Development and characterization of a sustainable iron carbonate binder using thermally treated red mud

ABSTRACT

This study investigates the development of a novel iron carbonate binder, utilizing red mud, an industrial byproduct of aluminum production, as a replacement for traditional components such as fly ash. This study compares the performance of binders made with raw, unsintered red mud (USRM) with those made with red mud that has been thermally activated by sintering at 750°C (SRM). The results demonstrate that sintering pre-treatment is a critical and transformative step. The SRM binder achieved a 28-day compressive strength of 24.12 MPa, a 72.5% improvement over the 13.98 MPa strength of the USRM binder. Furthermore, the SRM binder exhibited excellent long-term stability, maintaining its strength for 90 days, whereas the USRM binder showed lower strength than the SRM samples, indicating potential durability issues. Microstructural analysis via scanning electron microscopy (SEM) revealed that sintering enabled the formation of a dense, robust, and composite binder matrix, characterized by the presence of stable and cohesive calcium carbonate. In contrast, the USRM binder forms a porous, weakly consolidated structure. X-ray diffraction (XRD) analysis confirmed that sintering activated the red mud’s mineralogy, creating reactive cementitious phases essential for superior performance. Thermogravimetric analysis (TGA) indicated higher mass loss in unsintered red mud due to the presence of thermally unstable hydrated phases. In contrast, sintered red mud exhibited reduced mass loss and improved thermal stability, confirming phase transformation during sintering.

CO₂ utilization; Industrial waste; Mineral carbonation; Carbon capture; Microstructural analysis

1. INTRODUCTION

Global climate change has elevated the Earth’s temperature by over 1°C since the pre-industrial era, resulting in extreme weather events, rising sea levels, and negative impacts on global populations [¹, ²]. The cement industry is a significant contributor to global warming, accounting for 5.9% of greenhouse gas emissions [³]. It emits 776 kg of CO₂ per ton of cement and is responsible for 40% of energy consumption [⁴]. As a major component of carbon emissions, CO₂’s widespread use accelerates climate change, prompting researchers to find ways to reduce these emissions [⁵, ⁶]. The extensive use of concrete in large-scale urban infrastructure contributes to environmental harm through CO₂ emissions [⁷], leading construction companies to seek environmentally friendly alternatives to remain sustainable and competitive [⁸]. To address these challenges, one strategy is to replace clinker with alternative materials to reduce carbon emissions and work towards the cement and concrete industry’s goal of zero CO₂ emissions [⁹]. Therefore, it is crucial to find alternative materials to decrease the CO₂ emissions associated with traditional cement and enhance the sustainability of the construction industry [¹⁰]. The proposed solution involves using less cement and incorporating waste materials like industrial by-products and natural pozzolans [¹¹], which could lower costs and CO₂ emissions [¹²]. Carbon Capture and Storage (CCS) systems have been developed to mitigate CO₂ emissions [¹³]. This process involves capturing carbon dioxide from emission sources, compressing it, and then transporting it to a suitable site for long-term storage [¹⁴]. The sequestration of CO₂ plays a vital role in mitigating climate change. In recent years, utilizing industrial waste for CO₂ mineralization has emerged as a promising technique for reducing CO₂ emissions while improving the recycling of industrial waste [¹⁵,¹⁶,¹⁷]. Industrial wastes containing alkaline components such as CaO or MgO, including iron powder, red mud, coal fly ash, and various slags, have been assessed for CO₂ sequestration [¹⁸,¹⁹,²⁰]. Compared with natural minerals, industrial wastes are often highly reactive, inexpensive, and abundantly available, giving them significant potential for this application [²¹].

The interaction between iron and carbon dioxide, known as iron carbonation, results in the formation of iron carbonate products such as siderite (FeCO₃) [²², ²³]. This process, which can also occur indirectly when dissolved carbonate ions react with iron [²⁴], effectively sequesters CO₂ from industrial operations by converting it to stable iron carbonate compounds, and the increase in global steel production has led to a corresponding increase in iron waste [²⁵]. Primary steel manufacturing emits 1.75–1.85 tons of CO₂ per ton of crude steel produced, mainly from the use of coal in iron ore reduction [²⁶]. With global steel output projected to hit 2.19 billion tons by 2050 and 0.3 tons of iron waste generated per ton of crude steel, this presents a significant challenge [²⁷]. This steel waste, often iron powder from machining and grinding, can be repurposed as a filler in composite materials [²⁸]. This has led to the development of materials like ferrock, a sustainable, carbon-negative binder made of 90% waste materials, including iron powder, fly ash, metakaolin, and calcium carbonate [⁸, ²⁹, ³⁰]. In this binder, oxalic acid acts as an organic reducing agent, and its chelating properties help break down iron oxides, a reaction essential for the hardening and improvement of the binder’s mechanical characteristics [³¹]. The chemical reaction steps for the formation of the iron carbonate binder are as follows [³²]:

(A) $${\text{Fe + 2CO}}_{\text{2}}{\text{+ 2H}}_{\text{2}}\text{O}\to {\text{Fe}}^{\text{2+}}{\text{+ 2HCO}}^{\text{-}}{}_{\text{3}}{\text{+ H}}_{\text{2}}$$

(B) $${\text{Fe}}^{\text{2+}}{\text{+ 2HCO}}^{\text{-}}{}_{\text{3}}\to {\text{FeCO}}_{\text{3}}{\text{+ CO}}_{\text{2}}{\text{+ H}}_{\text{2}}\text{O}$$

The net reaction for the above equations is expressed as

(C) $${\text{Fe + CO}}_{\text{2}}{\text{+ H}}_{\text{2}}\text{O}\to {\text{FeCO}}_{\text{3}}{\text{+ H}}_{\text{2}}$$

Unlike conventional cement, iron-based binders have the potential to absorb more CO₂ than they emit, thus offering a carbon-negative solution [³³]. A significant co-benefit is the production of hydrogen gas as a byproduct, which can serve as a clean fuel source [³⁴]. This dual advantage of carbon sequestration and clean energy production aligns with global sustainability policies, making iron carbonate binders favorable materials for future construction [³⁵].

