An overview of mullite and its synthesis using iron mining waste

Abstract

Mullite is an aluminosilicate of great technological importance, characterized by its high-performance refractory properties, which enable its use in both traditional and advanced applications. This review provides an overview of the crystalline structure of mullite, describes its nucleation process and kinetics, and explores synthesis strategies that enable its formation at lower temperatures, along with notable application examples reported in recent literature. Special attention is given to the use of iron ore tailings (IOTs) as a raw material for mullite production, highlighting an economically viable and environmentally friendly alternative that adds value to these materials. The paper further analyzes the synthesis methods employed and the mechanical properties of the resulting ceramics as a function of composition and IOTs content, as well as their potential applications, with particular emphasis on the construction sector.

Keywords:
Mullite; Mullitization; Iron ore tailings (IOTs); Mullite seeds

INTRODUCTION

Mullite (3Al₂O₃·2SiO₂) stands out as one of the most important components in both traditional and advanced ceramics^{1), (2), (3), (4}, being primarily used in industrial applications as a refractory material due to its high-performance mechanical and thermal properties^{2), (5}. The synthesis process of mullite occurs through ionic diffusion mechanisms at high temperatures and the formation of intermediate liquid phases, which lead to the nucleation and growth of mullite crystals^{2), (6), (7}. Moreover, synthesis conditions, such as temperature and precursors used, directly influence the structure and properties of the obtained mullite.

The search for sustainable alternatives has led to the use of industrial and mining residues as part of the raw materials for mullite production. In particular, the use of IOTs has proven to be a cost-effective alternative. Iron ore is the most exploited mineral in the world, with an annual production estimated at 2.5 billion tons in 2024 and reserves estimated at 200 billion tons⁸. This level of production generates approximately 1.4 billion tons of waste annually, which can be reused in the manufacturing of ceramic components^{9), (10), (11}.

This approach aligns with the global push for sustainable practices in ceramics and waste management, as highlighted in recent reviews on environmentally friendly alternatives in ceramic synthesis, aimed at limiting environmental impact, optimizing resource use, and promoting sustainable solutions within a circular economy, with waste recycling as a central focus^{12), (13), (14), (15), (16), (17), (18), (19), (20}.

From a crystallographic perspective, mullite belongs to the compositional series of orthorhombic aluminosilicates, characterized by three mutually perpendicular crystallographic axes of unequal lengths. Its general composition is described by Eq. (A), where x represents the proportion of oxygen vacancies per unit cell^{2), (3), (4), (26), (57}.

$A{l}_{\left(4+2x\right)}S{i}_{\left(2-2x\right)}{O}_{\left(10-x\right)}$ (A)

The average structure of mullite can be derived from sillimanite (x=0), a closely related but structurally simpler phase consisting of AlO₆ octahedral chains connected at the edges to AlO₄ and SiO₄ tetrahedral chains^{3), (4}. Formally, mullite can be derived from sillimanite by a coupled substitution, where two silicon (Si⁴⁺) ions and one oxygen (O^2-) ion are replaced by two aluminum (Al³⁺) ions, creating an oxygen vacancy (□), as expressed in Eq. (B).

$2S{i}^{4+}+O^{2-}\to 2A{l}^{3+}+\square$ (B)

This reaction involves the removal of oxygen atoms from the structure, leading to oxygen vacancies and a subsequent rearrangement and disordering of the tetrahedral cations. Mullite consists of 0.20<x<0.67, with the most important representatives of this series being stoichiometric mullite 3:2 (x=0.25) and mullite 2:1 (x=0.40)⁵⁸, where 3:2 and 2:1 refer to the molar ratio between alumina (Al₂O₃) and silica (SiO₂). Among these, stoichiometric mullite is of the greatest commercial interest, as it can be obtained through solid-state sintering processes⁵ and is the only stable compound in the Al₂O₃-SiO₂ system under ambient pressure^{2), (59}.

The presence of oxygen vacancies promotes a structural rearrangement, displacing aluminum atoms within the tetrahedra and forming a tricluster, i.e., three tetrahedra sharing a common oxygen atom^{2), (60}, as shown in Fig. 1, which presents a model of the main structural elements of mullite along the c crystallographic axis.

Figure 1
Crystalline structure of mullite highlighting oxygen vacancies and triclusters (adapted from⁵⁸).

