixed samples increases from 9.69% to 11.39%.

Figure 3.

TG-DTA results: (a) TG and (b) DTA.

The TG curve shows that the final weight loss reaction of the samples occurs between 1100°C and 1210°C. However, the DTA curve displays an endothermic peak after 1210°C. The endothermic peaks for FA and LZT are observed at 1290°C and 1235°C, respectively, while the endothermic peaks for the mixed samples is between those of FA and LZT (1241°C–1264°C). This phenomenon indicates the formation of a liquid phase in the mixed samples within the temperature range of 1100°C to 1264°C. Considering the economic benefits as well as the addition of coal powder, the sintering temperature of the lead–zinc tailings-based ceramsite was set at 1050°C to 1250°C.

Surface morphology of ceramsite

Figure 4 displays the surface morphology of the ceramsite samples prior to and following the sintering process. With increasing sintering temperature, the volume of the ceramsite expands gradually, the colour deepens, and the surface roughness diminishes. The pellets that have not undergone the sintering process exhibit a silver–grey hue and a rough surface. At a sintering temperature of 1050°C, the ceramsite exhibits a pale yellow in colour, accompanied by a degree of shrinkage, and retained a rough surface. Some ceramsite display evidence of cracking, suggesting that the liquid phase at this temperature is constrained and exhibits low viscosity, which is unfavourable for ceramsite sintering. As the temperature rises above 1100°C, the ceramsite begins to expand. By 1150°C, the volume of the ceramsite exceeds that of the pellets and the glassy substance is found on the surface. At 1200°C, the ceramsite undergoes a colour change from yellow to black, accompanied by the formation of a glassy layer on the surface. At 1250°C, the ceramsite reaches its maximum volume, assuming a black colour, and the surface develops a smooth vitreous coating. At this stage, slight adhesion is observed between some ceramsite and between the ceramsite and the container, which suggests that the sintering temperature should not exceed 1250°C.

Figure 4.

Surface morphology of ceramsite samples before and after sintering.

Physical properties of the ceramsite

As shown in Figure 5(a), the sintering temperature exerts a pronounced impact on the apparent density and water absorption of LZT-based ceramsite. At 1050°C, the apparent density of ceramsite increases in comparison to the unsintered state, which may be attributed to the sintering reactions between particles.⁴² As the sintering temperature rises above 1100°C, metal oxides begin to melt and form viscous liquid phases. These liquid phases have the potential to encapsulate gases generated from the decomposition of carbonates and the reduction of FeO, causing the expansion of the ceramsite and the decrease in apparent density.⁴³ As shown in Figures 6 and 7, the coefficient of expansion of ceramsite increases from 9.2% to 49.7% and the apparent density of ceramsite decreases from 1597.5 to 1165.8 kg/m³ with the rise of sintering temperature from 1100°C to 1250°C. The most considerable decline in apparent density occurs when the sintering temperature is elevated from 1100°C to 1150°C. The impact of sintering temperature on water absorption exhibits a similar trend to that observed on apparent density. From 1050°C to 1250°C, the water absorption of ceramsite decreases from 17.5% to 3.1%. It is noteworthy that the greatest reduction in water absorption occurs when the sintering temperature is increased from 1100°C to 1150°C. As observed in Figure 4, an increase in sintering temperature results in a gradual densification of the ceramsite surface, effectively preventing water intrusion. Upon exceeding 1150°C, a glassy layer begins to form on the ceramsite surface, which serves to further reduce the water absorption. A negative correlation exists between the water absorption rate and the degree of sintering.⁴⁴ Consequently, at 1250°C, the ceramsite exhibits the highest degree of sintering. Figure 5(b) illustrates the cylinder compressive strength of the LZT-based ceramsite at different sintering temperatures. The sintering temperature has a marked effect on the cylinder compressive strength of ceramsite. At lower temperatures (1050°C and 1100°C), due to the limited generation of liquid phases within the ceramsite, the majority of particles remain connected by physical contact without forming a load-bearing structure. This results in the lower cylinder compressive strength. As the sintering temperature exceeds 1150°C, the cylinder compressive strength of ceramsite displays a precipitous increase, by more than 2 MPa for every 50°C increase in temperature. This phenomenon can be attributed to the fact that at elevated temperatures (above 1150°C), the quantity of molten material within the system grows, the contact area between particles expands, and the connection mode transitions from physical contact to atomic bonding.³⁵

Figure 5.