This study focused on using red mud in raw and thermally decomposed form (Sintered) forms as an alternative to fly ash to create an iron carbonate binder. Red mud, a byproduct of aluminum manufacturing, is a highly alkaline solid waste rich in iron oxide, which can be made more reactive by sintering, changing the crystalline structure to amorphous, making it an excellent feedstock for carbon sequestration [³⁶]. Annually, 120 million tons are produced globally, while its high alkalinity can pose environmental risks, such as groundwater pollution, red mud has a great capacity for CO₂ sequestration and is capable of storing up to 5.3 g of CO₂ per 100 g [³⁷,³⁸,³⁹,⁴⁰,⁴¹]. Atmospheric carbonation of red mud can trap six million tons of CO₂ annually, transforming it into a valuable asset for environmental remediation [⁴²]. Carbonating iron in red mud also improves its characteristics for use in sustainable buildings, wastewater treatment, and as a soil supplement [³⁷, ⁴³]. The carbonation enhances the mechanical properties and durability of iron carbonate-based construction materials, including their tensile strength, Young’s modulus, and fracture characteristics [⁴⁴]. A carbonation period of 1 to 4 days can reduce the pore volume and result in fracture energies that are three to five times greater [⁴⁵]. Furthermore, incorporating calcined red mud into cement improves its flexural performance [⁴⁶], and combining it with silica fume increases its compressive strength and water resistance [⁴⁷]. Carbonated steel slag also demonstrates strong mechanical properties and stability, making it suitable for large-scale construction [⁴⁸].

1.1. Research significance

In this study, carbon sequestration through mineral carbonation of both sintered and unsintered red mud was investigated, with the main aim of reducing CO₂ emissions from traditional cement manufacturing by developing a sustainable iron-rich material. This work provides critical insights into the feasibility of valorising thermally treated and untreated red mud in carbon-negative building materials by utilising red mud as an iron-rich industrial byproduct and promoting CO₂ mineral carbonation. Furthermore, this investigation aims to mitigate the environmental impact of the construction sector through the application of carbon capture, utilisation, and storage (CCUS) technologies, as well as promote sustainable and circular material practices. This experimental study aimed to evaluate the influence of a red mud treatment on carbonation efficiency, compressive strength, phase identification using X-ray diffraction, and microstructural characterization using scanning electron microscopy. To analyze binders composed of sintered red mud (SRM) and unsintered red mud (USRM) to understand their influence on strength development and carbonation behavior.

2. MATERIAL PROPERTIES

This study utilized several raw materials sourced from India. The primary component is iron powder, a by-product of structural steel production that is typically sent to landfills. The powder was collected from a scrap unit in Chennai and had a particle size of 43 µm. Its chemical composition was dominated by 87.32% iron (III) oxide (Fe₂O₃). Red mud, a residual substance from the Bayer process, was acquired from an Indian aluminum refinery. It has a particle size of 17 µm and a specific gravity of 2.7, with its main chemical components are 35.07% Fe₂O₃, 31.62% aluminum oxide (Al₂O₃), and 18.98% silicon dioxide (SiO₂). To help set a consistent binder paste, clay-derived metakaolin with an 8.5 µm particle size and a specific gravity of 2.4 was used. X-ray fluorescence (XRF) analysis revealed that its composition was primarily 52% SiO₂ and 46% Al₂O₃. Fine calcium carbonate powder, with a 9.4 µm particle size and a 2.6 specific gravity, was included to provide necessary nucleation sites for crystal growth. This powder was composed of 98% calcium oxide (CaO). Finally, oxalic acid (C₂H₂O₄), a weak organic acid, was used as a catalyst and chelating agent to improve the iron solubility, prevent oxidation, and promote the formation of siderite. All materials were prepared and mixed using portable water available in the laboratory at room temperature.

3. METHODOLOGY AND MIX PROPORTION

The red mud used in this study was sintered through the process of thermal decomposition. The red mud was heated to a temperature of 750°C for a period of 6 hours and then allowed to cool in the oven. The sintering process was carried out in a box furnace, where the crucible was rotated every 30 minutes to allow equal heat distribution through the processed red mud. The maximum temperature was reached in 3 hours and is maintained at 750°C for 6 hours, then returned to the room environment over the next 3 hours. The sample is tested for its chemical characteristics through XRD and presented in Figure 1. The presence of hematite (Fe₂O₃) and katoite is predominant in the available red mud. The decrease in characteristic hematite peak intensities for sintered red mud at 33.1° and 43.2° in 2θ suggests a mineralogical phase transformation during the sintering process. This suggests that crystalline hematite has either reacted to form new complex mineral phases or has been incorporated into an amorphous glassy matrix, rather than a reduction in the total iron oxide content [⁴⁹, ⁵⁰].

