Influence of FexOy and Al2O3 Contents on the Thermal Stability of Iron Ore-Waste Fibers: Key Mechanisms and Control

Traditional rock wool fibres are susceptible to crystallization and pulverization. To mitigate this, glass fibres were produced from iron ore waste (IOW). When the ratio of Fe2+ and Fe3+ is 1:3 and the Al2O3 content is 10 wt.%, increasing the FexOy content enhances the thermal stability.At an FexOy content of 17–19% and an Al2O3 content of 10–13%, the glass transition temperature (Tg) peaked. Increasing the FexOy content from 10% to 20% enhanced the stability of Si-O and Al-O bonds and increased bridged oxygen, stabilizing the structure. Here, Fe2+ balances structural charges, while Fe3+ replaces some Al atoms in the network. When the Al2O3 content is 10–13% and the FexOy content is 17–19%, the thermal stability of the IOW rock glass reaches its optimal level. At 20% FexOy content, the structure becomes three-dimensional and cyclic, increasing polymerization. Consequently, incorporating FexOy alongside a 10% Al2O3 content improves thermal stability, supporting the development of high-stability rock wool from IOW. This approach also enhances the refractory properties of rock wool fibres within the FexOy-Al2O3-SiO2-MgO-CaO system.


Introduction
Traditional rock wool fibres are made from basalt and consume natural resources.When heated by fire, they are prone to crystallisation and pulverisation, resulting in decreased strength.Therefore, long-term use of such fibres poses a safety hazard [1,2].However, the acidity coefficient of iron ore waste (IOW) is close to that of basalt for the production of rock wool; thus, it can potentially be used as a raw material for the production of rock wool fibres [3,4].IOW is rich in Fe and Al, which improves the thermal stability of glass [5].Therefore, the use of IOW to produce rock wool may replace traditional rock wool to avoid crystallisation and powdering during fires [6].
Al 2 O 3 is a typical amphoteric oxide that exhibits acidity in alkaline melts [7].Forming a [ AlO 4 ] tetrahedral structure through covalent bonding between Al and O leads to the formation of a three-dimensional network structure that acts as a network-forming agent.In the acid melt, Al 2 O 3 becomes alkaline, and an ionic bond is formed between Al and the anion, acting as a network modifier [8].In the (CaO-SiO 2 ) (1-x) (Al 2 O 3 ) x system where x = 0-30 mol%, an increase in the population of bridging oxygens and tricluster oxygens are observed with increasing Al 2 O 3 content, while the number of free oxygens tends to diminish.Regions with a high number of Al-O-Al linkages start to appear, which is also accompanied by an increase in the glass transition temperature and elastic moduli [9].However, in the CaO-Al 2 O 3 -SiO 2 system, a concentration of Al 2 O 3 more than 30% leads to a relative deficiency of Ca atoms and an increase in tri-coordinate O and five-coordinate Al content.This can lead to an increase in the self-diffusion coefficients of Al and O atoms, and the structure tends to become unstable [10].IOW rocks comprise a complex Fe x O y -Al 2 O 3 -SiO 2 -MgO-CaO(FASMC) system, with Fe 3+ and Fe 2+ coexisting in aluminosilicate glasses playing different roles in the microstructure [11].When the mass fraction of Al 2 O 3 is 15 wt.%., increasing the Fe 2 O 3 content from 2 wt.%. to 22 wt.%.provides charge compensation for the [AlO 4 ] tetrahedron, leading to a decrease in the coordination number of the Al-O bond and the formation of a more stable 4-coordinate Al structure [12].With the increase in FeO content from 5 wt.%. to 25 wt.%., free oxygen O 2− ions destroy the Si-O bond, leading to the depolymerization of the aluminosilicate structure.When the FeO content exceeds 23 wt.%.,the Al-centred tetrahedral structure in the SiO 2 -Al 2 O 3 -FeO system is depolymerized easily [13].The relative ratio of Fe 3+ to Fe 2+ determines the degree of polymerisation within the glass structure [14,15].
Controlling the changes in the Fe x O y and Al 2 O 3 contents provides a feasible approach to preparing highly thermally stable IOW fibres, which is a new way to improve the refractoriness of rock wool fibres based on the FASMC system.Existing studies have focused primarily on the effect on aluminosilicates when the Fe x O y content is low or only in a single valence state.There is a gap in the theoretical study of IOW fibres based on the FASMC structure regarding the influence of Fe x O y and alumina content in the range of 10-20% on the overall structure.
In this study, X-ray photoelectron spectroscopy (XPS) was used to analyse the detailed composition of IOW rocks for formula design, and a structural unit model was established.The roles of Fe x O y and Al 2 O 3 in the microstructure were analysed using molecular dynamics (MD) simulations.By comparing the measured and simulated data, we investigated the correlation between the changes in Fe and Al contents with the glass transition temperature (T g ) and thermal stability and verified the reliability of the simulation results.In order to optimise the thermal stability of the FASMC structures, the effects of iron and aluminium content on the radial distribution function (RDF), coordination number (CN), and bridging oxygen (BO) of the glass structures were analysed.Finally, Raman spectroscopy was used to confirm the effect of the Al/Fe content in the structure on the degree of polymerisation.These promising results provide a theoretical basis for the production of fibres with high thermal stability by controlling the Al/Fe content of IOW rocks.

Component of IOW Rocks
IOW rocks with Fe x O y and Al 2 O 3 contents in the range of 10-20% were tested using XPS (250 xi, Thermo Fisher, Waltham, MA, USA).The fitted results are shown in Figure 1.
structure tends to become unstable [10].IOW rocks comprise a complex FexOy-Al2O3-SiO MgO-CaO(FASMC) system, with Fe 3+ and Fe 2+ coexisting in aluminosilicate glasses pla ing different roles in the microstructure [11].When the mass fraction of Al2O3 is 15 wt.% increasing the Fe2O3 content from 2 wt.%. to 22 wt.%.provides charge compensation fo the [AlO4] tetrahedron, leading to a decrease in the coordination number of the Al-O bon and the formation of a more stable 4-coordinate Al structure [12].With the increase in Fe content from 5 wt.%. to 25 wt.%., free oxygen O 2− ions destroy the Si-O bond, leading the depolymerization of the aluminosilicate structure.When the FeO content exceeds 2 wt.%.,the Al-centred tetrahedral structure in the SiO2-Al2O3-FeO system is depolymerize easily [13].The relative ratio of Fe 3+ to Fe 2+ determines the degree of polymerisation with the glass structure [14,15].
Controlling the changes in the FexOy and Al2O3 contents provides a feasible approac to preparing highly thermally stable IOW fibres, which is a new way to improve the r fractoriness of rock wool fibres based on the FASMC system.Existing studies have focuse primarily on the effect on aluminosilicates when the FexOy content is low or only in a si gle valence state.There is a gap in the theoretical study of IOW fibres based on the FASM structure regarding the influence of FexOy and alumina content in the range of 10-20% o the overall structure.
In this study, X-ray photoelectron spectroscopy (XPS) was used to analyse the d tailed composition of IOW rocks for formula design, and a structural unit model was e tablished.The roles of FexOy and Al2O3 in the microstructure were analysed using mole ular dynamics (MD) simulations.By comparing the measured and simulated data, we i vestigated the correlation between the changes in Fe and Al contents with the glass tra sition temperature (Tg) and thermal stability and verified the reliability of the simulatio results.In order to optimise the thermal stability of the FASMC structures, the effects iron and aluminium content on the radial distribution function (RDF), coordination num ber (CN), and bridging oxygen (BO) of the glass structures were analysed.Finally, Rama spectroscopy was used to confirm the effect of the Al/Fe content in the structure on th degree of polymerisation.These promising results provide a theoretical basis for the pr duction of fibres with high thermal stability by controlling the Al/Fe content of IOW rock

Component of IOW Rocks
IOW rocks with FexOy and Al2O3 contents in the range of 10-20% were tested usin XPS (250 xi, Thermo Fisher, Waltham, MA, USA).The fitted results are shown in Figure  As shown in Table 1, Fe 2+ /Fe 3+ = 1/3.Therefore, when constructing a microstructural model of the IOW rocks, the ratio of Fe 2+ to Fe 3+ in Fe x O y should be 1/3.The initial model was constructed using Materials Studio, and the proportions of raw material components were obtained using the simplex method, as shown in Table 2.For the simulation, the Born-Mayer-Huggins potential function, which comprises the van der Waals force, long-range Coulomb interaction, and short-range repulsive interaction, was chosen to describe the interactions of each atomic pair [16].The Fe-O parameters were obtained from Belashchenko [17,18], and the other parameters were obtained from the literature [19][20][21].In the simulation, each model had 6000 atoms with a density of 2.7 g/cm 3 .The atoms were randomly placed in a model box.Creating an infinite system without boundaries using three-dimensional periodic boundary conditions yielded more accurate results.The truncation radius was set to 12.5 Å for the short-range force, the long-range Coulomb force was calculated using the Ewald summation method, the MD was calculated using the Verler velocity algorithm with an integration step of 1 fs, and the temperature and pressure were based on the Nose-Hoove and Barostat methods [21].

Molecular-Dynamics Simulation Process
The system was relaxed to 6000 K for 100 ps to remove the effects of the initial state of the atoms.The system was then cooled to 1773 K in 80,000 steps and finally relaxed for 500 ps at 1773 K before being further quenched from 1773 K to 300 K over 1 ns and equilibrated at 300 K for 500 ps to ensure that the atoms in the system were well mixed so that structural information, such as the RDF [22,23] and CN [24], could be obtained.The Neumann-Planck-Thompson ensemble was selected, and the structure was gradually heated from 300 K to 1500 K over 500 ps to obtain information such as the glass transition temperature.The density curves of the last smooth section at each temperature were averaged.

Glass Structure Preparation
Based on the proportions of the oxides listed in Table 2, the materials were fully ground with a sample grinder for 10 min and placed in a 100 mL alumina crucible and melted uniformly at 1500 • C for 1 h.The furnace chamber was opened and the sample was taken out together with the crucible and placed in cold water to obtain the samples.The vitreous sample melts at high temperatures and cools rapidly, resulting in a glassy state.The vitreous sample was dried and ground to a powder in a sample mill.The glass was analysed by X-ray diffraction (XRD), as shown in Figure 2. No significant sharp peaks were observed in the crystalline phase, nor in the crystalline phase.Within the characteristic diffraction peak range of 20-35 • , a clear amorphous characteristic envelope was observed, indicating that the sample has fine glass transition characteristics.

Glass Structure Preparation
Based on the proportions of the oxides listed in Table 2, the materials were fully ground with a sample grinder for 10 min and placed in a 100 mL alumina crucible and melted uniformly at 1500 °C for 1 h.The furnace chamber was opened and the sample was taken out together with the crucible and placed in cold water to obtain the samples.The vitreous sample melts at high temperatures and cools rapidly, resulting in a glassy state.The vitreous sample was dried and ground to a powder in a sample mill.The glass was analysed by X-ray diffraction (XRD), as shown in Figure 2. No significant sharp peaks were observed in the crystalline phase, nor in the crystalline phase.Within the characteristic diffraction peak range of 20-35°, a clear amorphous characteristic envelope was observed, indicating that the sample has fine glass transition characteristics.

The Characteristics of Glass
The produced glass was thoroughly ground to a powder and then passed through a 200 mesh sieve for structural testing.It was added to a crucible and heated from room temperature to 1200 °C at a rate of 10 °C/min under nitrogen atmosphere.The Tg values were simulated with a simultaneous thermal analyser (TG-DSC, SDT Q600, TA, USA) and measured by isobaric method [25].Five groups of glass were selected with ratios of 10% Al2O3 and 10%, 11.67%, 15%, and 20% FexOy for Raman spectroscopy analysis (Raman, Alpha300R, WITec, Ulm, Germany) [26].The results were obtained using Gaussian decomposition within the range of 800-1200 cm −1 .The integral area of the peaks was used to evaluate the content of the characteristic structural units in the system.

The Glass Transition Temperature (Tg)
Tg is an important parameter reflecting the thermal stability of a material [27].Currently, no unified explanation for this glass transition phenomenon exists [28].The free volume theory states that a molecular motion energy below Tg is low, resulting in frozen chain segments.With increasing temperature, the change in the free or void volumes of the polymers was very small.During the heating process, the particles within the rock wool fibre system were rearranged, causing the ion bonds to break and reform, thereby leading to crystallisation.Consequently, the volume of fibres expanded, causing crystallisation and powdering., and there is no obvious mineral crystalline phase appearing in the ten samples, which has good vitreous characteristics, indicating that the vitreous samples were successfully prepared.

The Characteristics of Glass
The produced glass was thoroughly ground to a powder and then passed through a 200 mesh sieve for structural testing.It was added to a crucible and heated from room temperature to 1200 • C at a rate of 10 • C/min under nitrogen atmosphere.The Tg values were simulated with a simultaneous thermal analyser (TG-DSC, SDT Q600, TA, USA) and measured by isobaric method [25].Five groups of glass were selected with ratios of 10% Al 2 O 3 and 10%, 11.67%, 15%, and 20% Fe x O y for Raman spectroscopy analysis (Raman, Alpha300R, WITec, Ulm, Germany) [26].The results were obtained using Gaussian decomposition within the range of 800-1200 cm −1 .The integral area of the peaks was used to evaluate the content of the characteristic structural units in the system.

The Glass Transition Temperature (T g )
T g is an important parameter reflecting the thermal stability of a material [27].Currently, no unified explanation for this glass transition phenomenon exists [28].The free volume theory states that a molecular motion energy below T g is low, resulting in frozen chain segments.With increasing temperature, the change in the free or void volumes of the polymers was very small.During the heating process, the particles within the rock wool fibre system were rearranged, causing the ion bonds to break and reform, thereby leading to crystallisation.Consequently, the volume of fibres expanded, causing crystallisation and powdering.
MD simulations can predict the initial crystallisation and pulverisation temperatures of rock wool fibres, thereby determining the T g value of the material and characterising the thermal stability of the microstructure.The higher T g value, the better the thermal stability and fire resistance of the rock wool fibres.The T g values simulated and measured by MD are shown in Figure 3a,b.
MD simulations can predict the initial crystallisation and pulverisation temperatures of rock wool fibres, thereby determining the Tg value of the material and characterising the thermal stability of the microstructure.The higher Tg value, the better the thermal stability and fire resistance of the rock wool fibres.The Tg values simulated and measured by MD are shown in Figure 3a,b.As shown in Figure 3a, when the Al2O3 content is relatively low and the FexOy content increases from 10% to 20%, Tg increases by 20% to 30%, resulting in a corresponding increase in thermal stability.As the FexOy content increases, Fe 3+ forms a tetrahedral structure of [FeO4] to compensate the discontinuous network in the glass matrix.Simultaneously, Fe 2+ acts as an alkaline earth metal ion and provides charge compensation.This forms more tetrahedral structures, thereby making the structure more stable.The highest range of the Tg was 1125-1165 K, with the FexOy and Al2O3 contents ranging from 15% to 20% and 10% to 15%, respectively.
The measured Tg values are shown in Figure 3b.When the Al2O3 content is maintained, the Tg value increases with increasing FexOy content.When the FexOy and Al2O3 contents are between 17 and 20% and 10 and 13%, respectively, the Tg ranges from 1130 K to 1142 K.A comparison of Figure 3a and b shows that the experimental value is marginally lower than the simulation value.This may be because the MD simulation time is relatively short, whereas the heating rate of the experimental environment gradually increases in seconds, and the movement of elements is more abundant.This implies that the molecular chain segments in the polymer material move more smoothly at high temperatures, and the intermolecular forces weaken.The movement of polymer segments makes exceeding the critical temperature easier, resulting in a decrease in the Tg [29].Therefore, the measured Tg value was marginally lower than the simulated value.In summary, the variation trends in the Tg obtained from the simulation and measurement were consistent.
To obtain the highest Tg, the ranges of the simulation and measurement results were overlaid, as shown in Figure 4.The highest Tg is indicated by the white area, with FexOy and Al2O3 contents ranging from 17% to 19% and 10% to 13%, respectively.Within this Tg range, the IOW rock wool exhibited the best thermal stability.As shown in Figure 3a, when the Al 2 O 3 content is relatively low and the Fe x O y content increases from 10% to 20%, T g increases by 20% to 30%, resulting in a corresponding increase in thermal stability.As the Fe x O y content increases, Fe 3+ forms a tetrahedral structure of [FeO 4 ] to compensate the discontinuous network in the glass matrix.Simultaneously, Fe 2+ acts as an alkaline earth metal ion and provides charge compensation.This forms more tetrahedral structures, thereby making the structure more stable.The highest range of the T g was 1125-1165 K, with the Fe x O y and Al 2 O 3 contents ranging from 15% to 20% and 10% to 15%, respectively.
The measured T g values are shown in Figure 3b.When the Al 2 O 3 content is maintained, the T g value increases with increasing Fe x O y content.When the Fe x O y and Al 2 O 3 contents are between 17 and 20% and 10 and 13%, respectively, the T g ranges from 1130 K to 1142 K.A comparison of Figure 3a and b shows that the experimental value is marginally lower than the simulation value.This may be because the MD simulation time is relatively short, whereas the heating rate of the experimental environment gradually increases in seconds, and the movement of elements is more abundant.This implies that the molecular chain segments in the polymer material move more smoothly at high temperatures, and the intermolecular forces weaken.The movement of polymer segments makes exceeding the critical temperature easier, resulting in a decrease in the T g [29].Therefore, the measured T g value was marginally lower than the simulated value.In summary, the variation trends in the T g obtained from the simulation and measurement were consistent.
To obtain the highest T g , the ranges of the simulation and measurement results were overlaid, as shown in Figure 4.The highest T g is indicated by the white area, with Fe x O y and Al 2 O 3 contents ranging from 17% to 19% and 10% to 13%, respectively.Within this T g range, the IOW rock wool exhibited the best thermal stability.

Bond Length Simulation of Atomic Pairs
Figure 5 shows the average bond length of each atom in an RDF diagram.The average bond length is the horizontal coordinate of the first peak in the RDF of the corresponding atom, representing the nearest neighbour bond length of the atom pair.A narrow and sharp RDF peak indicates that the atomic bond formed between the two atoms is more stable.The nearest-neighbour bond lengths of various atomic pairs are listed in Table 3.

Bond Length Simulation of Atomic Pairs
Figure 5 shows the average bond length of each atom in an RDF diagram.The average bond length is the horizontal coordinate of the first peak in the RDF of the corresponding atom, representing the nearest neighbour bond length of the atom pair.A narrow and sharp RDF peak indicates that the atomic bond formed between the two atoms is more stable.The nearest-neighbour bond lengths of various atomic pairs are listed in Table 3.     3 shows that the Ca-O bond length is shorter than that of Mg-O.This may be due to the fact that Ca 2+ has a larger radius than Mg 2+

Coordination Number Simulation of Atomic Bonds
The CNs of each atom were obtained by integrating the RDF curve.The wider and flatter the CN curve, the more stable the atomic bond.
Figure 7a showed that the average CNs of Si-O, Al-O, Fe-O(Fe2O3) and Fe-O(FeO), Ca-O, Mg-O were 3.99, 4.13, 4.50, 5.14, 5.46, and 5.23, respectively.These results are consistent with those previously reported [33][34][35].As is well known, the most stable structure in the aluminosilicate network is the tetrahedron.That is, the closer the CNs of Si-O or Al-O are to 4, the more stable the structure.Figure 7b shows that regardless of the changes in the Al2O3 and FexOy contents, the CN of Si-O remains at approximately 4. The corresponding experimental value is 3.99 Å, with a relative error of only 0.02% [36].This indicates that the four-coordination structure of Si-O is stable.As shown in Figure 7c, when the Al2O3 content is between 10% and 12%, FexOy content increases from 10% to 16%, and the CN of Al-O increases from 3.84 to 4.01.When the FexOy content increases from 16% to 20%, the CN of Al-O exceeds 4. Therefore, when the content of FexOy is 16%, the Al-O structure is the most stable.

Coordination Number Simulation of Atomic Bonds
The CNs of each atom were obtained by integrating the RDF curve.The wider and flatter the CN curve, the more stable the atomic bond.In conclusion, the stability of Si-O is higher than that of Al-O because the bond length of Fe-O (Fe 2 O 3 ) was close to that of Al-O, suggesting that Fe-O (Fe 2 O 3 ) can repair a network structure similar to that of Al-O and it replaces some Al atoms to some extent.As the Fe 2 O 3 content increases, the stability of the Si-O and Al-O bonds improves.Simultaneously, the CN of Si-O remains at approximately 4, indicating a dense structure with a high thermal stability.As shown in Figure 8, when the content of Al2O3 remains constant, the Ob content gradually increases with increasing FexOy content.This is because the Fe 2+ in FexOy balances the charge in the structure, providing the possibility for the formation of tetrahedra.However, Fe 3+ in FexOy acts as a ligand to strengthen the connection between the tetrahedra, forming more [FeO4], thereby gradually increasing the Ob content and stabilising the structure.When the Al2O3 content is 10-12% and the FexOy content is greater than 14%, the structure has a high amount of Ob with a higher degree of structural polymerisation.This may be because Al2O3, as an acidic oxide, forms a network structure with oxygen, and the essence of acidic oxides is the formation of functional groups by acidic cations and oxygen ions, thereby increasing the degree of polymerisation and Ob content.This enhances the structural stability of the system.In summary, when the FexOy content is 14-20% and the Al2O3 content is 10-12%, the highest content of Ob in the microstructure is 64%, resulting in a more stable structure and a higher Tg.

Characteristic Raman Spectroscopy of Glass
The characteristic peak near 850 cm −1 represents the Q 0 (Si) group, which is connected to four non-bridged oxygen atoms on each network-forming atom.The peaks in the region of 900-1190 cm −1 can be sequentially represented as the characteristic peaks of the Q 1 (Si), Q 2 (Si), Q 3 (Si), and Q 4 (Si) groups [39][40][41][42].The band in the region of 1120-1190 cm −1 represents Q 4 (Si), which is associated with the complete aggregation of the network structure [1].
Figure 9 shows the integration of the characteristic peaks.As no obvious characteristic peaks of Q 4 (Si) were obtained in any of the samples, only the percentages of Q 0 (Si), Q 1 (Si), and Q 2 (Si)+Q 3 (Si) are displayed.When the Al2O3 content is between 10% and 12%, the relative strengths of Q 0 (Si) and Q 1 (Si) gradually decrease with an increase in FexOy content.Q 1 (Si) represents the connection of three non-bridged oxygen atoms to each network-forming atom.Q 0 (Si) and Q 1 (Si) have simple polymer structures.When the content of Q 0 (Si) and Q 1 (Si) is lowered, they may form looser structures such as chains, layers, or fibres.This is mainly because a lower content results in weaker interaction forces, which As shown in Figure 8, when the content of Al 2 O 3 remains constant, the O b content gradually increases with increasing Fe x O y content.This is because the Fe 2+ in Fe x O y balances the charge in the structure, providing the possibility for the formation of tetrahedra.However, Fe 3+ in Fe x O y acts as a ligand to strengthen the connection between the tetrahedra, forming more [FeO 4 ], thereby gradually increasing the O b content and stabilising the structure.When the Al 2 O 3 content is 10-12% and the Fe x O y content is greater than 14%, the structure has a high amount of O b with a higher degree of structural polymerisation.This may be because Al 2 O 3 , as an acidic oxide, forms a network structure with oxygen, and the essence of acidic oxides is the formation of functional groups by acidic cations and oxygen ions, thereby increasing the degree of polymerisation and O b content.This enhances the structural stability of the system.In summary, when the Fe x O y content is 14-20% and the Al 2 O 3 content is 10-12%, the highest content of O b in the microstructure is 64%, resulting in a more stable structure and a higher T g .

Characteristic Raman Spectroscopy of Glass
The characteristic peak near 850 cm −1 represents the Q 0 (Si) group, which is connected to four non-bridged oxygen atoms on each network-forming atom.The peaks in the region of 900-1190 cm −1 can be sequentially represented as the characteristic peaks of the Q 1 (Si), Q 2 (Si), Q 3 (Si), and Q 4 (Si) groups [39][40][41][42].The band in the region of 1120-1190 cm −1 represents Q 4 (Si), which is associated with the complete aggregation of the network structure [1].
Figure 9 shows the integration of the characteristic peaks.As no obvious characteristic peaks of Q 4 (Si) were obtained in any of the samples, only the percentages of Q 0 (Si), Q 1 (Si), and Q 2 (Si)+Q 3 (Si) are displayed.When the Al 2 O 3 content is between 10% and 12%, the relative strengths of Q 0 (Si) and Q 1 (Si) gradually decrease with an increase in Fe x O y content.Q 1 (Si) represents the connection of three non-bridged oxygen atoms to each networkforming atom.Q 0 (Si) and Q 1 (Si) have simple polymer structures.When the content of Q 0 (Si) and Q 1 (Si) is lowered, they form looser structures such as chains, layers, or fibres.This is mainly because a lower content results in weaker interaction forces, which loosen the structure.As the Fe x O y content increases, the relative strength of Q 2 (Si)+Q 3 (Si) increases, and the vibration near Q 2 (Si) is caused by the vibration of each silicon containing two non-bridged oxygen atoms in the network-structure unit.The band near Q 3 (Si) is the result of the symmetrical vibrations in the network structure unit, where each networkforming atom is connected to only one non-bridged oxygen atom.When the content of Fe x O y is 20%, the relative intensity of the Q 2 (Si)+Q 3 (Si) group is the highest.Q 3 (Si) typically represents complex polymer structures.A higher content typically indicates a higher degree of structural aggregation, with the network structure being primarily three-dimensional or ring-shaped.This may be because as the content of Fe x O y increases, the contents of the network-modifying bodies, such as CaO and MgO, gradually decrease.Some of the FeO in Fe x O y can act as network-modifying bodies to balance the charges in the system.The [AlO 4 ] tetrahedral structure increases, and the network structure gradually transforms from tetrahedral monomers and island-like structures to three-dimensional and ring-shaped structures, resulting in an increased degree of polymerisation.
Materials 2024, 17, x FOR PEER REVIEW 10 of 14 degree of structural aggregation, with the network structure being primarily three-dimensional or ring-shaped.This may be because as the content of FexOy increases, the contents of the network-modifying bodies, such as CaO and MgO, gradually decrease.Some of the FeO in FexOy can act as network-modifying bodies to balance the charges in the system.The [AlO4] tetrahedral structure increases, and the network structure gradually transforms from tetrahedral monomers and island-like structures to three-dimensional and ring-shaped structures, resulting in an increased degree of polymerisation.In summary, when the Al2O3 content is 10-12% and the FexOy content is 20%, the composition of glass evolves from simple to complex, promoting the improvement of the polymerisation degree and enhancing the thermal stability of the structure.This finding is consistent with the fitting results of the MD software, proving that controlling the FexOy In summary, when the Al 2 O 3 content is 10-12% and the Fe x O y content is 20%, the composition of glass evolves from simple to complex, promoting the improvement of the polymerisation degree and enhancing the thermal stability of the structure.This finding is consistent with the fitting results of the MD software, proving that controlling the Fe x O y content can have a positive impact on the thermal stability of the IOW fibres.It can be seen that the iron ore waste rock wool fibres' surfaces are relatively smooth, a small amount of white material appears, and the changes in the fibres' diameters are not large.Under the electron microscope at 1000×, it was observed that the damaged fibres were distributed in short rods and did not have a long fibrous structure.The elongated fibres became powdery, and the surface was not smooth with the generation of pore defects, while the fibres' volume had a tendency to expand.It was speculated that crystalline substances might have been produced inside the fibres, leading to the coarsening of the fibre diameter.

Conclusions
The high-temperature glass body of rock wool fibres is easily crystallised and crushed, and rock wool made from IOW rich in iron and aluminium is expected to improve its thermal stability.
In this study, the effects of FexOy and Al2O3 on the refractory structure and FexOy-Al2O3-SiO2-MgO-CaO composition of glass were investigated using MD software.The Tg values obtained through simulation and measurement were consistent, indicating that the MD simulation was reliable.The highest Tg values were 17-19% for FexOy content and 10-13% for Al2O3 content; thus, IOW has a higher Tg and better thermal stability than ordinary rocks.With an increase in the FexOy content, the bond lengths of Si-O and Al-O increased, the average CN remained at approximately 4, and the amount of bridge oxygen increased.Simultaneously, the network structure gradually transformed from tetrahedral monomers and island-like structures to three-dimensional and ring-like structures, making it more polymerised.These results prove that the FexOy-Al2O3-SiO2-MgO-CaO structure is more stable when the Al2O3 content is 10-12% and the FexOy content is close to 20%, resulting in a higher Tg.
In summary, an increase in the FexOy content can improve the thermal stability of IOW fibres based on an FexOy-Al2O3-SiO2-MgO-CaO glass structure.In particular, when the Al2O3 content is 10-13% and the FexOy content is 17-19%, the thermal stability of the IOW rock glass reaches its optimal level.The results of this study not only provide data in support of the preparation of high-thermal-stability fibres from IOW, but also offer a new approach to the improvement of the refractory properties of rock wool fibres based on the FexOy-Al2O3-SiO2-MgO-CaO system.It can be seen that the iron ore waste rock wool fibres' surfaces are relatively smooth, a small amount of white material appears, and the changes in the fibres' diameters are not large.Under the electron microscope at 1000×, it was observed that the damaged fibres were distributed in short rods and did not have a long fibrous structure.The elongated fibres became powdery, and the surface was not smooth with the generation of pore defects, while the fibres' volume had a tendency to expand.It was speculated that crystalline substances might have been produced inside the fibres, leading to the coarsening of the fibre diameter.

Conclusions
The high-temperature glass body of rock wool fibres is easily crystallised and crushed, and rock wool made from IOW rich in iron and aluminium is expected to improve its thermal stability.
In this study, the effects of Fe x O y and Al 2 O 3 on the refractory structure and Fe x O y -Al 2 O 3 -SiO 2 -MgO-CaO composition of glass were investigated using MD software.The T g values obtained through simulation and measurement were consistent, indicating that the MD simulation was reliable.The highest T g values were 17-19% for Fe x O y content and 10-13% for Al 2 O 3 content; thus, IOW has a higher T g and better thermal stability than ordinary rocks.With an increase in the Fe x O y content, the bond lengths of Si-O and Al-O increased, the average CN remained at approximately 4, and the amount of bridge oxygen increased.Simultaneously, the network structure gradually transformed from tetrahedral monomers and island-like structures to three-dimensional and ring-like structures, making it more polymerised.These results prove that the Fe x O y -Al 2 O 3 -SiO 2 -MgO-CaO structure is more stable when the Al 2 O 3 content is 10-12% and the Fe x O y content is close to 20%, resulting in a higher T g .
In summary, an increase in the Fe x O y content can improve the thermal stability of IOW fibres based on an Fe x O y -Al 2 O 3 -SiO 2 -MgO-CaO glass structure.In particular, when the Al 2 O 3 content is 10-13% and the Fe x O y content is 17-19%, the thermal stability of the

Figure 1 .
Figure 1.(a) XPS graph of IOW rocks and (b) formula design based on the simplex method.

Figure 1 .
Figure 1.(a) XPS graph of IOW rocks and (b) formula design based on the simplex method.

Figure 2 .
Figure 2. XRD patterns of prepared IOW glass.The figure (a,b) shows the XRD diffraction patterns of 10 groups of vitreous, the diffraction patterns of the samples show a wide hump in the range of 20-35°, and there is no obvious mineral crystalline phase appearing in the ten samples, which has good vitreous characteristics, indicating that the vitreous samples were successfully prepared.

Figure 2 .
Figure 2. XRD patterns of prepared IOW glass.The figure (a,b) shows the XRD diffraction patterns of 10 groups of vitreous, the diffraction patterns of the samples show a wide hump in the range of 20-35 • , and there is no obvious mineral crystalline phase appearing in the ten samples, which has good vitreous characteristics, indicating that the vitreous samples were successfully prepared.

Figure 3 .
Figure 3. (a) Simulated Tg and (b) measured Tg thermal analysis of IOW fibres based on FASMC structure.

Figure 3 .
Figure 3. (a) Simulated T g and (b) measured T g thermal analysis of IOW fibres based on FASMC structure.

3. 2 .Figure 5
Figure5shows the average bond length of each atom in an RDF diagram.The average bond length is the horizontal coordinate of the first peak in the RDF of the corresponding atom, representing the nearest neighbour bond length of the atom pair.A narrow and sharp RDF peak indicates that the atomic bond formed between the two atoms is more

Figure 4 .
Figure 4. Superimposed contour plot of simulated and measured values.

Figure 4 .
Figure 4. Superimposed contour plot of simulated and measured values.

Figure 5 Figure 5 .
Figure 5 shows that the bond length of Fe-O(Fe2O3) was close to that of Al-O, suggesting that Fe-O(Fe2O3) can repair a network structure similar to that of Al-O and that Fe (Fe2O3) and Al can be transformed into each other.The Ca-O, Mg-O, and Fe-O(FeO) peaks were wide and weak, mainly because Ca 2+ , Mg 2+ , and Fe 2+ acted as modified ions in the silicate network structure.The Al-O bond lengths ranged from 1.745 to 1.785, which may

Figure 5
Figure 5 shows that the bond length of Fe-O(Fe 2 O 3 ) was close to that of Al-O, suggesting that Fe-O(Fe 2 O 3 ) can repair a network structure similar to that of Al-O and that Fe (Fe 2 O 3 ) and Al can be transformed into each other.The Ca-O, Mg-O, and Fe-O(FeO) peaks were wide and weak, mainly because Ca 2+ , Mg2+ , and Fe 2+ acted as modified ions in the silicate network structure.The Al-O bond lengths ranged from 1.745 to 1.785, which may have been due to the coordination of Al with Al IV , Al V , and Al VI in the structure[30][31][32].These coordination states affected the length of the Al-O bond.Table3shows that the Ca-O bond length is shorter than that of Mg-O.This may be due to the fact that Ca 2+ has a larger radius than Mg 2+ which has a wider range of structural activity.According to the RDF plot, the average bond lengths ofSi-O, Al-O, Fe-O(Fe 2 O 3 ), Fe-O(FeO), Ca-O, and Mg-O are close to the experimental values [27], indicating the reliability of the simulated results.More importantly, in the simulation of the glass, the first peak of Si-O is narrow and sharp, indicating that the short-range structural unit of Si-O is stable, whereas the structural unit stability of Al-O ranks second.Five sets of samples with Al 2 O 3 contents within 10-12% and Fe x O y contents within 10-20% were selected to simulate the RDFs of Si-O and Al-O, as shown in Figure 6.When the Al 2 O 3 content is between 10% and 12%, and Fe x O y content increases from 10% to 20%, the RDF peak of Si-O exhibits a significant upward trend.When the Fe x O y content reaches 20%, the RDF peak reaches its maximum value.This indicates that within the range of Al 2 O 3 content, the stability of Si-O bonding can be improved by increasing the Fe x O y content.A closer look at the changes in the Al-O bonds showed that as Fe x O y content increases, the average bond length and RDF peak increase.Similarly, the high RDF peak corresponding to 20% Fe x O y indicates the enhanced stability of Al-O.Because Fe x O y in glass mainly exists in the form of Fe 3+ , it substitutes some Al atoms and becomes part of the network structure.Therefore, a moderate increase in Fe x O y can enhance the stability of Si-O and Al-O bonds and promote an increase in the T g .
Figure 5 shows that the bond length of Fe-O(Fe 2 O 3 ) was close to that of Al-O, suggesting that Fe-O(Fe 2 O 3 ) can repair a network structure similar to that of Al-O and that Fe (Fe 2 O 3 ) and Al can be transformed into each other.The Ca-O, Mg-O, and Fe-O(FeO) peaks were wide and weak, mainly because Ca 2+ , Mg2+ , and Fe 2+ acted as modified ions in the silicate network structure.The Al-O bond lengths ranged from 1.745 to 1.785, which may have been due to the coordination of Al with Al IV , Al V , and Al VI in the structure[30][31][32].These coordination states affected the length of the Al-O bond.Table3shows that the Ca-O bond length is shorter than that of Mg-O.This may be due to the fact that Ca 2+ has a larger radius than Mg 2+ which has a wider range of structural activity.According to the RDF plot, the average bond lengths ofSi-O, Al-O, Fe-O(Fe 2 O 3 ), Fe-O(FeO), Ca-O, and Mg-O are close to the experimental values [27], indicating the reliability of the simulated results.More importantly, in the simulation of the glass, the first peak of Si-O is narrow and sharp, indicating that the short-range structural unit of Si-O is stable, whereas the structural unit stability of Al-O ranks second.Five sets of samples with Al 2 O 3 contents within 10-12% and Fe x O y contents within 10-20% were selected to simulate the RDFs of Si-O and Al-O, as shown in Figure 6.When the Al 2 O 3 content is between 10% and 12%, and Fe x O y content increases from 10% to 20%, the RDF peak of Si-O exhibits a significant upward trend.When the Fe x O y content reaches 20%, the RDF peak reaches its maximum value.This indicates that within the range of Al 2 O 3 content, the stability of Si-O bonding can be improved by increasing the Fe x O y content.A closer look at the changes in the Al-O bonds showed that as Fe x O y content increases, the average bond length and RDF peak increase.Similarly, the high RDF peak corresponding to 20% Fe x O y indicates the enhanced stability of Al-O.Because Fe x O y in glass mainly exists in the form of Fe 3+ , it substitutes some Al atoms and becomes part of the network structure.Therefore, a moderate increase in Fe x O y can enhance the stability of Si-O and Al-O bonds and promote an increase in the T g .

14 Figure 7 .
Figure 7. (a) The average CN curves in the system; (b) Si-O curves and (c) Al-O curves of CNs.In conclusion, the stability of Si-O is higher than that of Al-O because the bond length of Fe-O (Fe2O3) was close to that of Al-O, suggesting that Fe-O (Fe2O3) can repair a network structure similar to that of Al-O and it replaces some Al atoms to some extent.As the Fe2O3 content increases, the stability of the Si-O and Al-O bonds improves.Simultaneously, the CN of Si-O remains at approximately 4, indicating a dense structure with a high thermal stability.

Figure 7 .
Figure 7. (a) The average CN curves in the system; (b) Si-O curves and (c) Al-O curves of CNs.

Figure 8 .
Figure 8.The effect of Fe x O y and Al 2 O 3 changes on bridge oxygen.

Figure 9 .
Figure 9. (a-e) Peak fitting results in the range of 800-1200 cm −1 for Raman spectroscopy.(f) Peak area proportion of each group at Al2O3 content of 10-12% and FexOy content increased from 10% to 20%.

Figure 9 .
Figure 9. (a-e) Peak fitting results in the range of 800-1200 cm −1 for Raman spectroscopy.(f) Peak area proportion of each group at Al 2 O 3 content of 10-12% and Fe x O y content increased from 10% to 20%.

Figure 10 Figure 10 .
Figure 10 represents the microscopic morphology of (a) iron ore waste rock wool and (b) ordinary rock wool observed by SEM after 800 • C high-temperature treatment.

Figure 10 .
Figure 10.Microscopic morphology of rock wool fibres.(a) Iron ore waste rock wool fibres; (b) ordinary rock wool fibres.

Table 1 .
The XPS Gaussian peak resolution of IOW rocks.

Table 2 .
Composition and atomic numbers of IOW fibres based on FAMSC structure.