Effect of sintering temperature on physical properties of ceramsite: (a) apparent density and water absorption and (b) cylinder compressive strength.

Figure 6.

Effect of lead–zinc tailings content on physical properties of ceramsite: (a) apparent density and water absorption and (b) cylinder compressive strength.

Figure 7.

The expansion index of ceramsite at different sintering temperatures.

The effects of LZT content on the apparent density and water absorption of ceramsite are depicted in Figure 6(a). As the LZT content increases, the apparent density of ceramsite initially exhibits a decline, followed by an uptick. As the LZT content rises from 50% to 65%, the apparent density decreases from 1159.7 to 1045.2 kg/m³. This reduction is primarily due to the increased LZT content reducing the temperature at which the liquid phase forms (as shown in Figure 3(b)). At the same temperature, the increased amount of liquid phase and reduced viscosity lead to further expansion of the ceramsite.⁴⁵ When the LZT content increases to 70%, the apparent density increases instead. This phenomenon can be attributed to the production of an excess of low-viscosity liquid phase, which results in the collapse of the internal structure of the ceramite.³⁰ The escape of gas leads to the formation of interconnected pores both internally and on the surface of the ceramsite, which is the direct cause of the increase in water absorption.⁴⁶ As the LZT content rises from 50% to 65%, the rate of gas production in the samples increases (as shown in Figure 3(a)), resulting in higher water absorption. Upon reaching 70% LZT content, a notable rise in water absorption is observed, which is attributed to the collapse of the internal ceramsite structure and the outward escape of gas encapsulated by the liquid phase. The effect of LZT content on the cylinder compressive strength of ceramsite is depicted in Figure 6(b). As the LZT content increases from 50% to 70%, the cylinder compressive strength of ceramsite decreases significantly from 8.5 to 2.1 MPa. There are two main reasons for this phenomenon: (1) As the LZT content increases, the SiO₂-Al₂O₃ ratio rises, leading to a higher amount of liquid phase during sintering. Under the influence of gas pressure, the ceramsite expands, causing a transition from a dense internal structure to a porous structure.⁴⁷ (2) With the increase in LZT content, the amount of mullite provided by FA gradually decreases. Since mullite plays a crucial role in the load-bearing capacity of ceramsite, its reduction leads to a decrease in cylinder compressive strength.^31,48,49

XRD analysis

Given that the chemical composition of the samples (C1–C5) is primarily comprised of CaO, SiO₂ and Al₂O₃, it is possible to infer the phase transition behaviour during the ceramsite sintering process by utilising the CaO-SiO₂-Al₂O₃ system. The compositional range of the samples falls within the region delineated by the yellow circle in Figure 8, which represents the junction of the quartz (SiO₂), mullite (3Al₂O₃·2SiO₂) and anorthite (CaAl₂Si₂O₈) phases. The following inferences can be drawn: (1) The samples exhibit a melting point below 1500°C. (2) The final ceramsite product is composed of the aforementioned three crystalline phases. Inference 1 is corroborated by the TG-DTA analysis results (see Figure 3), which demonstrate the presence of a liquid phase prior to 1264°C. The discrepancy between the actual and theoretical temperatures may be attributed to the reaction of alkali metal salts or alkali metal oxides in the samples with SiO₂ to form low-melting-point silicates.⁵⁰ The XRD analysis results of the ceramsite samples (see Figure 9) corroborate inference 2.

Figure 8.

Phase diagram of CaO-SiO₂-Al₂O₃ ternary system.

Figure 9.

