Structural and Chemical Stability of TiO₂-Doped Basalt Fibers in Alkaline and Seawater Conditions

Abstract

Alkali resistance is a critical factor for the long-term performance of glass fibers in cementitious composites. While zirconium oxide doping has proven effective in enhancing the durability of basalt fibers, its high cost and limited solubility motivate the search for viable alternatives. This study presents the first systematic investigation of titanium dioxide (TiO₂) doping in basalt-based glasses across a wide compositional range (0–8 mol%). X-ray fluorescence and diffraction analyses confirm complete dissolution of TiO₂ within the amorphous silicate network, with no phase segregation. At low concentrations (≤3 mol%), Ti⁴⁺ acts as a network modifier in octahedral coordination ([TiO₆]), reducing melt viscosity and lowering processing temperatures. As TiO₂ content increases, titanium in-corporates into tetrahedral sites ([TiO₄]), competing with Fe³⁺ for network-forming positions and displacing it into octahedral coordination, as revealed by Mössbauer spectroscopy. This structural redistribution promotes phase separation and triggers the crystallization of pseudobrukite (Fe₂TiO₅) at elevated temperatures. The formation of a protective Ti(OH)₄ surface layer upon alkali exposure enhances chemical resistance, with optimal performance observed at 4.6 mol% TiO₂—reducing mass loss in NaOH and seawater by 13.3% and 25%, respectively, and improving residual tensile strength. However, higher TiO₂ concentrations (≥5 mol%) lead to pseudobrukite crystallization and a narrowed fiber-forming temperature window, rendering continuous fiber drawing unfeasible. The results demonstrate that TiO₂ is a promising, cost-effective dopant for basalt fibers, but its benefits are constrained by a critical solubility threshold and structural trade-offs between durability and processability.

1. Introduction

Alkali-resistant (AR) glass fiber has been specifically engineered to enhance the performance of cement-based composites in construction applications. This material plays a key role in glass fiber-reinforced concrete (GFRC) and textile-reinforced concrete (TRC), where its properties help improve durability and mechanical behavior [1]. In GFRC, short AR-glass fibers are commonly incorporated to mitigate early-age shrinkage cracking and enhance the fracture resistance of the otherwise brittle cement matrix [2]. Both TRC and GFRC are widely used in lightweight architectural elements and structural strengthening applications. A critical requirement for these materials is maintaining their high tensile strength and toughness over time, ensuring long-term performance in demanding environments [3].

There are several stages in the reaction process of the glass with alkaline solution. Initially OH- is adsorbed on the glass surface [4]. Then the following reactions occur:

$$-\mathrm{S}\mathrm{i}-O-\mathrm{S}\mathrm{i}-O-\cdots -\mathrm{S}\mathrm{i}- + O{H}^{-}+ m{H}_{2}O\to nSi{O}_2\cdot m{H}_{2}O+O{H}^{-}$$

$$-\mathrm{S}\mathrm{i}-OR + H_{2}O\iff -\mathrm{S}\mathrm{i}-OH + R^{+}+ O{H}^{-}$$

$$nSi{O}_2\cdot m{H}_{2}O+2qNaOH\to qN{a}_{2}O\cdot nSi{O}_2\cdot p{H}_{2}O+n{H}_{2}O$$

The final phase, reaction products are desorbed from the glass. The limited availability of active adsorption sites causes the corrosion process to stabilize, after which the rate of glass destruction is determined exclusively by how quickly these products dissolve [5].

The primary strategies for improving fiber alkali resistance include:

• Optimizing the glass composition [6,7,8];
• Developing new protective coatings [9,10];
• Employing additives in the cement or concrete mix [11].

Basalt fibers demonstrate superior alkali resistance compared to conventional E-glass fibers widely used in construction [12]. Our previous research has identified zirconium oxide doping as an effective method for enhancing the alkali stability of continuous basalt fibers [13].

We have thoroughly characterized the mechanism underlying this approach. When exposed to hydroxide ions, zirconium oxides form insoluble hydroxides (primarily Zr(OH)₄) on the fiber surface. This protective layer significantly impedes alkaline penetration, thereby preserving the structural integrity of the fibers [7,14].

While zirconium oxide is particularly effective due to the exceptional chemical stability of Zr(OH)₄ in alkaline environments, we have observed that its solubility in basalt glasses and fibers is inherently limited. To address this, we developed a novel method to increase zirconium incorporation. The resulting zirconium-enriched basalt fibers achieve performance characteristics comparable to commercial AR-glass fibers. The persistent zirconium hydroxide surface film—owing to its extremely low solubility in alkaline solutions—provides long-term protection by preventing solution infiltration and subsequent fiber degradation.