This structural configuration contributes to the stabilization of mullite at high temperatures due to increased configurational entropy³. Moreover, mullite has the ability to incorporate a wide variety of foreign cations into its structure, which can modulate its optical⁶¹, electrical, and magnetic properties^{62), (63}, enable the development of novel heavy metal adsorbents⁶⁴, and improve refractory production technologies⁶⁵.

2.2 Mullitization Process

The mullitization reaction, i.e., the formation of the mullite phase from aluminum oxide (Al₂O₃) and silicon oxide (SiO₂), occurs through nucleation and crystal growth mechanisms driven by the minimization of the system’s free energy^{6), (7}. As the temperature increases, diffusion processes become more pronounced, initially occurring at the particle interfaces^{2), (6}.

The nucleation of the mullite phase is predominantly driven by the solubility of Al³⁺ ions in the silica structure. This is attributed to the lower Al-O bond energy compared to Si-O, which renders alumina less thermodynamically stable⁶⁶. In contrast, the solubility of silica in the crystalline structure of alumina is negligible at temperatures below the mullite melting point and can therefore be disregarded⁷. Between 1200ºC and 1300ºC, Al³⁺ ions diffuse from alumina into silica particles, forming a metastable aluminosilicate liquid with eutectic composition (8 wt% Al₂O₃ and 92 wt% SiO₂)^{2), (6}, obtained from the alumina-silica phase diagram⁶⁷.

After the formation of the metastable liquid, the reaction kinetics are controlled by the dissolution rate of alumina into this medium, occurring between 1200ºC and 1550ºC⁶⁸, as the metastable liquid interacts with the surface of Al₂O₃ grains, promoting the breaking of Al-O bonds at the solid-liquid interface^{6), (68}. This interfacial dissolution process shares similarities with pore-flow or solution-friction mechanistic models, in which the dissolution and transport of Al³⁺ ions are governed by the thermodynamic driving force across the solid-liquid interface and by frictional interactions within the metastable liquid phase⁶⁹.

The process is driven by the thermodynamic favorability of forming the silico-aluminous liquid phase, since the formation of Si-O-Al bonds, predominant in the liquid, has a lower energetic state than the Si-O-Si and Al-O-Al bonds present in pure silica and alumina solids^{66), (70}. Therefore, the overall reaction is energetically advantageous. As the continuous dissolution of alumina in the liquid reaches the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the nucleation and crystal growth of mullite begin⁶.

Finally, at temperatures above 1550ºC, Al³⁺ ion diffusion enables the removal of alumina particles trapped within large mullite grains, which involves interdiffusion over relatively long distances⁶⁸. However, it is important to note that the described mullitization mechanism is a simplified model. It disregards possible intermediate reactions during heating, as well as the effects of ceramic powder morphology and potential impurities, which can lead to significant variations in the mullitization temperature^{2), (5), (35), (71}. Fig. 2 presents a simplified representation of the mullitization process.

Figure 2
Mullitization process.

Several techniques reported in the literature have been employed for the synthesis of mullite under different conditions.

High-temperature solution-assisted recrystallization, for example, enables the formation of single-crystal mullite. In this method, a standard mullite-silica ceramic containing approximately 60 mol% Al₂O₃ and single-crystal sapphire was submerged in a 50 mL platinum crucible containing a PbO-V₂O₅ liquid flux at 970ºC, under isothermal conditions with a temperature stability of ±0.1ºC for 10 and 18 hours. Next, the melt was poured off, and the crystallization products were washed with 25% nitric acid. This process resulted in the formation of essentially pure mullite crystals due to the nucleation and growth of microcrystals under the chemical equilibrium established between the starting materials and the flux⁷¹.

The sol-gel technique is also a widely employed method for mullite synthesis, as it is a versatile method capable of generating products with high purity and homogeneity, in addition to good reproducibility⁷². In one study, the mullite phase was obtained using nano-hydrated aluminum nitrate and tetraethyl orthosilicate (TEOS) as precursors, with an Al:Si molar ratio of 3:1, varying the solvents used, including ethyl alcohol, isopropyl alcohol, tetrahydrofuran, acidified water (pH≈2), and basified water (pH≈12). The synthesis was conducted at 1200ºC with a heating rate of 5ºC/min, resulting in the predominant formation of the mullite phase in all samples. However, those synthesized with ethyl alcohol and isopropyl alcohol exhibited the highest crystallinity indices, reaching 91.7% and 90.2%, respectively⁷³.

