Durability and Mechanical Analysis of Basalt Fiber Reinforced Metakaolin–Red Mud-Based Geopolymer Composites

Abstract

Cement is widely used as the primary binder in concrete; however, growing environmental concerns and the rapid expansion of the construction industry have highlighted the need for more sustainable alternatives. Geopolymers have emerged as promising eco-friendly binders due to their lower carbon footprint and potential to utilize industrial byproducts. Geopolymer mortar, like other cementitious substances, exhibits brittleness and tensile weakness. Basalt fibers serve as fracture-bridging reinforcements, enhancing flexural and tensile strength by redistributing loads and postponing crack growth. Basalt fibers enhance the energy absorption capacity of the mortar, rendering it less susceptible to abrupt collapse. Basalt fibers have thermal stability up to about 800–1000 °C, rendering them appropriate for geopolymer mortars designed for fire-resistant or high-temperature applications. They assist in preserving structural integrity during heat exposure. Fibers mitigate early-age microcracks resulting from shrinkage, drying, or heat gradients. This results in a more compact and resilient microstructure. Using basalt fibers improves surface abrasion and impact resistance, which is advantageous for industrial flooring or infrastructure applications. Basalt fibers originate from natural volcanic rock, are non-toxic, and possess a minimal ecological imprint, consistent with the sustainability objectives of geopolymer applications. This study investigates the mechanical and thermal performance of a geopolymer mortar composed of metakaolin and red mud as binders, with basalt powder and limestone powder replacing traditional sand. The primary objective was to evaluate the effect of basalt fiber incorporation at varying contents (0.4%, 0.8%, and 1.2% by weight) on the durability and strength of the mortar. Eight different mortar mixes were activated using sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) solutions. Mechanical properties, including compressive strength, flexural strength, and ultrasonic pulse velocity (UPV), were tested 7 and 28 days before and after exposure to elevated temperatures (200, 400, 600, and 800 °C). The results indicated that basalt fiber significantly enhanced the performance of the geopolymer mortar, particularly at a content of 1.2%. Specimens with 1.2% fiber showed up to 20% improvement in compressive strength and 40% in flexural strength after thermal exposure, attributed to the fiber’s role in microcrack bridging and structural densification. Subsequent research should concentrate on refining fiber type, dose, and dispersion techniques to improve mechanical performance and durability. Examinations of microstructural behavior, long-term durability under environmental settings, and performance following high-temperature exposure are crucial. Furthermore, investigations into hybrid fiber systems, extensive structural applications, and life-cycle evaluations will inform the practical and sustainable implementation in the buildings.

1. Introduction

Davidovits was the first to give birth to the term ‘geopolymer composites’ and use them as an alternative to cement, reducing this environmental pollution due to the 5–7% CO₂ [1] emissions of cement manufacturing [2]. Aluminosilicates are the primary material of these 3D pozzolanic geopolymer mortar binders, such as metakaolin, fly ash, and red mud waste materials activated with alkaline activators [1]. Regarding waste recycling materials, carbon dioxide emissions while producing OPC decreased, and they improved high durability and mechanical properties due to their compact and dense microstructure, according to OPC. Metakaolin is a waste pozzolanic material produced as a byproduct of the calcination of kaolin from 600–900 °C, a significant environmental issue using many new technological operations [3]. Geopolymer materials (metakaolin, fly ash, and red mud) have improved mechanical, microstructural, and physical properties and are environmentally friendly.

The sand used in construction, as a limited resource, has been replaced by other alternative waste materials such as limestone, marble, and basalt powder [4]. Moreover, in previous studies, different geopolymer waste materials have been used as binder and filler materials to replace OPC and sand. Geopolymer mortars were activated by adding sodium potassium, silicate, and hydroxide. According to other research, this study used sodium silicate and hydroxide in close amounts [5]. The byproducts used to replace sand as filler materials are derived from basalt rock, limestone. According to previous studies, industrial wastes, especially basalt powder, enhance manufactured geopolymer mortars’ durability and mechanical properties [6].

Over and above, the different fiber uses have important advantages in improving the properties of geopolymer mortars. Carbon, steel, basalt, PVA, and basalt glass fibers were recently used to manufacture geopolymer mortars [7]. Furthermore, newly polyvinyl alcohol PVA and nano-silica were used for the same purpose [8]. Proving high mechanical and durability properties by using basalt powder as a filler material in geopolymer mortar, replacing sand, and increasing the attempt properties by adding basalt fiber could, together, significantly contribute to this topic [9].