XRD patterns of ceramsite under different sintering temperatures and material ratios: (a) unsintered and 1050°C; (b) 1100°C to 1250°C and (c) sintered at 1150°C, C1 to C5. XRD: X-ray diffraction.

Figure 9 displays the XRD spectra of ceramsite at varying sintering temperatures with 50% LZT content. The results demonstrate that the crystalline phases in the ceramsite undergo decomposition and reconstruction following high-temperature sintering. The crystalline phases present in the unsintered sample include quartz (SiO₂), mullite (3Al₂O₃·2SiO₂), calcite (CaCO₃) and pyrite (FeS₂), which align with the mineral composition analysis of the raw materials. The crystalline phases identified in the sintered samples are quartz (SiO₂), mullite (3Al₂O₃·2SiO₂) and anorthite (CaAl₂Si₂O₈). As indicated in Figure 9(a), the diffraction peaks of calcite (CaCO₃) and pyrite (FeS₂) are no longer visible after sintering at 1050°C, which is in accordance with results of the TG analysis. The intensity of the diffraction peak of quartz (SiO₂) undergoes a notable decline, while the diffraction peak of anorthite (CaAl₂Si₂O₈) emerges. As shown in Figure 9(b), an increase in sintering temperature from 1100°C to 1250°C is accompanied by a continuous decrease in quartz peak intensity and a gradual increase in anorthite peak intensity. This phenomenon can be attributed to the fact that, as the sintering temperature increases, the Al₂O₃ content within the liquid phase rises, creating a greater number of opportunities for Al³⁺ to gain entry into the silica network and form tetrahedral AlO₄⁴⁺, which in turn results in the replacement of some SiO₄⁴⁺ groups. As the +4 charge required for tetrahedral structures is substituted by the +3 valence of Al, Ca²⁺ combines with AlO₄⁴⁺ groups to maintain electrical neutrality, resulting in the increased peak intensity of anorthite.⁴⁷ As shown in Figure 9(c), as FA content increases, the content of Al₂O₃ in the liquid phase rises. It can be observed that the peak intensity of Quartz continuously decreases, while that of Anorthite gradually increases.

SEM-EDS analysis

The internal microstructure of the ceramsite at different sintering temperatures with LZT content of 50% was observed using SEM, and the results are shown in Figure 10. At a sintering temperature of 1050°C, the internal structure of the ceramsite was characterised by a loose configuration, with the presence of unmelted intact particles. The aforementioned particles exhibited point contacts, which resulted in high water absorption and low cylinder compressive strength values for ceramites. As the sintering temperature was increased to 1100°C and 1150°C, it was observed that the particles were largely or completely fused. However, gaps were still visible within the ceramsite, indicating that the viscosity of the molten substance was relatively high and its fluidity poor. At 1150°C, the formation of pores was observed, with an average diameter of approximately 3 μm. As the sintering temperature increased to 1200°C, the internal structure of the ceramsite became smoother, accompanied by a notable rise in porosity.^51–53 The majority of the observed pores were elliptical in shape, with a diameter range of 2 to 5 μm, and an average diameter of approximately 8 μm. As the temperature was increased to 1250°C, the isolated pores gradually enlarged and merged, developing into irregular large pores with an average diameter of 15 μm. Furthermore, minor collapse zones were observed within the ceramsite, indicating that the optimal sintering temperature should be within the range of 1200°C to 1250°C. This conclusion is in accordance with the findings of the analysis of the ceramsite's physical properties.

Figure 10.

SEM images of ceramsite fired at different sintering temperatures. SEM: scanning electron microscope.

The results of the scanning electron microscope with SEM-EDS analysis for sample T3 are presented in Figure 11. Area 1 is relatively rough and represents the portion of the ceramsite that has not fully melted, with the main elements being O, Si and Al. In contrast, area 2 is a smooth plane with the primary elements being O and Si. This indicates that the presence of Al elements raises the melting point of the sample, which explains the leftward shift of the liquid phase peak with increasing LZT content, as shown in Figure 3(b).