This technique stands out for promoting good mixing and uniformity of the starting materials at the nanoscale. As a result, the aluminum and silicon atoms are in proximity, reducing the required diffusion paths and enabling the reaction to occur at relatively lower temperatures compared to conventional sintering methods⁷⁴. However, the technique has limitations, such as the high cost of raw materials and long processing times⁷⁵.

In parallel, recent advances have highlighted the use of polymeric and hybrid organic-inorganic precursors, which have gained increasing attention in the literature^{76), (77), (78), (79), (80), (81), (82), (83}. Recently, preceramic polymer resins were employed in the fabrication of reticulated mullite ceramic foams⁷⁹. In this study, a polyurethane (PU) foam template was immersed in three aluminosiloxane (AS) polymer resins synthesized by varying the aluminium tri secbutoxide (AsB) to TEOS molar ratio for 10 minutes, allowing infiltration of the resin into the porous structure. The infiltrated foams were then removed, dried at 80ºC for 2 hours, and heat treated at 200ºC for 2 hours to cure the resin.

The foams were heat-treated in an oxidizing furnace with flowing air at 1600ºC for 3 hours, with a heating rate and a cooling rate of 0.5ºC/min. The resulting lightweight mullite foams exhibited reasonable compressive strength (0.2 to 0.4 MPa) and very low thermal conductivity (0.15 to 0.19 W/(m·K) at porosities ranging from 83.0 to 97.0%, making them suitable for high-temperature thermal insulation applications⁷⁹.

Another approach investigated the influence of the type of precursor on mullite formation⁸⁴. Hydrated alumina oxide (HAO), obtained from the sulfuric acid processing of nepheline ((Na,K)AlSiO₄), and fine aluminum hydroxide (FAH), commercially sourced, were employed. Each material was mixed with amorphous silica, also obtained as a by-product of the (Na,K)AlSiO₄ leaching process, in a porcelain mortar for 6 h, using a slightly aluminum-rich proportion relative to the ideal mullite stoichiometry, aiming to minimize the formation of glassy phases. The samples were sintered at temperatures ranging from 800 to 1600ºC, with an initial heat rate of 10ºC/min up to 1000ºC and then 2ºC/min until the target temperature.

In the sample containing HAO, the mullite phase appeared starting at 1200ºC, as a result of the interaction between γ-Al₂O₃ and amorphous silica. However, using FAH, the presence of a residual ҡ-Al₂O₃ phase was recorded, and mullite formation was not observed below 1200ºC. Instead, sillimanite (Al₂O₃·SiO₂) was identified, which subsequently transformed into mullite above 1400ºC. This occurred because, when ҡ-Al₂O₃ interacts with amorphous SiO₂, the formation of sillimanite becomes energetically preferable⁸⁴.

The use of commercial kaolin and kaolin from rare-earth deposits, combined with γ-Al₂O₃ powder, has also been employed for the synthesis of high-performance mullite ceramics. The rare-earth-derived kaolin contained 60.45% SiO₂, 28.78% Al₂O₃, and trace amounts of rare-earth-rich minerals, whereas commercial kaolin, used as a reference, contained 53.54% SiO₂ and 43.58% Al₂O₃. These materials were milled in a ball mill for 4 hours and then sintered between 1320ºC and 1500ºC for 4 hours, with a heating rate of 5ºC/min²⁵. The formation of mullite was observed in both series starting at 1320ºC (Fig. 3).

Figure 3
Relative mullite content sintered at 1320 to 1500ºC for 4 hours (adapted from²⁵).

The authors suggest that differences in the initial composition and the presence of rare earth-rich minerals influenced the sintering process, as the excess silica formed a glassy phase that facilitated the secondary mullitization of the rare earth-derived kaolin.

2.3 Autocatalytic effects of mullite seed addition

An alternative for synthesizing mullite at relatively low temperatures is the addition of mullite seeds, i.e., pre-formed particles that act as heterogeneous nucleation sites, reducing the activation energy required for phase formation^{68), (85), (86), (87}. These seeds enable dissolved Al³⁺ ions to precipitate directly onto their surface, bypassing the need for metastable liquid saturation and accelerating the overall reaction⁶.

The use of seeds is not only cost-effective in mullite synthesis but is also widely reported in the literature for other materials. Examples include the synthesis of zeolites^{88), (89), (90), (91), (92}, molecular sieves for catalysts and adsorbents⁹³, radial TiO₂ homomesocrystals for plasmonic photocatalysts⁹⁴, metal-organic framework membranes for selective and permeable hydrogen separation⁹⁵, and flexible, durable, and anti-fouling maghemite-copper oxide nanocomposites⁹⁶.