Figure 1
X-ray diffraction analysis of red mud (unsintered and sintered at 750°C).

The sintering process in red mud increases the SiO₂ and CaO contents and reduces the Fe₂O₃ and Al₂O₃ contents. The density of unsintered red mud is 2.65 g/cm³, and the density of the red mud increases to 3.12 g/cm³ after the sintering process. The particle size distribution of all raw materials, along with the SEM and XRD analysis in an early study, ensures the powder matrix bond of all together and the possibility of formation of an iron carbonate binder [⁵¹]. The novel iron carbonate binder was prepared with conventional compounds like iron powder, calcium carbonate, metakaolin and oxalic acid as per earlier works [⁴⁵, ⁵²,⁵³,⁵⁴,⁵⁵], and the variation is that red mud was being used in this study instead of fly ash. The red mud used was adopted in two different forms: the iron carbonate binder with unsintered/raw red mud was denoted as the USRM sample, and the iron carbonate binder with sintered red mud was designated as the SRM sample. The USRM and SRM mixes utilize the same by weight proportion of all raw materials. The proportions of the constituent materials were based on the total batch size of 1 kilogram. The iron powder was 600 g (or 60% of the total mix), red mud was 200 g (or 20% of the total mix), calcium carbonate (CaCO₃) was 80 g (or 8% of the total mix), metakaolin was 80 g (or 8% of the total mix) and oxalic acid was 40 g (or 4% of the total mix). The mix proportions are presented in Table 1.

Table 1
Mix proportion of iron carbonate binders of SRM and USRM.

MIX ID	IRON POWDER	RED MUD	CALCIUM CARBONATE	METAKAOLIN	OXALIC ACID
SRM/USRM	600 g	200 g	80 g	80 g	40 g

• *SRM and USRM represent sintered and unsintered red mud, respectively.

Figure 2 shows the thermogravimetric analysis profiles of the USRM and SRM powders. The USRM exhibited a higher and continuous mass loss over a temperature range that indicated the presence of thermally unstable phases. At 150°C, the free water that is present in the red mud is removed, and between 200 and 400°C, the temperatures are attributed to the iron hydroxide phases and dehydroxylation of aluminium, mainly gibbsite and goethite [⁵⁶, ⁵⁷]. This temperature stage confirms that the USRM retains a significant amount of the hydroxylated and hydrated phases. The temperature between 400 and 700°C is due to the decomposition of carbonate phases. Overall, the USRM shows a higher mass loss with lower thermal stability. The SRM showed a smooth TGA curve and reduced mass loss because of the effect of the sintered red mud. No sharp mass loss in the temperature range of 200–400°C because the hydroxylated phases are decomposed during the sintering process [⁵⁸]. In this process, gibbsite and goethite are transformed into hematite, which is a more stable iron oxide phase. At higher temperatures, decarbonation was lower than that of the USRM. This indicates that the carbonate phases were more stable with amorphous calcium–iron–aluminosilicate phases [⁵⁹]. The potable water was used as the mixing agent at a proportion of 0.25 as the water/binder ratio. All the raw materials were mixed well for 5 minutes in a mortar mixer to achieve a consistent mix, and the mix was cast in a 50 mm cube. The molds containing fresh SRM and USRM binder paste were promptly placed in the carbonation curing chamber for a duration of six days. The CO₂ gas was replenished at a pressure of 50 kg/cm² at regular intervals to sustain the saturation level within the chamber. The temperature and relative humidity (RH) were regulated at 30 ± 2°C and 85 ± 5%, respectively, within the carbonation chamber. Following a duration of 6 days, the samples were subjected to atmospheric exposure and subsequently evaluated for compressive strength at intervals of 7, 14, 28, 56, and 90 days. The samples were cured for six days to examine the effect of the binder in a CO₂ atmosphere. Figure 3 shows the various stages of the sample casting regime, where the sample is prepared in a fresh state and then moulded and CO₂-cured.

Figure 2
Thermogravimetric analysis graph of USRM and SRM.

Figure 3
Red mud-based iron carbonate binder sample preparation, curing and sampling (A) iron carbonate binder in fresh state (B) iron carbonate binder casting (C) CO₂ curing (D) cured sample ready for testing.

3.1. Experimental methods and study methods

The methodology of this study is illustrated in Figure 4. According to IS: 516-2021, the compressive strength was assessed using a 50 mm cube specimen formulated with an SRM and USRM-based binder. The compressive strength test was performed at 7, 14, 28, 56 and 90 days using a compression testing machine with a maximum capacity of 2000 kN (Shimadzu, Concreto 2000X). The compressive strength was derived from the mean values of the three samples. The cube samples underwent compression testing at a loading rate of 0.5 MPa/s, with continuous pressure applied until total breakage of the specimens occurred. Morphological examinations of both SRM and USRM samples were conducted using high-resolution scanning electron microscopy (HR-SEM). The samples were analyzed using a Thermo Fisher FEI QUANTA 250 FEG scanning electron microscope. X-ray diffraction (XRD) analysis was performed with a Rigaku SmartLab diffractometer (3 kW) to examine the microstructure characteristics of both SRM and USRM samples. The samples were placed in metal holders after being ground into a powder using an agate mortar and pestle. The diffraction patterns were analysed using Cu Kα radiation at 40 kV and 40 mA to capture the diffraction patterns across a 2θ range of 5° to 80°, with a step increment of 0.02° and a scanning rate of 5° per minute.

