Next Article in Journal
Evaluation of the Alkali–Silica Reaction Potential of Korean Aggregates: Experimental Insights and Mitigation Strategies for Concrete Durability
Previous Article in Journal
Four-Dimensional-Printed Woven Metamaterials for Vibration Reduction and Energy Absorption in Aircraft Landing Gear
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Materials
Volume 18
Issue 14
10.3390/ma18143372
materials-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Preparation of Foamed Ceramic from Cr Slag and MSWI Fly Ash and Its Cr Leaching Inhibition

by
Hesong Li
,
Cheng Liu
,
Yikun Tang
and
Shilin Zhao
*
Hunan Engineering Research Center of Clean and Low-Carbon Energy Technology, School of Energy Science and Engineering, Central South University, Changsha 410083, China
*
Author to whom correspondence should be addressed.
Materials 2025, 18(14), 3372; https://doi.org/10.3390/ma18143372
Submission received: 13 June 2025 / Revised: 11 July 2025 / Accepted: 14 July 2025 / Published: 18 July 2025
(This article belongs to the Special Issue Functional Materials for Separation and Recovery of Rare Earth Elements and Heavy Metals)
Browse Figures
Versions Notes

Abstract

The sustainable utilization of solid waste is crucial for environmental protection. This work investigates the fabrication of foamed ceramics from Cr slag and municipal solid waste incineration (MSWI) fly ash, focusing on the effects of three inhibitors—NH2SO3H, ZnO·TiO2, and (NH4)2HPO4—on material properties and Cr leaching behavior. Experimental analysis, chemical thermodynamic calculations, and material characterization were all employed. Results show that the prepared foamed ceramics meet the JG/T 511-2017 standard for building materials, exhibiting excellent physical properties but significant Cr leaching. Among the inhibitors, (NH4)2HPO4 with a molar ratio of n(P)/n(Cr) = 1 shows the best performance, achieving a bulk density of 205 kg/m3, compressive strength of 0.850 MPa, Cr leaching concentration of 188 μg/L, and a 70.0% of Cr leaching inhibition rate. The improvement is attributed to the AlPO4 formation that enhancing the strength, and Ca2P2O7 that stabilizing Cr during sintering. This work provides a feasible method for the safe resource utilization of Cr-containing waste.

1. Introduction

1.1. Research Background

Cr slag and municipal solid waste incineration fly ash (MSWI-FA) are two major types of hazardous solid waste produced in large quantities in China. As the world’s largest producer and consumer of sodium dichromate [1], China generates approximately 2.50 × 105 tons of Cr slag annually, with a historical accumulation of nearly six million tons [2,3,4]. Cr slag contains Cr(VI), a highly mobile, toxic, and carcinogenic substance that poses significant threats to human health and the environment [5,6]. Meanwhile, MSWI-FA, a byproduct of the widely adopted waste-to-energy technology with over 10 million tons of that generated annually, presents similar environmental concerns [2,3,4]. Therefore, developing effective treatment and utilization methods for these wastes has become an urgent and enormous challenge.

1.2. Current Research Status and Limitations

In recent years, studies on the potential of utilizing Cr slag and MSWI-FA as raw materials for resource recovery have been explored. Liu et al. [7] prepared ceramic with a water absorption of 0.08% and a flexural strength of 124.78 MPa based on the SiO2-Al2O3-CaO-MgO (8 wt.%) phase diagram using different types of MSWI-FA and other solid wastes. Hou et al. [8] produced tiles using Cr slag and copper smelting waste slag, achieving a compressive strength of 127.2 MPa and a Cr (VI) leaching concentration of 3.31 mg/L, with a Fe/Cr ratio of 1.5, a sintering temperature of 1200 °C, and a holding time of 30 min. Hou et al. [9] utilized Cr slag as the main raw material to fabricate black ceramic tiles, achieving a compressive strength of 162.61 MPa and a Cr(VI) leaching concentration of 0.97 mg/L under optimal conditions (Fe/Cr/Mn molar ratio of 3:2:1, sintering at 1150 °C for 45 min). These studies achieve the resource conversion and utilization of waste Cr slag.
Among various solid waste-derived products, foamed ceramics have attracted increasing attention due to their excellent properties such as low density, thermal insulation, corrosion and oxidation resistance. It is often referred to as “solid waste gold”, which has emerged as a major research hotspot in the field of solid waste utilization, Ge at al. [10] developed closed-cell foam ceramic by using Cr slag and coal bottom ash as raw materials. With 30 wt.% Cr slag, the ceramic achieved a bulk density of 0.191 g/cm3, a compressive strength of 1.3 MPa, a pore diameter of 0.73 mm, and a Cr leaching concentration of 0.004 mg/L. Xu et al. [11] adopted Cr slag as the primary raw material for synthesizing microcellular foamed ceramics, with copper oxide and manganese oxide incorporated as additives. It facilitated the reuse of Cr slag, and made the foamed ceramics with enhanced thermal insulation properties. Liu et al. [12] utilized Fe-Cr slag, industrial alumina, and silica powder as raw materials to produce porous cordierite ceramic with a flexural strength of 47.26 ± 1.01 MPa, a coefficient of thermal expansion of 3.5 × 10−6/°C, and a Cr leaching rate of 35.0 mg/kg.
However, high-temperature treatment often induces oxidation of Cr(III) to Cr(VI), significantly increasing Cr leaching risk. Additionally, alkali metals and alkaline earth metals, such as Ca, Na, and K, are the key factors leading to Cr leaching during Cr slag incineration or other thermal treatments [13,14]. Gao et al. [15] used sulfates (NH2SO3H, NH4HSO4, NaHSO4) to mitigate Cr leaching during tannery sludge incineration, which found the reaction between sulfate and CaO effectively inhibited Cr leaching, with NH2SO3H showing the most significant effect. Nearly all Ca was bound to the CaSO4 phase, which retained the ability to oxidize Cr2O3 to CaCrO4. Zhao et al. [16] found adding ZnO or TiO2 reduced Cr leaching during the heat treatment of Cr-containing tannery sludge, where addition of ZnO·TiO2 produced a notably inhibitory effect. Mao et al. [17] demonstrated (NH4)2HPO4 could reduce Cr through chemical thermodynamic calculation, which achieved a detoxification rate of over 97% after heat treatment. It can be found that NH2SO3H, ZnO·TiO2, and (NH4)2HPO4 are effective in inhibiting Cr leaching for Cr-containing solid wastes during thermal treatment. However, there are no reports on the effects of these inhibitors on the physical and Cr leaching properties of foamed ceramic prepared from Cr slag and MSWI-FA (CS-MSWI-FA-FC). The optimal inhibitor and its ratio, and related inhibition mechanism need to be explored.

