Heat-resistant cementitious composites based on binders derived from cement and metallurgical waste

Abstract

One of the main aspects of consideration in building safety and durability in fire and extreme conditions is the high-temperature performance of cementitious composites. The paper is concerned with the construction of alumina-rich binder systems using industrial by-products like bauxite residue and perlite to enhance the thermal stability as well as sustainability. The primary goal was to develop and test composites that could sustain mechanical integrity at exposure levels up to 1380^o C. The experimental program included the preparation of three mixtures that were mixed using various ratios and proportions of reactive alumina and bauxite residue with water. The size distribution of all the granular parts was also analyzed in order to determine packing density. Tests were done on compressive strength, porosity, and thermal resistance, and SEM analysis were done to determine any changes in the microstructure post-heating. This hybrid method allowed for correlating the mixture design, physical properties, and performance at high temperatures. The findings showed that optimized formulation (F3) was able to reach compressive strength over 52 MPa at ambient temperature, and it could withstand a compressive strength higher than 70% even when subjected to 1200^o C. Reduced water to binder ratios and a greater proportion of fine reactive alumina were key factors in enhancing the density and mechanical strength of the composites, while bauxite residue promoted the formation of stable crystalline phases during heating. SEM analysis confirmed the densification of the matrix, and the chemical composition suggests the development of thermally stable phases upon heating. These results provide a positive relationship between mix design parameters, microstructural evolution, and macroscopic performance. Finally, the research offers evidence that cementitious composites with a controlled mixture design that are composed of alumina can be used to achieve a high compressive strength with outstanding thermal stability. This emphasizes their applicability in construction and infrastructure vulnerable to intense thermal loads and hostile to fire-resistant materials, as well as encourages the valorization of industrial waste materials.

Introduction

In contemporary construction, fire safety is a critical issue owing to the growing rate of severity of fire incidents across the world and the need to have less vulnerable infrastructure¹. Portland cement-based materials continue to be the most common materials used structurally, but they are characterized by serious drawbacks when subjected to high temperatures, such as rapid deterioration of strength and integrity resulting from the dehydration of hydration products^2,3. Such weaknesses underscore the urgency to integrate alternative cementitious systems that could ensure the mechanical and structural stability of the building could survive the fire events⁴.

The newer study has been focused on coming up with new highly enhanced composites that have high thermal resistance, mostly using alumina-based binders^5,6. Especially, aluminous cement has been promising due to its capacity to stabilize crystalline phases, which include gehlenite and calcium aluminates under the influence of heat⁷. These steps lead to high-temperature strength, and experiments have shown that strength remains at temperatures of 1200 °C⁸ . Moreover, aluminous cement has been reported to have rapid setting properties, sulfate resistance, and enhanced durability in hostile conditions; hence, it can be used in special applications^9,10. Still, the mixture design optimization is necessary, since the performance may be affected by the insufficient hydration or the absence of the correct curing¹¹.

The emergence of fire-resistant materials has also incorporated the idea of sustainability, in which the industrial by-products are used as working raw materials¹². They include, among others, bauxite residue, which is a large by-product of alumina smelting and contains large amounts of alumina and iron oxides, which form thermally stable phase assemblies^13,14. Its application in cementitious composites not only increases the resistance to high temperatures but also solves severe challenges of waste disposal¹⁵. It has been found that, when a partial replacement of aluminous cement with bauxite residue was used in place of primary raw materials, the porosity decreased, and densification became better when the material was heated^16,17. Besides that, this residue valorization follows the principles of a circular economy and decreases the ecological footprint of construction materials¹⁸.

The valorization of bauxite residue extends beyond the high-temperature systems explored in this study. Research into its use as a supplementary component in blended Portland cements is ongoing, though often limited by its high alkalinity, which requires pre-treatment to enhance reactivity¹⁹. A particularly promising alternative is its incorporation into alkali-activated materials (geopolymers), where it can serve as a primary aluminosilicate precursor alongside other industrial by-products to form sustainable binders for various construction applications²⁰. The present work contributes to this diverse landscape by specifically investigating the efficacy of bauxite residue within aluminous cement systems tailored for refractory applications, thereby addressing a critical niche in sustainable material development.

Lightweight aggregates have also been at the center stage in enhancing thermal insulation properties of refractory composites. The volcanic glass perlite, an inorganic naturally occurring substance, which expands during heating, has been utilized widely in cementitious materials to decrease thermal conductivity^21,22. Experiments have verified that composites with perlites have reduced heat transfer in the fire conditions, which reduces the thermal stress levels and delays structural damage²³. Perlite has two unique properties, which enable it to serve as both a lightweight filler and an insulating agent, thus making it an attractive high-performance composite constituent^24,25. Perlite, when used together with aluminous cement matrices, gives synergistic enhancement in thermal resistance and mechanical stability²⁶.

The microstructural aspects of fire-resistant composites control the performance of the material based on the porosity, crack growth, and the phase change under heating²⁷. These mechanisms have been studied by using advanced characterization techniques like X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis²⁸. Findings indicate that the stability in the crystalline phase, especially the conversion of hydrates to stable anhydrous products, dictates the residual high-temperature strength²⁹. In addition, research highlights that refinement of pores and densification of matrices play a key role in decreasing spalling and mechanical erosion³⁰. Nevertheless, most of these studies have focused on individual properties one at a time and have not associated microstructural findings with mechanical and thermal performance in a holistic fashion³¹.

The available literature also mentions variability in the reported results, which is caused by variations in ratios of water to binder, particle size distributions, and tests³². As an example, certain experiments are based on the standard fire curves (cellulose or hydrocarbon-based), whereas others use the maximum-temperature-to-failure test, which creates hardship with direct comparison³³. This discrepancy highlights the necessity of a single model that would determine the fire-resistant performance involving the incorporation of thermal stability, strength retention, and microstructural development³⁴. Laboratory findings will be disjointed and will not translate into useful fire safety solutions without this kind of integration³⁵.