Figure 4
Study methodology.

4. RESULTS AND DISCUSSIONS

The samples of USRM and SRM are studied for their strength and microstructural characterization through the compressive strength, scanning electron microscopy (SEM), XRD and TGA/DTA. The results are presented in Figures 5–13, and detailed discussions of the results are presented. The results showed a difference in the performance of iron carbonate binder with sintered and unsintered red mud as the source powder.

Figure 5
Compressive strength of red mud iron carbonate binder with SRM and USRM.

Figure 6
Scanning electron microscopy image of USRM.

Figure 7
Scanning electron microscopy image of SRM.

Figure 8
Color coded particle analysis on SRM composite microstructure (A) image colorization 1(B) image colorization 2 (C) particle analysis (D) pseudo color coded image.

Figure 9
Color coded particle analysis on USRM composite microstructure (A) image colorization 1(B) image colorization 2 (C) particle analysis (D) pseudo color coded image.

Figure 10
X-ray diffraction graph for USRM.

Figure 11
X-ray diffraction graph for SRM.

Figure 12
Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) graph of USRM.

Figure 13
Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) graph of SRM.

4.1. Compressive strength

The Figure 5 illustrates the compressive strength development for iron carbonate binders using SRM and USRM over 90 days. Following a six-day carbon curing period, SRM binders exhibited superior strength and more rapid reaction kinetics than USRM counterparts. Notably, SRM achieved an early-age strength of 22 MPa by day seven representing 91% of its 28-day peak of 24.12 MPa confirming its effectiveness as a rapidly reacting cementitious system. This level of performance is highly significant, as it is comparable to, and in some cases exceeds, the compressive strengths reported for other advanced binder systems derived from industrial wastes [⁵¹, ⁶⁰,⁶¹,⁶²,⁶³,⁶⁴]. For instance, research on alkali-activated materials combining red mud and slag has reported 28-day strengths over 24 MPa, highlighting the potential of sintered binders for practical construction applications. Beyond the 28-day mark, the most striking characteristic of the sintered binder was its exceptional stability. The compressive strength remained virtually unchanged, measuring 24.10 MPa at 56 days and 24.12 MPa at 90 days. This flat strength curve suggests that the binder matrix formed was chemically stable and microstructurally robust, reaching a state of equilibrium without undergoing any deleterious long-term changes. The combination of rapid strength gain and long-term stability is highly desirable for construction materials.

In contrast, the USRM binder exhibited significantly inferior mechanical performance. Its strength development was minimal, reaching only 13 MPa after 7 days. While using raw red mud alone in alkali-activated systems can result in strengths below 5 MPa, the 13 MPa achieved here suggests that the intended iron carbonation mechanism is indeed active, but its efficiency is severely limited without sintering pre-treatment [⁶⁵]. The USRM binder exhibited nominal strength evolution, characterized by inconsistent growth that suggests fundamental instability within its matrix. After a marginal increase to 13.25 MPa at 14 days, the material showed slight but steady gains, reaching 13.98 MPa at 28 days and 14.14 MPa by day 90. This uneven variation implies an incomplete binding mechanism, potentially resulting in a porous, inferior microstructure or thermodynamically unstable reaction products prone to deleterious secondary reactions. A quantitative comparison highlights the transformative impact of sintering: at the 28-day benchmark, the 24.12 MPa compressive strength of the sintered binder (SRM) represents a 72.5% increase over the 13.98 MPa measured for the unsintered USRM. This disparity signifies a fundamental shift in material performance rather than a mere incremental improvement. The USRM samples exhibited minimal strength development, increasing from 13 MPa at 7 days to 14.14 MPa at 90 days. Compared to the 7-day specimens, the compressive strength grew by 1.92%, 7.54%, 7.85%, and 8.77% at 14, 28, 56, and 90 days, respectively. In contrast, the SRM samples demonstrated significantly higher performance, rising from 22 MPa to 24.12 MPa over the same period. Their strength increased by 8.18%, 9.64%, 9.55%, and 9.64% at the 14, 28, 56, and 90-day intervals relative to the 7-day strength. The divergent performance of these two binders, one achieving high, stable strength and the other showing low, constant growth, suggests fundamentally different underlying reaction pathways and resulting binder products. The stability of the sintered binder (SRM) indicates the formation of a durable, likely crystalline matrix composed of thermodynamically stable reaction products, such as carbonates, which are absent or poorly formed in the USRM matrix [⁶⁶]. Conversely, the decay observed in the USRM binder suggests potential microstructural instability. This could arise from the formation of poorly crystalline or metastable phases that slowly reorganize over time, or it could be symptomatic of deleterious chemical reactions. The literature on alkali-activated red mud systems notes that the high free alkali content in raw red mud can lead to issues like efflorescence, which can create internal stresses and disrupt the binder matrix [⁶⁷, ⁶⁸]. Therefore, the performance difference is not merely quantitative (a matter of higher versus lower strength) but is deeply qualitative, reflecting the formation of a stable, engineered matrix in one case and an unstable, weakly bound composite in the other. The data unequivocally establishes that sintering is not an optional enhancement but a critical, enabling step that is essential for unlocking the cementitious potential of red mud in this iron carbonate binder system. This behavior suggests an underlying instability within the unsintered binder material. Based on the properties of raw red mud and cement chemistry, the USRM binder’s reduced strength compared to the SRM samples likely comes from incomplete carbonation and high free alkalinity. In the unsintered system, iron carbonation may be kinetically hindered, forming metastable or poorly crystalline intermediates like iron oxyhydroxides instead of stable siderite. Over time, these phases may recrystallize into less structurally effective forms, compromising cohesion. Furthermore, raw Bayer red mud contains significant soluble sodium compounds, such as residual sodium hydroxide and aluminates, which likely exert deleterious effects on the binder’s durability and matrix integrity. During sintering, these alkalis are chemically bound into stable mineral structures like nepheline or complex ferrites, effectively immobilizing them [⁶⁹]. In the unsintered material, these alkalis remained free within the pore solution. However, high alkali concentrations are known to be problematic in cementitious systems. Over time, the migration of these soluble sodium salts to the surface can lead to efflorescence, a phenomenon that can induce internal stresses and micro-cracking [⁶⁰]. While the observed strength loss is minor, it may be a macroscopic symptom of this ongoing, disruptive chemical activity within the pore structure.