1.3. Main Content and Objectives

In this study, the influence of inhibitor types (NH2SO3H, ZnO·TiO2, and (NH4)2HPO4) and additional amount on the physical properties and Cr leaching behavior of CS-MSWI-FA-FC was systematically investigated. Combining the thermal treatment experiments, acetic acid buffer leaching tests (HJ/T 300-2007 [18]), characterization techniques (scanning electron microscopy (SEM), X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD)), and chemical thermodynamic simulation (HSC Chemistry software, https://www.hsc-chemistry.com/), the influencing mechanism on the physical and Cr leaching of the CS-MSWI-FA-FC were systematically explored. It aims to obtain the optimal type and additional amount of inhibitors, and to reveal the inhibition mechanism, which hopes to provide technical support and theoretical guidance for the synergistic resource utilization of Cr slag and MSWI fly ash in the preparation of foamed ceramics.

2. Materials and Methods

2.1. Preparation Process of CS-MSWI-FA-FC

The preparing process of CS-MSWI-FA-FC is illustrated in Figure 1. The procedure involved weighing specified proportions of solid wastes, including Cr slag and MSWI-FA, etc., adding MgO, Fe2O3, and other materials to achieve the desired compositional balance for the foamed ceramic system. To study Cr leaching, three inhibitors including NH2SO3H, ZnO·TiO2, and (NH4)2HPO4 were introduced at comparable molar ratios. A small amount of SiC was added as a foaming agent. The solid–liquid volume ratio was set to 1:2, and the mixture was ball-milled for 1.50 h to ensure thorough homogenization. After drying, the material was sieved through a 60.0-mesh screen, poured into a high-temperature-resistant crucible, and placed in a muffle furnace. The temperature setting program was 25.0–600 °C for 1.00 h; 600–950 °C for 80 min; 950–1.20 × 103 °C for 90.0 min; and then maintained at 1.20 × 103 °C for 1.00 h and cooled naturally to room temperature. The final product was the CS-MSWI-FA-FC.

2.2. Toxicity Leaching Experiment

The leaching characteristics of Cr from the foamed ceramic were assessed using the acetic acid buffer solution method specified in HJ/T 300-2007 [18]. An acetic acid solution with a pH of 2.64 ± 0.05 was employed as the leaching solution, with a solid–liquid ratio of 1:20 (g/mL). The mixture was then oscillated on a flip-type oscillating device at 30 ± 2 r/min for 18.0 h. The Cr concentration in the leachate was determined using inductively coupled plasma optical emission spectrometer (ICP-OES, SPECTRO BLUE SOP, Kleve, Germany).
To further evaluate the inhibition effect of Cr leaching, the inhibition rate of Cr, denoted as P (%), was introduced, in addition to the Cr leaching concentration, as shown in Equation (1).
P = C x − C 0 C 0 × 100 %
where C0 denotes the leaching concentration of Cr in the raw sample (μg/L), and Cx (where x represented NH2SO3H, ZnO·TiO2, or (NH4)2HPO4) denotes the leaching concentration of Cr in the sample after adding the corresponding inhibitor (μg/L).

2.3. Chemical Thermodynamic Calculation

The chemical thermodynamics software named HSC Chemistry was used to calculate the reactions within the system. Based on the XRF results of Cr slag, MSWI-FA, and other solid wastes, the main components of the reaction inputs for the thermodynamic calculations were Na2O, MgO, Al2O3, SiO2, CaO, K2O, Fe2O3, Cr2O3, as well as ZnO and TiO2. Since (NH4)2HPO4 and NH2SO3H decomposed easily at high temperature into equivalent amount of H3PO4 and H2SO4, they were used as replacement inputs in the calculation. Thermodynamic data for some reaction products were supplemented from the Refs. [19,20,21]. The possible Cr forms in the reaction products are shown in Table 1. Considering the reactants and reaction conditions, the possible products of elemental P in the (NH4)2HPO4 inhibitor included Ca(PO3)2, Ca2P2O7 [22], Na3PO4, and K3PO4 [23], etc. The possible products of elemental S in the NH2SO3H inhibitor were Cr2(SO4)3, CaSO4, SO2(g) [15], etc.