The exceptional durability of zirconium-enriched basalt fibers in alkaline environments stems from the extremely low solubility of zirconium hydroxide (Zr(OH)₄), with a solubility product constant (Ksp)~10⁻⁵². This property allows the protective surface film to remain stable for extended periods, effectively blocking alkaline solution penetration and preventing fiber degradation. Comparative analysis of hydroxide solubility products reveals why basalt fibers inherently outperform conventional E-glass fibers: while Ca(OH)₂ (Ksp~10⁻⁶), Mg(OH)₂ (~10⁻¹⁰), and even Ti(OH)₄ (~10⁻²⁹) show progressively better stability, they cannot match the performance of Zr(OH)₄. The natural alkali resistance of basalt fibers is further enhanced by their high iron oxide content, as evidenced by the remarkably low Ksp of Fe(OH)₃ (~10⁻³⁸). Interestingly, titanium oxide emerges as a potentially cost-effective alternative to expensive zirconium oxide for fiber doping, given the intermediate but still substantial insolubility of Ti(OH)₄ in alkaline solutions. This solubility hierarchy explains both the superior performance of modified basalt fibers compared to E-glass and the potential for optimizing cost/performance ratios in fiber development.

This hypothesis finds experimental confirmation in practice studies [14,15], where the addition of 2 wt.% titanium oxide was shown to increase alkali resistance by 50%. However, a higher doping level of 4 wt.% resulted in less significant improvements, suggesting the existence of an optimal concentration range for titanium oxide additives.

The addition of TiO₂ to silicate melts leads to a decrease in their heat capacity and enthalpy, as well as a decrease in the activation energy of viscous flow, which indicates a weakening of the melt network due to the formation of weaker Ti–O bonds compared to Si–O [16]. The studies demonstrate the potential of TiO₂ as a functional filler not only for improving the mechanical and tribological properties of polymer matrices [14], but also as a possible candidate for modifying inorganic fibers, such as basalt, to expand their application in seawater [10].

The application of basalt fiber extends beyond conventional concrete reinforcement: it is successfully employed in self-compacting concretes blended with bagasse ash and metakaolin [17], in lightweight geopolymer composites based on pumice, and in environmentally conscious solutions where thermal performance and carbon footprint reduction are as critical as mechanical properties [18]. In all these applications, long-term alkali resistance remains a key factor determining composite durability and service life

In the present work, we conducted the first systematic investigation of titanium oxide doping effects in basalt fibers in wide range of composition. We produced a series of basalt glass samples with titanium oxide content varying up to 8 mol%, significantly expanding the concentration range studied in earlier research.

2. Materials and Methods

2.1. Glass Sample Preparation

The bulk glasses were prepared by adding reagent-grade TiO₂ to milled basalt batches. Each batch of mixture placed in a platinum crucible was heated in a high-temperature furnace at a rate of 300 °C/h to 1200 °C and at 50 °C/h in a range of 1200–1550 °C and then held at 1550 °C for 20 h. The bulk glass was quenched from 1550 °C to 20 °C by rapidly pouring the melt into water.

2.2. Continuous Glass Fiber Preparation

Continuous fibers were produced using a laboratory-scale system [4]. The temperature of fiber manufacturing was measured with an accuracy of ±5 °C. The fiber diameter was controlled by varying the rotation rate of the reel. Lower temperature limit (T_ll) was taken to be the temperature at which a continuous fiber with a diameter less than 20 μm was manufactured within 30 min. The upper temperature limit (T_ul) was determined as the temperature at which the die field started to be flooded and drop formation ceased.

2.3. XRD Studies

X-ray diffraction (XRD) was performed at room temperature on a Thermo ARL X’TRA powder diffractometer (Thermo Scientific, Switzerland), CuKα1 radiation, λ = 1.54060 Å; CuK α2 radiation, λ = 1.54443 Å). XRD patterns were collected in an angular range 2θ = 10–60° at a scan step 2θ = 0.02° and a scan rate of 1° (2θ)/min. The phases were iden-tified using the Crystallographica Search-Match (CSM) software (Version 2.0.3.1) and the International Center for Diffraction Data (ICDD) database. Quantitative Rietveld analysis was not performed, as the primary goal was qualitative phase identification to establish crystallization trends correlating with loss of spinnability.

2.4. X-Ray Fluorescence Analysis

X-ray fluorescence analysis of the specimens was performed on a PAN Analytical Axios Advanced spectrometer (PANalytical B.V., Almelo, The Netherlands). Characteristic X-rays were excited using a 4 kW Rh-anode X-ray tube. The excited radiation was recorded by a wavelength-dispersive spectrometer (PANalytical B.V., Almelo, The Netherlands) with five exchangeable analyzer crystals (PE_002_C, PX_1, GEIII_C, LIF_200, LIF_220) and a detector (flow proportional and scintillation). Measurements were made in trans-mission geometry in vacuum.

The concentration of elements was expressed as mol percentage (mol%) of the corresponding oxides. The accuracy of XRF determinations of elements was controlled by analyses of standard samples OOKO 301 and OOKO 201 (State certified reference material 5358-90/5367-90) and was within 0.6% for SiO₂, within 0.4% for Al₂O₃, within 0.2% for Fe₂O₃, within 0.3% for CaO, within 0.1% for MgO, within 0.1% for K₂O, within 0.2% for Na₂O, within 0.1% for TiO₂, and within 0.3% for P₂O₅.