The strength reduction in the USRM binder with respect to the SRM binder should be viewed as a critical behavior in terms of its long-term performance. This highlights the fundamental chemical instability that the sintering process effectively corrects. Sintering not only boosts the initial strength by creating reactive phases but also enhances long-term durability by immobilizing deleterious components like free alkalis. This elevates the importance of the sintering step from being merely a performance-enhancer, which is a critical requirement for producing a reliable and durable construction material from red mud.

4.2. Scanning electron microscopy

The profound difference in mechanical performance between iron carbonate binders prepared with USRM and SRM is directly reflected in their microstructural characteristics. An examination of the scanning electron microscope (SEM) images for both samples is presented in Figures 6 and 7, and this provides compelling visual evidence that corroborates the compressive strength data and supports the proposed mechanisms of binder formation.

Figure 6, the SEM image of the USRM sample, reveals a porous, loosely packed, and poorly consolidated microstructure. The overall texture was granular, with significant void spaces visible between the particle agglomerates. This high porosity is a clear indicator of an inefficient binding mechanism, where the reaction products fail to form a continuous, dense matrix capable of effectively holding the constituent particles together.

The magnified inset provides a detailed view of the crystalline product morphology formed during the carbonation process. This image is primarily characterized by clusters of flaky, rosette-shaped crystals. Such a distinct morphology, frequently described as acicular, fibrous, or spherulitic is highly indicative of chukanovite (Fe₂(CO₃)(OH)₂), an iron hydroxy-carbonate mineral typically found as a corrosion product of iron within carbonate-rich environments. While siderite (FeCO₃) is also a predicted byproduct, it generally manifests as equant, rhombohedral, or globular crystals. Therefore, the prevalence of this chukanovite-like morphology suggests it is a significant, if not dominant, crystalline phase in this specific system. This open and poorly interlocked microstructure directly accounts for the inferior mechanical properties observed in the USRM binder. Within this framework, the voids function as stress concentration points, while the weak connections between crystalline clusters offer minimal resistance to fracture, resulting in low overall strength and potential long-term structural instability. In contrast, the SEM image of the SRM sample in Figure 7 displays a dense, compact, and largely monolithic microstructure. At the same magnification, the material appears significantly more coherent and homogeneous, showing a marked reduction in visible porosity. Individual particles are thoroughly embedded within a continuous binder matrix, creating a solid, well-consolidated mass. The magnified inset reinforces this, revealing a dense, granular texture where individual crystalline phases are tightly intergrown and difficult to distinguish. This morphology is consistent with the formation of a sophisticated, multi-phase composite binder. As proposed in the mechanistic analysis, the thermal activation of red mud via sintering creates reactive phases such as dicalcium silicate (C₂S). Upon carbonation, these phases react alongside iron components to form a complex binder system. This system consists of iron carbonates acting as crystalline fillers, calcium carbonate precipitating as fine rhombohedral crystals that serve as densifying micro fillers, and hematite, which provides the primary adhesive cohesion. The dense, relatively featureless appearance of the SRM matrix in the SEM imagery serves as evidence of this composite system. Amorphous carbonates effectively fill the interstices between precipitated iron and calcium carbonate crystals, resulting in a tightly interlocked, low-porosity microstructure. These efficient pore-filling and robust interfacial bonding are directly responsible for the superior mechanical performance, specifically the 28-day compressive strength of 24.12 MPa and its excellent long-term stability.

A detailed examination of the particle analysis performed on SEM images of the SRM and USRM composites provided quantitative validation of the profound microstructural differences between the two materials. The application of threshold detection, particle segmentation, and color-coding deconstructs the complex morphologies, revealing the statistical and spatial signatures of the distinct formation pathways governing each sample.