One of the earliest studies reported in the ScienceDirect Database on the use of mullite seeds dates back to 1995. The work introduced a novel approach to synthesizing mullite powder using a diphasic Al₂O₃-SiO₂ gel seeded with a small amount of crystalline mullite particles⁹⁷. The methodology involved preparing a stoichiometric Al₂O₃-SiO₂ gel by dissolving aluminum chloride (AlCl₃) salts in distilled water and mixing the solution with fumed silica under ultrasonic treatment. The suspension was vigorously stirred during the addition of an ammonia solution until complete coprecipitation of aluminum hydrate sol and SiO₂gel occurred.

To enhance crystallization, 4 wt% of crystalline mullite particles (≈2 µm) were incorporated as seeds into the gel. Remarkably, the seeded gel yielded pure mullite after firing at 1300ºC for 2 h, whereas the unseeded gel under the same conditions retained minor phases of alumina and cristobalite alongside mullite. These results demonstrate that mullite seeds act as effective nucleation sites, promoting phase-pure mullite formation at lower temperatures compared to conventional methods.

Another foundational study investigated how initial microstructure and grain boundary chemistry can be tailored to achieve self-reinforced mullite microstructures⁹⁸. Diphasic sols were prepared from boehmite (γ-AlO(OH)) and silica sol and then sintered at 1600ºC for 5 h. A mullite seed dispersion was prepared by suspending commercial mullite powder in distilled water, with the pH adjusted to 3 using nitric acid. This dispersion was stirred for 3 days, followed by sonication and centrifugation at 2000 rpm for 30 minutes, after which the suspended particles were used for seeding.

An increase in the seed content led to a reduction in the mullite formation temperature, from 1345ºC to 1330ºC with 2 wt% seeding, and further down to 1318ºC with 10 wt% seeding. Moreover, specimens containing 2 wt% seeds developed anisotropic grains with aspect ratios of approximately 5-6. This microstructural evolution effectively eliminated most intragranular pores, resulting in a densification of approximately 97% of the theoretical value.

A method for the homogeneous addition of mullite seeds and their influence on the sintering characteristics of the mullite phase was investigated⁸⁶. The seeds were produced from a precursor gel synthesized using Al(NO₃)₃·9H₂O, TEOS, and ethanol at a molar Al₂O₃/SiO₂ ratio of 72/28.

The study evaluated two distinct approaches. In the first step, the precursor gel was heated to 1300ºC for 1 hour to form the seeds, which were then mechanically mixed with α-Al₂O₃ obtained by sintering AlNH₄(SO₄)₂·12H₂O at 900ºC for 2 hours and subsequently at 1200ºC for another 2 hours. The second approach involved mixing the precursor gel with an aqueous solution of AlNH₄(SO₄)₂·12H₂O and heating the mixture to 1200ºC for 2 hours to form diphasic alumina, consisting of α-Al₂O₃ and finer mullite seeds (~6.5 wt.%).

In both formulations, 28 wt% silica sol, a stable dispersion of nanometric amorphous silica particles that forms a three-dimensional network through the gelation process, was added⁹⁹. The mixtures were ball-milled for 36 hours, sieved through an 80-mesh screen, and compacted. The compacts were sintered at temperatures ranging from 1100ºC to 1550ºC for 2 hours in an air atmosphere.

The results demonstrated that the addition of mullite seeds significantly reduced the mullitization temperatures. In the samples containing diphasic alumina, mullite formation began at 1250ºC and was completed at 1450ºC. In contrast, in the approach where the seeds were mechanically mixed with α-Al₂O₃, mullitization occurred between 1300ºC and 1500ºC. Meanwhile, the reference sample, composed only of α-Al₂O₃ and silica sol, exhibited mullite formation in the range of 1400ºC to 1600ºC⁸⁶.

In the studies by^{100), (101}, mullite seeds were obtained from whiskers synthesized using γ-Al₂O₃, SiO₂, and aluminum fluoride trihydrate (AlF₃·3H₂O) at a molar ratio of Al₂O₃:SiO₂:AlF₃·3H₂O equal to 71.4:30:18. Initially, the reagents underwent a ball-milling process for approximately 15 hours using anhydrous ethanol as the dispersing medium, followed by heating to 1450ºC for 6 hours in covered crucibles. After heating, the formed whiskers were milled for 1 hour to obtain the seeds, which exhibited a columnar morphology and an aspect ratio of approximately 15:1.