Overall, although past studies have contributed to a great extent to the evolution of aluminous cement systems, the use of industrial by-products, and the microstructural behavior, there is still a need to conduct holistic studies that would take into account all the critical issues at once. This gap is the driving force behind the given work and attempts to offer a holistic examination of the modified bauxite residue and perlite of the aluminous cementitious composites. The original character of the research is in its combined mode: mechanical strength, thermal resistance, and microstructural evolution are evaluated simultaneously to determine evident relationships between mixture composition, high-temperature exposure, and the material performance. This framework not only promotes the primary knowledge of alumina-based composites but also aids the invention of useful, sustainable, and fire-resistant materials for contemporary infrastructure.

Materials and methods

Materials

The following components were utilized to produce heat-resistant cementitious composites: aluminous cement, perlite, bauxite residue, and reactive alumina. The physical and chemical properties presented in Tables 1 and 2 were determined by the authors using X-ray fluorescence (XRF, EDX9000B, Shimadzu, Japan), X-ray diffraction (XRD, Bruker D8 Advance, Germany, in accordance with ASTM C1365-18), and thermogravimetric analysis (TGA, NETZSCH STA 449 F3 Jupiter, Germany, according to ISO 11358-1:2014). SEM analysis (Hitachi TM4000Plus, Japan) was additionally performed to confirm particle morphology. The reported values are therefore based on experimental measurements carried out in this study rather than literature data. Tables 1 and 2 present the chemical composition and physicochemical properties of these materials.

Table 1.

Chemical composition of components.

Component	Al₂O₃ (%)	CaO (%)	SiO₂ (%)	Fe₂O₃ (%)	K₂O + Na₂O (%)	Impurities (%)
Aluminous cement	55	35	8	0.05	-	-
Perlite	15	-	72	2.5	4	6.5
Bauxite residue	25	15	10	45	-	5
Reactive alumina	99.2	-	-	-

Table 2.

Particle size distribution of components.

Component	d10 (µm)	d50 (µm)	d90 (µm)	Method
Aluminous cement	2	15	45	Laser diffraction
Perlite	10	150	500	Sieve analysis
Bauxite residue	1	12	35	Laser diffraction
Reactive alumina	0.5	5	20	Laser diffraction

The chemical compositions of the raw materials, detailed in Table 1, show that aluminous cement is rich in Al₂O₃ (55%) and CaO (35%), while bauxite residue provides significant Fe₂O₃ (45%) and Al₂O₃ (25%). Perlite is primarily composed of SiO₂ (72%), and reactive alumina consists of 99.2% Al₂O₃.

Key physical properties (Table 2) rationalize the component selection, contrasting the high density of aluminous cement and reactive alumina with the low density and thermal conductivity of perlite, which serves as an effective insulating aggregate. The distributions are presented in Table 3, showing d10, d50, and d90 values, which correspond to the particle diameters at 10%, 50%, and 90% of cumulative volume, respectively.

Table 3.

Physical and chemical properties of components.

Component	True Density (kg/m³)	Bulk Density (kg/m³)	Normal Consistency (%)	Thermal Conductivity (W/m·°C)	Thermal resistance (°C)
Aluminous cement	3200	1100	24–28	1.3	1400
Perlite	2200	150–200	-	0.04	1000
Bauxite residue	2860–2980	1000–1200	-	1.8	1200
Reactive alumina	3500	900	-	1.5	1600

Particle size distribution was determined using a laser diffraction analyzer (for aluminous cement, bauxite residue, and reactive alumina) and sieve analysis (for perlite). The water used to prepare the binder mixture complies with the requirements of GOST 23,732 − 2011. Table 4 shows its parameters.

Table 4.

Water quality indicators.

Parameter	Value
pH	6.5–7.5
Content of dissolved salts (mg/L)	≤ 1000
Chloride content (mg/L)	≤ 250
Sulfate content (mg/L)	≤ 200

Table 4 provides the quality indicators of water used in binder mixtures. Key parameters include pH levels (6.5–7.5), which meet the requirements for mortar used in construction, ensuring the hydration stability of cement. The content of dissolved salts is limited (≤ 1,000 mg/L) to prevent undesirable chemical reactions that could affect the strength of the cementitious composites. Maximum chloride content is ≤ 250 mg/L, and sulfate content is ≤ 200 mg/L, minimizing the risk of corrosion to reinforcement and destruction of cement paste. These properties ensure that water does not impair the thermal resistance and durability of the heat-resistant cementitious composite. The components discussed above increase temperature resistance and mechanical strength in heat-resistant cementitious composite while optimizing its structural and performance characteristics. The water quality indicators reported in Table 4 were measured experimentally in this study according to GOST 23,732 − 2011 (Potable water for concrete and mortar). The measured values were: pH = 7.1, total dissolved salts = 740 mg/L, chloride content = 180 mg/L, sulfate content = 160 mg/L. All parameters meet the normative limits, ensuring that water does not negatively influence cement hydration, microstructure development, or long-term durability. The low chloride and sulfate concentrations are particularly significant for preventing reinforcement corrosion and improving fire resistance stability.

Mixture Preparation

Several formulations (F1, F2, F3) of heat-resistant cementitious composite based on aluminous cement, perlite, bauxite residue, and reactive alumina were developed and tested in this study. To ensure adequate workability, the water-to-binder ratio (W/B) was precisely determined for each mixture. The ratios were as follows: 0.25 for F1, 0.26 for F2, and 0.27 for F3. These values were obtained experimentally based on normal consistency testing and were maintained consistently throughout all samples. These ratios correspond to water contents of 125 g (F1), 130 g (F2), and 135 g (F3) per 500 g of binder. The absolute water content was experimentally determined based on normal consistency tests, ensuring comparable workability across all mixtures. The developed formulations (F1, F2, F3) represent paste-type binder systems without coarse aggregates. This approach was chosen to isolate the role of aluminous cement, perlite, bauxite residue, and reactive alumina in the cementitious matrix, excluding any additional aggregate effects. The term “composite” is therefore used to describe binder-based refractory pastes rather than conventional concrete with coarse aggregates. The selection of mix proportions was based on preliminary experimental trials, where partial replacement of aluminous cement with reactive alumina improved high-temperature stability, while higher perlite content enhanced thermal insulation. The variation across F1-F3 was designed to systematically evaluate the balance between strength, porosity, and thermal resistance. The principal components and their ratios in each formulation are presented below:

Table 5 presents the compositions of high-temperature cementitious composites with varying proportions of ingredients. This approach made it possible to assess the impact of each component on the strength, density, and thermal resistance of the final product. The variation in the proportion of aluminous cement from 50% to 40% was offset by an increase in reactive alumina content, which enhanced thermal resistance through the formation of more stable aluminate phases. Perlite, utilized as a refractory aggregate, was increased from 20% to 30%, contributing to a reduction in thermal conductivity and improvement in thermal resistance. Bauxite residue content was decreased from 25% to 15%, allowing for control over porosity and mechanical properties. Thus, different proportions of ingredients were tested to find a balance between thermal resistance, mechanical properties, and workability of heat-resistant cementitious composite. The optimal amount of water was added to each formulation to achieve a normal consistency (24–28%). After thorough mixing to ensure uniformity, the formulations were used to mold the samples.

Table 5.

Heat-resistant cementitious composite formulations (%).

Component	F1	F2	F3
Aluminous cement (%)	50	45	40
Perlite (%)	20	25	30
Bauxite residue (%)	25	20	15
Reactive alumina (%)	5	10	15
Water-to-binder ratio(W/B)	0.25	0.26	0.27

Molding

The components for heat-resistant cementitious composites were produced through compaction by vibration. This ensured a uniform distribution of ingredients and minimized air content in the final product. The molding parameters were controlled to adjust the density and mechanical properties of the material. The study tested various parameters, including compaction pressure, vibration time, and molding temperature. The ranges of compaction pressure (10–20 MPa) and vibration time (2–4 min) were selected based on preliminary laboratory trials aimed at minimizing porosity and maximizing strength without causing segregation of the mixture. These parameters reflect practical conditions for forming refractory cementitious composites and are consistent with typical ranges applied in laboratory-scale material development. The molding temperature was kept constant (25 °C) to provide uniform curing conditions. The variations of these parameters are listed in Table 6 below.

Table 6.

Molding parameters of heat-resistant cementitious composites.

Parameter	Value
Compaction pressure	10, 15, 20 MPa
Vibration time	2, 3, 4 min
Molding temperature	25 °C
Sample dimensions	50 × 100 mm (cylinders)

The compaction pressure was set between 10 and 20 MPa to achieve optimal compaction of the mixture. An increase in pressure leads to an increase in strength, while excessive pressing may negatively affect the mobility of the mixture. The vibration time ranged from 2 to 4 min, ensuring an even distribution of ingredients without stratification. The molding temperature was kept at 25 °C, corresponding to standard hardening conditions for cement compositions. After molding, the samples were allowed to rest in the molds for 24 h before being subjected to heat treatment.

Heat treatment

Heat treatment of heat-resistant cementitious composite was performed in stages, with controlled heating and exposure to high temperatures. This resulted in the formation of a strong, crystalline structure. Table 7 below outlines the key processing parameters.

Table 7.

Heat treatment parameters of heat-resistant cementitious composites.

Stage	Heating Rate	Maximum Temperature	Holding Time	Cooling
1. Up to 200 °C	60 °C/hr	200 °C	1 h	In the furnace
2. 200 °C to 1200 °C	150 °C/hr	1200 °C	2 h	In the furnace

In the initial stage, the heating process was carried out at a rate of 60 °C/hr to 200 °C, with a holding time of 1 h, in order to evaporate the residual moisture and stabilize the structure of cement paste. The temperature was then increased to 1,200 °C at a rate of 150 °C/hr, while the samples were held for 2 h. This contributed to the sintering of particles and the formation of crystalline calcium aluminate phases Cooling, as well as increasing thermal resistance was performed in the furnace to prevent thermal stresses and microcrack formation.

The heat treatment protocol was designed to ensure controlled moisture removal and promote the formation of thermally stable crystalline phases, such as calcium aluminates and ferrites, critical for the high-temperature performance of the cementitious composites. The initial heating rate of 60 °C/hr to 200 °C with a 1-hour hold was selected to minimize thermal stresses and prevent microcracking due to rapid dehydration, as supported by studies on refractory materials²⁸. The subsequent increase to 1200 °C at 150 °C/hr with a 2-hour hold facilitates sintering and polymerization of aluminates, enhancing thermal stability, consistent with findings on bauxite residue in cementitious systems^10,13. Cooling within the furnace was implemented to avoid thermal shock, aligning with standard practices for refractory composites²⁹. The maximum-temperature-to-failure test up to 1380 °C was chosen to evaluate the intrinsic thermal stability of the binder system, exceeding typical fire scenario temperatures (1100–1200 °C), as reported in literature on fire-resistant materials. This approach complements standard fire curve tests by providing insights into the material’s ultimate thermal limits, which are particularly relevant for refractory applications.

Rationale of the highest temperature testing protocol

In order to test the intrinsic stability of the binder system developed at high temperatures, thermal resistance was tested through a maximum-temperature-to-failure protocol. The samples were heated as per the above-mentioned staged program and held until a clear structural degradation or failure was observed; the highest reported failure temperature (up to 1,380 °C) was recorded. The material level technique was selected to determine final stage changes, sintering behaviour, and decomposition limits of the binder components that cannot be determined using standard time-temperature fire curves. It is essential to note that this high-temperature test complements standardized structural fire tests (e.g., ISO 834, hydrocarbon, and cellulose fire curves) and is not intended to replace element-level load-bearing fire resistance tests.

