Comprehensive evaluation of impact strength and microstructural characteristics of geopolymer concrete reinforced with four types of natural fibers of varying lengths

Abstract

This study systematically studied the impact strength of Geopolymer concrete (GC) integrating fly ash, slag, and silica fume, reinforced with four natural fibers including sisal, jute, coir, and flax at varying aspect ratios. The primary objectives are to analyze the influence of fiber length on initial cracking number (J1), failure number (J2), crack-bridging mechanisms, and failure modes under impact loading. This study presents a unique contribution by thoroughly examining the impact of fiber aspect ratio in multi-binder GC, offering critical insights into optimizing fibrous GC for improved structural performance and resilience. The impact performance of GC was influenced by both fiber type and length. Coir fibers enhanced J1 by up to 66% and J2 by 171.43%, with optimal energy absorption at 60 mm. Flax and jute fibers showed peak performance at 40 mm, with J2 improvements of 75.82% and 100%, respectively, while longer lengths led to dispersion issues and diminished gains. Sisal fiber at 40 mm offered balanced enhancement (J1: 50%, J2: 117.58%), whereas the highest J2 (135.16%) was achieved with 60 mm, accompanied by a moderate drop in J1, indicating that excessive fiber lengths may hinder bonding efficiency.

Introduction

The increasing emphasis on eco-friendly construction solutions has highlighted the importance of alternative binders that not only promote environmental sustainability but also offer superior mechanical properties. Geopolymer concrete (GC) has attracted considerable attention due to its reduced carbon footprint relative to conventional Portland cement-based concrete¹. GC is produced by activating aluminosilicate sources: fly ash (FA), ground granulated blast furnace slag (GGBS), or metakaolin using alkaline solutions². A blend of alumina and silica undergoes polymerization causing the formation of a three-dimensional Si-O-Al-O polymeric network³. This phenomenon yielding a material known for its enhanced durability and mechanical characteristics. Extensive research on FA-based GC has highlighted its superior fresh, mechanical, and durability properties^4–6. A limitation of utilizing FA-based GC is the requirement for curing under higher temperatures⁷. To mitigate this limitation, various admixtures and additives have been incorporated into GC formulations, as evidenced by the literature⁸. Silica fume (SF) and GGBS are utilized as an substitute to FA⁹. Zargaleh et al.¹⁰ demonstrated that incorporating GGBS as a partial substitution up to 20% causing a rise in compressive strength. Nevertheless, further raising the substitution rate from 20 to 30% causing a decline in compressive strength. Moreover, raising the substitution rate of GGBS from 20 to 30% enhanced fracture toughness from 16.73 to 27.49 MPa√mm, indicating a significant improvement in the material’s resistance to crack propagation. Amin et al.¹¹ reported that GC with 500 kg/m³ slag exhibited a slump of 225 mm. The mix comprising 300 kg of slag and 200 kg of metakaolin achieved the maximum compressive strength (63.3 MPa early, 82.6 MPa later) and superior mechanical properties, including a modulus of elasticity of 37.68 GPa, flexural strength of 9.2 MPa and tensile strength of 6.2 MPa. Karthik et al.¹² stated that the combination of FA, GGBS, and SF resulted in a compressive strength of up to 41.3 MPa for the GC.

Impact resistance is a crucial performance attribute of GPC under dynamic loading conditions¹³. The drop weight impact strength, which evaluates the concrete’s capability to withstand high-energy impacts without substantial damage, is an essential parameter for structural design. However, GPC typically exhibits lower impact resistance compared to traditional concrete. To enhance this property, adding fibers such as steel, basalt, polypropylene, and glass has been found effective¹³. Fiber reinforcement improves toughness, crack control, and fracture resistance, offering benefits in mechanical performance, durability, and energy absorption¹⁴. Chen et al.¹⁵ established that adding steel fibers in the concrete matrix significantly advances impact resistance. The fibrous GC slabs exhibited an increased impact load-bearing capability of up to 463 kN, surpassing the 425 kN capacity of the plain GC slabs. Moreover, the fibrous GC slabs maintained superior structural integrity, with a reduction in peak deflection ranging from 27.0 to 66.5% compared to the plain slabs. This behaviour is ascribed to the fiber bridging effect, which effectively reduced concrete spalling and scabbing. Asrani et al.¹³ studied the impact behavior of fibrous GC using mono and hybrid fibers, including glass, steel and polypropylene. The hybrid mixes of steel (1%), polypropylene (0.3%), and glass fibers (0.3%) significantly increased maximum impact strength by 7.9 times for the first crack and 10.4 times for failure compared to fiber-free GC. Additionally, the use of mono and hybrid fibers enhanced impact ductility by 8.6–196.1%. Zhi et al.¹⁶ stated that the impact toughness of GC initially rises with the content of steel fiber, peaking at a certain level before decreasing. The peak impact toughness, recorded at a steel fiber content of 2%, was 7133.83 N·m. Additionally, the energy dissipation during the first fracture and ultimate crack exhibited substantial increases, with enhancements of 60 times and 606 times, respectively. A multitude of studies have examined the mechanical characteristics of GC incorporating with various fibers, such as steel, glass, polypropylene and basalt which enhance toughness, impact resistance, and crack control under static and dynamic loading. However, despite extensive research on synthetic and mineral-based fibers, the use of plant-based fibers in GC and other concrete type remains underexplored and warrants further investigation.