The obtained seeds were used in the synthesis of anorthite/mullite whiskers through the freeze-casting method. X-ray Diffraction (XRD) analysis indicated an increase in diffraction intensity at the (110) plane of the mullite phase, as well as an increase in the mullite-to-anorthite peak ratio compared to the reference sample, which did not contain seeds. This result demonstrates that the presence of seeds favored the formation of the mullite phase in the composite. Furthermore, Scanning Electron Microscopy (SEM) images showed that the seeds significantly increased the number of whiskers formed while simultaneously reducing their diameter, resulting in a higher aspect ratio.

Although the use of seeds to catalyze nucleation and, consequently, promote mullite formation has proven to be an effective strategy, this approach remains underexplored in the literature. A search for the term “mullite seed” in the Web of Science and ScienceDirect databases, filtered by Title, Abstract, and Keywords, identified only 116 and 48 results, respectively, over the entire research period. In the past five years, the number of publications was even lower, totaling only 17 and 8, respectively. This scarcity of studies underscores a promising opportunity to advance research into cost-effective mullite synthesis routes through optimized seeding mechanisms.

RECENT APPLICATIONS OF MULLITE

As previously discussed, mullite has applications in various fields. An analysis of the ScienceDirect journal database using the search term “mullite application” and filtering for research articles only revealed a trend of increasing publications over the years (Fig. 4a). Notably, the years 2008, 2018, and 2024 showed significant increases of 38.5%, 30.7%, and 30.8%, respectively, compared to the previous year (Fig. 4b). A similar trend was observed in the Web of Science journal database, further highlighting the growing relevance of mullite in scientific research.

Figure 4
Journal publications on mullite applications from 2001 to 2024 (a) and annual variance percentage (b).

Recent research has highlighted mullite as a material with high-performance applications in various industrial fields^{102), (103), (104), (105}. A notable example is its use as a substrate in CO₂ capture systems¹⁰⁶. In the study, mullite ceramic substrates were produced via additive manufacturing using the Digital Light Processing (DLP) technique. Two distinct geometries were employed: a lattice architecture, widely used in gas filtration systems, and a triply periodic minimal surface (TPMS) configuration with a Schwartz primitive structure. The latter features smooth surfaces and high porosity and is obtained through Eq. (C)¹⁰⁷.

$\cos \left(x\right)+\cos \left(y\right)+\cos \left(z\right)=c$ (C)

The ceramic pieces were functionalized with a type of Metal-Organic Framework (MOF) known as MOF-199 or Cu-BTC, a highly porous material composed of copper (Cu²⁺) nodes coordinated with benzene-1,3,5-tricarboxylate (BTC), recognized for its CO₂ adsorption capacity.

Compared to the standalone application of the MOF, the functionalized ceramic pieces demonstrated significantly higher adsorption capacity, with a CO₂ capture increase of 40.1% for the lattice structure and 51.3% for the TPMS structure. This result highlights the synergy between mullite and MOFs for selective adsorption applications, particularly in CO₂ capture¹⁰⁶.

Mullite has also been explored for industrial wastewater treatment. In the study by¹⁰⁸, the aim was to produce mullite-based ceramic membranes. A colloidal sol of titania-zirconia (TiO₂-ZrO₂) was prepared using the sol-gel method and coated onto the ceramic supports via dip-coating.

The efficiency of the membranes was evaluated based on pure water permeability under pressures ranging from 1 to 7 bar, salt removal in saline solutions, and filtration of synthetic oily wastewater. The results demonstrated a filtration efficiency of 98.65% for oily wastewater, as well as improved salt retention in acidic and basic environments compared to neutral conditions, indicating the applicability of this material for oily wastewater treatment and in environments with high acidity and alkalinity.

The development of porous ceramics for thermal insulation and sound absorption has also been investigated. To this end, porous mullite ceramics incorporating whiskers into the structure were developed to improve mechanical properties and reduce heat and sound conduction by creating tortuous paths for thermal transfer¹⁰⁹.