Adding an appropriate amount of basalt fibers enhances the compressive strength of geopolymer mortar. Furthermore, cracks in geopolymer mortar allow for the utilization of fibers as an adsorption medium for bacterial metabolic sediment, enhancing the adhesion of dense products and improving self-healing capabilities. The mortar specimen with a minimal self-healing agent and an adequate proportion of basalt fibers exhibited the highest compressive strength in the 28-day compressive strength test. The synergistic impact of fibers has markedly enhanced the performance and compressive strength of geopolymer mortar with this ratio. Modifying the fiber ratio may enhance mortar performance, achieving a self-healing effect for 100% of the crack area. The XRD data unequivocally indicated that calcium carbonate was the primary constituent of the healing agent within the fissure of the self-healing polymer mortar. SEM and EDS analyses indicated that the bacteria could endure in the mortar environment facilitated by EP (geopolymer), with the healing products being cuboidal and amorphous calcite [10].

Geopolymers are frequently advocated as environmentally sustainable substitutes for conventional cement-based materials, primarily because of their superior thermal stability and fire resistance. Assessing their performance at elevated temperatures facilitates the evaluation of their capacity to endure fire or intense heat while maintaining mechanical strength and structural integrity. Elevated temperatures can induce microstructural alterations in geopolymers, including phase transitions, shrinkage, cracking, or chemical breakdown. Comprehending these alterations is crucial for evaluating durability and sustained performance in conditions of anticipated heat exposure. Numerous uses of geopolymers—such as refractory linings, fireproof panels, or construction materials in high-temperature settings—necessitate that the material retains its qualities post-heat exposure. Investigating high-temperature effects guarantees that geopolymers fulfill these requirements. In contrast to OPC concrete, which often experiences considerable strength degradation and spalling when exposed to heat [11], geopolymers often demonstrate superior property preservation. Tests involving high-temperature exposure corroborate these benefits and facilitate the broader implementation of geopolymer technology [12].

The increase in high temperature significantly reduced the residual compressive and flexural strengths in the basalt fiber-added geopolymer mortar samples. Basalt fibers significantly reduced crack formation through a bridging effect, resulting in geopolymer samples with basalt fiber exhibiting more incredible residual compressive and flexural strengths compared to the control geopolymer sample [13].

The increased fiber content and fiber length enhanced ductility during post-peak strain, softening the compressive stress–strain curve and elevating peak strains. Adding 2% 35 mm long basalt fiber to geopolymer mortar increased compressive toughness and the compressive toughness index by up to 126% compared to the baseline geopolymer mortar. A model was developed to predict basalt fiber-reinforced geopolymer mortar’s compressive stress–strain behavior. This model considers the impacts of basalt fiber content and length, showing strong alignment with the test results. Enhancing the content and length of basalt fiber improves the compressive and flexural performance of basalt fiber-reinforced geopolymer mortar, particularly in compressive and flexural toughness (energy absorption capacities). The influence of fiber content on performance is more significant than fiber length [14].

The weight of the geopolymer mortar is made with limestone aggregate. The unit weight of the mortar decreased by approximately 53% with the incorporation of glass aggregate. The incorporation of basalt fibers did not significantly influence the unit weight of the mixture. At 900 °C, the strength of limestone-based mortar decreased by approximately 40%, whereas the strength of mortars incorporating lightweight aggregate significantly increased. The increases were 76.4% for fiber-free lightweight mortar and mortars containing 0.1%, 0.2%, and 0.4% basalt fiber [15].

The values used were 0.4, 0.8, and 1.2%; according to previous studies, the optimal amount is between 0.6% and 1.0% due to the results obtained with high-temperature tests of geopolymer mortars manufactured with basalt or PVA fibers. Basalt fiber is formed by melting basalt rocks at 1400 °C with no additives, improved at less cost than others, and is efficient, insulating, and non-toxic. [16] Basalt fiber is cheaper than other fiber types due to the lower energy cost during production and the fact that no additives are used [17]. Moreover, many researchers ensure its significant workability in the development of strength. Many studies have confirmed the positive effects of basalt fiber on geopolymer mortars, with better performance observed using 24 mm fibers compared to 12 mm, as concluded from the results of durability tests [18].