The integration of natural fibers: jute¹⁷, sisal¹⁸, coir¹⁹, flax²⁰, kenaf²¹ and bamboo²² into concrete has emerged as an effective method to enhance its mechanical properties. These renewable, biodegradable fibers provide an eco-friendly alternative to synthetic options, improving fracture toughness, impact resistance, and energy absorption. In concrete, natural fibers help bridge micro-cracks, redistribute stresses, and increase the materials capacity to resist low-velocity impact forces²³. Studies have shown that these fibers improve post-cracking behavior, ductility, and structural integrity under dynamic loading. Gao et al.²⁴ detected that adding 2.0% sisal fibers (18 mm in length) increased the flexural strength of ultra high performance concrete by 16.7% and toughness by 540.0%. An increase in fiber length enhanced both the pull-out load and energy absorption, with the fibers exhibiting uniform dispersion within the matrix. These results suggest that sisal fibers can enhance UHPC’s mechanical properties while offering potential for cost-effective and energy-efficient production. Rahimi et al.²⁵ confirmed that adding treated flax fibers led to a compressive strength enhancement of up to 12%, ascribed to the elimination of contaminants such as soluble sugars, waxes, pectin, lignin. The elimination of these substances increased the fiber’s surface roughness. Ahmad Farooq et al.²⁶ reported a 6–8% rise in compressive strength by adding coconut fiber. However, further increases in fiber content beyond 0.1% did not yield significant improvements. Thus, a fiber volume of 0.1–0.2% was identified as the optimal range for enhancing compressive strength. Guruswamy et al.¹⁹ determined that coir fibrous concrete exhibited a peak compressive strength of 34 MPa when incorporating fibers of 1.0 cm length at a dosage of 4.0%. However, compression test results indicated a progressive decline in compressive strength as the fiber dosage elevated from 4 to 12%. This reduction was primarily associated with diminished interfacial adhesion between the cementitious matrix and aggregate particles, ascribed to the heightened moisture uptake potential of coir fibers.

While many studies have explored the mechanical behavior of natural fibrous concrete, research focusing on the impact resistance of GC incorporating natural fibers remains scarce. Ramaswamy et al.²⁷ carried out the impact tests on coir fibrous concrete slabs (0.5% fiber volume) and beams (1% fiber volume). The tests showed a 10–20% improvement in impact strength in comparison with plain concrete. The incorporation of 2% fiber content and 40 mm fiber length in concrete has been found to optimize the material’s impact energy absorption capacity²⁸. Conversely, Hwang et al.²⁹ recommended a coir fiber content of 4% as optimal for achieving the best impact resistance in concrete. Ramakrishna and Sundararajan²⁸ stated that adding jute fibers to conventional concrete slabs significantly improved their impact strength, with observed enhancements ranging from three to eighteen times. Naraganti et al.³⁰ demonstrated that adding steel fibers into concrete markedly enhanced its resistance to early cracking under impact loading, with a tenfold increase observed at a fiber dosage of 1.5%. In contrast, the use of sisal fibers at the same fiber dosage resulted in a more modest improvement, enhancing early cracking resistance by a factor of 3.5. Ayeni et al.³¹ observed that the impact strength of GC by adding oil palm, coconut fiber and kenaf fiber fiber increased by 2.09%, 3.77% and11.35%, respectively, as the fiber dosage was augmented from 0 to 0.75%. Also, when the fiber content was further raised from 0.75 to 1%, the impact resistance of the corresponding fibrous GC improved by 88.33%, 98.33% and 122.11% respectively. These results underscore the significant enhancement in impact resistance achieved through the integration of natural fibers into GC.

Despite promising findings, there is a notable void in the available literature on the impact strength of natural fibrous GC. Further studies are needed to optimize factors such as fiber content, type, and their interaction with geopolymer matrices. This research is crucial for developing sustainable, high-performance materials and expanding the use of GC in structural engineering, particularly for applications requiring enhanced impact resistance and sustainability. Therefore, further exploration into the impact strength of these materials is essential for their practical application.

The present research undertakes a detailed evaluation of the influence exerted by fiber aspect ratio on the dynamic mechanical performance, crack control capacity, and failure progression in GC reinforced with natural fibers. While earlier investigations such as those conducted by Ramakrishna and Sundararajan²⁸ and Wang and Chouw⁴⁵ have examined fiber length effects in conventional cementitious systems, the novelty of this study lies in its emphasis on coir, flax, jute, and sisal fibers embedded within a hybrid-binder geopolymer framework. This relatively unexplored material configuration facilitates a refined understanding of how fiber geometry interacts with the microstructural and chemical characteristics of geopolymer matrices under impact loading. Moreover, the research is underpinned by a comprehensive suite of microstructural characterizations, including thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR), thereby offering a multi-scale interpretation of mechanical responses grounded in materials science principles.

Materials and methods

Raw materials

The binder materials used in this research included Class F FA, sourced from a thermal power plant in Astana, Kazakhstan; GGBS from Chelyabinsk, Russia; and SF obtained from Pavlodar, Kazakhstan. The chemical composition of FA, GGBFA and SF is depicted in Table 1. FA, with its high SiO₂ (61.23%) and Al₂O₃ (28.94%) content, exhibits strong pozzolanic properties suitable for geopolymerization. GGBS, characterized by a high CaO concentration (38.41%), enhances early strength but has lower SiO₂ (34.48%) and Al₂O₃ (11.81%) than FA. SF, with an exceptional SiO₂ content (96.3%), is highly reactive and improves concrete microstructure through secondary pozzolanic reactions. The graphical illustration of these materials is shown in Fig. 1. The Fig. 2a and b clearly illustrates that FA particles exhibit a spherical morphology, whereas GGBS particles possess an angular shape in comparison, similar observation recorded by Sidhu and Kumar³². SEM analysis of SF reveals ultrafine, spherical particles (Fig. 2c) that enhance packing density and reduce voids in geopolymer matrices. Its high surface area promotes secondary pozzolanic reactions, creating additional C-S-H gel, which improves GC strength.