2.5. Mechanical Properties

The mechanical properties of the fibers were determined on a Hounsfield H100K-S universal tensile testing machine (Hounsfield Test Equipment Ltd., Redhill, UK). Specimens were mounted in paper support frames using epoxy. The gauge length was 10 mm, and the crosshead speed was 5 mm/min (ISO 5079 [19]). Fibers for mechanical testing were produced at the midpoint of each composition’s fiber-forming temperature range (ΔTff) to ensure consistent processing conditions. Statistical analysis included 50 independent measurements per composition, with narrow confidence intervals (95% CI < ±5% of mean strength) con-firming data robustness.

2.6. Optical Analysis

Optical analysis of the fibers and fiber diameter measurements were performed at magnifications of 200× to 1000× on an Olympus BX51TRF modular optical microscope (12V100WHAL lamp (Philips 7724) in transmission, and U-LH75XEAPO xenon lamp in reflection) equipped with an Olympus C-5060 camera (Olympus Optical Co., Tokyo, Japan). The linear dimensions of the fibers were determined by analyzing their images using Image Scope Color software (ver. 10.1).

2.7. SEM

A JEOL JSM-6390LA microscope was used for the scanning electron microscope (SEM) analysis (JEOL, Tokyo, Japan). The accelerating voltage was set to 20 kV. The morphology of the fibers was determined in secondary electron imaging (SEI) mode. Energy-dispersive X-ray (EDX) analysis was carried out on a JEOL EX-54175 JMH system. Before the examinations all the fibers were coated with a conducting carbon layer.

2.8. Mössbauer Spectra

Mössbauer spectra were collected using a 57Co single-line source embedded in Rh. Isomer shifts were referred to metallic α-Fe. All spectra were first collected on a large velocity scale between −10 and +10 mm s⁻¹. Spectra showed no indication of magnetic interaction, were re-collected on a smaller velocity scale between −4 and +4 mm s⁻¹ to improve resolution. Parameters of the Mössbauer spectra obtained at 298 K (σ—isomer shift, Δ—quadrupole splitting, Г—line width at half maximum). Fits were performed using Univem MS software with high statistical confidence (typical error margins: ±0.02 mm/s for σ, ±0.05 mm/s for Δ), sufficient to reliably distinguish coordination states.

2.9. Alkali and Seawater Resistance

The alkali and seawater resistance of basalt fibers was evaluated using a standardized corrosion protocol with controlled surface area exposure. Fiber samples with an average diameter of about 11 μm were used to ensure consistency in surface-to-volume ratio. To achieve a specific surface area of 5000 cm² per test, the required fiber mass was calculated using the following formula:

$$m={\displaystyle \frac{\rho \cdot S\cdot d}{\mathrm{40,000}}},$$

where:

• m—mass of fibers (g);
• S = 5000—specific surface area (cm²);
• d—fiber diameter (μm);
• ρ = 2.65 g/cm³.

The calculated mass of fibers was placed in a plastic flask, and 250 mL of 2 M NaOH solution was added to simulate highly alkaline cement environments. The mixture was refluxed for 3 h in a water bath using a condenser to prevent evaporation and maintain constant solution concentration. For seawater resistance testing, an artificial seawater solution was prepared according to the ASTM D1141-98 standard, with the following approximate composition (g/L): NaCl (24.0), MgCl₂·6H₂O (5.0), Na₂SO₄ (4.0), CaCl₂ (1.2), KCl (0.7), NaHCO₃ (0.2). The fiber samples were immersed in 250 mL of this solution and similarly refluxed for 3 h.

After exposure, the fibers were thoroughly rinsed multiple times with deionized water to remove residual salts and alkali, then dried in air until constant mass was achieved. The mass loss was calculated as:

$$\Delta m={\displaystyle \frac{m_H-m_K}{m_H}},$$

• m_н—initial mass, g;
• m_к—mass after treatment, g;
• Δm—mass loss, %.

3. Results

3.1. Glass Composition

The chemical compositions and specimen designation of fibers are presented in

Table 1. Glass samples chemical composition (in mol%).

Sample	SiO2	Al2O3	TiO2	FeO + Fe2O3	FeO	Fe2O3 okt	Fe2O3 tet	CaO	MgO	Na2O	K2O
Ti0	61.1	9.9	0.9	5.9	0.30	0.00	0.70	10.7	7.4	2.9	1.1
Ti2	60.7	9.6	2.7	5.8	0.24	0.00	0.76	10.1	6.8	2.9	1.2
Ti4	60.1	9.9	4.6	5.7	0.28	0.41	0.31	9.8	5.9	2.9	1.2
Ti6	60.5	10.1	5.8	5.3	0.29	0.34	0.37	9.2	5.1	2.9	1.1
Ti8	59.5	10.1	7.8	5.4	0.34	0.45	0.21	8.9	4.6	2.7	1.1

. Total iron content is restated as Fe₂O₃. The XRD patterns of all obtained fibers confirm their amorphous nature.

3.2. XRD

Samples of the glasses were heat-treated at different temperatures—700, 800, 900, and 1000 °C—for 24 h in air atmosphere. After thermal treatment, the samples were analyzed by X-ray diffraction (XRD) to investigate phase evolution and crystallization behavior. The results are presented in Figure 1, Figure 2 and Figure 3, that show the XRD patterns of the TiO₂-doped basalt fibers across the temperature range studied.