Previous researchers discussed the effect of metakaolin material as a binder on geopolymer mortar, but a few studied the additive of fibers with different waste filler materials. This work’s primary goal is to study basalt fiber’s effect on metakaolin and red mud geopolymer mortar and develop it with limestone and basalt powder as waste filler materials, replacing river sand with 50%. Mechanical tests (compressive and flexural strength, ultrasonic pulse velocity) and high-temperature durability tests were performed using a combination of limestone and basalt powder as a filler material used to manufacture metakaolin and red mud geopolymer mortar.

Research Significance

In order to meet the pressing demand for environmentally friendly and sustainable building materials, this study investigates the usage of geopolymer composites reinforced with basalt fiber and based on metakaolin and red mud. The research helps lessen dependency on conventional cement, which lowers carbon emissions and environmental impact using natural fibers and industrial wastes. The results show notable increases in mechanical strength and durability, especially in hot weather, which is essential for resilient infrastructure. This study promotes the development of long-lasting, environmentally friendly, and economically viable solutions for contemporary building problems by providing insightful viewpoints on the valuable application of fiber-reinforced geopolymers.

2. Materials and Methods

2.1. Materials

This research used metakaolin and red mud-based geopolymer mortar as binder materials with a ratio of 50–50%. The chemical compositions of MK, RM, and slag are shown in

Table 1. The chemical composition of metakaolin, red mud, and slag [4].

Compositions	CaO	SiO2	Al2O3	Fe2O3	MgO	K2O	Na2O	P2O5	MnO
Metakaolin	0.19	56.1	40.23	0.85	0.16	0.51	0.24	0.07	–
Red Mud	3.22	17.38	24.52	35.25	0.42	0.43	8.45	–	0.07
Slag	35.58	40.55	10.11	12.83	5.87	–	0.79	0.08	1.14

. Basalt and limestone powder were used as filler material to replace river sand by 50%. The manufacturing materials used are shown in Figure 1. Basalt fiber diameters used in this study range from 1 to 300 µm, with an average of 120 µm and 2800 density/kg·m⁻³, as illustrated in

Table 2. The mechanical and physical characteristics of basalt fiber.

Type	Density (kg·m−3)	Diameter (µm)	Length (mm)	Nominal Strength (MPa)	Elastic modulus (GPa)
Basalt fiber	2800	18–30	12	300–450	3–3.5

. Geopolymer production and pozzolanic materials should be activated with an alkali activator. This process was conducted using sodium silicate Na₂SiO₃ and sodium hydroxide NaOH. The NaOH concentration used for effective polymerization was 12 Mol. The mass ratio of sodium silicate to sodium hydroxide solution generally varies from 1.0 to 2.5. In the manufacturing samples of this study, the value is around 2:1 (Na₂SiO₃:NaOH). This work utilized an alkaline activator-to-binder ratio of 1.0, above the conventional range documented in the literature (0.4 to 0.8). This modification was deliberately implemented to augment workability and facilitate the dissolving of solid precursors, such as metakaolin and red mud, which exhibit comparatively poor reactivity. An increased activator component enhances the completeness of polymerization by supplying enough alkaline solution to dissolve aluminosilicate phases, efficiently resulting in superior mechanical performance. Moreover, initial trials demonstrated that an increased activator dose enhanced homogeneity and decreased setting time without negatively impacting the composite’s durability. The elevated ratio may facilitate the accommodation of the distinct properties of the red mud employed, which may need augmented alkali content for effective activation. The chemical compositions of limestone and basalt powder are given in

Table 3. The mechanical and physical characteristics of limestone powder and basalt powder.

Chemical Analysis (%)	SiO2	Al2O3	Fe2O3	TiO2	CaO	CaO2	K2O	Na2O	SO3
Limestone powder (LS)	3.3	0.82	0.58	-	-	92.9	-	-	1.18
Basalt powder (BS)	56.9	17.6	8.1	0.9	7	-	1.9	3.8	-

.

2.2. Sample Preparation

Table 4. The Mix design (g).

Mix ID	Metakaolin	Red Mud	Slag	Na2SiO3	NaOH (12 mol)	River Sand	Limestone Powder	Basalt Powder	Basalt Fiber
Control1	500	500	133	667	333	1000	1000	-	-
4BL	500	500	133	667	333	1000	1000	-	0.40%
8BL	500	500	133	667	333	1000	1000	-	0.80%
12BL	500	500	133	667	333	1000	1000	-	1.20%
Control2	500	500	133	667	333	1000	-	1000	-
4BB	500	500	133	667	333	1000	-	1000	0.40%
8BB	500	500	133	667	333	1000	-	1000	0.80%
12BB	500	500	133	667	333	1000	-	1000	1.20%