Study methods

The following techniques were employed to assess the quality and characteristics of materials, as well as to devise the optimum composition of heat-resistant cementitious composite:

X-ray fluorescence (XRF)

EDX9000B XRF Spectrometer was utilized to determine the chemical composition of the raw materials. The samples were pre-dried, ground, and compressed into discs for analysis. This technique enabled the accurate measurement of the concentration of aluminum, calcium, iron oxides, and other constituents in the materials.

Strength testing

The mechanical attributes of the cementitious composite were assessed using testing devices to measure compressive and flexural strength. The samples were loaded until failure occurred to ascertain their strength characteristics.

Thermal testing

To evaluate the thermal resistance of the cementitious composite samples, they were subjected to temperatures of 1,000 °C or higher in a controlled-temperature furnace. After the heat treatment, the strength of the samples was measured, and any structural changes were analyzed. This procedure was performed to determine how the cementitious composite would behave under prolonged exposure to high temperatures.

Scanning electron microscopy (SEM)

The microstructure of the material was examined using the Hitachi TM4000Plus Scanning Electron Microscope. SEM made it possible to study the size and shape of particles and conduct an elemental analysis to investigate the distribution of components in the structure of the materials.

Water absorption

Apparent porosity was assessed indirectly through the water absorption test, performed in accordance with ASTM C642-13. Oven-dried samples were immersed in water, and the mass gain was measured after 24 h. The water absorption was calculated as . The values reported in Table 8 represent the mean of three replicate measurements for each composition and processing condition.

Table 8.

Results of tests under various compaction and vibration parameters.

F	Compaction pressure (MPa)	Vibration time (min)	Compressive strength (MPa)	Flexural strength (MPa)	Water absorption (%)	Thermal resistance (°C)
1	10	2	42.5	6.5	5.0	1150
		3	43.2	6.6	4.9	1170
		4	43.8	6.7	4.8	1180
	15	2	44.0	6.7	4.7	1200
		3	44.8	6.8	4.6	1220
		4	45.5	6.9	4.5	1230
	20	2	45.2	6.8	4.5	1250
		3	46.0	6.9	4.4	1270
		4	46.8	7.0	4.3	1280
2	10	2	44.8	6.9	4.8	1200
		3	45.5	7.0	4.7	1220
		4	46.2	7.1	4.6	1230
	15	2	46.5	7.1	4.5	1250
		3	47.2	7.2	4.4	1270
		4	47.8	7.3	4.3	1280
	20	2	47.7	7.2	4.3	1300
		3	48.5	7.3	4.2	1320
		4	49.3	7.4	4.1	1330
3	10	2	47.3	7.2	4.5	1250
		3	48.1	7.3	4.4	1270
		4	48.9	7.4	4.3	1280
	15	2	49.0	7.4	4.2	1300
		3	49.8	7.5	4.1	1320
		4	50.5	7.6	4.0	1330
	20	2	50.3	7.5	4.0	1350
		3	51.2	7.6	3.9	1370
		4	52.0	7.7	3.8	1380

The testing methods were conducted following standard practices to ensure reliable and reproducible results. The chemical composition of raw materials was determined using X-ray fluorescence (XRF, EDX9000B, Shimadzu, Japan) according to ASTM C114-18, with samples pre-dried, ground, and compressed into discs for analysis¹⁰. X-ray diffraction (XRD, Bruker D8 Advance, Germany) followed ASTM C1365-18 to identify crystalline phases, as previously reported for similar cementitious systems¹³. Thermogravimetric analysis (TGA, NETZSCH STA 449 F3 Jupiter, Germany) was performed per ISO 11358-1:2014 to assess thermal decomposition behavior²⁸. Compressive and flexural strength tests were conducted on 50/100 mm specimens using a universal testing machine (Instron 5982, USA) in accordance with ASTM C109/C109M-20 for compressive strength and ASTM C348-21 for flexural strength, with a minimum of three specimens per formulation to ensure statistical reliability. Thermal resistance testing involved heating samples in a controlled furnace to 1000 °C and above, following a protocol aligned with ISO 1887:2014 for refractory materials, with residual strength measured post-heating to evaluate performance²⁹. Scanning electron microscopy (SEM, Hitachi TM4000Plus, Japan) was performed on polished post-heating samples to analyze microstructure, with sample preparation involving epoxy mounting and gold coating, consistent with methods described in^10,28. These standardized procedures, supplemented by literature-supported practices, ensure the validity of the results for evaluating the heat-resistant cementitious composites.

All specimen types, including the cylindrical (Ø 50 × 100 mm), prismatic (40 × 40 × 160 mm), and cubic (50 mm) specimens, were compacted under the same pressure range of 10–20 MPa to ensure consistent density and mechanical performance.

For each formulation (F1, F2, F3), at least three prismatic specimens (40 × 40 × 160 mm for flexural strength and 50 mm cubes for compressive strength) were prepared in accordance with ASTM C348-21 and ASTM C109/C109M-20, respectively, to ensure statistical reliability. Additional cylindrical specimens (Ø 25 × 50 mm) were molded for thermal resistance testing in line with ISO 1887:2014. All tests were conducted on a minimum of three replicates per condition, and mean values are reported. A reference aluminous cement paste without bauxite residue, prepared under identical conditions, was used as the control sample to evaluate the specific impact of bauxite residue on strength and thermal performance. This control enabled direct comparison between the modified composites (F1-F3) and conventional aluminous cement binders.

Mechanical strength testing methodology

Mechanical tests were carried out at the accredited Construction Materials Laboratory of the Department of Industrial and Civil Engineering, L. N. Gumilyov Eurasian National University (Astana, Kazakhstan). Compressive strength was determined on 50 mm cube specimens according to ASTM C109/C109M-20, while flexural strength was measured on 40 × 40 × 160 mm prisms following ASTM C348-21. The specimens were loaded using a universal testing machine (Instron 5982, USA) with a loading rate of 2.4 kN/s for compressive strength and 50 N/s for flexural strength, in accordance with the specified standards. At least three replicates were tested for each formulation (F1, F2, F3) and for the control aluminous cement paste. The results are reported as mean values with corresponding standard deviations to provide statistical reliability.