2.4. Testing and Characterization of Foamed Ceramic Properties

The prepared ceramic samples were analyzed by XRD (Rigaku SmartLab SE, Tokyo, Japan) to determine their main mineral phases under Cu Kα radiation, with a 2θ range of 5–90° and a step size of 0.02°. The bulk density of the product was measured using the Archimedes drainage method, with water as the buoyant medium. The sample mass was determined using an analytical balance with a sensitivity of 10−4 g [27]. The SEM (TESCAN MIRA LMS, Shanghai, China) was used to observe the surface morphology of the foamed ceramic at various scales. The compressive strength and compressive modulus of the foamed ceramic were measured by using an electronic universal testing machine (MTS System CMT6103, Shanghai, China) with an entrance force of 1.00 N and a tensile rate of 10.0 mm/min. According to the performance requirements for foamed ceramic insulation board IIw without an axial surface in JG/T 511-2017 Foamed Ceramic Thermal Insulation Board in Building [28], the bulk density should be between 180 and 230 kg/m3, and the compressive strength should be ≥0.500 MPa.

3. Results and Discussion

3.1. General Information of Cr Slag, MSWI-FA, and Other Solid Wastes

The XRF results of Cr slag, MSWI-FA, and other solid wastes are presented in Table 2. Cr slag primarily consists of SiO2, Al2O3, and Fe2O3, with Cr2O3 as the main Cr form. To enhance the representativeness of the experimental results, pure Cr2O3 is added to the Cr slag, increasing its total content to 10.0 wt.% [29]. The MSWI-FA, dried after washing, mainly contains CaO, SiO2, and small amounts of MgO and Fe2O3. Kaolin, feldspar, and fluorspar tailings are predominantly composed of SiO2. In addition, kaolin and feldspar contain high contents of Na2O and K2O, which play a fluxing role in the preparation of foamed ceramic.

3.2. Physical and Cr Leaching Properties of CS-MSWI-FA-FC Without Inhibitors

The CS-MSWI-FA-FC (without inhibitors) and their corresponding SEM images are shown in Figure 2. It shows the CS-MSWI-FA-FC exhibit a well-developed pore structure without inhibitor. The compressive strength, bulk density, and compressive modulus are 1.74 MPa, 209 kg/m3, and 27.7 MPa, respectively, which meet the performance requirements of JG/T 511-2017 building material standard [28]. This indicates their potential to be used in the production of foamed ceramic thermal insulation boards for buildings.
The microstructure of the prepared foamed ceramic is shown in Figure 2b, which reveals the pore distribution and size are uniform, with a pore diameter of approximately 1 mm. This uniformity ensures both the mechanical strength and thermal insulation performance. SiC powder is commonly used as a blowing agent in the foamed ceramic producing. At temperatures above 900 °C, SiC typically undergoes an oxidation reaction, as shown in Equation (2). The toxic leaching of Cr from the prepared foamed ceramic is 628 μg/L, which complies with the GB 16889-2008 Standard for Pollution Control on the Landfill Site of Municipal Solid Waste [30]. However, there is still potential for further reduction in its leaching.
SiC + O 2 → Si O 2 + C O 2 ↑ + CO ↑

3.3. Effects of Inhibitor Types on the Physical and Cr Leaching Properties

Three inhibitors, namely, NH2SO3H, ZnO·TiO2, and (NH4)2HPO4, are selected based on previous studies. The ratios for the inhibitors are as follows: NH2SO3H: n(S)/n(Cr) = 2 [15]; ZnO·TiO2: n(Zn)/n(Cr) = 1, n(Ti)/n(Ca) = 0.8 [31]; and (NH4)2HPO4: n(P)/n(Cr) = 2 [17]. The prepared samples after adding inhibitors and their corresponding SEM images are shown in Figure 3. It shows the pore distribution in the foamed ceramic prepared with NH2SO3H and ZnO·TiO2 is not uniform, with some pores exceeding 5 mm. While other smaller pores, approximately 1 mm, are also present. This may be attributed to the significant difference between foaming temperature and formation temperature of the molten phase with the addition of these inhibitors. As a result, the molten phase in the sample is insufficient to encapsulate the gas generated by the blowing agent, leading to the small bubbles rising and eventually merging into larger bubbles [32]. In contrast, the overall pore distribution in the samples prepared with (NH4)2HPO4 is uniform, with small pores ranging from 0.2 to 1 mm.
The physical properties of the CS-MSWI-FA-FC prepared with different inhibitors are shown in Figure 4a. The structural properties of the ceramic material deteriorate after adding NH2SO3H, resulting in a bulk density of 150 kg/m3 and a compressive strength of only 0.170 MPa. This may be attributed to two factors: (1) the pores within the ceramic structure become uneven, with an increase in pore diameter and a reduction in pore wall thickness; and (2) the sulfate formed during the process decomposes at high temperature, producing SO2, which increases internal gas pressure and promotes bubble coalescence [33]. The compression modulus remains relatively unchanged at 28.7 MPa, indicating that the addition of NH2SO3H has minimal effect on the material’s plasticity. After the addition of ZnO·TiO2, the bulk density, compressive strength, and compressive modulus decrease to 57.7 kg/m3, 2.00 × 10−2 MPa, and 2.52 MPa, respectively, resulting in the inability to form a ceramic material with sufficient structural strength. This may be because the addition of ZnO·TiO2 changes the main components of the system, resulting in large differences in foaming temperature and formation temperature of the molten phase, and the bubbles become larger [10]. After adding (NH4)2HPO4, the volume density is 377 kg/m3, the pores are smaller, and the volume density is nearly doubled compared to the raw sample. The compressive strength and compression modulus are significantly increased to 3.70 MPa and 73.5 MPa, respectively. This shows the structural strength of the samples is stronger and the deformation degree is smaller when squeezed after adding (NH4)2HPO4. This may be because the generated phosphate has a higher density and also has a higher compressive strength at room temperature [34,35]. In summary, the addition of NH2SO3H and ZnO·TiO2 results in incomplete sample forming, uneven bubbles and poor material properties. However, the sample obtained after adding (NH4)2HPO4 is more in line with the standard of foamed ceramic insulation board for construction in terms of material performance indicators, the pores are more uniform and the compressive strength is more advantageous.
The Cr leaching characteristics of the CS-MSWI-FA-FC prepared with different inhibitors are shown in Figure 4b. Compared to the foamed ceramic without adding inhibitors (raw sample), all three inhibitors exhibit better inhibition on Cr leaching, with the Cr leaching concentration below 100 μg/L and inhibition rate exceeding 80.0%. Among the inhibitors, NH2SO3H demonstrates the least inhibition effects, with a Cr leaching concentration of 95.5 μg/L and an inhibition rate of 84.7%. (NH4)2HPO4 has the best inhibitory effect on the CS-MSWI-FA-FC, with a Cr leaching concentration of 63.3 μg/L and an inhibition rate of nearly 89.9%. Considering the physical properties of foamed ceramic and the inhibitory effect on Cr leaching, (NH4)2HPO4 is the best inhibitor.