For the natural basalt (Ti0) and low-TiO₂ (Ti2) samples, the main crystalline phases identified after heat treatment were magnetite (M, Fe₃O₄), pyroxene-type silicates (Py), hematite (H, α-Fe₂O₃), and plagioclase feldspar (Pl). With increasing TiO₂ content (Ti4, Ti6, and Ti8), new diffraction peaks emerge, which can be attributed to pseudobrukite (Ps, Fe₂TiO₅). This phase begins to form at 900 °C in the Ti4 sample and becomes more pronounced at 1000 °C in the Ti6 and Ti8 compositions. The appearance of pseudobrukite suggests that titanium is actively participating in the crystallization process, incorporating into the lattice in octahedral coordination as part of [TiO₆] octahedra.

3.3. Alkali and Seawater Resistant

Table 2. Mechanical properties and chemical resistance of TiO₂-doped basalt fibers. Tensile strength values are mean ± SD (n = 50); coefficient of variation (CV) < 4.5% for all compositions. Mass loss values are mean ± SD (n = 3). Differences in mass loss between Ti0, Ti2, and Ti4 are statistically significant (p < 0.01, two-tailed Student’s t-test).

Sample	Tensile Strength	Modulus	Mass Loss After Sea Water Treatment		Mass Loss After NaOH Treatment		Tensile Strength After Sea Water Treatment		Tensile Strength After NaOH Treatment
Unit	MPa	GPa	%	mg/cm2	%	mg/cm2	MPa	Residual %	MPa	Residual %
Ti0	1915 ± 41	64 ± 1	5.2 ± 0.1	0.035	9.8 ± 0.2	0.066	1466 ± 31	76.6	656 ± 55	34.3%
Ti2	2345 ± 75	68 ± 1	4.1 ± 0.1	0.027	9.0 ± 0.2	0.060	1898 ± 73	80.9	884 ± 38	37.7%
Ti4	2100 ± 66	67 ± 1	3.9 ± 0.1	0.026	8.5 ± 0.1	0.057	1772 ± 51	84.4	918 ± 51	43.7%

presents the tensile strength, Young’s modulus, mass loss after exposure to artificial seawater and 2 M NaOH solution, and the residual tensile strength of basalt fibers with varying TiO₂ content. Mass loss is given in both relative (%) and absolute (mg/cm²) terms. Residual strength is expressed as a percentage of the initial value.

3.4. Mössbauer Spectra

All glass samples synthesized in this study were characterized by Mössbauer spectroscopy. The resulting spectra are presented in Figure 4, revealing the iron oxidation states and local coordination environments within the glass network. The analysis provides crucial insights into the structural modifications induced by TiO₂ doping across different temperature regimes.

4. Discussion

The incorporation of titanium dioxide (TiO₂) into the composition of continuous basalt fibers represents a promising strategy for enhancing their chemical and alkali resistance. Basalt fibers inherently exhibit superior durability in alkaline environments compared to conventional E-glass fibers, primarily due to the high iron oxide content in natural basalt and the extremely low solubility of iron hydroxides—particularly Fe(OH)₃ (Ksp~10⁻³⁸) [12]. This natural resistance can be further improved through chemical modification of the glass network. As previously demonstrated in our studies, doping basalt glass with zirconium oxide (ZrO₂) significantly enhances fiber stability in aggressive environments [7,13]. The protective mechanism involves the formation of an insoluble zirconium hydroxide layer (Zr(OH)₄, Ksp~10⁻⁵²) on the fiber surface upon exposure to alkali, which acts as a diffusion barrier, effectively preventing further penetration of hydroxide ions and subsequent glass degradation [15].

However, despite its excellent protective properties, zirconium oxide has notable practical limitations. It is a high-melting-point compound, requiring elevated processing temperatures, and is relatively expensive, which may hinder its widespread industrial application [20]. Moreover, its solubility in basalt melts is limited, restricting the achievable concentration and potentially leading to phase separation or crystallization during fiber production [21].

In this study, titanium oxide is explored as a cost-effective alternative to zirconium oxide. Although titanium hydroxide (Ti(OH)₄, Ksp~10⁻²⁹) is more soluble than its zirconium counterpart, it still exhibits very low solubility in alkaline media, making it a viable candidate for forming a protective surface layer. The hypothesis is that, similar to Zr⁴⁺, Ti⁴⁺ ions can integrate into the glass network and, upon alkali exposure, precipitate as insoluble hydroxides, thereby improving long-term chemical stability.

For the first time, this work systematically investigates TiO₂ doping across a broad compositional range—from 2 to 8 mol%—in basalt-based glasses. X-ray fluorescence (XRF) analysis confirms that titanium oxide dissolves completely in the basalt melt within this concentration range. This is further supported by X-ray diffraction (XRD) data, which show no crystalline reflections attributable to free TiO₂ phases (Figure 5) in the quenched glass samples, indicating full incorporation of Ti⁴⁺ into the amorphous silicate network. The absence of phase segregation suggests that titanium is not merely present as an inert filler but actively participates in the glass structure, likely in both network-forming and modifying roles depending on concentration and local coordination.