Table 1.

Chemical properties of materials.

Composition	Na2O	MgO	Al2O3	SiO2	P2O5	K2O	CaO	TiO2	MnO	FeO	SO3
FA (%)	0.41	0.45	28.94	61.23	0.54	0.74	1.28	1.09	0.15	5.18	-
GGBS (%)	0.65	9.98	11.81	34.48	-	0.84	38.41	1.45	0.27	0.47	1.66
SF (%)	0.55	0.63	0.57	96.3	-	1.17	0.24	-	-	0.54	-

Fig. 1.

Raw material (a) FA, (b) GGBS, (c) SF, (d) fine aggregate and (e) coarse aggregate.

Fig. 2.

SEM analysis of raw materials (a) FA, (b) GGBS and (c) SF.

The XRD pattern of Class F FA displays a broad hump between 20° and 35° (2θ) (Fig. 3a), confirming its predominantly amorphous nature due to disordered aluminosilicate glass, which actively contributes to geopolymer gel formation. Superimposed crystalline peaks of Quartz (20.8°, 26.6°, 36.5°) and Mullite (16.4°, 26.1°, 40.8°), originating from coal combustion, are largely inert and minimally involved in the geopolymerization process. The XRD pattern of GGBS exhibits a broad hump between 25° and 35° (2θ) (Fig. 3b), representing its predominantly amorphous, glassy structure formed by rapid cooling. Minor crystalline peaks of Calcite, Dolomite, Magnetite, calcium carbide, and calcium silicate reflect limited crystallization during cooling or exposure. The XRD pattern of SF shows a broad hump between 20° and 30° (2θ), indicating its largely amorphous silica composition (Fig. 3c). This amorphous nature, mainly due to opaline and non-crystalline SiO₂, enhances its reactivity in pozzolanic and alkali-activated systems. Quartz shows peaks at 20.8°, 26.6°, and 36.5° (2θ), reflecting its crystalline, low-reactivity nature. Cristobalite (Cr) appears as a weak peak around 36°–37° (2θ), suggesting partial crystallization during thermal processing. Opal (O), an amorphous hydrated silica, contributes to the broad hump in the XRD pattern, indicating high pozzolanic reactivity.

Fig. 3.

XRD analysis of raw materials (a) FA, (b) GGBS and (c) SF.

In the present study, the fine aggregate was selected as river-derived natural sand, demonstrating a specific gravity of 2.60 and a fineness modulus of 2.71, fully conforming to the standards prescribed in IS: 383–2016³³. Crushed granite, serving as the coarse aggregate, was characterized by a maximum nominal size of 12.5 mm, a specific gravity of 2.66, and a water absorption value of 0.6%, thereby ensuring suitability for structural concrete applications.

The pozzolanic binder was activated using a dual-alkali system of NaOH (12 M) and sodium silicate at a mass ratio of 1.45. After dissolving the pellets in distilled water, the NaOH solution was created, followed by adding sodium silicate. The activator was permitted to stabilize for 24 h before casting to ensure uniform reactivity.

Four distinct natural fibers coir, flax, jute, and sisal were incorporated into the GC mixtures at a uniform dosage of 0.5% by volume of the GC, and the visual representation of these fibers are illustrated in Fig. 4. The physical and mechanical characteristics of these fibers are comprehensively presented in Table 2. SEM analysis of coir fiber reveals a rough, irregular surface with notable fibrillation and exfoliation (Fig. 5a), indicating strong potential for mechanical interlocking, improved fiber–matrix bonding, and enhanced stress transfer and crack bridging in the composite. Flax fiber exhibits a smoother, more aligned surface with longitudinal striations and slight irregularities (Fig. 5b), offering moderate matrix adhesion but reduced mechanical interlocking compared to coir due to its uniform texture. Jute fiber exhibits a smooth, cylindrical morphology with fine longitudinal ridges (Fig. 5c), indicating low surface roughness and reduced pull-out strength, though its uniform structure may support consistent mechanical performance in composites. Sisal fiber exhibits a moderately rough surface with clear striations and fine fibrils (Fig. 5d), offering a balance between interlocking ability and fiber integrity, which supports stable bonding in the geopolymer matrix. This study is limited to coir, flax, jute, and sisal fibers used at lengths of 20, 40, and 60 mm with a fixed dosage of 0.5% by volume. Longer fibers (> 60 mm) and higher dosage (> 0.5%) showed a tendency to entangle during mixing, forming clusters that reduce workability and lead to uneven fiber distribution^34,35. The high viscosity of geopolymer binders further impedes uniform dispersion, creating zones of fiber accumulation and under-reinforcement, which compromise mechanical performance. Increasing fiber content also demands more intensive mixing, but excessive mixing may degrade the natural fibers due to their sensitivity to alkaline environments and mechanical stress. Balancing effective dispersion with fiber integrity remains a key challenge at higher lengths and dosages.