3.4. Effect of (NH4)2HPO4 Amount on the Physical and Cr Leaching Properties

The CS-MSWI-FA-FC prepared with different additional amounts of (NH4)2HPO4 and their corresponding SEM results are shown in Figure 5. With the increase in added (NH4)2HPO4 quantity, the volume of the samples gradually decreases. The color of the foamed ceramic gradually changes from brown to black (the white substance on the surface is the high-temperature resistant aluminum silicate fiber between the material and the crucible during sintering). The SEM results show that the pore size reaches 2 mm and the pore distribution is uniform when n(P)/n(Cr) is 1. When n(P)/n(Cr) ≥ 1.5, the pore size decreases are all less than 1 mm, and the small pores on the pore wall are larger. This may be due to the difference between the melting phase and foaming temperature, which causes the pores to not merge during the foaming process.
The physical properties of the samples with different additional amounts of (NH4)2HPO4 are shown in Figure 6a. Compared with the raw sample, the volume density gradually increases with the increase in quantity of additional (NH4)2HPO4, indicating the pores gradually decrease, which is consistent with Figure 5. Compared with the raw sample and n(P)/n(Cr) = 1, the compressive strength decreases slightly from 1.74 MPa with no addition to 0.850 MPa, and the compression modulus also decreases from 27.7 MPa to 5.95 MPa accordingly. The material properties at this condition still meet the JG/T511-2017 standard [28]. When n(P)/n(Cr) further increases, the compressive strength and compression modulus firstly increase and then decrease, reaching a peak value of 3.70 MPa and 73.5 MPa, respectively, at n(P)/n(Cr) = 2. The addition of (NH4)2HPO4 may cause the oxygen bonds in the SiO2 structure to dissociate after sintering at 1.20 × 103 °C, which in turn leads to the transformation of a small amount of quartz to cristobalite. It will lead to volume expansion, density reduction, and deterioration of mechanical properties of SiO2 [35]. However, the addition of (NH4)2HPO4 may lead to the formation of phosphates during the preparation, which is beneficial to improving its compressive strength and compression modulus. Therefore, the volcano shape changes in compressive strength and compression modulus with the increase in n(P)/n(Cr) is the game result between these two reasons.
Cr leaching characteristics of CS-MSWI-FA-FC prepared with different additional amounts of (NH4)2HPO4 is shown in Figure 6b. As the additional amount of (NH4)2HPO4 increases, the Cr leaching concentration gradually decreases and the inhibition rate gradually increases. When n(P)/n(Cr) = 1, the Cr leaching concentration and inhibition rate is 188 μg/L, and 70.0%, respectively, while they are 35.6 μg/L, and 94.3% at n(P)/n(Cr) = 2.5. As the amount of (NH4)2HPO4 increases further, the Cr leaching does not change significantly. The addition of (NH4)2HPO4 has a good inhibitory effect on Cr leaching. Considering the physical properties and the Cr leaching characteristics of the CS-MSWI-FA-FC, the optimal additional amount of (NH4)2HPO4 is n(P)/n(Cr) = 1.