The ceramics were produced using the gel-casting method with foam and freeze-drying, employing construction waste supplemented with Al₂O₃ to achieve the stoichiometric mullite ratio. Additionally, 12 wt% AlF₃ was added as a catalyst for whisker formation, 4 wt% B₂O₃ as a sintering additive, 4 wt% Na₆O₁₈P₆ as a dispersant, and a variable amount of sodium dodecyl sulfate (SDS) as a foaming agent.

The best results were obtained at a sintering temperature of 1050ºC and 0.10 wt% SDS, resulting in a bulk density of 0.24 g/cm³, 90.52% porosity, and a compressive strength of 1.27 MPa. Sound absorption and thermal conductivity tests showed a sound absorption coefficient of 0.81 at a frequency of 2000 Hz and a thermal conductivity of 0.0627 W/(m·K).

These results demonstrate that the proposed method represents a cost-effective approach for manufacturing porous ceramics for construction applications. Comparatively, the developed material has a density similar to expanded polystyrene but with compressive strength approximately 6.5 times higher¹¹⁰.

SYNTHESIS OF MULLITE FROM IOTS

The reuse of waste from industrial, mining, and construction processes has become a key focus of scientific research and social interest¹¹¹. This practice aligns with modern waste valorization strategies that aim to transform these residues into valuable resources within a circular economy framework^{112), (113), (114), (115}. This practice not only reduces energy consumption and raw material usage but also conserves natural resources^{40), (116} and helps mitigate socio-environmental impacts, a principle that is core to green chemistry and life cycle assessment (LCA) methodologies^{117), (118}. These impacts include risks to human health, deterioration of air quality, alterations in the dynamics of aquatic ecosystems, changes in fauna and flora populations, and increased soil erosion, among other global challenges for safeguarding the environment and human population^{20), (119), (120), (121), (122}.

Studies have evaluated different types of waste for producing mullite-containing ceramics. Among them, notable examples include agricultural and food industry residues, such as silica from rice husks²⁶; spent cracking catalysts¹²³; aluminum manufacturing waste¹²⁴; byproducts from the ceramics and bauxite refining industries¹¹¹; fly ash from thermal power plants¹²⁵, among others. However, this study specifically focuses on the use of IOTs.

The economic viability of using IOTs is a critical factor for their large-scale adoption. A promising route lies in the high residual iron content still present in many tailing dams, making their reprocessing through methods such as magnetic separation and reverse cationic flotation economically attractive¹¹¹. This process generates a silica-rich by-product, which can drastically reduce the cost of SiO₂ as a raw material for mullite production and, consequently, incorporate SiO₂ into the value chain as a co-product of an already profitable iron recovery operation.

The composition of IOTs varies considerably depending on the mineralogy of the region^{40), (126), (127}. In general, these residues predominantly consist of SiO₂ (24-63%), Fe₂O₃ (32-72%), Al₂O₃ (1-18%), and small amounts of CaO, MgO, K₂O, among others⁴⁰.

The quantity and type of impurities directly influence the anisotropic growth of mullite grains by reducing surface energy in specific crystallographic directions, which affects crystal morphology^{116), (128}. In particular, the (001) plane is most favored by the presence of hematite (Fe₂O₃), the primary impurity of the IOTs in the context of mullite production¹²⁹.

Although the presence of impurities promotes the mullitization process through heterogeneous nucleation^{127), (130), (131), (132), (133}, mullite ceramics obtained from wastes generally exhibit multiple residual phases and higher porosity compared to those produced with high-purity materials¹¹⁶. This can impact the mechanical and thermal performance of these materials^{3), (6), (7), (116), (127), (134}.

Despite these limitations, various studies in the literature explore the reuse of IOTs for the production of ceramics containing, either fully or partially, the mullite phase^{40), (127}. The use of these residues typically promotes the formation of mullite, cristobalite, and hematite in the sintered product¹²⁷, making it necessary to evaluate alternatives for removing iron-rich phases. Techniques such as gravity separation¹³¹, magnetic separation¹³², and chemical leaching¹³⁰ can be applied, either individually or in combination, to reduce or eliminate these phases from the final ceramic product.

IOTs from the Shaoguan region, China, were used as part of the raw materials for producing glass-ceramics¹³³. The residue was composed of 45.25% Fe₂O₃, 27.1% SiO₂, and 14.27% Al₂O₃, along with small amounts of MgO, K₂O, CuO, ZnO, PbO, and MnO, as well as the mineral phases mayenite ((CaO)₁₂(Al₂O₃)₇) and braunite ((Mn₂O₃)₃MnSiO₂). Samples containing different proportions of this IOT, along with high-purity Al₂O₃ and SiO₂, were prepared by sintering at 1450ºC for 2 hours, with a heating rate of 5ºC/min.