4.3. Analysis of the sintered red mud (SRM) composite microstructure

The particle analysis of the SRM composite (Figure 8(A)–(D)) reveals a fundamentally heterogeneous, bimodal microstructure consisting of large crystalline growths embedded in a fine-particulate matrix. Software segmentation based on contrast thresholds (Figure (A) and (B)) identified 13,465 particles, yielding a high density of 8.25 million particles per mm². Despite this density, surface coverage is only 36.03%, with a small mean particle area of 0.04372 µm². These values quantitatively confirm a loosely consolidated matrix where extensive void spaces were correctly excluded from analysis. Color-coding in Figure 8 clarifies this bimodality: large, rosette-like crystalline growths are highlighted in red as the coarse phase, while the smaller green and blue-coded particles represent the background matrix. This mapping visualizes the sample’s heterogeneity, showing discrete, in-situ grown crystals within a porous, fine-grained environment. Furthermore, the pseudo-color topographical image (Figure 8(D)) shows the rosettes in prominent yellow and red hues, indicating they project significantly from the recessed, blue-toned matrix. This supports a solution-precipitation mechanism where large crystals nucleate and grow into available pore spaces, creating a complex 3D architecture.

4.4. Analysis of the unsintered red mud (USRM) composite microstructure

Particle analysis of the USRM composite (Figure 9(A)–(D)) reveals a more homogeneous, densified microstructure consistent with high-temperature sintering and fusion. Segmentation (Figure 9(A) and (B)) identifies granular, equiaxed particles fused into large agglomerates. Although the total particle count is higher (17,769), the density is significantly lower than the SRM sample at 2.72 million particles per mm². This lower density is explained by a higher surface coverage (40.05%) and a mean particle area of 0.1473 µm² over three times larger than the SRM sample. These metrics confirm that the identified units are larger grains formed during successful sintering rather than discrete fine powders. Color-coding (Figure 9(C)) reinforces this interpretation; unlike the bimodal SRM, the USRM shows a continuous size distribution where red, green, and blue particles are interspersed uniformly. This lack of phase separation signifies a single, continuous phase typical of thermal agglomeration. Furthermore, the topographical map (Figure 9(D)) shows a relatively uniform surface. While slight variations exist, the features blend smoothly into a consolidated, planar structure where fused particles have effectively eliminated extreme height differences and porosity.

The quantitative particle analysis and color-coding provide compelling, data-driven evidence for the two divergent formation mechanisms. The SRM composite analysis is characterized by high particle density, low mean area, and a bimodal color distribution dominated by large red rosettes quantitatively describes a porous, heterogeneous structure formed by the in-situ crystallization of a new phase. The USRM composite analysis is characterized by a lower particle density, a significantly larger mean area, and a homogeneous color distribution that quantitatively describes a dense, fused structure formed by an ineffective activation or product formation process due to its unreactivity.

4.5. X-ray diffraction analysis

The XRD pattern of the USRM sample, presented in Figure 10, reveals the presence of three distinct and well-defined crystalline phases superimposed on a relatively noisy background. The identification of these phases was based on the labelled peaks of calcite (C), siderite (S) and chukanovite (Ch). In calcite, the most prominent feature in the diffractogram was a sharp, high-intensity peak located at approximately 29.5° 2θ. This peak corresponds to the (104) crystallographic plane of calcite (CaCO₃), which is the most thermodynamically stable polymorph of calcium carbonate under ambient conditions [⁵⁸]. Calcite crystallizes in a trigonal system (space group R3ˉc) and is a common mineral that may be present in the original red mud or added as a fluxing agent. The siderite (S) has several distinct peaks of moderate intensity that are identified as siderite (FeCO₃), labeled ‘S’. Key reflections are visible at approximately 32.0°, 42.5°, and 61.0° 2θ. Siderite is an iron carbonate mineral that is isostructural with calcite, belonging to the same trigonal crystal system and space group (R3ˉc). Its presence confirms the incorporation of a primary iron carbonate phase into the binder formulation. Chukanovite has a third set of peaks, labeled ‘Ch’, and is attributed to the mineral chukanovite (Fe₂(CO₃)(OH)₂). Characteristic peaks for this phase are observed at approximately 24.0°, 34.0°, and 47.0° 2θ. Chukanovite is a ferrous hydroxy-carbonate, meaning its structure contains both carbonate and hydroxyl functional groups [⁷⁰, ⁷¹]. It crystallizes in the monoclinic system (space group P21/a), which is structurally distinct from the calcite group minerals. The presence of hydroxyl groups suggests that this phase exhibits different thermal decomposition behavior compared to anhydrous carbonates. The overall pattern of the USRM sample shows sharp, well-defined peaks, indicating good crystallinity of the additive phases. The elevated and noisy background is characteristic of red mud, which contains a mixture of crystalline phases (such as hematite, goethite, and quartz) and poorly crystalline or amorphous aluminosilicate components. A direct visual comparison between the XRD patterns of the USRM sample in Figure 10 and the SRM sample in Figure 11 reveals a profound change in the mineralogy of the material. The most striking observation is that while all three original crystalline phases, calcite, siderite, and chukanovite, were still identifiable in the SRM sample, the intensities of their corresponding diffraction peaks have been significantly reduced. This indicates that a substantial portion of each phase was consumed or transformed during the sintering process.

To provide a more rigorous, semi-quantitative assessment of these changes, the characteristics of the principal diffraction peaks for each phase in both samples are compared in Table 2. The relative intensity change confirmed that all three carbonate phases have undergone partial decomposition.

Table 2
Comparison of principal XRD peak characteristics for USRM and SRM binders.