3.5. Mechanism Analysis

The chemical thermodynamic calculation results at different temperatures in the sintering system of the raw sample and that with three inhibitors are shown in Figure 7. It shows that the Cr in the raw sample is easily oxidized to form Cr (VI) (CaCrO4, K2CrO4, Na2CrO4) at ≤1000 °C [17]. However, a small amount of Cr (III) (CaCr2O4) is generated under a high temperature, which is prone to Cr leaching [26]. In this work, the Cr forms are mainly Cr2O3 and MgCr2O4 when the temperature rises, which are not easy to leach with good Cr stability. For CaCrO4, the inhibition of Cr (VI) occurs at a lower temperature, while the Cr form does not change significantly at high temperature after adding NH2SO3H and (NH4)2HPO4. After adding ZnO·TiO2, the Cr morphology changes significantly at temperature above 400 °C, which generates a spinel phase (ZnCr2O4) with poor Cr leaching. It is consistent with the experimental results of Zhao et al. [16] and Yang et al. [31].
The XRD results of the raw sample before and after sintering, and that after sintering with different inhibitors, are shown in Figure 8. The main components are SiO2, CaCr2O4, MgCr2O4, CaSiO3, CaMgSiO6, after sintering of the raw sample, where CaCr2O4 is the main Cr leaching substance. MgO can combine with Cr to form spinel phase MgCr2O4, which is consistent with the chemical thermodynamic calculation results. It has a certain Cr fixation effect [36]. According to the ionic theory of slag, the diffusion degree between the reactants is determined by the activity of ion transfer. The more alkaline substances there are, the more O2− ions are ionized, and the greater the probability of Cr being oxidized. In this work, CaO and SiO2 are alkaline and acidic substances, respectively. Therefore, SiO2 mainly combines with CaO to form CaSiO3 to prevent it from participating in the oxidation reaction of Cr in the raw sample.
In addition to the main substances of the raw sample after sintering, the respective addition of NH2SO3H, ZnO·TiO2, and (NH4)2HPO4 also increases some substances, such as CaSO4, ZnCr2O4, CaTiSiO5, CaTiO3, Ca2P2O7, and AlPO4. NH2SO3H and (NH4)2HPO4 can combine with alkaline oxides to generate CaSO4 and Ca2P2O7. The acidity of H2SO4 is stronger than that of H3PO4. NH2SO3H has a stronger ability to inhibit Cr leaching in theory. However, the experimental result shows (NH4)2HPO4 has a better inhibitory effect. It may be that CaSO4 decomposes at 1.20 × 103 °C to produce SO2 and produces free Ca2+, which will further form CaCr2O4 that is easy to leach [17]. Meanwhile, the generated SO2 may be the main reason for the pore increased in the sample [33]. ZnO, as a divalent metal oxide, can combine with trivalent metal oxides such as Cr2O3 to form a spinel phase. TiO2 is an amphoteric metal oxide, so it can also combine with CaO to form CaTiO3, which thereby fixes Ca and inhibits Cr leaching. Judging from the inhibition effect of Cr leaching, (NH4)2HPO4 has stronger reducing properties compared to SiO2 in the raw sample [17]. Moreover, the liquid H3PO4 decomposed from (NH4)2HPO4 increases the contact area between CaCrO4 and phosphate and promotes Ca2P2O7 formation. Therefore, increasing the additional amount of (NH4)2HPO4 can significantly inhibit the Cr leaching [22]. From the physical properties of CS-MSWI-FA-FC, adding (NH4)2HPO4 can significantly affect the pore structure, where the generated phosphate AlPO4 will significantly improve the compressive strength that meets the building material standard. The influence mechanism of three inhibitors on the physical and Cr leaching properties of CS-MSWI-FA-FC is summarized in Figure 9.

4. Conclusions

The foamed ceramic prepared by Cr slag and MSWI-FA exhibit an (excellent) compressive strength, bulk density and compression modulus of 1.74 MPa, 209 kg/m3 and 27.7 MPa, respectively, meeting the JG/T 511-2017 building materials standard. However, the Cr leaching is relatively high at 628 μg/L, which is mainly caused form CaCr2O4. (NH4)2HPO4 has the best inhibitory effect on Cr leaching of the CS-MSWI-FA-FC among the three inhibitors. As the additional amount of (NH4)2HPO4 increases, the volume density gradually increases, the compressive strength and compression modulus have volcano-shaped changes, and the inhibition effect of Cr leaching gradually improves. Considering the physical requirements of foamed ceramic and Cr leaching, (NH4)2HPO4 and n(P)/n(Cr) = 1 are the best inhibitor type and additional amount. The difference between the temperatures of the molten phase and the foaming affects the foaming process and the final physical properties. (NH4)2HPO4 will promote the formation of phosphate AlPO4, etc., thereby improving the compressive strength. It also has a stronger reducibility and combines with Ca to form Ca2P2O7 during sintering to inhibit Cr leaching.
In future work, it is necessary to consider the long-term Cr leaching stability of foamed ceramics, and evaluate their environmental impact and economic feasibility from a life cycle perspective, so as to provide guidance for the industrial application of the resource utilization technology of foamed ceramics prepared with Cr slag and MSWI fly ash.

Author Contributions

H.L.: writing—original draft, conceptualization, project administration, supervision. C.L.: investigation, formal analysis, data curation, software, writing—review and editing. Y.T.: formal analysis. S.Z.: conceptualization, writing—review and editing, supervision, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Science and Technology Innovation Program of Hunan Province [No. 2023RC3043], the Changsha Major Science and Technology Project of Unveiling and Leading [No. kq2301007], and the High Performance Computing Center of Central South University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