Figure 11.

SEM and EDS analysis of T3. SEM: scanning electron microscope.

Analysis of heavy metal solidification

Figure 12 shows the changes in heavy metal leaching concentration of ceramsite prepared at different sintering temperatures. It can be seen from the graph that after LZT is sintered at high temperature to make ceramsite, the heavy metal leaching concentration of ceramsite decreases significantly compared with the LZT raw material, and the heavy metal concentration shows a downward trend with the increase of sintering temperature. The leaching concentrations of Pb, Zn and Mn in the ceramsite all fall below the standard limits, as shown in Table 3, achieving the effect of heavy metal solidification. This indicates that the sintering of LZT into ceramsite is of great significance for the harmless treatment of heavy metals.

Figure 12.

Leaching concentration of heavy metals with different sintering temperature.

The mechanism of ceramsite formation

The specific mechanism of mineral formation during the sintering process of ceramsite was further clarified through the TG-DTA curve (Figure 3). As can be seen from the figure, the raw embryo of ceramsite loses weight at 100°C to 460°C, with a mass decrease of 1.75%. This is mainly due to the shedding of free water and crystalline water in the raw materials and the combustion of organic matter. The weight loss between 460°C and 580°C is attributed to the decomposition reaction of pyrite (FeS₂) in this temperature range, which generates FeS and gaseous S. Obvious exothermic and endothermic peaks occur in the range of 750–800°C. The appearance of the exothermic peak is due to the fact that CaCO₃ begins to decompose in large quantities in this temperature range, generating CO₂, and at the same time, the mass in the TG curve decreases significantly. The appearance of the endothermic peak is due to the slightly increased heating rate during the melting process, which led to some impurity oxides not being fully melted. When conducting differential thermal analysis, some residual impurities continued to melt, thus generating the endothermic peak. Around 1250°C, a distinct endothermic peak emerged, which was due to the formation of anorthite and mullite during the sintering process of the ceramsite.

In conclusion, this study has revealed two parallel paths of the preparation mechanism of ceramsite. The first path is that CaO, SiO₂ and Al₂O₃ in LZT and FA react at high temperatures to form strength mineral phases such as anorthite and mullite, which are also the main mineral phases for enhancing the strength of ceramsite. The second path: during the sintering process of ceramsite, the carbon dioxide, gaseous S and water vapour released cause the ceramsite to expand, forming a pore structure, which endows the ceramsite with a relatively high expansion index.

Conclusions

In this study, ceramsite was prepared by high-temperature sintering using lead–zinc tailings as the main raw material and fly ash as the supplementary aluminum source. The impact of sintering temperature and lead–zinc tailings content on the properties of the ceramsite was examined. The intrinsic factors affecting ceramsite performance were analysed using techniques such as TG-DTA, XRD, and SEM. The conclusions reached are as follows:

When the sintering temperature is 1250°C and the lead–zinc tailings content is 50%, the prepared ceramsite exhibit lower apparent density and water absorption, higher cylinder compressive strength, and well-developed internal pore structure, all of which exceed the performance requirements specified by the Chinese standard (GB/T 17431.1–2010). Therefore, high-temperature sintering for the preparation of ceramsite is an effective approach for the high-value utilization of lead–zinc tailings.

Sintering temperature influences the melting amount and viscosity of substances such as SiO₂ and Al₂O₃, thereby affecting the overall performance of lead–zinc tailing-based ceramsite. As the sintering temperature increases from 1050°C to 1250°C, the apparent density and water absorption of the lead–zinc tailing-based ceramsite decrease, while the cylinder compressive strength increases.

The decomposition of calcite in the lead-zinc tailings at high temperatures generates CO₂, which effectively promotes the expansion of the ceramsite. However, when the lead–zinc tailing content reaches 70%, an excessive amount of molten material can lead to internal collapse of the ceramsite, causing gas escape, which in turn results in an increase in the apparent density of the ceramsite.