Fig. 4.

Fibers (a) coir, (b) flax, (c) jute, and (d) sisal.

Table 2.

Fiber properties.

Properties	Coir	Flax	Jute	Sisal
Fiber length (mm)	20, 40, 60	20, 40, 60	20, 40, 60	20, 40, 60
Diameter (µm)	500	20.5	22.0	18.0
Density (g/cm3)	1.21	1.50	1.45	1.46
Tensile strength (MPa)	250	1000	660	650

Fig. 5.

SEM analysis of fibers (a) coir, (b) flax, (c) jute, and (d) sisal.

Mixing combinations and specimen Preparation

A total of thirteen distinct GC mix formulations were developed in the present study, incorporating four diverse types of natural fibers, as outlined in Table 3. The selected fibers coir, flax, jute, and sisal were uniformly introduced into the GC matrix at a fixed dosage of 0.5% by volume of the binder. Each fiber type was utilized in three diverse lengths: 20 mm, 40 mm, and 60 mm. A plain control mix without any fiber reinforcement was designated as G-0. The mixes containing coir fibers of varying lengths were labeled as G-C20, G-C40, and G-C60, corresponding to fiber lengths of 20 mm, 40 mm, and 60 mm, respectively. In a similar manner, the remaining mixes were denoted based on the fiber type flax (‘F’), jute (‘J’), and sisal (‘S’) with numerical suffixes representing the respective fiber lengths incorporated into each formulation. The fabrication of GC incorporating FA, GGBS, SF, and natural fibers is carried out through a sequential and controlled mixing protocol aimed at achieving uniform dispersion and enhanced mechanical integrity. The NaOH solution is combined with a Na₂SiO₃ solution at a defined mass ratio of 1:1.45, and kept ready for integration into the mixture. In the initial mixing phase, the dry pozzolanic binders FA, GGBS, and SF are introduced into a pan-type mixer and blended for approximately 3 min to promote homogeneity. Following this, the fine and coarse aggregates are added to the dry blend, and the combined materials are mixed for an extra 3 min to achieve uniform dispersion. Subsequently, natural fibers are incrementally incorporated into the dry matrix. Care is taken to prevent fiber agglomeration, and mixing continues until the fibers are uniformly distributed (Fig. 6a). The solution, previously prepared, is then slowly introduced into the dry composite while continuous mixing is performed for roughly 2 min (Fig. 6b). This step facilitates the formation of a cohesive and workable geopolymer matrix. The resulting fresh GC is transferred into molds and subjected to vibration to eliminate entrapped air voids. The appearance of the finished cubical and cylindrical specimens is shown in Fig. 6 (c) and (d). After an initial curing duration of 24 h, the specimens are demolded and subsequently cured under ambient environmental conditions until the designated testing intervals. The compressive strength tests were conducted on cube specimens measuring 100 mm, following the procedures outlined in IS:516³⁶. The concrete specimen is positioned centrally in a calibrated compression testing apparatus, and a uniform axial load is applied at a controlled rate. The peak load sustained prior to failure is recorded to calculate the compressive strength in accordance with standard testing protocols. For impact resistance testing, cylindrical samples with a diameter of 152 mm and a height of 64 mm were used, in accordance with the guidelines provided by ACI Committee 544-2R³⁷.

Table 3.

Mixing combinations of GC (kg/m³).

Mixture id	FA	GGBS	SF	Na2SiO3	NaOH	Fiber type	Fiber length (mm)	Fiber dosage (%)	SP (%)
G-0	204	163	41	70	101	-	0	-	0.5
G-C20	204	163	41	70	101	Coir	20	0.5	0.5
G-C40	204	163	41	70	101	Coir	40	0.5	0.5
G-C60	204	163	41	70	101	Coir	60	0.5	0.5
G-F20	204	163	41	70	101	Flax	20	0.5	0.5
G-F40	204	163	41	70	101	Flax	40	0.5	0.5
G-F60	204	163	41	70	101	Flax	60	0.5	0.5
G-J20	204	163	41	70	101	Jute	20	0.5	0.5
G-J40	204	163	41	70	101	Jute	40	0.5	0.5
G-J60	204	163	41	70	101	Jute	60	0.5	0.5
G-S20	204	163	41	70	101	Sisal	20	0.5	0.5
G-S40	204	163	41	70	101	Sisal	40	0.5	0.5
G-S60	204	163	41	70	101	Sisal	60	0.5	0.5

Fig. 6.

GC (a) dry mix, (b) wet mix, (c) cube specimens and (d) cylindrical specimens.

Drop weight impact test setup

The impact strength of GC was assessed using the drop-weight impact test as per ACI Committee 544-2R³⁷. This testing method is relatively simple, as it does not necessitate continuous monitoring of load application, deformation responses, or vibrational effects. Instead, the primary parameters recorded include the number of impact blows needed to initiate the first visible crack (J1) and the total number of blows causing ultimate specimen failure (J2). Figure 7 provides a experimental setup utilized in this research. The instrument employed in this study was designed to drop a standard weight of 4.45 kg from a fixed height of 457 mm onto the specimen surface. The test configuration comprised a steel hammer that delivered impact forces onto a steel ball positioned directly over the specimen. To maintain stability and prevent lateral movement during testing, each specimen was firmly secured using positioning lugs and a steel disc. The initiation of cracking (J1) was identified by the emergence of visible surface cracks, which progressively extended toward the bottom surface until complete structural failure occurred (J2). The impact resistance of the specimens was observed by recording the total number of impact blows endured before failure. Each impact event delivered a consistent amount of energy, allowing for a quantifiable assessment of the GC’s capacity to withstand impact-induced damage. The impact energy absorbed by the GC was computed with the Eq. (1), providing a measure of the GC’s resistance to drop weight impact loading.