shows the sample proportions in the mix. Pre-tests of workability were carried out. The 0.4 percent, 0.8 percent, and 1.2 percent volume concentrations of fibers were found to be the correct dose for basalt fiber, taking reinforcement, workability, and economics into account (BF) [19]. The samples were prepared the same way cement mortar was prepared according to ISO 679:2009 [20]. The fiber was first immersed in an alkali activator solution, then a sufficient amount of water was mixed in a stirring jar. The mixture was swirled for 65 s at required speed after the metakaolin, red mud, and slag powder were poured into the pot. Then, within 30 s, an adequate amount of sand and limestone powder or basalt powder was added to the pot, followed by 60 s of high-speed mixing. After stopping for 90 s, the whirling pot took only 15 s to remove the mortar from the leaves [21]. The mixture was churned for another 60 s after scraping the mortar. After that, the mortar was put into the 40 × 40 × 160 mm and 50 × 50 × 50 molds, and bubbles were removed by vibrating it multiple times on the vibration table. After a single day of curing, the samples were taken out of the mold and allowed to reach the desired ages in a standard curing environment. Following a 28-day curing period, the samples were analyzed and exposed to extreme heat (high-temperature test) [22].

2.3. Experimental Program

The experimental program was developed following ASTM and EN standards to examine basalt fiber-reinforced geopolymer mortar’s mechanical properties and thermal stability. The main aim was to assess the effects of basalt fiber addition on compressive strength, flexural strength, ultrasonic pulse velocity (UPV), and mass loss of metakaolin and red mud-based geopolymer mortars before and after exposure to high temperatures. Unless stated otherwise, all tests were performed at room temperature (22 ± 2 °C) and 50% ± 5% relative humidity.

2.3.1. Materials and Specimen Preparation

Geopolymer mortars were prepared using metakaolin and red mud as precursors and activated with a suitable alkaline solution. Basalt fibers were incorporated at a predetermined volume fraction to assess their influence on mechanical performance. The fresh mix was cast into molds to produce two types of specimens:

• Cubes: 50 × 50 × 50 mm³ for compressive strength and mass change tests.
• Prisms: 40 × 40 × 160 mm³ for flexural strength and ultrasonic pulse velocity (UPV) tests.

All specimens were demolded after 24 h and cured at room temperature (20 ± 2 °C) for 7 and 28 days prior to testing [23].

2.3.2. Mechanical Testing Before and After Durability Exposure

Mechanical tests were conducted twice, initially after curing (at 7 and 28 days) and again following exposure to elevated temperatures (200, 400, 600, and 800 °C) to evaluate thermal resistance.

For each experimental condition, three replicate samples (n = 3) of geopolymer mortar were prepared and tested to ensure reproducibility and assess data variability. Mechanical testing was conducted using a universal testing machine following the procedures specified in the ASTM standards. Compressive strength was measured according to ASTM C39/EN 12390-3 [24,25] using a universal testing machine with a controlled loading rate Following ASTM C348/EN 12390-5 [26,27], flexural strength was assessed on prismatic specimens. Ultrasonic pulse velocity (UPV) tests were carried out at 20 °C using a non-destructive testing device to monitor internal damage before and after thermal exposure (Figure 2). Mass loss was determined by recording specimen weights before and after heating, and the percentage change was calculated [28].

2.3.3. High-Temperature Durability Test

The specimens were subjected to elevated temperatures of 200, 400, 600, and 800 °C in an electric furnace. The heating rate was maintained at [5 °C/min], and specimens were held at the target temperature for 2 h before being cooled to room temperature naturally (Figure 3). Post-exposure, [4] the same set of mechanical tests was repeated to quantify strength degradation and internal damage [29].

2.3.4. Replication and Data Analysis

For each test condition, a minimum of three specimens were tested. Average values and standard deviations were calculated and reported. Outlier results, if any, were identified and excluded based on (20 ±10% variation threshold).

3. Results and Discussions

3.1. Mechanical Tests

Mechanical tests, including compressive strength, flexural strength, and ultrasonic pulse velocity (UPV), were conducted at 7 and 28 days to evaluate the influence of basalt fiber (BF) on geopolymer mortars incorporating basalt powder (BB series) and limestone powder (BL series) as fillers [30]. Compressive Strength: Figure 4 and

Table 5. Compressive, flexural strength, and UPV results.

Mix ID	Compressive Strength (MPa)		Flexural Strength (MPa)		UPV (m/s)
	7 Days	28 Days	7 Days	28 Days	7 Days	28 Days
Control 1	50.82	51.16	7.34	7.79	3040	3065
4BL	51.86	52.33	7.57	7.80	3164	3200
8BL	52.19	52.84	7.93	8.15	3230	3285
12BL	52.77	53.16	8.22	8.61	3301	3378
Control 2	59.33	60.08	8.59	8.87	3120	3196
4BB	60.24	61.03	8.87	9.12	3186	3213
8BB	61.54	62.19	9.268	9.68	3265	3300
12BB	62.46	63.12	10.15	10.76	3367	3401

present the compressive strength results for control and BF-reinforced samples. For the BL series at 7 days, compressive strength values were 51.86 MPa (4BL), 52.19 MPa (8BL), and 52.77 MPa (12BL). At 28 days, the obtained results were 52.33 MPa (4BL), 52.84 MPa (8BL), and 53.16 MPa (12BL). For the BB series, the compressive strength values at 7 days were 60.24 MPa (4BB), 61.54 MPa (8BB), and 62.46 MPa (12BB). At 28 days, 61.03 MPa (4BB), 62.19 MPa (8BB), and 63.12 MPa (12BB) [31]. In all cases, the inclusion of basalt fiber improved compressive strength compared to the control samples, with the most significant enhancement observed at the 1.2% fiber content, likely due to improved crack bridging and a denser matrix. Flexural Strength: Flexural strength followed a similar trend (Figure 5). For the BL series at 7 days: 7.57 MPa (4BL), 7.93 MPa (8BL), and 8.22 MPa (12BL). At 28 days: 7.79 MPa (4BL), 7.80 MPa (8BL), and 8.15 MPa (12BL). For the BB series at 7 days: 8.87 MPa (4BB), 9.26 MPa (8BB), and 10.15 MPa (12BB). At 28 days: 9.12 MPa (4BB), 9.68 MPa (8BB), and 10.76 MPa (12BB) Table 5. Flexural strength improvements were more pronounced than compressive strength, especially at higher fiber content. The 1.2% BF samples consistently performed the best due to enhanced stress transfer and microcrack control [5,32].

The compactness and internal quality of the geopolymer mortars were evaluated using UPV testing. According to the inverse relationship between UPV and internal voids, samples with higher UPV values exhibited a denser matrix. The results shown in Figure 6 indicate that BF incorporation reduced internal porosity, with 1.2% BF specimens achieving the highest UPV values (Table 5 [33]). The improvement attributed to basalt fibers’ filling and bridging effect helped reduce microvoids and improve matrix continuity. Although BF had no direct effect on the chemical homogeneity of the mix, its presence promoted a more compact microstructure, enhancing both mechanical and durability properties [34]. The improved strength and compactness of the BF-reinforced samples are due to better stress distribution and microcrack prevention, increased nucleation sites from the basaltic content, and reduced porosity, leading to enhanced UPV and mechanical performance. These findings confirm the effectiveness of basalt fiber in strengthening metakaolin–red mud-based geopolymer composites, particularly at 1.2% fiber content.

3.2. Durability Tests

High Temperature

Geopolymer concrete undergoes substantial physical and chemical changes when subjected to 200, 400, 600, and 800 °C temperatures. At these temperatures, the geopolymer experiences thermal deterioration, microcracking, dehydration, and phase changes which affect its mechanical qualities and durability. Substituting natural sand with other materials, or adjusting its percentage, modifies the aggregate-matrix contact, influencing geopolymer concrete’s thermal stability and mechanical properties. The substitution of sand can affect thermal conductivity, expansion characteristics, and the composite’s overall resistance to thermal degradation. Advanced characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA), are crucial for examining these alterations. These approaches facilitate the identification of phase transitions, microstructural changes, differences in chemical bonding, and weight loss resulting from heat degradation. Exposure to elevated temperatures can induce alterations in aluminosilicate phases within the geopolymer matrix, resulting in either crystallization or amorphization, contingent upon temperature and composition. The phase transitions correspond with observable mechanical qualities and durability alterations, which may be measured and comprehended by the previously indicated characterization techniques. The performance of geopolymer mortar specimens exposed to elevated temperatures was evaluated and compared with specimens cured at room temperature for 28 days. Durability was assessed using mechanical strength, ultrasonic pulse velocity (UPV), and weight loss tests before and after exposure to elevated temperatures of 200 °C, 400 °C, 600 °C, and 800 °C [8]. The results are summarized in

Table 6. Compressive and Flexural strength results after exposure to high-temperature test.

Mix ID	Compressive Strength (MPa)					Flexural Strength (MPa)
	28 Days	200 °C	400 °C	600 °C	800 °C	28 Days	200 °C	400 °C	600 °C	800 °C
Control 1	52.82	44.39	40.19	21.01	8.69	8.79	7.47	6.76	4.67	2.26
4BL	51.86	44.82	40.38	21.76	8.85	7.80	6.82	5.57	3.84	1.71
8BL	52.19	45.37	41.58	22.88	9.64	8.15	7.05	5.8	3.74	2.06
12BL	52.77	46.58	45.87	29.67	9.97	8.61	7.12	6.26	4.07	2.13
Control 2	61.33	47.93	42.96	30.93	11.79	10.87	9.85	7.79	3.63	2.46
4BB	60.24	48.29	43.73	32.72	12.64	9.12	7.78	6.27	3.29	2.01
8BB	61.54	49.64	43.87	33.94	14.27	9.68	8.36	6.58	3.36	2.18
12BB	62.46	50.87	45.56	34.27	15.53	10.76	9.26	7.11	3.52	2.27

. Mechanical Strength After High-Temperature Exposure: The experimental results demonstrate a pronounced reduction in compressive and flexural strengths of geopolymer mortar specimens with increasing exposure temperature, corroborating previous investigations [35,36]. The highest mechanical performance (Figure 7) was observed in the 12BB samples, while the lowest belonged to the 4BL series. For specimens exposed to 800 °C, compressive strengths for 4BB, 8BB, and 12BB were 12.64 MPa, 14.27 MPa, and 15.53 MPa, respectively. Corresponding flexural strengths were 2.01 MPa, 2.18 MPa, and 2.27 MPa. The mechanical strength reduction became more evident from 400 °C due to endothermic dehydration and water vaporization. However, our data show a slightly improved strength retention compared to these prior results, which can be linked to the reinforcing effect of basalt fibers. Basalt fibers enhance matrix cohesion and effectively bridge microcracks, delaying the onset of failure—a phenomenon also observed by K Yu et al. (2024) and Uysal et al. [37,38]. These thermal effects caused internal vapor pressure, which led to the formation of microcracks and thermo-chemical degradation of the geopolymer matrix (Figure 8). Crystallization stresses and pore expansion from evaporating water further contributed to strength loss [35]. Notably, flexural strength decreased less than compressive strength, indicating higher thermal resilience in tensile response [39]. The vapor action highlighted reduced flexural and compressive strength with increasing temperature [29]. Weight loss analysis revealed that specimens with higher basalt fiber content had significantly better resistance to mass reduction under high temperatures. As shown in Figure 9, the control sample exhibited the highest weight loss, while 12BB had the least. This improvement was attributed to BF’s ability to limit thermal cracking, thereby reducing volatile escape pathways and structural deterioration [40]. The weight loss increased sharply after 400 °C (

Table 7. Weight loss results after exposure to high temperature.

Mix ID	Weight Loss (%)
	200 °C	400 °C	600 °C	800 °C
Control 1	1.8	4.8	7.5	8.5
4BL	1.4	4.6	7.1	8.2
8BL	1.3	4.4	7	8
12BL	1.1	4.2	6.8	7.8
Control 2	1.7	4.6	7.1	8.3
4BB	1.3	4.3	6.9	8.1
8BB	1.2	4.2	6.6	7.9
12BB	1	4	6.5	7.5

), reinforcing the mechanical findings. The basalt fibers helped stabilize the matrix and maintain structural integrity under thermal stress, contributing to lower porosity and improved durability [41]. The internal pressure increased gradually as the temperature soared over 100 °C. UPV and Damage Degree Assessment: To quantify internal degradation, UPV tests were conducted on all samples before and after high-temperature exposure. The results are shown in

Table 8. UPV results after exposure to high temperature.

Mix ID	UPV (m/s)
	200 °C	400 °C	600 °C	800 °C
Control 1	3060	2635	1916	1516
4BL	3062	2663	1948	1520
8BL	3066	2678	2052	1672
12BL	3069	2714	2117	1693
Control 2	3080	2725	2120	1723
4BB	3089	2765	2165	1787
8BB	3097	2787	2196	1834
12BB	3112	2830	2223	1869

and Figure 10. UPV values decreased with increasing temperature due to rising pore volume and crack formation, consistent with the observed mechanical deterioration [42]. For Control 1 samples, UPV values at 200 °C, 400 °C, 600 °C, and 800 °C were 3060 m/s, 2635 m/s, 1916 m/s, and 1516 m/s, respectively. For Control 2 samples, UPV values at 200 °C, 400 °C, 600 °C, and 800 °C were 3080 m/s, 2725 m/s, 2120 m/s, and 1723 m/s, respectively. This improved thermal resistance is further supported by ultrasonic pulse velocity (UPV) measurements, which demonstrated less internal damage and higher integrity in basalt-reinforced samples, consistent with findings from Xiao et al. (2021) [43].

Basalt is a volcanic rock that exhibits thermal stability and is abundant in silica (SiO₂), alumina (Al₂O₃), and iron oxides. Limestone mainly consists of calcium carbonate (CaCO₃), which undergoes decomposition at high temperatures (about 700–900 °C) [44]. Basalt maintains stability and enhances the geopolymer network under elevated temperatures owing to its high melting point (~1000–1200 °C) [45]. Limestone experiences decarbonization (CaCO₃ → CaO + CO₂) at around 700–800 °C, resulting in mass loss, pores, fissures formation, and a matrix weakening. The mineral phases of basalt exhibit exceptional thermal stability, maintaining the integrity of the geopolymer matrix [46].

Decarbonated limestone results in the disruption of matrix continuity, hence diminishing mechanical performance and durability. Basalt may engage more significantly in the geopolymerization reaction, providing additional reactive silica and alumina. Limestone mainly serves as a filler and does not chemically augment the geopolymer gel. The substitution of natural sand with basalt powder improves thermal stability due to its high melting point (~1000–1200 °C) and contributes additional reactive silica and alumina that participate in geopolymerization, enhancing the matrix’s chemical and mechanical stability under heat. Conversely, limestone substitution results in decarbonation at around 700–800 °C, leading to pore formation and matrix weakening, as evidenced by the larger weight loss and greater strength degradation in limestone-based samples [42,44]. These microstructural differences underscore the importance of filler selection for high-temperature applications.

For the 12BB samples, UPV values at 200 °C, 400 °C, 600 °C, and 800 °C were 3112 m/s, 2830 m/s, 2223 m/s, and 1869 m/s, respectively [47]. For 12BL samples, UPV values at 200 °C, 400 °C, 600 °C, and 800 °C were 3069 m/s, 2714 m/s, 2117 m/s, and 1693 m/s, respectively. Microstructural Synergy Involving Basalt Fibers [48]: Basalt fibers and basalt powder derive from the same source material, so they exhibit analogous thermal expansion behavior, possess comparable thermal expansion coefficients, and thus eliminate fiber-matrix discrepancies at elevated temperatures, hence decreasing thermal stress and fiber pull-out [45]. Conversely, basalt fibers and limestone powder exhibit differential thermal expansion, resulting in debonding and microcracking [4]. Enhanced fiber-matrix cohesion is achieved using basalt powder during thermal cycling. Research indicates that geopolymers, including basalt-based fillers, have superior compressive strength following exposure to temperatures exceeding 600 °C [49]. Mortars containing limestone often exhibit a more significant loss of strength and integrity following heat treatment.

The reduction in compressive strength following exposure to 800 °C corresponds with the findings of [36], who linked analogous decreases to microcracking and matrix degradation. The observed gradual strength loss in fiber-reinforced samples aligns with the findings of [37], indicating that basalt fibers may effectively bridge cracks and postpone the onset of failure [50,51]. The observed trends in strength degradation following thermal exposure are likely attributable to matrix dehydration and internal microcracking, as indicated by the SEM analyses in prior studies [52].

In contrast, the control samples recorded the lowest UPV values, indicating more significant internal damage [5]. Damage levels were quantified using previous studies’ relative reduction in UPV values. A clear correlation was found between temperature increase and damage degree, with damage progressively increasing as temperature rose [6]. However, samples with higher basalt fiber content showed lower damage levels, highlighting basalt fiber’s role in enhancing high-temperature resistance by reducing crack propagation and maintaining matrix integrity [40]. This condition resulted from evaporation in the internal structure, which also caused the increase in pores and decrease in values [53]. The values were almost consistent when comparing the UPV and the mechanical findings [54]. In every series, a proportionate association was found between the increase in high temperatures and the damage degrees [49]. The fracture breadth and number of metakaolin-based geopolymer samples increased as the temperatures rose [35]. The level of damage was assessed using the UPV values. Ref. [8] employed the UPV values in their earlier research to calculate the damage degree. The UPV test following fiber incorporation indicates enhanced microstructural integrity. Basalt fibers appear to inhibit the development of microcracks under thermal stress, a phenomenon similarly reported by [55], who observed that fibers could create mechanical interlocks that counteract shrinkage-induced cracking. The relative damage:

$${\mathrm{D}}_{\mathrm{d}}=1-(\frac{{\mathrm{v}}_{\mathrm{a}}}{{\mathrm{v}}_{\mathrm{b}}})$$

(1)

caused by the UPV (ultrasonic pulse velocity) response in a geopolymer mortar is a key indicator of the material’s internal integrity and homogeneity. Unlike traditional cement mortars, geopolymer mortars may exhibit more variable UPV readings due to differences in raw material composition, curing conditions, and the formation of microcracks or voids during synthesis. Lower UPV values generally indicate potential internal defects such as pores, cracks, or incomplete polymerization, which can compromise the mechanical performance and durability of the mortar. Conversely, higher UPV values suggest a denser, more uniformly bonded matrix, reflecting better structural quality. Therefore, comparing UPV results across samples or formulations can assess the relative damage or imperfections within geopolymer mortars, helping optimize mix designs and predict long-term performance more accurately. The relative damage was calculated according to Equation (1) [17]; the results are illustrated in

Table 9. Relative Damage Index (D_d (1)) after exposure to high temperature.

Mix ID	Relative Damage Index (Dd)
	200 °C	400 °C	600 °C	800 °C
Control 1	0.0016	0.1402	0.3748	0.5053
4BL	0.0431	0.1678	0.3912	0.525
8BL	0.0666	0.1847	0.3753	0.4910
12BL	0.0914	0.1965	0.3732	0.4988
Control 2	0.0362	0.1473	0.3366	0.4608
4BB	0.0385	0.1394	0.3261	0.4438
8BB	0.0615	0.1554	0.3345	0.4442
12BB	0.0849	0.1678	0.3463	0.4504

.

3.3. Microstructure Characteristics

Although advanced microstructural techniques such as SEM or XRD were not employed in this study, the observed mechanical behavior of the composites allows for some interpretation of the internal structure, based on the established literature. The enhanced compressive and flexural strength in basalt powder and fiber mixtures suggests improved matrix densification and fiber-matrix interaction. Due to its finer particle size and angular shape, basalt powder likely contributed to better particle packing and reduced porosity within the geopolymer matrix compared to limestone powder. Adding basalt fibers is assumed to have limited microcrack propagation by bridging cracks, a mechanism widely supported in fiber-reinforced geopolymer literature. The basalt fiber led to better stress distribution and higher strength and durability. While these insights are indirect, they align with known microstructural behaviors reported in similar geopolymer systems. Future studies are planned to include SEM and XRD analyses for direct validation.

4. Conclusions

This study investigated the mechanical and durability performance of metakaolin–red mud-based geopolymer mortar reinforced with varying contents of basalt fiber. The primary aim was to evaluate how basalt fiber affects compressive and flexural strength, ultrasonic pulse velocity (UPV), and resistance to elevated temperatures. The key conclusions can be summarized as follows:

• Incorporating basalt fiber significantly enhanced compressive and flexural strength, with the most notable improvement observed at 1.2% fiber content.
• Flexural strength’s increase was more pronounced than compressive strength, suggesting that basalt fiber efficiently bridges cracks and improves tensile behavior.
• After exposure to elevated temperatures (up to 800 °C), specimens with basalt fiber showed better retention of mechanical strength, lower damage levels, and reduced weight loss compared to control samples.
• UPV results confirmed improved matrix compactness and lower internal damage in fiber-reinforced specimens, particularly at higher basalt-fiber contents.

These results indicate that basalt fiber is an effective additive for improving geopolymer mortars’ mechanical performance and thermal durability. The fiber enhances the internal structure, delays crack propagation, and reduces degradation under thermal stress.

Recommendations for Future Research

Incorporating microstructural characterization tools to improve mechanical measurements and clarify failure processes will be a focus of future studies. Investigating the impacts of different curing regimens, different kinds of fiber, and field-scale applications would be advantageous. Furthermore, cost–benefit analyses and life-cycle evaluations can assess the viability of using basalt-based geopolymers in construction, as well as their long-term resilience to severe environmental factors and cyclic thermal stress. SEM and XRD are microstructural studies that can help better understand how the fiber and matrix interact. Fiber length and dispersion are optimized for better performance.