The phases of lead–zinc tailing-based ceramsite are composed of quartz, mullite, and anorthite. As the sintering temperature increases, quartz and mullite tend to transform into anorthite. Mullite provides substantial support for the cylinder compressive strength of the ceramsite.

The leaching concentration of heavy metals in LZT-based ceramiste is lower than 5.0 mg/L, meeting the limit value of standard (US EPA 1311). This means that the sintering of LZT into ceramsite is of great significance for the solidification of heavy metals.

Footnotes

Acknowledgement

The supplementary experimental works regard to the expansion property and the leaching toxicity of ceramsites and the revision of original manuscript by Ouyang Jie are greatly appreciated.

Author contributions

CZ were involved in conceptualisation, methodology, writing-review & editing and funding acquisition; XB in investigation, data curation, visualisation and writing – original draft; WH in investigation; ZC in supervision, funding acquisition and writing – review & editing; QL in visualisation, formal analysis and funding acquisition; and DJ in investigation.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the the Hunan Province Natural Science Foundation of China, the Hunan Provincial Key Research and Development Projects, the National Key Research and Development Project of China, the Doctoral Research Start-up Fund of University of South China, the Key Projects of Hunan Provincial Department of Education (grant number 2023JJ50133, 2024TT7433, 2025JK2091, 51678286, 703_210XQD025, 21A0286).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

Data will be made available on request.

References

Maruthupandian

Chaliasou

Kanellopoulos

. Recycling mine tailings as precursors for cementitious binders – methods, challenges and future outlook. Constr Build Mater 2021; 312: 125333.

Liu

Zhang

Wang

, et al. Pb-Zn metal resources situation and suggestion for Pb-Zn metals industry development in China. China Min Mag 2015; 24: 6–9.

Wang

. Resource utilization of lead-zinc tailings. Min Res Dev 2017; 38: 12–15. 10.39696/j.issn.1000-6532.2017.01.003

Wen

Tang

. Research progress on comprehensive utilization of lead-zinc tailings as resources. Nonferrous Metals (Mining Sect) 2024; 76: 17–22.

Wen

Zhou

Zheng

, et al. Geochemical properties, heavy metals and soil microbial community during revegetation process in a production Pb-Zn tailings. J Hazard Mater 2023; 463: 132809–132809.

Lei

Yan

Chen

, et al. Recovery of metals from the roasted lead-zinc tailings by magnetizing roasting followed by magnetic separation. J Cleaner Prod 2017; 158: 73–80.

Zhao

Wen

Feng

, et al. Study on recovering iron from the lead-zinc flotation tailings. Nonferrous Metals (Mineral Proc Sect) 2014; 66: 56–59. 10.3969/j.issn.1671-9492.014

Cui

Hou

Sun

, et al. Application research on the recovery of barite from lead-zinc tailings. Multipurpose Util Miner Res 2011; 32: 47–49. 10.3969/j.issn.1000-6532.2011.03.012

Chen

Zhang

, et al. Recycling lead–zinc tailings for cemented paste backfill and stabilisation of excessive metal. Minerals 2019; 9: 710–710.

10.

. Dry stacking gene of tailings. Min Equi 2015; 5: 26–27+14. cnki:sun:kyzb.0.2015-07-011

11.

Yin

Lin

. Research status and prospects for the utilization of lead–zinc tailings as building materials. Buildings 2023; 13: 150.

12.

Shi

Xue

. Progress on domestic investigation of building materials conversion from lead-zinc tailing. Bull Chinese Ceram Soc 2018; 37: 508–512.

13.

Yang

. Experimental study on the influence of shale ceramsite size on strength and thermal properties of concrete. Concrete 2020; 11: 52–56. cnki:sun:hltf.0.2020-11-016

14.

Zhang

Wang

Mao

. Study and development of composite clay ceramsite. Sci Technol Innov 2020; 24: 17–19. cnki:sun:hlkx.0.2020-03-009

15.