1

Fig. 7.

Impact testing device.

where h: height of drop hammer, m: the weight of the striking hammer., j: the count of applied impacts. and g: the gravity of acceleration (9.81 m/s²).

The characterization techniques employed in this study included XRD, performed using a DRON-3 diffractometer (Russia), and FTIR spectroscopy, conducted with a Shimadzu spectrometer (Japan). TGA was carried out using a Setaram LABSYS Evolution TG-DTA/DSC system (France). Microstructural observations were conducted via SEM using a Zeiss Crossbeam 540 instrument (Germany).

Discussions

Compressive strength of GC

The compressive strength of the GC with varying fibers and length, as depicted in Fig. 8. The control GC specimen without coir fiber reinforcement (designated as G-0) demonstrated a compressive strength of 43.07 MPa. The compressive strength of GC > 40 MPa observed due to the complementary reactions among these materials. FA forms N-A-S-H gel for long-term strength, while GGBS contributes calcium to develop calcium-alumino-silicate-hydrate (C-A-S-H) gel, enhancing early strength. SF improves packing density and supplies reactive silica, minimizing pore volume and enhancing microstructural density. The combined development of C-A-S-H and N-A-S-H gels, along with better bonding and fewer unreacted particles, led to a denser matrix and significantly improved compressive strength¹².

Fig. 8.

Effect of fibers on the compressive strength of GC.

The incorporation of coir fibers influenced the compressive performance, with recorded strengths spaning from 43.52 MPa to 39.95 MPa as the fiber length raised from 20 mm to 60 mm (Fig. 8a). The mix containing 20 mm fibers (G-C20) exhibited a marginal increase in compressive strength, attaining 43.52 MPa representing a 1.04% improvement over the control. This enhancement is likely associated with the uniform dispersion and effective crack-bridging ability of short fibers, which can restrict crack propagation without disrupting matrix continuity. However, further extension of the fiber length to 40 mm and 60 mm led to a systematic decline in compressive strength. The G-C40 and G-C60 mixes recorded values of 41.58 MPa and 39.95 MPa, corresponding to decreases of 3.45% and 7.24%, respectively, in comparison to the reference mix. The decline in performance with increased fiber length is attributed to issues such as poor fiber dispersion, reduced workability, and increased porosity, all of which adversely affect the structural integrity of the hardened matrix. Jamshaid et al.³⁸ noticed that adding 0.5% coconut fiber improved concrete’s compressive strength by 9%, highlighting its positive impact on mechanical performance. The incorporation of flax fiber into GC significantly affected compressive strength based on fiber length, as shown in Fig. 8b. The G-F20 mix with 20 mm fibers showed the highest strength of 45.87 MPa, a 6.50% increase over the control, likely due to improved stress transfer and crack resistance without compromising workability. Rahimi et al.²⁵ recorded that treating flax fibers to reduce impurities increased concrete’s compressive strength by up to 12%, emphasizing the value of fiber treatment for performance enhancement. In contrast, Page et al.³⁹ detected a 17% drop in compressive strength by adding 0.3 vol% flax fiber to concrete. As fiber length increased, compressive strength declined, with G-F40 and G-F60 recording 4.22% and 0.74% improvements, respectively. Despite exhibiting maximum compressive strength than the control mix, the declining trend with increasing flax fiber length indicates potential challenges analogous to those encountered with coir fiber, including fiber entanglement, inefficient dispersion, and elevated porosity. Conversely, Perrot et al.⁴⁰ and Khelifi et al.⁴¹ have demonstrated that greater fiber volume fractions and extended fiber lengths contribute to enhanced mechanical robustness, thereby revealing divergent findings regarding the influence of adding flax fiber on the compressive performance of concrete.

The integration of jute fibers into GC revealed a fiber length-dependent influence on compressive strength, with values spaning from 43.65 MPa to 40.72 MPa (Fig. 8c). The G-J20 specimen, incorporating 20 mm fibers, demonstrated a modest improvement of 1.35% over the control mix, indicating that shorter jute fibers may contribute positively to the matrix by promoting effective crack-bridging and enhancing stress distribution. Nevertheless, extending the fiber length to 40 mm and 60 mm causing a consistent decrease in strength, with G-J40 and G-J60 exhibiting compressive strengths 3.09% and 5.46% lesser than the reference specimen, respectively. Mohammed et al.⁴² detected that adding 35 mm jute fibers in GC led to a progressive lessening in compressive strength with increasing fiber volume fractions. Nevertheless, the mix containing 0.70% fiber by volume reached the highest 28-day compressive strength of 31.5 MPa, suggesting the existence of an optimal fiber content that maximizes mechanical performance despite the overall declining trend. A comparable pattern was noted in GC reinforced with sisal fibers. The G-S20 specimen, containing 20 mm fibers, achieved the maximum compressive strength within its group, showing a 4.76% enhancement relative to the control (Fig. 8d). This result implies that shorter sisal fibers offer a favorable balance between mechanical interlocking and matrix cohesion. Shcherban et al.⁴³ recorded that adding 1.0% sisal fiber to GC improved compressive strength to 25.6 MPa, a 12.8% improvement over the control, highlighting the beneficial effect of natural fiber reinforcement in GC⁴⁴. However, further increases in fiber length led to performance reductions, as evidenced by G-S40 and G-S60, which presented slight and moderate declines of 0.54% and 3.28%, respectively. Among the fiber-reinforced GC, flax fiber mixes displayed the highest compressive strength, succeeded by those with sisal, jute, and coir fibers. Flax fiber GC exhibited the highest compressive strength due to its superior tensile properties, high cellulose content, and strong fiber-matrix bonding. Sisal fiber offered moderate gains, though limited by its coarse texture. Jute’s high moisture absorption weakened the interface, while coir showed the lowest performance due to poor dispersion and low strength.

Impact strength of GC

The impact strength of the thirteen GC mixtures incorporating various fiber types and lengths is presented in Table 4. The presented values reflect the mean outcomes obtained from three specimens for each mix, and these averages were used for subsequent analysis and discussion (Figs. 9, 10, 11, 12 and 13).

Table 4.

Impact strength results of all mixtures.

Mixture id	(Impact strength) J1						(Impact strength) J2
	S1	S2	S3	Mean	SD	COV	S1	S2	S3	Mean	SD	COV
G-0	15	13	18	15	2.52	16.41	34	33	24	30	5.51	18.16
G-C20	24	25	26	25	1.00	4.00	73	60	66	66	6.51	9.81
G-C40	24	22	27	24	2.52	10.34	63	71	76	70	6.56	9.37
G-C60	23	29	24	25	3.21	12.69	72	80	95	82	11.68	14.18
G-F20	15	18	20	18	2.52	14.24	39	47	53	46	7.02	15.16
G-F40	14	19	23	19	4.51	24.16	42	63	55	53	10.60	19.87
G-F60	17	18	20	18	1.53	8.33	56	41	49	49	7.51	15.42
G-J20	18	21	20	20	1.53	7.77	49	62	55	55	6.51	11.76
G-J40	17	24	22	21	3.61	17.17	51	60	71	61	10.02	16.51
G-J60	22	18	23	21	2.65	12.60	59	48	64	57	8.19	14.36
G-S20	20	23	19	21	2.08	10.07	60	71	52	61	9.54	15.64
G-S40	21	25	23	23	2.00	8.70	62	67	69	66	3.61	5.46
G-S60	24	23	19	22	2.65	12.03	66	72	76	71	5.03	7.06

S1, S2, S3: specimen 1, 2 and 3; SD: standard deviation, COV: Coefficient of variance.

Fig. 9.

Impact strength of coir fibers GC.

Fig. 10.

Impact strength of flax fibers GC.

Fig. 11.

Impact strength of jute fibers GC.

Fig. 12.

Impact strength of sisal fibers GC.

Fig. 13.

Impact strength of GC with varying fibers.

Influence of Coir fiber on impact strength of GC

The Fig. 9 depict the effect of coir fiber incorporation on the fracture parameters J1 and J2, specifically highlighting their percentage increase relative to the control mix (G-0). A progressive enhancement in J1 values is observed with the inclusion of coir fibers, rising from 15 in the G-0 specimen to 25 in the G-C20 specimen (a 63.04% increment), attaining a value of 24 in G-C40 specimen (an 58.69% increase), followed by a slight increase to 25 in G-C60 specimen (an increase of 65.21%), as shown in Fig. 9a. The observed enhancement in J1 can be ascribed to the crack-bridging mechanism, wherein the fibers span developing cracks, thereby impeding their early propagation. Additionally, the interfacial bonding between the geopolymer matrix and coir fibers the likely contributes to improved mechanical interlocking, enhancing overall fracture resistance. Nonetheless, excessive fiber content, as in the G-C60 mix, may reduce workability, and promote fiber entanglement, thereby diminishing the reinforcing efficiency of the fibers. In contrast to the trend observed for J1, the J2 demonstrates a continuous and progressive increase with rising coir fiber length from 20 to 60 mm. Specifically, J2 increases from 30 in the G-0 specimen to 66 in G-C20 specimen (reflecting a 118.68% enhancement), further to 70 in G-C40 specimen (a 130.76% increase), and reaches a peak value of 8 in G-C60 specimen (a 171.43% increment), as shown in Fig. 9b. The increase in the J2 parameter is attributed to coir fibers bridging microcracks, preventing early propagation and enhancing impact resistance. Wang and Chouw⁴⁵ reported that coir fibers with lengths of 25 mm and 50 mm provided higher impact resistance in concrete compared to longer fibers measuring 75 mm, suggesting that excessively long fibers may reduce reinforcement efficiency.