Thank you to the editor and anonymous reviewers for providing valuable suggestions and opinions to improve the quality of this article.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Wang, X.; Li, L.; Yan, X.; Meng, X.; Chen, Y. Processes of chromium (VI) migration and transformation in chromate production site: A case study from the middle of China. Chemosphere 2020, 257, 127282. [Google Scholar] [CrossRef]
  2. Huang, B.; Gan, M.; Ji, Z.; Fan, X.; Zhang, D.; Chen, X.; Sun, Z.; Huang, X.; Fan, Y. Recent progress on the thermal treatment and resource utilization technologies of municipal waste incineration fly ash: A review. Process Saf. Environ. Prot. 2022, 159, 547–565. [Google Scholar] [CrossRef]
  3. Li, X.; Sun, Y.; Li, W.; Nie, Y.; Wang, F.; Bian, R.; Wang, H.; Wang, Y.; Gong, Z.; Lu, J.; et al. Solidification/stabilization pre-treatment coupled with landfill disposal of heavy metals in MSWI fly ash in China: A systematic review. J. Hazard. Mater. 2024, 478, 135479. [Google Scholar] [CrossRef] [PubMed]
  4. Luo, Z.; Zhi, T.; Liu, L.; Mi, J.; Zhang, M.; Tian, C.; Si, Z.; Liu, X.; Mu, Y. Solidification/stabilization of chromium slag in red mud-based geopolymer. Constr. Build. Mater. 2022, 316, 125813. [Google Scholar] [CrossRef]
  5. Aharchaou, I.; Py, J.S.; Cambier, S.; Loizeau, J.L.; Cornelis, G.; Rousselle, P.; Battaglia, E.; Vignati, D.A.L. Chromium hazard and risk assessment: New insights from a detailed speciation study in a standard test medium. Environ. Toxicol. Chem. 2018, 37, 983–992. [Google Scholar] [CrossRef] [PubMed]
  6. Den Braver-Sewradj, S.P.; Van Benthem, J.; Staal, Y.C.M.; Ezendam, J.; Piersma, A.H.; Hessel, E.V.S. Occupational exposure to hexavalent chromium. Part II. Hazard assessment of carcinogenic effects. Regul. Toxicol. Pharmacol. 2021, 126, 105045. [Google Scholar] [CrossRef]
  7. Liu, Y.; Tang, J.; Li, M.; Zhang, Q.; Zhang, W. Production of high-strength eco-conscious ceramics exclusively from municipal solid waste. Ceram. Int. 2024, 50, 47851–47863. [Google Scholar] [CrossRef]
  8. Hou, Y.; Yu, J.; Li, Z.; Hai, Y.; Xu, J.; Zheng, D. Preparation of black ceramic tiles with chromium slag and copper smelting waste slag. Metals 2023, 13, 537. [Google Scholar] [CrossRef]
  9. Hou, Y.; Yu, J.; Zheng, D.; Xu, J.; Ma, G.; Khojiev, S.; Kadirov, N. Preparation and chromatic performance of black ceramic tiles from chromium slag, copper slag and silicon manganese slag. J. Ceram. Process. Res. 2025, 26, 139. [Google Scholar]
  10. Ge, X.; Zhou, M.; Wang, H.; Liu, Z.; Wu, H.; Chen, X. Preparation and characterization of ceramic foams from chromium slag and coal bottom ash. Ceram. Int. 2018, 44, 11888–11891. [Google Scholar] [CrossRef]
  11. Xu, X.; Gao, W.; Bai, X. A Novel Foamed Ceramic and Its Preparation Method. CN Patent CN118812240A, 22 October 2024. [Google Scholar]
  12. Liu, C.; Liu, L.; Tan, K.; Zhang, L.; Tang, K.; Shi, X. Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag. Ceram. Int. 2016, 42, 734–742. [Google Scholar] [CrossRef]
  13. Luo, J.; Sheng, B.; Shi, Q. A review on the migration and transformation of heavy metals influence by alkali/alkaline earth metals during combustion. J. Fuel Chem. Technol. 2020, 48, 1318–1326. [Google Scholar] [CrossRef]
  14. Zhao, G.; Tian, C.; Wu, P.; Zhang, X.; Wang, Z.; Chen, X.; Xiong, Z.; Zhao, Y.; Zhang, J. Effect of oxide interactions on chromium speciation transformation during simulated municipal solid waste incineration. J. Environ. Sci. 2024, 142, 11–20. [Google Scholar] [CrossRef]
  15. Gao, B.; Jiang, H.; Chen, H.; Peng, M.; Zhang, W.; Hu, L.; Mao, L. The introduction of sulfates to suppress Cr(III) oxidation during incineration of tannery sludge and reduce leachability toxicity of incineration residue. J. Clean. Prod. 2023, 382, 135272. [Google Scholar] [CrossRef]
  16. Zhao, R.; Guo, Y.; Huang, X.; Qian, J.; Wu, Y.; Li, Z.; Lu, S. Synergistic immobilization of chromium in tannery sludge by ZnO and TiO2 and the oxidation mechanism of Cr(III) under alkaline in high temperature. J. Hazard. Mater. 2022, 424, 127290. [Google Scholar] [CrossRef]
  17. Mao, L.; Su, P.; Huang, B.; Zhang, W. Detoxification of solid waste containing Cr(VI) with phosphate by thermal treatment. Chem. Eng. J. 2017, 314, 114–122. [Google Scholar] [CrossRef]
  18. HJ/T 300-2007; Solid Waste-Extraction Procedure for Leaching Toxicity-Acetic Acid Buffer Solution Method. Ministry of Ecology and Environment of the People’s Republic of China: Beijing, China, 2007.