MINERAL PHASE	LABEL	APPROX. 2θ (°)	RELATIVE PEAK HEIGHT (USRM, ARBITRARY UNITS)	RELATIVE PEAK HEIGHT (SRM, ARBITRARY UNITS)	INTENSITY DECREASE (%)
Calcite	C	29.5	100	65	~35%
Siderite	S	32.0	38	23	~40%
Chuknovite	Ch	24.0	25	15	~40%
Siderite	S	42.5	30	18	~40%
Siderite	S	61.0	35	20	~43%

The data in Table 2 clearly show a substantial decrease in the crystalline content of all three primary phases. The intensity of the main siderite and chukanovite peaks decreased by approximately 40% in SRM, while the calcite peak, although still the most prominent in the SRM pattern, was diminished by about 35%. This uniform reduction across all phases is direct and compelling evidence of widespread thermal decomposition initiated by the sintering process.

Given the clear evidence for the decomposition of the initial carbonate phases, a careful examination of the SRM diffractogram was conducted to identify any new crystalline phases that may have formed. The expected solid products from the thermal decomposition of calcite, siderite, and chukanovite are calcium oxide (CaO) and various iron oxides, such as hematite (α−Fe₂O₃) or magnetite (Fe₃O₄). The primary diffraction peaks of crystalline CaO are expected at 2θ values of approximately 32.2°, 37.3°, and 53.9° [⁷²]. The most intense peaks for hematite were expected around 33.2°, 35.6°, and 49.5° 2θ [⁷³]. The characteristic peaks for magnetite occured at approximately 30.1°, 35.5°, 43.1°, and 62.6° 2θ [⁷⁴].

Despite the significant decomposition of the primary minerals, the SRM pattern in Figure 11 does not show the emergence of any new, sharp, high-intensity peaks that could be clearly assigned to these oxide products. Some of the expected peak positions for the new oxides overlapped with the residual peaks of siderite and calcite, and the main CaO peak at 32.2° was very close to the main siderite peak at 32.0°, which could mask their presence at low concentrations. However, the absence of other distinct, non-overlapping peaks for these phases is a critical finding. This suggests that either the oxide products are present in very small quantities, or, more likely, they exist in a poorly crystalline or amorphous state, which does not produce sharp diffraction peaks. This observation is central to understanding the true nature of the transformations that occur during sintering.

4.6. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA)

The differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were conducted to evaluate the phase transformation, phase reaction mechanisms, and thermal stability of the red mud iron binder, and the results are presented in Figures 12 and 13. Figure 12 shows the DTA and TGA curves of the USRM sample, and Figure 13 shows the DTA and TGA curves of the SRM sample. The TGA analysis was conducted from 30°C to 900°C to assess the mass loss of the binder with respect to the temperature increase. This TGA test indicated the highest temperature at which a specimen could retain its stability. DTA was used to determine the endothermic and exothermic types of reactions associated with the chemical transformation of the binder. The USRM binder exhibited a higher mass loss than the SRM binder, which also occurred at multiple phases across various temperature ranges. This indicates that the USRM binder contains hydrated and thermally unstable phases. The initial mass loss at 150°C was due to loosely and physically bound water in binder materials with a porous structure, containing partially crystalline and amorphous phases [⁷⁵]. The second mass loss peak occurred in the temperature range of 200^oC to 450°C, resulting from the dehydroxylation of hydroxide-bearing mineral phases such as gibbsite and goethite, as well as the interaction between oxalic acid and iron powder, which leads to the partial decomposition of iron oxalate complexes [⁷⁶]. This phase exhibited endothermic DTA peaks, indicating that energy was absorbed during the removal of structural water and breakdown of bonds. The third mass reduction occurs in the temperature range of 600 to 800°C, where a significant mass reduction is primarily due to the decomposition of calcium carbonate into carbon dioxide and CaO, as well as the breakdown of siderite, which disrupts the carbonate residual phases [⁵]. The DTA peaks of around 800°C showed a strong endothermic peak in this region with excessive decarbonation reactions. Overall, the higher mass loss observed in the USRM iron carbonate binder confirms the presence of unreacted hydroxides from the thermally unstable phases, which include poorly crystalline structures [⁷⁷]. The SRM binder showed a reduced mass loss with few distinct thermal transformation stages and endothermic/exothermic reactions. These results confirm that the binder was more thermally stable when compared to USRM binders. The first minor weight loss was at a temperature below 150°C, which shows that only a smaller percentage of free water is present in the binders because in the sintering process of red mud, the moisture has been removed earlier and more densely [⁷⁸]. At temperatures above 300°C, there was less mass loss, and the DTA peaks were weak. These data confirm that the hydroxide phases were broken down during the sintering of the red mud. As a result, the minerals present in the red mud have changed into more stable iron oxide phases, such as hematite, so there are fewer dehydroxylation phases and therefore further heating [⁵⁷]. At high temperatures, the mass loss was less when compared to USRM binders. This effect is because the carbonate decomposes and undergoes structural changes, while some carbonate phases are partially decomposed by the sintering process of red mud. The DTA peaks were weaker and wider in the endothermic range, indicating that the changes in the phases occurred gradually rather than through decomposition [⁷⁹]. As a result, SRM binders have more stable carbonated phases with partially amorphous calcium iron aluminosilicate phases.