The feasibility of fiber drawing from the prepared glass compositions was evaluated by determining the fiber-forming temperature range (ΔT_ff), defined as the interval between the lower limit (T_ll)—the minimum temperature at which a continuous fiber with diameter below 20 μm could be drawn—and the upper limit (T_ul)—the temperature at which the spinneret begins to flood and fiber formation ceases. As shown in Figure 6, the fiber-forming window progressively narrows with increasing TiO₂ content more than 2 mol %. In the undoped (Ti0) and low-doped (Ti2) samples, a sufficiently wide ΔT_ff allows stable fiber production. However, with further TiO₂ addition—starting from approximately 4 mol% (Ti4)—the processing window becomes significantly reduced. For compositions with 6 mol% TiO₂ (Ti6) and higher, the temperature interval is so narrow that continuous fiber drawing becomes practically unfeasible under standard conditions.

Moreover, the onset temperature for fiber formation (T_ll) increases with TiO₂ content, more than 2 mol % indicating a rise in the viscosity of the melt at high temperatures. This trend is contrary to earlier experimental findings [15], where titanium oxide was observed to act as a network modifier, typically associated with a reduction in melting temperature and enhanced glass melt fluidity in silicate systems. For instance, in a study on basalt melts, it was demonstrated that the addition of TiO₂ leads to a significant decrease in viscosity due to the depolymerization of the silicate network, which can effectively lower the processing temperature and reduce production costs [22]. Within certain concentration limits, this effect is beneficial for fiber manufacturing, as reduced viscosity improves melt homogeneity and drawing stability.

The role of TiO₂ is system-dependent. In simple silicate melts, Ti⁴⁺ predominantly remains in octahedral coordination and acts as a network modifier even at high concentrations. However, in complex multicomponent basalt glass—rich in Si, Al, and Fe—Ti⁴⁺ competes effectively for tetrahedral sites with Fe³⁺. Our Mössbauer data (

Table 3. Parameters of the Mössbauer spectra obtained at 298 K (σ—isomer shift, Δ—quadrupole splitting, Г—line width at half maximum).

Sample	Components	σ	Δ	Г	Area	Fe Coordination
Units		mm/s−1	mm/s	mm/s	%
Ti0	Fe3+ (doublet)	0.29	1.27	0.76	69.8	tet
	Fe2+ (doublet)	1.03	1.95	0.77	30.2	okt
Ti2	Fe3+ (doublet)	0.28	1.32	0.76	76.4	tet
	Fe2+ (doublet)	1.03	1.90	0.75	23.6	okt
Ti4	Fe3+ (doublet)	0.36	1.39	0.79	69.9	tet
	Fe3+ (doublet)	0.40	0.66	0.56	30.1	okt
	Fe2+ (doublet)	0.96	2.07	0.78	30.8	okt
Ti6	Fe3+ (doublet)	0.31	1.41	0.76	40.8	tet
	Fe3+ (doublet)	0.35	0.77	0.58	28.4	okt
	Fe2+ (doublet)	1.02	1.97	0.78	37.3	okt
Ti8	Fe3+ (doublet)	0.29	1.30	0.78	34.1	tet
	Fe3+ (doublet)	0.38	0.82	0.60	28.6	okt
	Fe2+ (doublet)	0.98	1.99	0.78	20.8	okt

) show a clear decrease in tetrahedral Fe³⁺ and increase in octahedral Fe³⁺ with rising TiO₂ content, indicating Ti⁴⁺ incorporation into [TiO₄] units at higher concentrations (>4 mol%). This aligns with studies on glass-ceramics where TiO₂ acts as a nucleating agent due to its ability to form tetrahedral units [22], and explains both the improved alkali resistance and the narrowed fiber-forming window observed here. Specifically, when TiO₂ content exceeds approximately 1 wt% (equivalent to 2.33 mol%), the melt viscosity above the crystallization temperature decreases below the optimal range of 10–30 Pa·s required for stable continuous fiber formation. This reduction in viscosity leads to excessive melt fluidity, resulting in frequent fiber breakage during drawing [21]. Our results extend these observations by systematically investigating higher TiO₂ concentrations up to 8 mol%. We observe that the fiber-forming temperature window progressively narrows with increasing TiO₂ content, starting from approximately 4.6 mol%, and becomes unfeasibly narrow at 5.8 mol% and above, leading to complete loss of spinnability. These findings are consistent with the literature, demonstrating that while low TiO₂ levels act as network modifiers, enhancing melt fluidity, higher concentrations induce structural changes that compromise processability

Our data indicate that, in the basaltic system studied here, higher TiO₂ loading promotes the incorporation of Ti⁴⁺ into tetrahedral network-forming positions, thereby enhancing polymerization and increasing melt rigidity—contrary to the depolymerizing effect observed at lower concentrations in other systems. This network-forming behavior is consistent results reported in other studies on continuous basalt fibers [14,22], where an increase in glass transition temperature with TiO₂ addition was observed. Although the authors did not interpret this effect in terms of titanium coordination, we attribute this trend to the incorporation of Ti⁴⁺ ions into the glass network as network formers, which enhances structural rigidity. The combined evidence supports a dual role for TiO₂: as a network modifier at low concentrations, reducing viscosity, and as a network former at higher concentrations, strengthening the glass structure at the expense of processability.