This integrated approach to examining the properties of the materials enabled the development of an optimal formulation for a heat-resistant cementitious composite based on bauxite residue.

Results

The chemical composition of the heat-resistant cementitious composites was determined using X-ray fluorescence (XRF). The clustered column chart illustrates the variation in the content of key oxides across formulations F1, F2, and F3, with Al₂O₃ increasing from 40.2% in F1 to 55.1% in F3, and Fe₂O₃ decreasing from 20.5% in F1 to 14.2% in F3. These compositional changes reflect the optimized use of reactive alumina and reduced bauxite residue, contributing to the formation of thermally stable phases that enhance high-temperature performance (Fig. 1).

Fig. 1.

Chemical composition of heat-resistant cementitious composites.

XRF analysis revealed that the principal component of all the formulations is aluminum oxide (Al₂O₃), with its content increasing from 40.2% to 55.1%. This finding confirms the high-temperature resistance of cementitious composite, as aluminum oxide forms compounds that are resistant to high temperatures. Iron oxide (Fe₂O₃) decreases as aluminum oxide increases, reducing the likelihood of thermal cracking. Calcium oxide (CaO), ranging from 15.2% to 20.1%, provides strength characteristics to the cement matrix. Silicon dioxide (SiO₂), ranging between 10.5% and 12.9%, enhances thermal cyclic strength. Alkaline oxides (K₂O + Na₂O) are present in small quantities (2.1–2.8%), minimizing the risk of alkaline corrosion. MgO and TiO₂ affect mechanical properties, though their proportions are small. Overall, the formulations are optimized for use in high-temperature environments. X-ray fluorescence (XRF) analysis was conducted to determine the chemical composition of the heat-resistant cementitious composites, as presented in Table 8, to confirm the presence and proportions of key oxides (Al₂O₃, Fe₂O₃, CaO, SiO₂) critical for achieving high thermal resistance and mechanical strength, aligning with the study’s objective of developing sustainable, high-performance materials using bauxite residue. The high Al₂O₃ content (40.2–55.1%) and Fe₂O₃ (from bauxite residue) promote the formation of thermally stable aluminate and ferrite phases. These phases enhance the composites’ ability to withstand temperatures up to 1380 °C, as evidenced by thermal testing results. To further verify structural changes, X-ray diffraction (XRD, ASTM C1365-18) confirmed the formation of crystalline phases such as calcium aluminates and ferrites post-heating, which contribute to the observed thermal stability. Thermogravimetric analysis (TGA, ISO 11358-1:2014) revealed minimal mass loss at high temperatures, indicating robust thermal decomposition behavior. The combination of XRF, XRD, and TGA analyses provides a comprehensive understanding of the chemical and structural evolution of the composites, directly supporting the study’s goal of optimizing mixture design for enhanced fire resistance and mechanical performance. The particle size distribution data demonstrate that reactive alumina and aluminous cement are the finest components, which enhances packing density and reduces porosity.

In contrast, perlite has a coarser distribution, contributing to the overall thermal insulation but increasing total porosity. The intermediate particle sizes of bauxite residue enable it to fill voids between larger particles, thereby improving both packing density and thermal stability. Table 8 presents the results of tests on heat-resistant cementitious composite formulations at various compaction and vibration conditions. The data cover key mechanical and thermal properties, including compressive strength, flexural strength, water flexural, and thermal resistance.

The data in Table 8 demonstrate that an increase in compaction pressure from 10 to 20 MPa resulted in a marked increase in both compressive and flexural strength. The observed variations in porosity and strength can be attributed not only to compaction and vibration parameters but also to the different W/B ratios of the formulations. Higher W/B values (as in F3 = 0.27) resulted in slightly increased porosity, yet this was offset by the enhanced thermal resistance due to the formation of stable aluminate phases. The observed variations in strength can be attributed not only to compaction and vibration parameters but also to the different W/B ratios and compositions of the formulations. Furthermore, the systematic increase in perlite content from F1 to F3 (20% to 30%) introduced a more porous lightweight phase, which contributed to the improved thermal insulation properties and influenced the overall mechanical performance. This can be explained by the improved density and reduced porosity of the binder mixture. For instance, F1 with a pressure of 10 MPa exhibited a compressive strength of 42.5 MPa, while at 20 MPa, this value increased to 46.8 MPa. Similarly, the flexural strength increased from 6.5 to 7.0 MPa as the pressure increased. Additionally, an increase in vibration time from 2 to 4 min resulted in a slight enhancement in strength, although the effect was less pronounced than that of changes in the compaction pressure. Optimal vibration time lies between 3 and 4 min, as this range ensures an even distribution of components and minimizes porosity. In turn, excessive vibration may cause stratification of the mixture. An important aspect is the reduction in water absorption with increased compaction pressure and vibration time. This is attributed to the improved structure of the material: at a pressure of 10 MPa, water absorption was 5.0%, but at 20 MPa, it decreased to 4.0%. The data also confirms the positive effect of bauxite residue on thermal resistance. Samples with their addition demonstrated improved thermal characteristics compared to traditional heat-resistant cementitious composite. For example, F3, at a pressure of 20 MPa and a vibration time of 4 min, reached a thermal resistance of 1,380 °C, which was 230 °C higher than that of the same formulation at 10 MPa. The high content of aluminum and iron oxide in bauxite residue is known to facilitate the formation of heat-resistant phases, such as hercynite and calcium aluminates, in cementitious systems upon heating^13,28. Thermal resistance testing was conducted to evaluate the performance of the cementitious composites under high-temperature exposure, aligning with the study’s objective of developing a heat-resistant material suitable for extreme thermal loads. The samples were subjected to temperatures of 1000 °C and above in a controlled furnace, with key measurements including residual compressive strength, maximum failure temperature, and structural integrity post-heating. Observations indicated that formulation F3, with 40% aluminous cement, 30% perlite, 15% bauxite residue, and 15% reactive alumina, maintained over 70% of its compressive strength (approximately 36 MPa) after exposure to 1200 °C, reaching a maximum failure temperature of 1380 °C. Minimal cracking and low spalling were observed, which is consistent with the development of a stable microstructure upon heating. The chemical composition (high Al₂O₃ and Fe₂O₃) suggests the potential formation of stable aluminate and ferrite phases, as reported in similar systems^10,13. These results demonstrate the superior thermal stability of the composites, particularly due to the incorporation of bauxite residue, which promotes the formation of high-temperature-resistant phases, as supported by previous studies. The high failure temperature and retained strength confirm the suitability of the developed composites for refractory applications, directly supporting the study’s goal of creating sustainable, high-performance materials for fire-prone infrastructure.

To complement the tabulated results presented in Table 8 and to improve the visual interpretation of quantitative trends across the three formulations (F1-F3), graphical summaries of the mechanical, physical, and thermal properties were additionally developed. These graphs allow a clearer comparison of key performance indicators and directly address the reviewer’s request for improved result visualization. Figure 2 presents the compressive and flexural strength of the three formulations.

Fig. 2.

Compressive and flexural strength of F1-F3 formulations.

The results confirm the progressive improvement in mechanical performance with increasing reactive alumina content and optimized proportions of aluminous cement and perlite. Compressive strength increased from 46.0 MPa in F1 to 51.2 MPa in F3, while flexural strength increased from 6.9 MPa to 7.6 MPa, respectively. These trends are consistent with the densification behavior described earlier and align with values reported for alumina-rich refractory binders. The water absorption results, presented in Fig. 3, illustrate the inverse relationship between the formulation density and porosity.

Fig. 3.

Water absorption of F1-F3 formulations.

Water absorption decreased from 4.4% in F1 to 3.9% in F3, reflecting the reduced pore connectivity associated with the higher proportion of reactive alumina and improved packing density. This trend is consistent with the thermal and mechanical improvements observed, as lower water absorption typically correlates with enhanced microstructural stability. Thermal stability is summarized in Fig. 4, which shows the maximum failure temperatures for each formulation under elevated thermal exposure.

Fig. 4.

Thermal resistance of F1-F3 formulations.

The thermal resistance increased from 1270 °C in F1 to 1370 °C in F3, demonstrating the significant effect of reactive alumina and bauxite residue on the formation of thermally stable aluminate and ferrite phases. The superior performance of F3 aligns with its highest mechanical strength and lowest porosity, confirming the internal consistency of the dataset.

Collectively, these graphical representations strengthen the interpretation of the experimental results and provide an accessible comparison of the three formulations. The observed relationships between mixture composition, mechanical performance, water absorption, and high-temperature resistance demonstrate a coherent internal structure of results. The graphical trends also support the microstructural observations presented in the following section, where SEM analysis further confirms the densification and crystalline phase formation responsible for the enhanced properties of the optimized F3 formulation.

A comparison of the formulations revealed the superiority of F3, which exhibited the highest strength and thermal resistance, making it the most suitable for use in high-temperature conditions. Nevertheless, F2 also demonstrated excellent performance, particularly when subjected to increased compaction pressure; therefore, it is a viable alternative. Overall, the findings in Table 8 indicate that optimized compaction and vibration parameters constitute a balanced approach to achieving strength, thermal resistance, and water absorption. The use of bauxite residue was found to enhance the performance of thermal-resistant cementitious composite, making it more effective in extreme temperature environments.

The results suggest that the optimal mixture composition for producing the highest quality heat-resistant cementitious composite is F3, which includes 40% aluminous cement, 30% perlite, 15% bauxite residue, and 15% reactive alumina. This formulation provides the best compressive strength (up to 52 MPa), thermal resistance (up to 1,380 °C), and low water absorption (3.8%). To prepare the mixture, a compaction pressure of 20 MPa and a vibration time of 3–4 min are required, ensuring uniform distribution of components and minimal porosity. After molding, the samples should undergo heat treatment: heating at a rate of 60 °C/hr to 200 °C with a 1-hour hold to remove moisture, followed by heating to 1200 °C at a rate of 150 °C/hr with a 2-hour hold to form a stable crystalline structure. Cooling should be performed in the furnace to prevent thermal stresses. These composition and processing conditions make the cementitious composite ideal for applications under extreme temperatures, such as furnace linings or industrial units. The microstructure of the heat-resistant cementitious composite produced under these conditions was analyzed using scanning electron microscopy (SEM). For comparison, it is important to note that before heating, the microstructure of similar aluminous binder systems is typically characterized by partially hydrated phases and higher open porosity, as reported in previous studies^10,13. In contrast, the post-heating SEM image obtained in this study demonstrates a denser structure with crystalline phases and reduced porosity, which is consistent with the observed mechanical and thermal improvements. Note that the maximum-temperature-to-failure values reported above represent material-level thermal stability and are not directly equivalent to standard fire-resistance ratings derived from time-temperature fire curve tests. Nevertheless, the observed failure temperature (1,380 °C) substantially exceeds typical peak temperatures reported for common fire scenarios (cellulose and hydrocarbon fires, which normally peak near 1,100-1,200 °C), indicating strong intrinsic thermal stability and potential suitability for refractory or high-temperature applications. Figure 5 shows the results.

Fig. 5.

Comparative SEM micrographs of heat-treated cementitious composites (F1-F3) at 1200 °C showing progressive densification of the matrix and reduction of pores with increasing reactive alumina content. (a) F1 - porous, partially sintered matrix with visible cracks; (b) F2 - transitional microstructure with reduced cracking; (c) F3 - dense crystalline matrix with minimal pores.

The microstructure of the heat-treated cementitious composites (F1-F3) was analyzed using scanning electron microscopy (SEM) to evaluate matrix densification and phase formation after exposure to 1200 °C. Polished sections of each formulation were embedded in epoxy, ground, and coated with gold to ensure conductivity. The analysis was carried out on a Hitachi TM4000Plus microscope at 15 kV using both secondary electron (SE) and backscattered electron (BSE) modes. Elemental mapping was additionally performed by EDX to confirm the spatial distribution of Al, Fe, and Ca phases.

A clear microstructural evolution is observed: F1 shows a porous, partially sintered matrix with numerous inter-particle cracks; F2 demonstrates reduced porosity and partial densification; F3 exhibits a compact crystalline matrix with well-bonded grains and minimal pores. These observations correlate with the mechanical and thermal results of formulation F3, having the highest reactive alumina content, achieves the densest structure and superior thermal stability. The micrographs confirm that increasing reactive alumina promotes sintering and the formation of stable aluminate and ferrite phases responsible for enhanced heat resistance.

Discussion

The experimental results, as detailed in the Results section, demonstrate a clear relationship between the mixture design, processing parameters, and the performance of the heat-resistant cementitious composites. The XRF analysis highlights the critical role of high Al₂O₃ and Fe₂O₃ content in forming stable phases, which directly contributes to the superior thermal resistance observed in formulation F3. The mechanical and thermal improvements are attributed to the fine particle size of reactive alumina and aluminous cement, reducing porosity, and the incorporation of bauxite residue, which enhances phase stability at high temperatures, consistent with literature findings.

To quantify the performance improvements, formulation F3 was compared to standard aluminous cement-based mixtures without bauxite residue. Typical standard mixtures exhibit compressive strength of 40–45 MPa, flexural strength of 5–6 MPa, and thermal resistance up to 1100–1200 °C, with water absorption around 5–6%. In contrast, F3 achieved 52 MPa compressive strength, 7.7 MPa flexural strength, 1380 °C thermal resistance, and 3.8% water absorption, representing a 15–20% increase in strength, 15% higher thermal limit, and 30% lower water absorption. These enhancements are attributed to bauxite residue forming stable aluminate and ferrite phases and perlite improving thermal insulation. Additionally, replacing traditional aggregates with bauxite residue reduces material costs by 15–20%, enhancing economic viability while maintaining superior performance.

This research has shown that the use of bauxite residue in heat-resistant cementitious composites results in high levels of thermal resistance and mechanical strength. The findings confirm that bauxite residue, a by-product of the metallurgical industry, can be effectively utilized as a component of construction materials. Its high Al₂O₃ and Fe₂O₃ content promotes the development of a thermally stable microstructure, likely through the formation of refractory phases such as hercynite (FeAl₂O₄) and calcium aluminates, as supported by the literature^13,14. This approach has the potential to solve environmental and economic challenges associated with industrial waste disposal. This approach has the potential to solve environmental and economic challenges associated with industrial waste disposal. Comparative SEM analysis of the three formulations (F1-F3) confirms the progressive densification of the cementitious matrix with increasing reactive alumina content. In F1, the microstructure remains porous with interconnected voids, while F2 displays intermediate compaction. The optimized F3 composition reveals a dense crystalline network with minimal microcracks, consistent with its highest compressive strength and residual strength retention after 1200 °C. Backscattered electron imaging and EDX elemental mapping verified a homogeneous distribution of Al- and Fe-rich phases, confirming the formation of thermally stable aluminates and ferrites that account for the superior thermal resistance. These findings provide direct microstructural evidence supporting the mechanical and thermal performance trends discussed above.

While the maximum-temperature testing demonstrates the intrinsic thermal stability of the binder system, practical fire performance of structural elements requires demonstration of load-bearing capacity over time under standardized fire exposures. Standardized tests (e.g., ISO 834, EN 13501-2, or national equivalents) apply prescribed time–temperature curves and measure time to loss of load-bearing capacity, spalling, and other failure modes relevant to structures. The present material-level results are therefore complementary: they indicate that the binder chemistry and phases remain stable to temperatures above common fire peaks, but they do not capture element-scale phenomena such as thermal gradients, moisture-driven pore pressure, reinforcement interaction, or progressive structural failure. We therefore recommend subsequent element-level testing (e.g., loaded beam or slab tests under standard fire curves) to quantify time-to-failure, spalling risk, and performance when the binder is used within concrete matrices or refractory linings.

Water content plays a decisive role in defining porosity and, consequently, the mechanical and thermal performance of cementitious composites. As confirmed in our study, a lower W/B ratio (F1 = 0.25) contributed to reduced porosity and higher compressive strength, whereas a higher ratio (F3 = 0.27) led to increased pore formation but also improved thermal resistance. These findings align with previous studies emphasizing the critical influence of water content on the microstructure and durability of heat-resistant binder systems.

The results of this study are consistent with data presented in the scientific literature. For instance, the studies by Salim et al.¹⁰ and Wang et al.³⁶ confirm that adding bauxite residue to concrete mixtures enhances their thermal resistance and mechanical properties. Specifically, the researchers have also noted that iron oxide and aluminum oxide present in bauxite residue lead to the formation of high-temperature-resistant phases, improving the compressive and flexural strength of cementitious composite. In our study, the compressive strength reached 52 MPa, which aligns with the findings of Li et al.¹³ and Yang et al.³² These scientists have reported improvements in the strength characteristics of cementitious composite with metallurgical waste additions. In the present study, the results of thermal tests show that the thermal resistance of cementitious composite with bauxite residue can reach 1,380 °C, which is 200–300 °C higher than that of traditional heat-resistant cementitious composites. This finding supports the data presented in the works of Ghosh et al.,²⁸ Hlystov et al.,²⁹ and Mukhametov et al.³⁷ who also found that the use of metallurgical waste enhances the thermal resistance of construction materials.

The microstructural behavior of the composites can be directly related to particle size distribution and component composition. A finer distribution (aluminous cement and reactive alumina) ensures better packing and higher strength, while the coarser perlite fraction, being a highly porous lightweight aggregate, significantly increases the overall porosity and enhances thermal insulation. The synergy between the dense fine fraction and the insulating coarse fraction explains the observed optimal balance of mechanical strength and heat resistance in the F3 composition, where a high perlite content (30%) is effectively compensated by a dense and stable binder matrix.

One of the key advantages of using bauxite residue in heat-resistant cementitious composites is its ability to enhance both the mechanical and thermal properties of the material. As noted above, due to its high content of iron and aluminum oxides, bauxite residue promotes the formation of high-temperature-resistant phases, making the cementitious composite more durable under extreme thermal loads. Additionally, the use of bauxite residue reduces the cost of concrete production by replacing expensive natural materials such as sand and limestone. This is particularly important for large-scale construction projects, where reducing material costs can significantly impact overall economic efficiency³⁸.

Another significant advantage is the environmental aspect. The utilization of bauxite residue in the construction industry mitigates the environmental impact by decreasing the volume of waste that must be stored in landfills. This issue is especially relevant for Kazakhstan, where over 2.5 million tons of bauxite residue are produced annually. The use of this material in construction not only minimizes environmental risks but also contributes to the sustainable development of the region.

Despite the positive results, the use of bauxite residue in heat-resistant cementitious composites has certain limitations. One of the main challenges is the need for careful control of the residue’s chemical composition, as the presence of impurities can negatively affect the properties of the cementitious composite. Thus, high levels of alkaline compounds can cause reinforcement corrosion and reduced durability of structures. Therefore, further research is required to optimize the concrete composition and develop methods for activating bauxite residue before this technology can be widely adopted.

Moreover, it is important to note that the properties of bauxite residue may vary depending on its source. This necessitates a tailored approach to designing cementitious composites for each specific case. In the future, more detailed studies should be conducted to investigate the influence of various parameters, such as heat treatment temperature, vibration time, and compaction pressure, on the properties of the cementitious composite. This will help optimize the production process and improve the quality of the final product.

Another potential direction for future research is the study of the durability of cementitious composites containing bauxite residue under real operating conditions. It is necessary to evaluate how such cementitious composites perform under prolonged exposure to high temperatures and thermal cyclic loading. Detailed insights into these aspects are particularly important for applications in industrial furnaces and other high-temperature installations.

The use of bauxite residue in the production of heat-resistant cementitious composite has considerable economic potential. Replacing traditional aggregates with bauxite residue can reduce the cost of cementitious composite by 15–20%, making it more competitive in the construction materials market. Moreover, the utilization of metallurgical waste reduces storage and processing costs, further enhancing economic efficiency. At the same time, a full assessment of economic feasibility must consider the costs of transporting and preparing bauxite residue, as well as potential additional expenses for quality control and optimization of the cementitious composite. Overall, the use of bauxite residue in the construction industry could be a decisive step toward sustainable development, combining environmental and economic benefits.

While the results demonstrate superior thermal resistance and strength of composites containing bauxite residue, several limitations should be acknowledged. First, the variability of bauxite residue composition across production sites poses a challenge for reproducibility and requires strict quality control protocols. Second, the current findings are based on laboratory-scale paste systems; translation to full-scale concrete applications with coarse aggregates may introduce additional factors such as differential thermal gradients, spalling risk, and reinforcement interaction that must be systematically addressed. Third, although economic benefits are suggested, practical implementation depends on the logistics of residue collection, transportation, and processing, which may reduce the anticipated cost savings. Finally, long-term durability under cyclic thermal loading and aggressive environmental conditions remains to be investigated. Addressing these challenges is essential for ensuring that the proposed composites can move from laboratory validation to reliable use in industrial and structural applications.

Conclusions

The goal of this study was to develop a heat-resistant cementitious composite with enhanced physical and mechanical properties by utilizing bauxite residue as an alternative component. This approach not only reduced the production cost of construction materials but also addressed the issue of industrial waste disposal, which is crucial for environmentally sustainable development. The research methodology included the chemical composition analysis of the components, the development of optimal concrete formulations, mechanical and thermal tests, and microstructural analysis of the samples. The experiments revealed that the optimal composition consists of 40% aluminous cement, 30% perlite, 15% bauxite residue, and 15% reactive alumina. To achieve the best properties, a compaction pressure of 20 MPa and a vibration time of 3–4 min were applied, ensuring uniform distribution of components and minimal material porosity. The results indicate that the developed formulation of heat-resistant cementitious composite exhibits the following properties: compressive strength of 52 MPa, flexural strength of 7.7 MPa, thermal resistance up to 1,380 °C, and water absorption of 3.8%. These properties surpass those of traditional heat-resistant cementitious composites, confirming the feasibility of using bauxite residue in construction materials. The particle size distribution of the components was identified as a key factor in achieving the optimal balance of porosity, strength, and thermal resistance. Although the material-level tests demonstrate thermal stability up to 1,380 °C, future work must include standardized fire-resistance testing on representative structural elements to confirm compliance with building-code fire performance requirements and to evaluate spalling and load-bearing behaviour under realistic fire scenarios.

The limitations of the study are associated with variability in the chemical composition of bauxite residue, which requires additional quality control. There is also the need to assess the durability of the cementitious composite under real operating conditions. Future research should focus on further optimizing heat treatment parameters. It is also necessary to study the behavior of the cementitious composite under thermal cyclic loading, as well as to develop methods to improve adhesion to other construction materials. This technology has the potential to reduce industrial waste volumes, enhance the sustainability of the construction industry, and maintain a balance between economic and environmental factors. The study highlights the importance of controlling the water-to-binder ratio, with an optimal value of 0.26–0.27, ensuring a balance between strength and thermal resistance. The post-heating SEM observations, interpreted together with literature data on pre-heating microstructures, confirmed that densification and crystalline phase formation were responsible for the superior mechanical and thermal performance of the developed composites.

Author contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by AZ, MS, LA, AB. The first draft of the manuscript was written by SA. All authors read and approved the final manuscript.

Funding

This research is funded by a grant from the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (grant no. AP26197151 Technology of efficient fine-grained fiber concrete based on waste for 3D printing).

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.