  19. Degterov, S.; Pelton, A.D. Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CrO-Cr2O3-SiO2-CaO system. Metall. Mater. Trans. B-Proc. Metall. Mater. Proc. Sci. 1997, 28, 235–242. [Google Scholar] [CrossRef]
  20. Klemme, S.; van Miltenburg, J.C.; Javorsky, P.; Wastin, F. Thermodynamic properties of uvarovite garnet (Ca3Cr2Si3O12). Am. Mineral. 2005, 90, 663–666. [Google Scholar] [CrossRef]
  21. Ziemniak, S.E.; Anovitz, L.M.; Castelli, R.A.; Porter, W.D. Thermodynamics of Cr2O3, FeCr2O4, ZnCr2O4, and CoCr2O4. J. Chem. Thermodyn. 2007, 39, 1474–1492. [Google Scholar] [CrossRef]
  22. Pan, Y.; Shao, H.J. Preparation of β-TCP with high thermal stability by solid reaction route. J. Mater. Sci. 2003, 38, 1049–1056. [Google Scholar] [CrossRef]
  23. Yagmurlu, B.; Zhang, W.; Heikkilä, M.J.; Koivula, R.T.; Friedrich, B. Solid-state conversion of scandium phosphate into scandium oxide with sodium compounds. Ind. Eng. Chem. Res. 2019, 58, 14609–14620. [Google Scholar] [CrossRef]
  24. Pérez, F.J.; Hierro, M.P.; Pedraza, F.; Gómez, C.; Carpintero, M.C.; Trilleros, J.A. Kinetic studies of Cr and Al deposition using CVD-FBR on different metallic substrates. Surf. Coat. Tech. 1999, 122, 281–289. [Google Scholar] [CrossRef]
  25. Zhao, S.; Zhang, S.; Liang, X.; Li, J.; Liu, C.; Ji, F.; Sun, Z. Transformation and environmental chemical characteristics of hazardous trace elements in an 800 t/d waste incineration thermal power plant. Sci. Total Environ. 2024, 918, 170693. [Google Scholar] [CrossRef] [PubMed]
  26. Hu, H.; Shi, M.; Yang, Y.; Liu, H.; Xu, M.; Shen, J.; Yao, H. Further insight into the formation and oxidation of CaCr2O4 during solid fuel combustion. Environ. Sci. Tech. 2018, 52, 2385–2391. [Google Scholar] [CrossRef] [PubMed]
  27. Li, X.; Li, S.; Yin, Z.; Shi, W.; Tao, M.; Liu, W.; Gao, Z.; Ma, C. Foam-gelcasting preparation and properties of high-strength mullite porous ceramics. Ceram. Int. 2023, 49, 6873–6879. [Google Scholar] [CrossRef]
  28. JG/T 511-2017; Foamed Ceramic Thermal Insulation Board in Building. Ministry of Housing and Urban-Rural Development of the People’s Republic of China: Beijing, China, 2017.
  29. Li, C.; Zhang, P.; Zeng, L.; Yu, L.; Li, D. Study on preparation of glass-ceramics from municipal solid waste incineration (MSWI) fly ash and chromium slag. J. Build. Eng. 2023, 68, 106080. [Google Scholar] [CrossRef]
  30. GB 16889-2008; Standard for Pollution Control on the Landfill Site of Municipal Solid Waste. Ministry of Ecology and Environment: Beijing, China, 2008.
  31. Yang, Y.; Zhu, C.; Wang, H.; Hao, Y.; Yan, L.; Zhang, Z.; Yan, H.; Chen, X.; Ma, H. Synergistic immobilization of Cr from real tannery sludge by formation of spinel phases with TiO2 and ZnO. J. Environ. Chem. Eng. 2022, 10, 108679. [Google Scholar] [CrossRef]
  32. König, J.; Petersen, R.R.; Yue, Y.; Suvorov, D. Gas-releasing reactions in foam-glass formation using carbon and MnxOy as the foaming agents. Ceram. Int. 2017, 43, 4638–4646. [Google Scholar] [CrossRef]
  33. Zhou, M.; Ge, X.; Wang, H.; Chen, L.; Chen, X. Effect of the CaO content and decomposition of calcium-containing minerals on properties and microstructure of ceramic foams from fly ash. Ceram. Int. 2017, 43, 9451–9457. [Google Scholar] [CrossRef]
  34. Jia, C.Y.; Ding, R.; Liu, J.C.; Liu, A.F.; Teng, D.X. Effects of Al(OH)3 on the structure and properties of foam ceramics cemented by phosphate. Adv. Mater. Res. 2015, 1120–1121, 21–26. [Google Scholar] [CrossRef]
  35. Khamkongkaeo, A.; Bootchanont, A.; Klysubun, W.; Amonpattaratkit, P.; Boonchuduang, T.; Tuchinda, N.; Phetrattanarangsi, T.; Nuntawong, N.; Kuimalee, S.; Lohwongwatana, B. Effect of phosphate compound on physical and mechanical properties of SiO2 ceramic. Ceram. Int. 2019, 45, 1356–1362. [Google Scholar] [CrossRef]
  36. Kavouras, P.; Pantazopoulou, E.; Varitis, S.; Vourlias, G.; Chrissafis, K.; Dimitrakopulos, G.P.; Mitrakas, M.; Zouboulis, A.I.; Karakostas, T.; Xenidis, A. Incineration of tannery sludge under oxic and anoxic conditions: Study of chromium speciation. J. Hazard. Mater. 2015, 283, 672–679. [Google Scholar] [CrossRef]
Materials 18 03372 g001
Figure 1. Preparation process of CS-MSWI-FA-FC.
Figure 1. Preparation process of CS-MSWI-FA-FC.
Materials 18 03372 g001
Materials 18 03372 g002
Figure 2. CS-MSWI-FA-FC (without inhibitors): (a) physical properties; (b) SEM images.