Both alkali resistance and seawater durability improve with increasing TiO₂ content up to 4.6 mol% (Ti4 composition). The higher tensile strength observed at 2 mol% TiO₂ (2345 MPa) compared to 4.6 mol% (2100 MPa) reflects titanium’s concentration-dependent structural role: at low concentrations (≤3 mol%), Ti⁴⁺ in octahedral coordination ([TiO₆]) acts as a network modifier, improving melt homogeneity and reducing flaw density, thereby enhancing mechanical strength. At 4.6 mol%, increased incorporation into tetrahedral sites ([TiO₄]) strengthens the network against chemical attack but may introduce subtle heterogeneities that slightly reduce intrinsic tensile strength. Compared to ZrO₂-doped fibers [7,13], where solubility is limited to ~2–3 mol%, TiO₂ offers higher amorphous solubility (up to 8 mol%) but exhibits a similar functional limit (~4–5 mol%) due to induced crystallization—aligning with literature optima around 2 wt.% (~4.7 mol%).

As shown in Table 1, the mass loss after exposure to NaOH solution decreases from 9.8% (Ti0) to 8.5% (Ti4), while the mass loss in seawater drops from 5.2% to 3.9%. Similarly, the residual tensile strength after seawater exposure increases from 76.6% (Ti0) to 84.4% (Ti4), and after NaOH treatment—from 34.3% to 43.7%. These results indicate that moderate TiO₂ doping enhances the chemical stability of basalt fibers in aggressive environments (Table 2).

The mechanism of improved alkali resistance is believed to be analogous to that previously observed in zirconium-modified basalt fibers. In this scenario, Ti⁴⁺ ions are released from the glass network upon exposure to alkaline solutions and hydrolyze to form an insoluble titanium hydroxide (Ti(OH)₄) layer on the fiber surface. This protective film acts as a diffusion barrier, limiting the penetration of OH⁻ ions and slowing down the degradation of the silicate network.

This hypothesis is supported by SEM micrographs (Figure 7), which reveal the presence of a dense, continuous surface layer on TiO₂-doped fibers after alkali treatment. The morphology of this layer is consistent with the formation of a passivating hydroxide-rich film, similar to the Zr(OH)₄ layers reported in our earlier studies. A similar protective mechanism and results were shown previously [14].

Thus, while TiO₂ doping up to 4 mol% effectively improves both chemical resistance and residual mechanical properties, higher concentrations are not feasible due to processing limitations. This optimal range represents a favorable balance between enhanced functional performance and technological viability in fiber manufacturing.

To elucidate the structural origin of the observed changes in fiber performance, the synthesized glasses were systematically investigated using Mössbauer spectroscopy and X-ray diffraction (XRD) after heat treatment at 700, 800, 900, and 1000 °C for 24 h in air. These analyses provide critical insights into the coordination environment of iron, the role of Ti⁴⁺ in the glass network, and the crystallization behavior, all of which govern the functional and technological properties of the fibers.

Mössbauer spectra of all TiO₂-modified glass samples exhibit a superposition of contributions from Fe²⁺ and Fe³⁺ cations, consistent with the redox equilibrium established during high-temperature melting in air. The isomer shift (σ) and quadrupole splitting (Δ) values, along with the relative spectral areas, allow for the identification of the local oxygen coordination of iron species (Table 3). In the undoped (Ti0) and low-TiO₂ (Ti2) samples, the spectra are well-fitted by two doublets: a Fe³⁺ component with σ ≈ 0.28–0.29 mm/s and Δ ≈ 1.27–1.32 mm/s, characteristic of tetrahedrally coordinated Fe³⁺ (Fe³⁺_tet), and a Fe²⁺ component with σ ≈ 1.03 mm/s and Δ ≈ 1.90–1.95 mm/s, assigned to octahedrally coordinated Fe²⁺ (Fe²⁺_okt). The Fe³⁺_tet/Fe²_okt area ratio is approximately 2:1, which aligns well with literature data for natural basaltic glasses [23], where Fe²⁺ content typically does not exceed 30 wt.% of total iron after redox equilibration [24].

A significant structural transition is observed starting at 4 mol% TiO₂ (Ti4). The Mössbauer spectrum of the Ti4 sample reveals the emergence of a third doublet with σ = 0.40 mm/s and Δ = 0.66 mm/s—characteristic of octahedrally coordinated Fe³⁺ (Fe³⁺_okt). This component becomes more prominent in the Ti6 and Ti8 samples, where the relative area of Fe³⁺_okt increases while the Fe³⁺_tet fraction decreases. This shift indicates a progressive change in the local environment of iron ions, driven by the increasing TiO₂ content.

The decrease in Fe³⁺tet content suggests that tetrahedral sites are being occupied by another high-field-strength cation—most likely Ti⁴⁺—which competes effectively for these network-forming positions at higher concentrations. This interpretation aligns with the observed evolution of melt properties. At low TiO₂ concentrations (≤2–3 mol%), the deacresing fiber forming temperature (Figure 6) indicate a network-modifying role of Ti⁴⁺, likely in octahedral coordination ([TiO₆]), leading to depolymerization and reduced viscosity. However, as the TiO₂ content increases beyond ~4 mol%, the Fe³⁺tet fraction decreases, and the onset of pseudobrukite crystallization suggests that Ti⁴⁺ begins to incorporate into tetrahedral sites ([TiO₄]), strengthening the network and promoting phase separation. Thus, the dual role of titanium is concentration-dependent: at low levels (≤3 mol%), it acts primarily as a network modifier in octahedral coordination ([TiO₆]), reducing viscosity; at higher concentrations (≥4 mol%), it increasingly occupies tetrahedral sites ([TiO₄]), displacing Fe³⁺ into octahedral positions and reinforcing the glass network. This conclusion is supported by convergent evidence: (1) Mössbauer data showing Fe³⁺tet → Fe³⁺okt redistribution, (2) XRD detection of pseudobrukite (requiring octahedral Ti⁴⁺/Fe³⁺) upon heat treatment, and (3) non-monotonic change in fiber-forming temperature. Direct confirmation via ⁴⁷Ti NMR is planned for future work. This transition explains both the initial improvement in chemical resistance and the subsequent deterioration in spinnability.

The phase transformation processes in basaltic amorphous systems are well-established in the literature. In natural basalt glasses, crystallization typically initiates with the spontaneous formation of magnetite (Fe₃O₄), which is promoted by initial liquid–liquid phase separation [25]. These magnetite nanocrystals act as heterogeneous nucleation sites for the subsequent precipitation of pyroxene-type silicate phases. The overall crystallization kinetics in bulk basalt glasses have been widely interpreted as a three-dimensional, diffusion-controlled growth of pyroxene-like phases on a fixed number of nuclei. At higher temperatures, further crystallization leads to the formation of plagioclase phase [26].

In the present study, this classical crystallization pathway is clearly observed for the undoped (Ti0) and low-TiO₂ (Ti2) samples. XRD patterns after heat treatment reveal the sequential appearance of magnetite, pyroxene, hematite, and plagioclase phases between 700 and 1000 °C, confirming that the base basalt system follows the expected thermodynamic evolution. However, a significant deviation occurs with increasing TiO₂ content. For compositions Ti4, Ti6, and Ti8, a new crystalline phase—pseudobrukite (Fe₂TiO₅)—emerges upon heat treatment. This phase is first detected at 900 °C in the Ti4 sample and becomes increasingly dominant at 1000 °C in Ti6 and Ti8, indicating a strong concentration-dependent crystallization tendency (Figure 8).

Studies of the CaO–SiO₂–TiO₂ system with the addition of 10 wt.% Al₂O₃ showed that at 1300–1400 °C, solid phases of perovskite (CaO TiO₂), rutile (TiO₂), wollastonite and sphene (CaO SiO₂ TiO₂) exist in equilibrium with the liquid phase, indicating the complex participation of TiO₂ in crystallization processes [27]. In glass-ceramic systems with zero coefficient of thermal expansion, titanium plays a key role as a crystallization initiator, while its coordination state (4, 5, 6) directly affects the crystal nucleation mechanism and the stability of the amorphous network [28]. Pseudobrukite, like other TiO₂ polymorphs (rutile, anatase, brukite), is built upon [TiO₆] octahedra. The formation of Fe₂TiO₅ requires the availability of Ti⁴⁺ in octahedral coordination, either residual from the glass structure or formed during heat treatment, as well as local enrichment of Fe³⁺ in octahedral sites—consistent with the Mössbauer-observed increase in Fe³⁺okt. This structural distortion arises from the partial substitution of Ti⁴⁺ by Fe³⁺ ions, which occupy irregular lattice positions due to differences in ionic radius and charge. The formation of Fe₂TiO₅ demonstrates that titanium is not merely a passive network constituent but actively participates in phase separation and crystallization, particularly when its concentration exceeds the network’s capacity for homogeneous dispersion.

The emergence of pseudobrukite at relatively low temperatures (≥900 °C) has critical implications for fiber manufacturing. Although quantitative phase analysis (e.g., Rietveld refinement) was not performed, the strong correlation is evident: pseudobrukite is first detected at 900 °C in Ti4 (where ΔTff begins to narrow significantly) and becomes dominant at 1000 °C in Ti6/Ti8 (where fiber drawing becomes impossible). Since fiber drawing occurs at 1380–1550 °C—well above these crystallization temperatures—pseudobrukite nucleation in the melt at the spinneret inevitably increases viscosity and causes die clogging, directly linking crystallization to loss of spinnability. Since continuous fiber drawing occurs at temperatures between 1380 °C and 1550 °C, any tendency toward early crystallization significantly increases the risk of nucleation and growth of solid phases in the molten glass at the spinneret. This leads to increased melt viscosity, die clogging, and surface defects—ultimately resulting in fiber breakage. This explains the dramatic narrowing of the fiber-forming temperature window (ΔT_ff) with increasing TiO₂ content and the complete inability to produce continuous fibers at Ti6 and Ti8.

Based on the combined Mössbauer and XRD results, we conclude that the structural role of Ti⁴⁺ evolves with concentration. At low levels (≤3 mol%), Ti⁴⁺ predominantly occupies octahedral sites ([TiO₆]), acting as a network modifier that reduces melt viscosity and lowers processing temperatures. At higher concentrations (≥4 mol%), increasing incorporation of Ti⁴⁺ into tetrahedral coordination ([TiO₄]) strengthens the network and displaces Fe³⁺ from tetrahedral to octahedral sites. This redistribution promotes local clustering and provides the structural prerequisites for pseudobrukite nucleation, ultimately compromising spinnability. While direct spectroscopic probes of Ti coordination (e.g., Raman, XPS, NMR) were not employed in this study, future work will include ⁴⁷Ti NMR analysis to directly confirm the tetrahedral/octahedral site occupancy of Ti⁴⁺.

To overcome this limitation, we propose two potential strategies. First, following our earlier work with zirconium, titanium could be introduced in the form of a pre-formed silicate (e.g., titanosilicate) to enhance its compatibility with the basalt network and promote tetrahedral coordination. Second, the addition of fluxing agents (e.g., B₂O₃, F⁻, or alkali borates) may lower the melting and processing temperatures, thereby suppressing premature crystallization and improving melt homogeneity. The latter approach is particularly promising from a technological standpoint, as it directly addresses the processing challenges identified in this study.

While a full cost–benefit analysis is beyond the scope of this study, the economic advantage of TiO₂ is clear: it is 5–10 times cheaper than ZrO₂. Although 4.6 mol% TiO₂-doped fibers show slightly lower alkali resistance than ZrO₂-modified ones—due to the higher solubility of Ti(OH)₄ (Ksp~10⁻²⁹) versus Zr(OH)₄ (Ksp~10⁻⁵²)—the cost difference makes TiO₂ doping highly attractive for applications requiring sufficient, not maximum, durability. This is especially relevant in large-scale construction, where material cost dominates decision-making. At scale, the advantage grows: lower raw material cost, compatibility with existing basalt fiber production, and no need for complex processing additives make TiO₂ a practical, cost-effective alternative.

These considerations are the subject of our ongoing research. The goal is to extend the solubility limit of Ti⁴⁺ in the basalt matrix while maintaining a stable amorphous structure, thereby decoupling functional enhancement from processability constraints.

5. Conclusions

This study demonstrates that titanium dioxide (TiO₂) is a viable and cost-effective alternative to zirconium oxide for enhancing the alkali and seawater resistance of basalt fibers. Systematic doping with TiO₂ up to 8 mol% results in complete incorporation of Ti⁴⁺ into the amorphous basalt glass network, as confirmed by XRF and XRD analyses. The improved chemical durability—particularly at 4.6 mol% TiO₂—is attributed to the formation of an insoluble Ti(OH)₄ surface layer that acts as a diffusion barrier against aggressive alkaline environments. This leads to a notable reduction in mass loss and a significant increase in residual tensile strength after exposure to NaOH and seawater.

However, higher TiO₂ concentrations (≥5.8 mol%) induce structural changes that compromise fiber production. At low concentrations (≤3 mol%), Ti⁴⁺ primarily acts as a network modifier in octahedral coordination ([TiO₆]), reducing melt viscosity and lowering processing temperatures. As the TiO₂ content increases, titanium increasingly incorporates into tetrahedral sites ([TiO₄]), competing with Fe³⁺ for network-forming positions and displacing it into octahedral coordination. This structural redistribution promotes phase separation and triggers the crystallization of pseudobrukite (Fe₂TiO₅) at elevated temperatures, as confirmed by Mössbauer and XRD data. The early onset of crystallization narrows the fiber-forming temperature window and ultimately prevents stable fiber drawing.

Thus, while TiO₂ doping up to ~4–5 mol% offers an optimal balance between enhanced durability and processability, exceeding this threshold introduces significant technological challenges due to the saturation of the glass network and loss of amorphous stability. The narrowing of the fiber-forming temperature window (ΔTff) beyond 4 mol% TiO₂ presents a significant industrial constraint, increasing sensitivity to temperature fluctuations and reducing production yield. To mitigate this, two strategies are proposed: (1) introducing titanium as a pre-reacted titanosilicate to promote homogeneous tetrahedral incorporation, and (2) adding fluxing agents (e.g., B₂O₃, F⁻) to lower melt viscosity and suppress pseudobrukite crystallization. While 4.6 mol% TiO₂-doped fibers do not match the absolute alkali resistance of commercial ZrO₂-containing AR-glass (due to Ksp(Ti(OH)₄)~10⁻²⁹ vs. Ksp(Zr(OH)₄)~10⁻⁵²), the 5–10× lower cost of TiO₂ makes them a cost-effective, mid-performance alternative for applications where sufficient durability—not maximum—is required, particularly in large-scale construction. These findings provide a foundation for the rational design of high-performance, economically viable basalt fibers for durable construction materials.