4.7. Overall discussion

This research demonstrates that thermal sintering of red mud at 750°C is a critical, transformative step in the development of high-performance iron carbonate binders. The results of the mechanical, microstructural, and mineralogical analyses showed a strong correlation, establishing sintering as an essential activation process rather than a mere enhancement. The SRM binder achieved a 28-day compressive strength of 24.12 MPa, a 72.5% improvement over the 13.98 MPa recorded for the USRM binder. SEM analysis revealed a dense, monolithic matrix in which individual particles were thoroughly embedded, resulting in low porosity and high cohesion in the SRM samples, whereas the USRM binder exhibited a porous, loosely packed, and granular texture. The presence of voids acting as stress concentration points directly correlates with their inferior mechanical performance and limited early-age strength. XRD analysis confirms that sintering fundamentally alters the red mud mineralogy to create reactive phases [⁴⁹, ⁵⁸, ⁶⁹]. The sintering induces the formation of reactive compounds that work synergistically with iron carbonation to create a complex, multi-phase binder of iron and calcium carbonates. The sintered binder showed exceptional long-term stability, maintaining strength for 360 days, whereas the USRM binder exhibited less strength achievement, likely due to high free alkalinity and the formation of metastable phases like chukanovite. The strength development of sintered red mud samples, even though it has less red mud, 20% red mud produces reactive Fe, which in turn is activated during carbon curing and forms stable iron carbonate phases, thereby producing strength enhancement and better microstructural behavior [⁴⁰, ⁴², ⁶⁷]. The DTA and TGA results show that the sintering process improves the reactivity and thermal behaviour of the red mud. The SRM iron carbonate binder was thermally more stable, with less bound water and the presence of amorphous reactive phases. Conversely, the USRM binders are highly unstable thermally hydroxides and have a higher mass loss and significant heat-absorbing reactions. It has been already studied and established through Life Cycle Assessment on iron carbonate binder with fly ash that approximately 85% lower fossil-based global warming potential and over 80% reductions in water consumption compared to Ordinary Portland Cement, demonstrating a potable-water-free, resource-efficient binder suitable for circular and climate-resilient infrastructure [⁸⁰]. Further, it was observed through cradle-to-gate Life Cycle Assessment (LCA) following ISO 14040/44 guidelines on iron carbonate binder that the binder is carbon negative through CO₂ sequestration with reductions of 30% in human health impacts, 37% in ecosystem damage, and 34% in resource depletion, also it improves the air quality by negative photochemical ozone formation [⁸¹]. This indicates the iron carbonate binder is an effective green material, and the red mud-based iron carbonate binder shall have enhanced LCA, and even though the sintering process reduces the greener impact, the possibility of a carbon-negative binder is high.

5. CONCLUSION

A study of an iron carbonate binder comprising conventional materials along with sintered and unsintered red mud showed that the sintering process effectively consolidates the performance of the binder.

1. The sintering pre-treatment method is a highly effective and essential step for producing high-performance binders from red mud.

2. The SRM samples significantly improve the mechanical performance of the binder, achieving the 28-day compressive strength of the binder by more than 85%, increasing its strength from 12.98 MPa (USRM) to 24.12 MPa (SRM), which is attributed to the formation of a dense, composite binder matrix. It demonstrates the highly effective improvement in strength development.

3. The SEM analysis revealed that the USRM samples were highly porous and had poor particle interlocking with a weakly bonded structure and poorly interlocked, flaky crystals, resulting in low and unstable strength, whereas the SRM samples formed a dense, monolithic matrix and compact composite matrix with the formation of carbonate phases such as calcite and siderite, which contributed to enhancing the mechanical performance of the red mud iron binder.

4. The dense microstructure effectively minimized porosity and maximized cohesion, leading to a high, stable compressive strength. The SEM images show that sintering pre-treatment is the critical factor that enables the formation of a high-performance, composite binder matrix, transforming the red mud from a simple component into a reactive precursor for a durable construction material.

5. XRD analysis revealed the formation of CaCO₃, FeCO₃, and chukanovite in both the SRM and USRM samples. In the carbonated SRM samples, the reduction in diffraction peak intensities indicates partial thermal decomposition and the formation of poorly crystalline or amorphous reactive phases, which contribute to the improved binder strength development.

6. The long-term strength stability of the sintered system indicates the formation of a robust and durable final product. In contrast, the slight but consistent strength retrogression observed in the unsintered system highlights potential long-term durability issues, which are likely linked to the high concentration of free, untransformed alkalis inherent in raw red mud.

7. The principal result of this study was not the creation of a single mixture of residual carbonates and well-crystallised simple oxides. The newly formed and highly reactive oxides (CaO, Fe₂O₃, and Fe₃O₄) produced during decomposition rapidly reacted with the active aluminosilicate and titanate constituents of the red mud matrix.

8. In summary, it was concluded that the sintered red mud provides effectively enhances the strength and microstructural characterisation.

6. ACKNOWLEDGMENTS

The authors acknowledge the Vellore Institute of Technology, Chennai, for their essential support in executing this work. Also, we extend our sincere appreciation to Dr. Shanmugasundaram M, Associate Professor, School of Civil Engineering, Vellore Institute of Technology, Chennai, for his invaluable insights that greatly contributed to the completion of this work.

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Publication Dates

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

History

• Received
  27 Aug 2025
• Accepted
  04 Mar 2026