Lin

Song

. Preparation and mechanism of high-intensity ceramsite with Zambia copper tailing. J Ceram 2016; 37: 404–408. 10.13957/j.cnki.tcxb.2016.04.015

16.

Deng

Yang

Gao

. Preparation of lightweight and high-strength fly ash ceramsite and its concrete performance. J Qingdao Univ Technol 2009; 30: 70–74.

17.

Weng

Zhang

Cao

. Characteristics and sintering technology of haydite made of sewage sludge. J Zhejiang Univ (Eng Sci) 2011; 45: 1877–1883. cnki:sun:zdzc.0.2011-10-032

18.

Tao

. Experimental study on preparation of high strength haydite from gangue. Non-Metallic Mines 2010; 33: 20–22. cnki:sun:fjsk.0.2010-03-007

19.

Wang

Yang

, et al. The formation of porous light ceramsite using Yellow River sediment and its application in concrete masonry production. Case Stud Constr Mater 2022; 17: e01340.

20.

Wang

Jian

, et al. Preparation of water storage ceramsite via dredged silt and biomass waste: pore formation, water purification and application. Sci Total Environ 2023; 859: 160314–160314.

21.

Marella

Tao

, et al. The application of ceramsite ecological floating bed in aquaculture: its effects on water quality, phytoplankton, bacteria and fish production. Water Sci Technol: J Int Assoc Water Pollut Res 2018; 77: 2742–2750.

22.

Bayer Ozturk

Pekkan

Tasci

, et al. The effect of granulated lead–zinc slag on aesthetic and microstructural properties of single-fired wall tile glazes. J Aust Ceram Soc 2020; 56: 609–617.

23.

Bayer Ozturk

Engur

. Exploring the potential of slag waste generated after zinc metal recovery in geopolymer mortar production. Estuscience 2024; 25: 308–319.

24.

Bao

Zhang

, et al. Preparation of non-sintered sewage sludge based ceramsite by alkali-thermal activation and hydration mechanism. Ceram Int 2022; 48: 31606–31613.

25.

Chen

Wang

Hao

. Preparation process of gangue ceramisite based on the orthogonal optimization method. Bull Chinese Ceram Soc 2015; 34: 841–845. 10.16552/j.cnki.issn1001-1625.2015.03.033

26.

Qin

Feng

Fang

. Effect of SiO₂ on structural strength and porosity of sludge sintered ceramsite. New Build Mater 2019; 46: 117–122. cnki:sun:xxjz.0.2019-09-028

27.

Zhang

Chen

. Preparation of porous ceramsite with sludge and gangue. J Ceram 2015; 36: 58–63. 10.13957/j.cnki.tcxb.2015.01.013

28.

Sun

Zhang

Liu

. Preparation of ceramsite filter materials for water treatment by tailings with high intensity magnetic separation from red mud. Bull Chinese Ceram Soc 2016; 35: 2270–2275. 10.16552/j.cnki.issn1001-1625.2016.07.050

29.

Wang

Chu

. Study on preparation and performances of building ceramisite with low silica iron tailings. New Building Mater 2016; 43: 36–38+51. https://cnki:sun:xxjz.0.2016-02-013

30.

, et al. Utilization of municipal sewage sludge and waste glass powder in production of lightweight aggregates. Constr Build Mater 2020; 256: 119413.

31.

Xiao

Fan

Zhou

, et al. Preparation of ultra-lightweight ceramsite from waste materials: using phosphate tailings as pore-forming agent. Ceram Int 2024; 50: 15218–15229.

32.

Long

Yang

, et al. Preparation of high-strength ceramsite from municipal solid waste incineration fly ash and clay based on CaO-SiO2-Al2O3 system. Constr Build Mater 2023; 368: 130492.

33.

Zhao

Wang

Zhou

. Study on preparation and properties of super-strong ceramsite based on iron tailings. Non-Metallic Mines 2023; 46: 92–94+99. cnki:sun:fjsk.0.2023-04-021

34.