Figure 2. CS-MSWI-FA-FC (without inhibitors): (a) physical properties; (b) SEM images.
Materials 18 03372 g002
Materials 18 03372 g003
Figure 3. CS-MSWI-FA-FC after adding inhibitors and their corresponding SEM images. (a) NH2SO3H; (b) ZnO·TiO2; (c) (NH4)2HPO4.
Figure 3. CS-MSWI-FA-FC after adding inhibitors and their corresponding SEM images. (a) NH2SO3H; (b) ZnO·TiO2; (c) (NH4)2HPO4.
Materials 18 03372 g003
Materials 18 03372 g004
Figure 4. Physical properties and Cr leaching characteristics of CS-MSWI-FA-FC prepared with different inhibitors: (a) physical properties; (b) Cr leaching properties.
Figure 4. Physical properties and Cr leaching characteristics of CS-MSWI-FA-FC prepared with different inhibitors: (a) physical properties; (b) Cr leaching properties.
Materials 18 03372 g004
Materials 18 03372 g005
Figure 5. CS-MSWI-FA-FC prepared with different additional amounts of (NH4)2HPO4 and their corresponding SEM results. (a) n(P)/n(Cr) = 1; (b) n(P)/n(Cr) = 1.5; (c) n(P)/n(Cr) = 2; (d) n(P)/n(Cr) = 2.5; (e) n(P)/n(Cr) = 3.
Figure 5. CS-MSWI-FA-FC prepared with different additional amounts of (NH4)2HPO4 and their corresponding SEM results. (a) n(P)/n(Cr) = 1; (b) n(P)/n(Cr) = 1.5; (c) n(P)/n(Cr) = 2; (d) n(P)/n(Cr) = 2.5; (e) n(P)/n(Cr) = 3.
Materials 18 03372 g005
Materials 18 03372 g006
Figure 6. Physical properties and Cr leaching characteristics of CS-MSWI-FA-FC prepared with different additional amounts of (NH4)2HPO4: (a) Physical properties; (b) Cr leaching characteristics.
Figure 6. Physical properties and Cr leaching characteristics of CS-MSWI-FA-FC prepared with different additional amounts of (NH4)2HPO4: (a) Physical properties; (b) Cr leaching characteristics.
Materials 18 03372 g006
Materials 18 03372 g007
Figure 7. Chemical thermodynamic calculation results at various temperatures within the sintered system for the CS-MSWI-FA-FC and that with three inhibitors. (a) Raw sample; (b) NH2SO3H; (c) ZnO·TiO2; (d) (NH4)2HPO4.
Figure 7. Chemical thermodynamic calculation results at various temperatures within the sintered system for the CS-MSWI-FA-FC and that with three inhibitors. (a) Raw sample; (b) NH2SO3H; (c) ZnO·TiO2; (d) (NH4)2HPO4.
Materials 18 03372 g007
Materials 18 03372 g008
Figure 8. XRD results of the raw sample before and after sintering, and after sintering with the inhibitors.
Figure 8. XRD results of the raw sample before and after sintering, and after sintering with the inhibitors.
Materials 18 03372 g008
Materials 18 03372 g009
Figure 9. Influence mechanism of three inhibitors on the physical and Cr leaching properties of CS-MSWI-FA-FC.
Figure 9. Influence mechanism of three inhibitors on the physical and Cr leaching properties of CS-MSWI-FA-FC.
Materials 18 03372 g009
Table 1. Possible Cr forms in the reaction products.
Table 1. Possible Cr forms in the reaction products.
PhaseComponentsRef.
Gas phaseCr, CrO, CrO2, CrO3, CrCl4, CrCl3, CrCl2[24,25]
Solid phaseCa3Cr2(SiO4)3, CaCrO4, CaCr2O4, Cr2O3, MgCr2O4, K2CrO4, Na2CrO4, CrO2, FeCr2O4, Cr2(SO4)3[15,25,26]
Table 2. XRF results of Cr slag, MSWI-FA, and other solid wastes (wt.%).
Table 2. XRF results of Cr slag, MSWI-FA, and other solid wastes (wt.%).
Na2OMgOAl2O3SiO2ClK2OCaOTiO2Cr2O3Fe2O3Others
Cr slag1.321.9015.462.60.05002.600.6700.5600.070014.10.750
MSWI-FA0.9202.181.639.550.07000.56072.80.3100.2901.3910.4
Kaoline7.431.4415.966.20.004.283.880.06000.02000.6200.210
Feldspar6.770.16013.870.40.01007.880.3300.06000.02000.2600.350
Fluorspar tailings0.1600.3103.6391.80.04001.101.010.000.02000.8601.10
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Li, H.; Liu, C.; Tang, Y.; Zhao, S. Preparation of Foamed Ceramic from Cr Slag and MSWI Fly Ash and Its Cr Leaching Inhibition. Materials 2025, 18, 3372. https://doi.org/10.3390/ma18143372

AMA Style

Li H, Liu C, Tang Y, Zhao S. Preparation of Foamed Ceramic from Cr Slag and MSWI Fly Ash and Its Cr Leaching Inhibition. Materials. 2025; 18(14):3372. https://doi.org/10.3390/ma18143372

Chicago/Turabian Style

Li, Hesong, Cheng Liu, Yikun Tang, and Shilin Zhao. 2025. "Preparation of Foamed Ceramic from Cr Slag and MSWI Fly Ash and Its Cr Leaching Inhibition" Materials 18, no. 14: 3372. https://doi.org/10.3390/ma18143372

APA Style

Li, H., Liu, C., Tang, Y., & Zhao, S. (2025). Preparation of Foamed Ceramic from Cr Slag and MSWI Fly Ash and Its Cr Leaching Inhibition. Materials, 18(14), 3372. https://doi.org/10.3390/ma18143372

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop