Strength and durability characteristics of basalt fiber-based engineered geopolymer composites under elevated temperature

ABSTRACT

This study explores the mechanical properties and environmental sustainability of basalt fiber-based Engineered Geopolymer Composites (BFEGC). Trial mixes, were developed and analyzed to determine their flowability and compressive strength, leading to the identification of an optimal formulation with a compressive strength of 55.34 MPa. The optimal mix was also found to exhibit split tensile and flexural strengths of 15.5 MPa and 5.13 MPa, respectively. Durability assessments of this mix were conducted through water absorption, sorptivity, and chloride penetration tests, confirming compliance with codal requirements. Thermal resistance evaluations were performed under varying conditions: low (300°C for 30 minutes), moderate (600°C for 20 minutes), and high (900°C for 15 minutes), followed by both rapid and gradual cooling. The minimal compressive strength degradation was observed for the optimal mix (Mix 4) at 300°C, with a decrease of only 19.40%. Additionally, the strain hardening behavior of BFEGC samples at elevated temperatures was studied, revealing microstructural stability up to 300°C during gradual cooling. TGA results indicated that the geopolymer matrix and basalt fibers work synergistically, resulting in a minor weight loss of only 2–5% at 300°C. The environmental impact of BFEGC was also assessed, demonstrating its potential to significantly reduce carbon emissions and embodied energy. BFEGC finds application in strengthening structural elements in fire-prone environments due to its thermal stability, low shrinkage, and strength retention at high temperatures.

Keywords:
Engineered geopolymer composite; Basalt fiber; Elevated temperature; Strength; Durability; Sustainability

1. INTRODUCTION

Recent research has focused on Engineered Geopolymer Composites (EGCs) as environmentally friendly substitutes for conventional cementitious materials. According to DAVIDOVITS [¹], the primary objective is to enhance structural performance while reducing the carbon footprint associated with construction [¹, ²]. EGCs are considered superior binders due to their exceptional adherence, durability, and resistance to acid attacks and freeze-thaw cycles when used in construction [³]. A study on EGC-based retrofitting methods [⁴] has shown that they outperform Engineered Cementitious Composites (ECC) in terms of long-term durability and shrinkage. Additionally, the interfacial transition zone properties demonstrated improvements in bond strength. Research by NEMATOLLAHI et al. [⁵, ⁶] has provided evidence that EGCs can achieve tensile properties significantly higher than those of conventional fiber-reinforced cementitious composites. They attribute this enhancement to the unique structure of the engineered geopolymer matrix and its interaction with the reinforcing fibers. Recent findings [⁷] have also highlighted the potential of EGCs, showcasing their ability to exhibit multiple cracking and tensile strain hardening behavior under uniaxial tensile stress. However, inorganic fibers have shown limited strain-hardening capacity due to poor bond-slip characteristics between the fibers and the matrix [⁸, ⁹, ¹⁰]. The flexural strength of the nano-modified composites decreased more gradually from room temperature to 600°C, indicating improved mechanical performance and improved resistance to thermal deterioration [¹¹]. This limitation affects the use of inorganic fibers in EGC systems. To address this challenge, more effective surface treatment methods for fibers [¹², ¹³] have been developed to improve the bond-slip properties between the inorganic fibers and the EGC matrix. Additionally, incorporating hybrid inorganic and organic fibers could strengthen the EGC system, potentially enhancing its resistance to elevated temperatures, as well as improving its economic viability and sustainability.

Previous research conducted by OHNO and LI [¹⁴] demonstrated the impressive characteristics of short polyvinyl alcohol fibers, revealing average crack widths of 45 μm and maximum crack widths of 117 μm. Researchers have identified various methods to enhance the compressive strength of EGC, focusing on factors such as alkali sources, precursors, curing processes, and elevated temperatures [¹⁵]. Earlier studies explored the effects of different alkali activator solutions on polyvinyl alcohol fiber-reinforced EGC [¹⁶, ¹⁷]. This investigation involved four types of activators: two sodium silicate liquids, a potassium-based liquid, and a lime-based powder. Among these, the sodium silicate (Na₂SiO₃) based activator emerged as a promising option. It consists of an 8M sodium hydroxide liquid (approximately 28.6%) and water glass (around 71.4%), with a SiO₂/Na₂O ratio of 2. This formulation not only proved cost-effective but also achieved a compressive strength of 60 MPa and a fiber bridging stress of 4.7 MPa [¹⁸]. Moreover, the sodium-based EGC exhibited superior matrix fracture properties and a better stress-performance index compared to other alkali sources in EGC and ECC [¹⁹]. While incorporating industrial by-products as alternative binders promotes sustainability, their lower reactivity compared to cement can reduce early-age compressive strength. This underscores the need to optimize replacement ratios to maintain the mechanical performance of ECC [²⁰]. Studies also indicate that the FRP-to-UHS-ECC bond plays a minor role, whereas the UHS-ECC-to-concrete interface strongly affects flexural behaviour [²¹]. Although such high-performance materials offer great promise for modern infrastructure, their reliance on high cement content still contributes significantly to carbon emissions [²²]. The impact of varying volume fractions of slag on the micro characteristics of EGC at ambient temperature was also a focal point of study [²³]. When compared to the control mix, hybrid polypropylene fibers helped reduce shrinkage by up to 15%, while macro-PP fibers only managed to reduce shrinkage by up to 6%. The study also examined the degree to which current literature and standards match experimental predictions of tensile strength and elastic modulus [²⁴]. In Alkali-activated slag’s (AAS) capacity for self-healing it affects the mechanical performance of AAS mortars are investigated. The efficiency of several self-healing techniques, their real-world uses, and the unique difficulties they provide are taken into account and compared [²⁵].

In terms of building safety, fire resistance is crucial, as structural damage caused by fires poses significant risks. Consequently, retrofitting structures affected by fire has gained significant attention, particularly through the use of composite materials. Research indicates that geopolymer composites perform significantly better than traditional cement-based materials when exposed to elevated temperatures, while still maintaining structural integrity. By using dangerous waste cathode ray tube (CRT) glass as heavy aggregate in geopolymer concrete, this study reduces carbon emissions by 34% while increasing γ-ray shielding by 9% and preserving a compressive strength above 30 MPa [²⁶]. It also creates high-performance radiation shielding concrete (HPRSC), which is perfect for high-radiation situations and has a strength of over 70 MPa and improved shielding thanks to heavy aggregates and lead fibers [²⁷]. Additionally, geopolymer foam (FRGF) with hollow microspheres exhibits better performance with 50% microspheres and 0.6% basalt fiber, improving mechanical strength, thermal insulation, and high-temperature stability [²⁸]. The high fire resistance of EGC is attributed to its inorganic aluminosilicate structure, which undergoes beneficial changes at elevated temperatures. For retrofit applications, combining EGC binders with Fibre Reinforced Polymers (FRP) provides improved fire resistance compared to conventional systems [²⁹, ³⁰]. In studies examining the behavior of EGC-FRP composites at high temperatures, it was found that the EGC matrix offers the FRP reinforcement excellent thermal insulation, which helps retain mechanical properties. AL-MAJIDI et al. [³¹] noted that EGC-FRP systems exhibit lower thermal expansion and are more compatible with concrete substrates at high temperatures, thereby reducing the risk of delamination during fire exposure. Furthermore, ZHANG et al. [³²] demonstrated that, compared to epoxy-based FRP systems, EGC-FRP maintain greater flexural strength and stiffness, even under extreme temperatures. Overall, findings collectively show that EGC-based composite systems outperform traditional ECC or epoxy-based systems in fire-prone conditions, providing enhanced safety and durability for fire-resistant retrofitting applications [³³].

Limited research has been conducted on the fire resistance of Engineered Geopolymer Composites (EGC) under elevated temperatures and various cooling regimes. Although engineered geopolymer composites have demonstrated potential in structural applications, little is known about how BFEGC (basalt fiber-based Engineered Geopolymer Composite) would behave specifically at high temperatures. Furthermore, the benefits of such composites for sustainability particularly in terms of lower energy and carbon emissions are rarely examined in the literature. This study fills these gaps by assessing BFEGC’s high-temperature performance and investigating its potential as a sustainable substitute for conventional cement-based systems. The fire-resistant behavior and thermal stability of BFEGC under high temperatures, followed by two cooling methods: gradual and rapid cooling. In this investigation, two precursors of Ground Granulated Blast Furnace Slag (GGBFS) and manufactured sand (M-sand) were used, along with alkaline solutions and basalt fibers, to produce BFEGC. The research focused on analysing the mechanical properties, microstructure, durability, and sustainable characteristics of these composites.

2. EXPERIMENTAL ANALYSIS

2.1. Material properties

The study utilized Ground Granulated Blast Furnace Slag (GGBFS) and manufactured sand (M sand) as precursors for Basalt Fiber Enhanced Geopolymer Concrete (BFEGC). The fineness of the GGBFS was measured at 390 m²/kg, with a specific gravity of 2.85. The chemical composition of the GGBFS is detailed in Table 1. The physical properties of basalt fiber, as provided by the supplier’s manual, are illustrated in Tables 2. The basalt fiber-reinforced polymer composite has a glass transition temperature of around 166.9°C. Basalt fibers are known to remain thermally stable above 600°C, however between 120°C and 200°C, mechanical degradation, particularly in tensile strength, becomes noticeable [³⁴]. The melting point of the fiber, which is normally near 1350°C, is preceded by this deterioration, the physical properties of basalt sheet as shown in Table 3. Figure 1 shows the particle size distribution of the precursors. The micro steel fiber has a length of 13 mm with the aspect ratio of 65. The specific gravity and tensile strength are 7.8 and 2200 MPa (as per supplier’s data). A commercially available NaOH flakes of 99.76% purity and a Na₂SiO₃ solution that contained 0.004% SiO₂, 0.004% Na₂SO₄ 0.21 percent NaCO₃, and 0.020% NaCl by mass (suppliers manual), the alkaline activation of M-sand and GGBFS was accomplished. The ratio of SiO₂ to Na₂O, or the silica modulus, was found to be 2.19. A viscous modifying agent (VMA) based on polycarboxylic superplasticizers (PSP), supplied by Astra Chemicals, Chennai, Tamil Nadu, India, was used in the BFEGC mix to maintain flowability. The VMA has a specific gravity of 1.10, appears as a light viscous liquid, contains 45% total solids, and has a pH value of 6.

Table 1
Chemical composition of GGBFS (suppliers manual).

OXIDES (%)	MgO	SO3	SiO2	MnO	Al2O3	CaO	INSOLUBLE RESIDUE	LOI
Value	7.73	0.5	36.2	0.12	12.42	32.37	0.49	0.26

Table 2
Physical properties of basalt fiber (suppliers manual).

PROPERTIES	ULTIMATE TENSILE STRENGTH (MPa)	FIBER MODULUS (GPa)	ULTIMATE TENSILE ELONGATION (%)	FIBER THICKNESS (mm)	FIBER DENSITY (g/cm3)	FIBER WEIGHT (g/m2)
Values	4900	230	2.1	0.227	1.76	400

Table 3
Physical properties of basalt sheet (suppliers manual).

PROPERTIES	DENSITY (kg/dm3)	MELTING POINT 0C	SPECIFIC SURFACE WEIGHT (g/m2)	WIDTH (mm)	THICKNESS (mm)	MOISTURE CONTENT (FABRIC) (%)
Values	2.7	1350	200	1000	0.2	0.4-0.6

Figure 1
Particle size distribution.

Although basalt fibers melt around 1350°C, significant mechanical degradation begins near 900°C, due to oxidation and embrittlement [³⁵, ³⁶]. This does not indicate melting but reflects a loss of structural integrity. Between 600°C and 900°C, the geopolymer matrix undergoes phase transformations, including destabilization of calcium- and magnesium-rich phases, affecting matrix–fiber bonding and contributing to strength loss. The term “slag dissolution” has been clarified to reflect this chemical breakdown.

Figure 2 shows the thermogravimetric analysis (TGA) of chopped basalt fiber. The TGA was conducted using a Thermogravimetric-Differential Thermal Analyzer (TG-DTA) by TA Instruments, with a testing temperature range from 0°C to 700°C. Samples weighing approximately 60 mg were placed in an alumina crucible. At 300°C Figure 2a, minimal weight loss of less than 1% was observed, indicating that the material has high thermal stability. The breakdown of the chopped basalt fibers began at 600°C (refer to Figure 2b), where more significant structural failure was noted, contributing to further weight loss and altering the fiber’s thermal degradation profile [³⁴]. At 600°C, a slight weight loss of 2–3% occurred due to the onset of oxidation and structural changes in the basalt fibers as indicated in [³⁷, ³⁸]. The fibers completely break down around 900°C illustrated in Figure 2c, leaving only minimal remnants.

Figure 2
Thermogravimetric analysis of chopped basalt fiber a) 300°C, b) 600°C, c) 900°C.

2.2. Mix proportion for trial mixes

The trial mixes for the Basalt Fiber Enhanced Geopolymer Concrete (BFEGC) were designed to achieve a target compressive strength of 50 MPa. The proportions for different trial mixes are shown in Table 4. An alkali source comprising sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) was added at a ratio of 1:1.5 and sodium silicate (Na₂SiO₃) solution with a modulus of 2.5. The weight ratio of sodium silicate to sodium hydroxide was maintained at 2.0 across all mixtures. This specific ratio was selected based on previous research [³⁹,⁴⁰,⁴¹], which indicated that it provides satisfactory workability and strength for the geopolymer matrix. The study examines two variables: the type of fiber used and the amount of fiber added, ranging from 0% to 2.0% by volume. The alkaline activator, which was different from the original sodium silicate solution, was made by mixing 12M sodium hydroxide with a commercial sodium silicate solution. To enhance the workability of the composite, a viscous modifying agent was incorporated. Initially, the required amount of sodium hydroxide flakes was weighed according to the desired molarity (12M) and then dissolved in distilled water. During the mixing of the BFEGC, an exothermic reaction occurred, causing the liquid to become quite warm. The solution was kept at room temperature until it reached an equilibrium state before use. Typically, the NaOH solution was prepared at least 24 hours in advance of casting the BFEGC ingredients. The batching of trial mixes for BFEGC is illustrated in Figure 3. The alkali source and precursors were gently added after the Ground Granulated Blast Furnace Slag (GGBFS) and manufactured sand (M sand) had been mixed thoroughly for one minute. This was followed by an additional mixing process for two minutes. The basalt fibers were then gradually introduced into the matrix [⁴²], and the mixture was blended for an additional ten minutes.

Table 4
Design mix for BFEGC specimens (kg/m³).

MIX LABEL	MATERIALS							FIBER VOLUME FRACTION (%)
	GGBFS	NAOH	Na2SiO3	FINE AGGREGATE	CHEMICAL ADMIXTURE	VMA	FIBER TYPE
Mix 1	470	83	83	625.07	4.7	0.0062	–	–
Mix 2	470	83	83	625.07	4.7	0.0062	B + S	1 + 1
Mix 3	470	83	83	625.07	4.7	0.0062	B + S	1 + 2
Mix 4	450	77.3	116.6	337.28	9	0.0062	B	1

• *B-Basalt fibre; S-Steel fibre

Figure 3
Batching of trial mixes.

2.3. Flowability test

According to ASTM C1437-15 [⁴³], a flowability test was conducted to assess the flow rate of fresh mixes. The fresh BFEGC mixes were first placed into a truncated conical mold with a height of 50 mm, a bottom diameter of 100 mm, and a top diameter of 70 mm. After completing the test, the mold was carefully removed, and the flow table was tapped approximately 25 times before measuring the spread diameter. Three tests were performed for each mixture to ensure accurate assessment of flowability, which can be calculated using the following Equation 1.

(1) $$\text{Flow}\text{=}\left|\frac{{\text{D}}_{\text{avg}}{\text{-D}}_{\text{b}}}{{\text{D}}_{\text{b}}}\right|\text{×100}$$

Where D_avg is the spread diameter, D_b is the bottom diameter of the truncated conical mould.

The hardened properties of BFEGC can be influenced by the presence of fibers in the fresh mixtures, which are often viscous and less flowable. The volume of fibers affects the internal structure and compactness of the mixture, particularly concerning the distribution of the fibers. This results in greater total shear resistance, primarily due to the contact between the fibers and the enhanced absorption of moisture on their surfaces [³⁶]. Furthermore, the reduction in flowability may be influenced by the characteristics of the fibers [⁴⁴, ⁴⁵].

2.4. Casting of BFEGC specimens

A total of 36 cubic samples, each measuring 50 mm × 50 mm × 50 mm, were cast to determine the compressive strength characteristics of BFEGC. Mix 4 achieved the target strength and was designated as the optimum mix. Additionally, eighteen cubes of the same size were cast to evaluate the degradation of compressive strength under three different elevated temperatures: 300°C for 30 minutes, 600°C for 20 minutes, and 900°C for 15 minutes, followed by a cooling regime. The elevated temperatures were based on the ISO 834 fire curve [⁴⁶], accompanied by both gradual and rapid cooling methods. Three cylinders (diameter: 50 mm, height: 100 mm) and three prisms (50 mm × 50 mm × 100 mm) were also cast to assess the tensile and flexural strength of the optimum mix. The durability properties of Mix 4 were examined by casting nine discs (100 mm × 50 mm) for tests including water absorption, sorptivity, and Rapid Chloride Penetration. All specimens were demolded after one day and subjected to ambient curing prior to testing. To investigate the strain hardening behavior, coupon samples measuring 10 mm × 50 mm × 450 mm were cast. A basalt sheet was sandwiched between a single layer of BFEGC. Additional layers were applied to the loading edges to prevent stress concentration. The test coupons were allowed to undergo ambient curing before testing.

2.5. Insight to elevated temperature

This study investigated the behavior of BFEGC subjected to various temperatures. Three intensity of temperature was fixed to follow three different ranges of thermal effects low (300°C for 30 minutes); medium (600°C for 20 minutes); high (900°C – 15 minutes) as per ISO834 fire curve. Gradual and rapid cooling regime was followed post heating. Since rapid cooling was adopted, the duration of sustaining the temperature is limited to avoid total disintegration of the specimens. The practical scenario to put off of fire in buildings by fire fighters would be within half an hour, the same is followed for the study. Previous studies [⁴⁷, ⁴⁸, ⁴⁹] on geopolymer mortar have shown that temperatures below 400°C can enhance strength and tensile ductility. This improvement is attributed to the geopolymerization reaction, which is exothermic in nature. Heating up to 400°C can lead to the formation of additional geopolymeric gel, thereby densifying the matrix through precipitation and improving mechanical properties. All cubic and coupon samples were heated in a furnace facility at the Metal Care unit – Trichy. The furnace has a loading capacity of up to 2 tonnes, with a temperature limit of 1150°C. The increment of the temperature was followed as per ISO834 fire curve and the intensity was sustained before cooling, as shown in Figure 4a. The specimens placed inside the furnace and the temperature maintained are displayed in Figure 4b and Figure 4c. After the heating period, the fire-loaded samples were allowed to cool gradually to ambient temperature in the laboratory. For rapid cooling the specimens were immersed in pool of water without further delay.

Figure 4
a) Temperatures varying over time in accordance with the ISO fire standard (b) sSpecimens under furnace and (c) temperature.

2.6. Testing of specimens

The BFEGC cubes and cylinders were tested using a compression testing machine (CTM) as per Is 516-1959 [⁵⁰] with a capacity of 3000 kN, as shown in Figures 5a and 5b. The prism and tensile tests for coupon samples were conducted using a universal testing machine (UTM) with a capacity of 1000 kN, depicted in Figures 5c and 5d. In accordance with ASTM D3039/D3039M-2008 and BS EN ISO 527-5:2009 [⁵¹], the loading rate was maintained at a minimum of 0.5 kN/min to prevent brittle failure. The setup for water absorption, sorptivity, and RCPT tests is illustrated in Figure 5e. To assess the water absorption of the BFEGC specimens, as per ASTM C642 [⁵²], three cubes from the Mix 4 series were first weighed and then oven-dried at 85°C. The samples were subsequently immersed in water for 24 hours, after which the surface-dry weight of the specimens was recorded. The percentage of water absorption (%) was calculated using following Equation 2.

(2) $$\text{Water absorption(\%)}\text{=}\frac{W_2-W_1}{W_1}\times 100$$

Figure 5
(a) Cube compression of BFEGC (b) split-tensile of BFEGC (c) flexural strength of BFEGC (d) tensile strength of BFRP based BFEGC specimen (e) durability test specimen of water absorption, sorptivity and RCPT.

where W₂ is the weight of the disc following after immersion (g), W₁ is the weight of the disc following before immersion (g).

The gradient of the curve representing the sorptivity of the optimum Mix 4 provides a continuous series of water absorption measurements in accordance with the standard ASTM C 1585-20 [⁵³]. The surfaces of the disc specimens were sealed and then immersed in water to a depth of 5 mm. After being removed from the water at regular intervals, the specimens were allowed to dry, after which their surfaces were cleaned and weighed. Sorptivity (S) can be estimated using following Equation 3.

(3) $$S=\frac{I}{t^{1/2}}$$

where S – sorptivity; I – change in mass divided by the product of the cross-sectional area of the test specimen and the density of water; t – time in minutes or seconds.

The RCPT was conducted following ASTM standards C1202 [⁵⁴]. The test setup consisted of two chambers separated by a 50 mm thick concrete disc specimen. One chamber was filled with a 3% sodium chloride (NaCl) solution (cathode), while the other contained a 0.3M sodium hydroxide (NaOH) solution (anode). A potential difference of 60 V was applied across the specimen for six hours. The total charge that passed through the specimen was measured in coulombs, which indicates the resistance to chloride ion penetration in BFEGC specimens.

2.7. Microstructure and sustainability analysis of BFEGC

Microstructure analysis of BFEGC was conducted using X-ray diffraction (XRD) to characterize the BFEGC mixtures. This testing was performed on both control specimens and those that showed minimal degradation of strength at elevated temperatures. Scanning Electron Microscopy (SEM) was utilized to examine the morphology of these samples and identify their functional groups. The thermal stability of the resulting mixtures was evaluated through Thermogravimetry Analysis (TGA). To assess the sustainability of the optimum Mix 4, evaluated the Embodied Energy (EE) and Carbon Emission (CE). Both the carbon output and the overall energy consumption during production of BFEGC were examined in order to assess the material’s environmental performance. The material quantities needed for one cubic meter of ECC and BFEGC were evaluated using the corresponding economic and emission factors.

3. RESULTS AND DISCUSSION

3.1. Strength characteristics of BFEGC mix

It has been observed that as the surface area of the fibers increases, the flowability of BFEGC tends to decrease. This decrease can lead to a tendency for fiber balling once the fiber content exceeds a certain threshold. As observed in, optimum composition of Mix 4 achieved a flow percentage of 115%, satisfying the requirement [⁵⁵]. The flowability of BFEGC is shown in Figure 6. and the compressive strength of the BFEGC trial mixes is shown in Figure 7. Mix 4, which includes basalt fiber and increased molarity, has achieved the desired characteristic strength. Figure 8 compares the degradation of Mix 4 strength when exposed to elevated temperatures for varying durations and cooling patterns. Particularly, Mix 4 exhibits less degradation when subjected to a temperature of 300°C for 30 minutes, followed by gradual cooling (referred to as 300_GC). Additional strength characteristics for Mix 4 are summarized in Table 5. The split tensile strength and flexural strength of this mix meet the necessary code requirements.

Figure 6
Flow range of BFEGC trial mixes.

Figure 7
Compressive strength of BFEGC at ambient temperature.

Figure 8
Compressive strength of BFEGC at elevated temperature.

Table 5
Strength characteristics of Mix 4.

TEST	SPECIMEN & PARAMETERS	FORMULA	TRIAL MIX	VALUES
Compression test(ASTM C109)	Cube	$\frac{\text{F}}{\text{b×d}}$ *F – Ultimate load (kN)b – Breadth (mm)d – Depth (mm)	Curing	7 days	14 days	28 days
			Mix1	26.45	29.23	31.48
			Mix2	27.23	36.34	40.66
			Mix3	28.34	45.45	45.79
			Mix4	35.97	49.81	55.34
Split tension test(ASTM C496, (2004))	Cylinder	$\frac{\text{2F}}{\pi \text{DL}}$ *F – Ultimate load (kN)D – Diameter (mm)L – Depth (mm)	Mix4	15.5
Flexure test(ASTMC78)	Prism	$\frac{\text{3FL}}{\text{2}b{d}^{\text{2}}}$ *F – Ultimate load (kN)b – Breadth (mm)d – Depth (mm)L – Span (mm)	Mix4	5.13

3.1.2. Tensile strength of coupon samples under elevated temperature

The tensile strain hardening behavior of the coupon samples was examined under various thermal loading and cooling regimens. The dimensions of the sandwiched coupon are illustrated in Figure 9a. The specimens were secured at both ends using appropriate fixtures, as shown in Figure 9b, and a controlled loading rate of 0.5 kN/min was applied until the samples failed. The failure mode exhibited a ductile pattern, characterized by the elongation of fibers prior to breaking, as depicted in Figure 9c.

Figure 9
(a) Basalt fiber sheet-based BFEGC (b) Test setup and (c) Failure mode.

Figure 10 shows the effects of elevated temperatures on the tensile behavior of BFEGC samples that were exposed to fire and subsequently cooled. The strain-hardening observed in these samples at elevated temperatures aligns with trends seen in compressive strength. A noticeable degradation of strength occurred in the samples, except for those tested at 300°C. The observed inability to regain stiffness after cooling may explain this behavior. At temperatures between 600°C and 900°C, the coupon samples lost their strain-hardening ability, leading to significant cracking, as previously reported [⁴⁵].

Figure 10
Tensile strain hardening behavior of basalt sheet-based BFEGC.

3.2. Durability characteristics of optimum mix

3.2.1. Water absorption

The observed water absorption result of BFEGC mix samples are displayed in Table 6. The percentage of water absorption for the samples is within the allowable limit of 3% to 6% [⁵²].

Table 6
Water absorption of BFEGC mixes.

MIX	SAMPLE	DRY WEIGHT (G)	WET WEIGHT (G)	WATER ABSORPTION (%)	MEAN (%)	STD DEV (%)
Mix 1	1	670	687	2.54
	2	675	693	2.67
	3	615	634	3.09	2.77	0.29
Mix 2	1	666	689	3.46
	2	655	678	3.52
	3	657	679	3.35	3.44	0.09
Mix 3	1	701	722	2.99
	2	705	723	2.55
	3	679	701	3.24	2.93	0.35
Mix 4	1	713	744	4.35
	2	735	773	5.17
	3	756	789	4.37	4.63	0.47

3.2.2. Sorptivity

The results of the sorptivity test are presented in Figure 11. It was observed that Mix 4 was impermeable, as it absorbed minimal surface water. The gradient of the curve, representing the magnitude of sorptivity, falls within the permissible range for water absorption according to the standard [⁵³].

Figure 11
Sorptivity behaviour of BFEGC.

3.2.3. Rapid chloride permeability test

The Rapid Chloride Permeability Test (RCPT) is a crucial durability assessment for EGC [⁵⁵]. In a study conducted by YASWANTH et al. [⁴⁵], RCPT was performed on Mix 4 to evaluate its resistance to chloride ion penetration. The results indicated that the current did not pass through the sample. The presence of fibers and alkali activators resulted in low permeability, recorded at fewer than 1,000 coulombs, which satisfies the code requirements, as shown in Table 7. This low permeability may be attributed to the basalt fibers, which can act as insulators by effectively arresting or absorbing electrical charge, thus preventing it from flowing. This behavior is consistent with the semi-conductive properties of basalt fibers in certain alkaline environments. Additionally, as established by the scanning electron microscopy (SEM) results, the incorporation of alkali sources in the BFEGC mix resulted in a dense microstructure. This enhanced microstructure demonstrated excellent resistance to chloride diffusion and functioned effectively as an insulator, owing to the bridging action of the basalt fibers present in the BFEGC matrix.

Table 7
RCPT test for BFEGC

MIX 4 SERIES	CHARGE PASSED (COULOMBS)	PENETRABILITY RATE (ASTM C 1202)
Sample 1	945	100-1000	Very Low
Sample 2	823	100-1000	Very Low
Sample 3	745	100-1000	Very Low
Sample 4	767	100-1000	Very Low

3.3. Microstructure characteristics

3.3.1. Thermo-gravimetric analysis (TGA)

A detailed thermal evaluation of Mix 4 was performed using the SDT Q600 V20.9 Build 20 instrument, which combines thermogravimetric analysis (TGA) and differential thermal analysis (DTA) to assess the composite’s stability across a wide temperature range. The sample, composed of a basalt fiber-based engineered geopolymer composite (BFEGC), was subjected to a controlled heating regime to monitor both mass loss and thermal events associated with decomposition and phase transformations. The TGA curve revealed three major weight loss stages, each corresponding to distinct thermal phenomena within the material structure. In the first stage, around 300°C, the sample exhibited a mass reduction of approximately 2–5% was observed in Figure 12a. This loss is primarily due to the evaporation of free and physically bound water, along with the initial breakdown of hydroxyl groups present in the geopolymer gel structure [²⁵]. This dehydration process is endothermic, as confirmed by the associated DTA peak, indicating energy absorption due to water removal and minor structural changes.

Figure 12
Thermo-Gravimetric Analysis (TGA) of BFEGC (a) 300°C (b) 600°C and c) 900°C.

As the temperature increased to approximately 600°C as shown in Figure 12b, a second mass loss of 5–10% was observed. This corresponds to continued dehydroxylation, as well as structural reorganization of aluminosilicate chains within the geopolymer matrix. Previous studies have shown that slag particles predominantly dissolve at temperatures ranging from 600°C to 900°C [³⁸]. During this stage, the onset of basalt fiber degradation is also initiated, which contributes to the composite’s internal breakdown. The DTA curve displayed a noticeable endothermic shift, indicating increased energy involvement due to matrix destabilization [⁵⁶, ⁵⁷]. By 900°C, a third and more pronounced mass loss of around 5–7% occurred as shown in Figure 12c, likely resulting from oxidation of the basalt fiber components and the softening or partial melting of the fibers. This stage signifies the limit of thermal stability for both the fibers and matrix, and the associated DTA trace shows a peak linked to these exothermic degradation processes. The combined TGA–DTA results suggest that the geopolymer matrix acts as a thermal barrier, delaying the degradation of basalt fibers, while the fibers enhance the overall structural integrity of the matrix under extreme heat [⁵⁸]. This mutual reinforcement highlights the synergistic behavior of the BFEGC system, making it a promising candidate for fire-resistant and high-temperature structural applications.

3.3.2. X-ray diffraction (XRD) analysis

The X-ray diffraction (XRD) assessment of Mix 4, illustrated in Figure 13a and 13b, highlights significant shifts in crystalline structure between ambient and elevated temperature conditions. At room temperature (Figure 14a), the detection of trikalsite (Ca₂SiO₄·H₂O), periclase (MgO), and tridymite (a polymorph of SiO₂) confirms the formation of standard geopolymeric reaction products [²⁹]. Upon heating to 300°C, the appearance of lazurite, albite, haycockite, and brucite (Figure 14b) indicates elemental redistribution and new phase formation, suggesting ion movement and recrystallization within the matrix. The absence of trikalsite at this stage implies decomposition of calcium silicate hydrates, whereas the emergence of albite and lazurite reflects the generation of more thermally stable aluminosilicate phases, consistent with previously reported findings on phase transitions in geopolymeric systems [⁵⁹]. Based on the sustainable ion exchange with aluminate phases and surface complexation with C-A-S-H were used to stabilize borate [⁶⁰]. The binding efficiency was increased by the presence of calcined clay with a high aluminum content, with the integration of borate into C-A-S-H. The addition of barite resulted in a denser structure and better interface in ultra-high-performance concrete, as microstructural observations verified [⁶¹]. These microstructural changes correspond with the enhanced mechanical strength observed at 300°C, particularly in specimens subjected to gradual cooling. The formation of brucite (Mg (OH)₂) could also be indicative of post-heating hydration, which may contribute to partial strength recovery. At 600°C and 900°C, further phase breakdown and structural degradation are evident, aligning with the progressive decline in compressive and flexural strength. Overall, the XRD results provide a clear link between microstructural evolution and the macroscopic performance of BFEGC under thermal loading, highlighting the importance of phase stability in determining high-temperature durability.

Figure 13
X-ray diffraction (XRD) plot for test samples at elevated temperature (a) Control and (b) 300GC.

Figure 14
SEM morphology for test sample at elevated temperature (a) Control and (b) 300GC.

3.3.3. Scanning Electron Microscopy (SEM) analysis

The SEM analysis for both Mix 4 and 300_GC is presented in Figure 14a and Figure 14b present the SEM images of Mix 4 and the 300_GC sample, respectively. The control sample shows a relatively uniform matrix with moderate porosity, while Mix 4 reveals a denser microstructure with partially reacted precursor particles and finely distributed pores, consistent with findings in previous studies [¹⁹]. The presence of MgO particles contributes to matrix densification by acting both as micro-fillers and as reactive agents in the geopolymerization process. Exposure to 300°C appears to promote secondary geopolymerization reactions, leading to further matrix consolidation and reduction in pore volume [⁶²]. This refined microstructure explains the improved compressive and flexural strength observed at 300°C, particularly under gradual cooling conditions. The controlled cooling facilitates structural reorganization, allowing partial healing of microcracks and contributing to a more stable and cohesive matrix. Moreover, the interaction between MgO and aluminosilicate phases at this temperature likely leads to the formation of new crystalline products, such as magnesium-aluminosilicate hydrates, which enhance the matrix integrity [⁶³]. As reported by RASHAD et al. [⁶⁴], moderate thermal exposure can initiate beneficial mineral transformations within the geopolymer matrix, reinforcing its mechanical performance and thermal stability. By correlating these microstructural developments with mechanical test results, it becomes evident that the thermal exposure at 300°C, combined with gradual cooling, creates conditions favorable for strength enhancement in BFEGC. These results underscore the importance of microstructural evolution and phase formation in governing the macroscopic behavior of the composite.

3.4. Sustainability of BFEGC

The environmental impact of Bio-based Fiber Enhanced Geopolymer Concrete (BFEGC) has been primarily assessed in terms of carbon emissions (CE) and embodied energy (EE). The quantities of materials used per cubic meter for ECC and BFEGC have been multiplied by the coefficients and different binder material shown in Table 8 by the following equations.

Table 8
Sustainability and cost analysis of BFEGC.

MATERIALS	BFEGC	ECC [65]	COI (kgCO2/kg)	EEI (MJ/kg)	BFEGC		ECC
					COI (kg)	EE (MJ)	COI (kg)	EE (MJ)
Cement [66]	0.00	450.00	0.95	5.50	0.00	0.00	427.50	2475.00
Flyash [67]	0.00	854.00	0.03	0.00	0.00	0.00	25.62	0.00
M sand [68]	337.28	469.00	0.01	0.04	3.37	13.49	4.69	18.76
PVA [67, 69]	0.00	26.00	3.60	106.54	0.00	0.00	93.60	2770.04
Basalt fiber [70]	3.3	0.00	0.70	18.00	23.10	59.4	0	0
Super plasticier [71]	9.00	0.68	0.72	18.30	6.48	164.70	0.49	12.44
Water [72]	0.00	326.00	0.00	0.01	0.00	0.00	0	3.26
GGBFS [73]	450.00	0.00	0.05	0.86	24.40	387	0.00	0.00
NaOH [74]	77.30	0.00	1.92	20.50	148.42	1584.65	0.00	0.00
Na2SiO3 [62]	116.60	0.00	1.22	5.37	142.25	626.14	0.00	0.00
				Total	348.02	2835.38	551.9	5279.5

Table 9
Sustainability values for different binder material.

FACTORS	BFEGC	ECC
COi (kg)	348.02	551.9
EE (MJ)	2835.38	5279.5
Compressive strength (MPa)	55.34	30.00
${\text{CO}}_{\mathrm{i}}\frac{kg}{MPa}$	6.28	18.39
$\text{EE}\frac{MJ}{MPa}$	51.23	175.98

(12) $$\text{CE}\text{=}{\displaystyle {\sum }_{\text{i=1}}^{\text{n}}{\text{M}}_i}\text{×}C{O}_i$$

(13) $$\text{EE}\text{=}{\displaystyle {\sum }_{\text{i=1}}^{\text{n}}{\text{M}}_i}\text{×}E{E}_i$$

where n is the total number of materials used, M represents the material type, i denotes the material index, and CE and EE are the CO₂ emission and embodied energy factors associated with each material, respectively. The sustainability parameters for various binder materials were optimized, and the compressive strength was determined for both BFEGC and ECC as shown in Table 9. Previous research indicates that geopolymer concretes can reduce CO₂ emissions by 44% to 64% compared to ordinary Portland cement (OPC) concretes [⁶⁶]. The embodied energy of EGC varies depending on the precursors and activators used, but it generally shows improvements over traditional cement-based materials. A detailed analysis conducted [⁶⁷] by embodied energy involved in geopolymer cement production and found that using ground granulated blast furnace slag (GGBFS) as a precursor could reduce embodied energy by up to 59% compared to OPC. The economic viability of EGCs [⁶³] is a crucial factor influencing their adoption. Furthermore, the elimination of cement in BFEGC contributes to achieving sustainable goals. Material performance can be improved by using steel fibers, fly ash, slag, bottom ash, biomass ash, and graphite powder, according to research on thermoelectric energy harvesting and power plant waste management [⁷⁵]. By acting as efficient supplemental cementing agents, these materials enhance sustainability. Their adoption encourages eco-friendly building practices and supports the concepts of the circular economy, particularly in emerging economies [⁷⁶].

4. CONCLUSION

The study demonstrates the strength, durability and sustainability characteristics of Basalt Fiber-based Engineered Geopolymer Composites (BFEGC) when exposed to elevated temperatures. The following points were observed:

• BFEGC with trial Mix 4 exhibited excellent flowability at 115% and achieved a characteristic compressive strength of 55 MPa. The split tensile strength reached 15.5 MPa, while the flexural strength was recorded at 5.13 MPa. The degradation of compressive strength for Mix 4 was minimal at 300GC and the percentage decreased as 19.40%.

• The water absorption percentage of the samples falls between 3% and 6%, which is an acceptable range. Furthermore, the sorptivity curve’s gradient conforms to the acceptable thresholds for water absorption. This result supports the suitability of basalt fibers for use in structural applications by proving their semi-conductive nature in specific alkaline settings.

• BFEGC samples exposed to elevated temperatures showed negligible spalling. The presence of chopped basalt fibers played a key role in mitigating spalling and enhancing the composite’s resistance to thermal damage, contributing to the overall fire performance of the matrix.

• The tensile test conducted on thermally exposed coupon specimens reinforced with basalt sheet-based BFEGC revealed that the sample labeled 300GC, which was subjected to 300°C, exhibited the highest tensile strain, reaching up to 8%. This significant strain capacity indicates the material’s excellent ability to undergo deformation without failure at moderate temperatures, likely due to improved fiber-matrix bonding and microstructural refinement at this exposure level.

• Microstructural analysis using X-ray diffraction (XRD) and scanning electron microscopy (SEM) of BFEGC confirmed the formation of new compounds such as haycockite, albite, lazurite, brucite at 300GC compared to control specimens.

• Thermogravimetric analysis (TGA) of the basalt fiber indicated degradation at 600°C and 900°C, whereas the TGA of BFEGC demonstrated improved resistance under elevated temperatures. A slight weight loss of 2-5% occurred at 300°C, mainly attributed to the evaporation of physically adsorbed water and the early stages of dihydroxylation within the geopolymer matrix.

• BFEGC showed significant environmental benefits, reducing carbon emissions and embodied energy by 2.92 times and 3.44 times, respectively, compared to cement-based materials.

4.1. Future scope and recommendations

Further investigation into Basalt Fiber-based Engineered Geopolymer Composite (BFEGC) systems presents significant potential for advancing fire-resilient retrofitting strategies. While current studies have demonstrated the thermal stability of BFEGC, there remains a need to explore its performance under more complex thermal conditions, including fluctuating temperatures, prolonged exposure durations, and cyclic thermal loading, which more accurately simulate real fire events. Future research could also focus on evaluating the influence of increased FRP layering, varied aspect ratios, and geometry-dependent heat distribution on the composite’s structural response. Additionally, testing BFEGC in high-stress zones and non-uniform loading conditions, such as corners, beam-column joints, and cantilevered sections, would provide a deeper understanding of its mechanical reliability. Investigating different fiber orientations and hybrid composite configurations may further improve its adaptability for a range of repair and retrofitting applications across structural systems exposed to extreme environments.

4.2. Applications

Particularly in fire-prone infrastructure and the post-fire rehabilitation of reinforced concrete parts, the use of Basalt Fiber-based Engineered Geopolymer Composite (BFEGC) in structural retrofitting offers substantial promise for practical applications. BFEGC is ideally suited for reinforcing columns, beams, and slabs in buildings, tunnels, and industrial facilities where exposure to intense heat is a design problem because of its intrinsic thermal stability, low shrinkage, and good compressive strength retention at increased temperatures. When industrial by-products such fly ash and GGBS are utilized as precursors, BFEGC provides benefits in terms of cost-effectiveness, durability, and environmental sustainability from the standpoint of engineering design. It is a good choice for green building techniques because it doesn’t need cement, which lowers carbon emissions. This study’s compression, flexural, and tensile behavior suggest that BFEGC performs promisingly when compared to traditional FRP systems in terms of both mechanical recoveries following thermal exposure and thermal resistance. The geopolymer matrix in BFEGC offers superior confinement and strength retention, particularly in the gradual cooling conditions that follow a fire, in contrast to conventional FRP systems.

5. ACKNOWLEDGMENTS

The authors express their heartfelt appreciation to the Vice Chancellor and Management of SASTRA deemed University for funds provided through the TRR research grant.

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DATA AVAILABILITY

The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

Publication Dates

• Publication in this collection
  11 Aug 2025
• Date of issue
  2025

History

• Received
  22 Jan 2025
• Accepted
  14 May 2025