Shao

Zhang

, et al. Preparation of municipal solid waste incineration fly ash-based ceramsite and its mechanisms of heavy metal immobilization. Waste Manage 2022; 143: 54–60.

35.

Liu

Guo

Pan

, et al. Preparation of ceramsite from low-silicon red mud (LSRM): effects of Si–Al ratio and sintering temperature. Ceram Int 2023; 49: 34191–34204.

36.

Liu

Lin

Liu

, et al. Phase evolution, pore morphology and microstructure of glass ceramic foams derived from tailings wastes. Ceram Int 2018; 44: 14393–14400.

37.

Wang

Qin

okeke

, et al. Revealing the intrinsic sintering mechanism of high-strength ceramsite from CFB fly ash: focus on the role of CaO. Ceram Int 2024; 50: 24281–24292.

38.

Zhan

Wang

, et al. Roasting mechanism of lightweight low-aluminum–silicon ceramisite derived from municipal solid waste incineration fly ash and electrolytic manganese residue. Waste Manage 2022; 153: 264–274.

39.

Liu

Wang

, et al. Effect of SiO₂ and Al₂O₃ on characteristics of lightweight aggregate made from sewage sludge and river sediment. Ceram Int 2018; 44: 4313–4319.

40.

Huang

Chang

, et al. Production of lightweight aggregates from mining residues, heavy metal sludge, and incinerator fly ash. J Hazard Mater 2007; 144: 52–58.

41.

Pei

Pan

, et al. Preparation and characterization of ultra-lightweight ceramsite using non-expanded clay and waste sawdust. Constr Build Mater 2022; 346: 128410.

42.

. Influence of sintering temperature on performance of red mud lightweight ceramsite. Adv Mat Res 2013; 662: 323–326.

43.

Bernhardt

Justnes

Tellesbø

, et al. The effect of additives on the properties of lightweight aggregates produced from clay. Cem Concr Compos 2014; 53: 233–238.

44.

Wei

Lin

, et al. Preparation of low water-sorption lightweight aggregates from harbor sediment added with waste glass. Mar Pollut Bull 2011; 63: 135–140.

45.

Dondi

Cappelletti

D’Amore

, et al. Lightweight aggregates from waste materials: reappraisal of expansion behavior and prediction schemes for bloating. Constr Build Mater 2016; 127: 394–409.

46.

Liu

Guan

, et al. Low-cost and environment-friendly ceramic foams made from lead–zinc mine tailings and red mud: foaming mechanism, physical, mechanical and chemical properties. Ceram Int 2016; 42: 1733–1739.

47.

Liu

. Effect of the ratio of components on the characteristics of lightweight aggregate made from sewage sludge and river sediment. Process Saf Environ Prot 2017; 105: 109–116.

48.

Zahide

İsa

. Mechanical and microstructural characteristics of geopolymer mortars at high temperatures produced with ceramic sanitaryware waste. Ceram Int 2022; 48: 12932–12944.

49.

Luo

Guo

Liu

, et al. Preparation of ceramsite from lead-zinc tailings and coal gangue: physical properties and solidification of heavy metals. Constr Build Mater 2023; 368: 130426.

50.

Zhang

Chen

Lin

, et al. Kinetics and fusion characteristics of municipal solid waste incineration fly ash during thermal treatment. Fuel 2020; 279: 118410.

51.

Chenxi

Yuping

, et al. Preparation of high-strength ceramsite from coal gangue, fly ash, and steel slag. RSC Adv 2025; 15: 4332–4341.

52.

Shuo

Haihong

Yuxiang

, et al. Preparation of lightweight ceramsite from solid waste using SiC as a foaming agent. Materials (Basel) 2022; 15: 325.

53.

Yanlong

Mingyuan

Quan

, et al. Study on the properties and heavy metal solidification characteristics of sintered ceramsites composed of magnesite tailings, sewage sludge, and coal gangue. Int J Environ Res Public Health 2022; 19: 11128.