The formation of mullite varied with the Al₂O₃/SiO₂/IOT ratio. The maximum mullite content, 83.7 wt%, was obtained at the 6/2/3 ratio, corresponding to the conversion of approximately 95% of Al₂O₃ and SiO₂ into mullite. When the proportion of IOT in the mixture increased beyond this ratio, the mullite content decreased, and the amorphous phase became predominant, rising from 16.3% at 6/2/4 to 67.2% at 6/2/6. This behavior can be attributed to the interaction between hematite and aluminosilicates in the IOT, which promoted the formation of glassy phases and inhibited crystallization.

In terms of compressive strength, the sample with the 6/2/3 ratio exhibited a value of 907.6 MPa. However, despite containing the highest mullite content, the maximum strength was achieved at a 6/2/6 ratio, reaching 1061.6 MPa. It is suggested that the higher amount of amorphous phase contributed to reducing the sample’s porosity, thereby enhancing its mechanical strength¹³³.

Recent studies have investigated the use of IOT in combination with other residues, such as coal gasification slag (CGS), for the production of geopolymers. In one study¹³⁴, samples containing different proportions of IOT and CGS were prepared to evaluate their effect on the structural and mechanical properties of geopolymers sintered at 650ºC, 850ºC, and 1050ºC. The IOT, collected from Shaanxi, China, had the following chemical composition: 43.56% SiO₂, 11.11% Al₂O₃, 15.02% Fe₂O₃, 10.80% CaO, and 7.32% MgO, along with trace amounts of TiO₂, Na₂O, and K₂O. The CGS was primarily composed of 52.51% SiO₂, 16.46% Al₂O₃, 14.19% CaO, and 6.30% Fe₂O₃. The combination of 80% IOT and 20% CGS resulted in the highest compressive strength of 35.5 MPa after sintering at 1050ºC with a heating rate of 5ºC/min for 2 hours. According to the authors, the presence of the mullite phase was one of the primary factors responsible for the improved mechanical strength of the geopolymer¹³⁴.

The application of IOT has also been evaluated in the formulation of multicomponent refractory cementitious materials, focusing on high-temperature resistance¹³⁵. The samples were formulated with fixed compositions of Portland cement (11.0%), lime powder (13.0%), dihydrate gypsum (2.4%), potassium feldspar powder (2.4%), quartz (1.0%), alumina powder (1.0%), and metakaolin (11%), while varying the proportions of fly ash and IOT in different formulations. The samples were heated in a muffle furnace at 200ºC, 300ºC, 550ºC, 850ºC, 1200ºC, and 1300ºC, with a heating rate of 10ºC/min.

The compressive strength tests showed a progressive reduction in strength up to approximately 850ºC, where the minimum value was recorded for most samples. Between 200 and 400ºC, the decrease was attributed to the gradual removal of water from the calcium silicate hydrate (C-S-H) structure, the primary phase formed during Portland cement hydration¹³⁶. Between 400ºC and 850ºC, the decomposition of calcium hydroxide and calcium carbonate further aggravated the structural deterioration of the material, resulting in a sharp decline in strength^{137), (138), (139}.

Above 850ºC, a significant increase in mechanical strength was observed up to 1300ºC. This behavior was attributed to the formation of mullite, which contributed to the recovery of the material’s mechanical properties. Among the tested formulations, the sample containing 10.7% IOT exhibited the best performance at high temperatures, achieving a compressive strength of 28 MPa¹³⁵.

The technical feasibility of using IOT, blast furnace slag (BFS), foundry sand (FS), and natural clay (NC) as partial substitutes for conventional raw materials was analyzed¹³⁹. Table I presents the chemical composition of the residues used. The samples, containing different proportions of these materials, were sintered at temperatures ranging from 900 to 1200ºC for 3 hours, with a heating rate of 10ºC/min.

Table I
Chemical composition of the residues used by¹³⁹.

Residue	SiO2(%)	Fe2O3(%)	Al2O3(%)	CaO(%)
IOT	58.6	35.4	3.2	-
BFS	39.7	-	12.9	32.8
FS	91.2	1.2	2.3	-
NC	53.3	6.1	24.7	-

The chemical composition of the IOT, particularly its Fe₂O₃ and SiO₂ content, have comparable data with the characterization of tailings from similar iron ore processing operations reported in recent literature^{140), (141), (142), (143}.

The sample containing 40% IOT, 20% BFS, 10% FS, and 30% NC exhibited the highest flexural strength, reaching 12.19 MPa at 1200ºC, surpassing the minimum requirement of 12 MPa established by the Brazilian standard NBR 13818¹⁴⁴, which sets the technical specifications for ceramic tile production. This result highlights the practical potential of this composition. However, it was observed that increasing the IOT content led to a reduction in flexural strength modulus, demonstrating the need for a balanced proportion of materials to optimize mechanical performance.

Additionally, XRD analysis revealed the predominance of quartz, mullite, cristobalite, and hematite phases, with mullite formation attributed to the decomposition of metakaolinite into mullite and cristobalite, a process that begins around 900ºC and reaches its peak reaction near 1300ºC¹³⁹.

In the study by¹²⁷, the potential of IOTs collected in the Minas Gerais region, Brazil, was investigated for the production of ceramic materials, focusing on tile manufacturing. The tailings, sourced from four high-environmental-risk dams, underwent a dry segregation process, resulting in three distinct fractions: iron ore concentrate, clay fraction, and sand fraction.

The characterization of these fractions revealed significant differences in granulometry, morphology, chemical composition, and mineralogy. The clay fraction, composed predominantly of Al₂O₃ (16.22%), Fe₂O₃ (62.23%), and SiO₂ (18.31%) in the form of kaolinite and goethite minerals, was identified as a promising raw material. Meanwhile, the sand fraction, with a high concentration of SiO₂ (87.04%) and high granulometric and mineralogical uniformity, proved suitable for replacing natural sand in various construction applications.

The clay fractions were directly used in the production of ceramic pieces, which were pressed and sintered at 1200ºC for 2 hours with a heating rate of 10ºC/min. During sintering, the formation of mullite and glassy phases was observed, contributing to the densification of the ceramic pieces. The final products met the flexural strength requirements established by ISO 13006¹⁴⁵ and NBR 13818¹⁴⁴ for pressed ceramic tiles.

These results highlight the great potential of using IOT as a sustainable and high-value raw material for the ceramics industry, contributing to the reduction of environmental impacts associated with tailings dams.

CONCLUSION

Mullite is a technologically significant ceramic phase, widely employed in advanced and refractory ceramics. From a thermodynamic perspective, its formation occurs through the diffusion of Al³⁺ ions into the silica structure, inducing structural instability and resulting in the formation of a metastable liquid with a eutectic composition. Once saturated, the nucleation process of the mullite phase begins.

Various techniques and precursors for mullite synthesis, aiming for a high degree of purity, are described in the literature. Notable synthesis methods include high-temperature solution-assisted recrystallization, the sol-gel method using nano-hydrated aluminum nitrate and TEOS as precursors with ethyl or isopropyl alcohol as solvents, sintering of fine aluminum hydroxide combined with silica, and the use of kaolin as a raw material.

Although still underexplored in the literature, the addition of mullite seeds is an effective strategy to lower synthesis temperatures and accelerate phase formation. These seeds act as heterogeneous nucleation sites, eliminating the need for saturation of the metastable liquid.

An analysis of recent advances in mullite applications highlights its versatility across various industries, including CO₂ capture, wastewater treatment, and the development of porous ceramics for thermal and acoustic insulation.

The use of waste materials, particularly IOTs, for mullite synthesis represents a sustainable alternative and contributes to mitigating the social and environmental impacts associated with their management and disposal. However, due to the diverse composition of these residues and the inherent presence of impurities, the final properties of the resulting ceramics can vary significantly depending on the proportion and nature of the waste used. Finally, the literature presents various synthesis approaches that use IOTs partially or entirely to produce ceramic tiles, geopolymers, and refractory materials. In these applications, the mullite phase plays a key role in enhancing mechanical strength.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of the Department of Materials Engineering of the Federal Center for Technological Education of Minas Gerais.

DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author, upon reasonable request.

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Edited by

• AE:
  Daniel Zanetti de Florio

Publication Dates

• Publication in this collection
  17 Apr 2026
• Date of issue
  2026

History

• Received
  31 Mar 2025
• Reviewed
  11 July 2025
• Reviewed
  04 Sept 2025
• Reviewed
  28 Jan 2026
• Accepted
  19 Feb 2025