Under impact loading, the fibers stretch, absorb energy, and redistribute stress within the matrix. Longer coir fibers (40 mm and 60 mm) are especially effective in bridging wider cracks, thereby improving fracture resistance and post-impact load-bearing capacity. Additionally, the fiber pullout mechanism significantly enhances impact resistance in fiber-reinforced geopolymer composites by dissipating energy through friction during loading. Longer coir fibers (40 mm and 60 mm) offer greater embedment, improving pullout resistance and impact performance. The analysis clearly indicates that GC reinforced with 60 mm coir fibers exhibited the highest J2 value, reflecting superior post-crack energy absorption. While previous studies have shown that coir fiber-reinforced concrete containing 2% fiber content with a 40 mm fiber length achieved maximum energy absorption capacity²⁸Hwang et al.²⁹ reported that a higher fiber coir dosage of 4% was optimal for maximizing impact resistance in concrete. The findings of this study align with the observations reported by Ramakrishna and Sundararajan²⁸who observed that coir fiber-reinforced concrete exhibits a markedly improved impact energy absorption capacity showing a fourfold increase during the initial impact stage and up to a twentyfold enhancement at the ultimate fracture stage compared to conventional concrete. Sekar and Kandasamy⁴⁶ stated that an rise in the aspect ratio of coir fibers corresponded with a proportional enhancement in the J1 and J2 of conventional concrete, indicating improved resistance to crack initiation and propagation.

Influence of flax fiber on impact strength of GC

Figure 10 elucidates the effect of flax fiber length on the impact resistance of GC, with particular emphasis on J1 and J2. A progressive enhancement in both parameters was apparent with extending fiber length up to 40 mm. Specifically, the inclusion of 20 mm and 40 mm long flax fibers in the G-F20 and G-F40 mixtures resulted in J1 improvements of 15.21% and 21.74%, respectively, in comparison to the control mix (G-0). The superior J1 value observed in the G-F40 mix indicates that fibers of this length effectively resisted crack initiation and propagation under impact loading through enhanced crack-bridging mechanisms. However, further extending the fiber length to 60 mm (G-F60) led to a marginally lower increase in J1 (19.56%). A similar trend was evident in the J2 results, wherein the G-F20 and G-F40 mixes demonstrated notable enhancements of 52.74% and 75.82%, respectively, over the control. In contrast, the G-F60 mix yielded a relatively lower J2 increment of 60.43%, further substantiating the presence of an optimal fiber length beyond which the mechanical benefits diminish due to dispersion-related deficiencies. The reduced J1 and J2 observed with 60 mm flax fibers, compared to 20 mm and 40 mm lengths, is primarily due to poor dispersion, fiber agglomeration, and misalignment within the viscous geopolymer matrix. These factors create weak zones and limit effective crack bridging. Additionally, the excessive fiber length surpasses the optimal threshold for load transfer, leading to reduced interfacial bonding due to a lower surface area-to-volume ratio, ultimately diminishing the reinforcement efficiency. Rahimi et al.²⁵ findings revealed that adding flax fibres into concrete matrices causing a marked enhancement in energy absorption capacity, exhibiting an increase between 22% and 105% in comparison to the control specimens.

Influence of jute fiber on impact strength of GC

Figure 11 depicts the influence of varying jute fiber lengths on the impact performance of GC, with a specific focus on both J1 and J2 energy absorption capacities. Adding jute fibers resulted in a non-linear enhancement in both parameters, depending on fiber length. Notably, specimens reinforced with 20 mm and 40 mm fibers (designated as G-J20 and G-J40) exhibited substantial increases in primary impact resistance, with J1 values rising by 28.26% and 36.95%, respectively, relative to the unreinforced control specimen (G-0). The G-J40 mixture recorded the highest J1 value among all configurations, signifying its superior capacity to withstand the initial phase of impact loading. When the fiber length was extended to 60 mm (G-J60), a J1 amounting to 36.95% above the control was observed. A comparable trend was observed in the J2, representing the material’s post-crack energy absorption capability. The J2 values for G-J20 and G-J40 increased markedly by approximately 82.41% and 100%, respectively demonstrating the beneficial role of jute fibers in enhancing ductility and energy dissipation. Nevertheless, the use of 60 mm fibers (G-J60) resulted in a reduced improvement of 87.91%, further suggesting that excessively long fibers may adversely affect post-crack behavior due to issues such as misalignment, increased curvature, and reduced interfacial bonding with the binder matrix. Ramakrishna and Sundararajan²⁸ informed that adding jute fibres to plain concrete slabs significantly enhanced their impact resistance, achieving improvements between 3 and 18 times relative to slabs without fibre reinforcement. Hussain and Ali reported⁴⁷ a substantial rise in the impact strength of concrete integrating jute fibres. Specifically, the jute fibrous concrete exhibited a 6.5-fold rise in performance at an impact drop height of 60 cm and a six-fold rise at 90 cm, relative to conventional plain concrete.

Influence of Sisal fiber on impact strength of GC

Figure 12 displays the influence of sisal fiber on the impact strength of GC, reveals a significant variation in both J1 and J2. Incorporating 20 mm sisal fibers (G-S20) resulted in a noticeable improvement, with J1 and J2 values increasing by 34.78% and 101.09%, respectively, in comparison to the control specimen (G-0) without fibers. This enhancement is ascribed to the fiber’s ability to bridge microcracks and delay crack proliferation during impact loading. Further extending the fiber length to 40 mm (G-S40) led to a more pronounced improvement, achieving 50% and 117.58% increases in J1 and J2, respectively. This suggests that longer fibers are more effective in energy dissipation due to enhanced fiber-matrix interlocking and crack-bridging capabilities. However, increasing the fiber length beyond 40 mm did not yield a consistent benefit. Although the G-S60 specimen, incorporating 60 mm fibers, showed a marginal reduction in J1 (43.47%) compared to G-S40, it exhibited the highest increase in J2 (135.16%). This divergence indicates that excessively long fibers may lead to agglomeration, reduced workability, or uneven dispersion, thereby limiting their effectiveness in J1, but still contributing positively to J2 due to fiber pull-out mechanisms. The extended length of 60 mm sisal fibers facilitates more efficient interaction with the geopolymer matrix, thereby enhancing fiber anchorage and promoting improved stress transfer between the fibers and the surrounding material. This interaction contributes to an increased resistance to crack propagation under impact loading conditions. In this study, a dosage of 0.5% sisal fiber was utilized, demonstrating enhanced impact resistance in GC compared to its fiber-free counterpart. Previous research by Al Rawi et al.⁴⁸ indicated that a 1.5% inclusion of sisal fiber significantly enhanced the impact strength of concrete. Naraganti et al.³⁰ stated that incorporating 1.5% sisal fibers enhanced the resistance to initial cracking by approximately 3.5 times compared to plain concrete.

Comparison of impact strength with various fibers

The evaluation of impact strength across various fiber-reinforced geopolymer mixtures reveals distinct trends related to the type and length of fibers, particularly in terms of J1 and J2, as shown in Fig. 13. The reference mixture without fiber reinforcement (G-0) exhibited the lowest impact resistance, with J1 and J2 values of 15 and 30, respectively, demonstrating the limited toughness of unreinforced geopolymer matrices. In contrast, the incorporation of natural fibers enhanced the impact performance, with the degree of improvement varying based on fiber characteristics. Among the tested fibers, coir reinforcement produced the most significant enhancement in failure impact resistance. The G-C60 mixture, incorporating 60 mm coir fibers, recorded the highest J2 value of 82, while maintaining a consistent J1 value of 25 across different fiber lengths. This behavior is primarily attributed to the high elongation potential and surface roughness of coir fibers, which facilitate effective crack-bridging and energy dissipation mechanisms. Sisal fiber mixtures also demonstrated substantial improvement, particularly at longer lengths. The G-S60 mix exhibited a J2 value of 71 and a J1 of 22, indicating the effectiveness of sisal’s balanced mechanical properties namely, moderate ductility and stiffness in resisting crack propagation and absorbing impact energy. Jute fiber-reinforced mixtures displayed moderate performance, with J1 values consistently ranging from 20 to 21 and J2 values increasing incrementally from 55 to 61 with longer fibers. The improvement is likely due to enhanced fiber-matrix interlocking, although the relatively low ductility of jute fibers may limit further energy absorption. Flax fiber mixtures, on the other hand, showed the least enhancement in J1 (18–19) and a limited increase in J2 (maximum of 55). This subdued performance is attributed to flax’s brittle behavior and limited ability to deform under impact loading. Across all fiber types, longer fibers exhibited a more pronounced influence on J2 than on J1, suggesting that fiber length plays a more critical role in post-crack impact resistance than in initial crack prevention. Coir and sisal fibers, particularly at 60 mm, proved to be the most effective in improving impact strength due to their favorable mechanical behavior and interfacial bonding properties. Coir and sisal fibers enhance impact strength due to their superior mechanical properties, strong fiber-matrix bonding, and durability in alkaline geopolymer environments. Coir’s high elongation and toughness, along with sisal’s balanced strength and ductility, allow effective energy absorption through fiber stretching, crack bridging, and pull-out. Using 60 mm fibers further improves performance by spanning wider cracks and enhancing post-cracking energy dissipation.

Natural fibers such as flax, sisal, coir, and jute, when utilized in lengths of approximately 60 mm, present notable challenges related to dispersion and workability within composite and cementitious matrices. Although these fibers contribute positively to mechanical properties, including enhanced tensile strength and crack resistance, their practical application is often constrained by issues such as fiber entanglement, agglomeration, and non-uniform distribution. These dispersion-related problems can result in localized weaknesses and the formation of voids within the matrix. Furthermore, the inherently hydrophilic nature of certain fibers particularly coir and jute exacerbate these difficulties under wet mixing conditions. In addition, the incorporation of longer fibers significantly reduces the workability of fresh mixes, impeding effective molding, compaction, and pumping processes. Such limitations increase labor demands and processing time, thereby restricting the feasibility of these fibers for large-scale or structural applications. To overcome these barriers, further investigations into the dispersion characteristics of long fibers in various binder systems are necessary. Potential mitigation strategies include fiber surface modification, partial length reduction, or the use of hybrid fiber systems. Ultimately, optimizing mix design parameters and establishing standardized processing methods are essential to facilitate the broader adoption of long natural fibers in sustainable construction materials.

Impact ductility index of GC

The evaluation of impact ductility index (IDI) across the different mixture compositions revealed a notable improvement in ductility characteristics with the inclusion of fibers, as shown in Fig. 14. The control mixture (G-0) recorded the lowest IDI of 1.98, serving as the reference point for comparative analysis. Adding natural fibers into the GC matrix causing a notable rise in IDI, reflecting enhanced energy absorption capacity and post-crack ductile behavior. Among the fiber-reinforced composites, mixes incorporating coir fibers exhibited the highest IDI values, ranging from 2.65 to 3.25 as fiber length increased from 20 to 60 mm. This trend highlights coir fiber’s effective impact energy dissipation, despite their relatively lower stiffness. In contrast, flax fiber-reinforced mixes showed a narrower and slightly lower IDI range of 2.62 to 2.86. The marginal reduction in ductility performance may be attributed to the propensity of flax fibers to undergo brittle fracture at greater lengths or possible challenges related to fiber dispersion within the matrix. Jute and sisal fiber-reinforced GC demonstrated superior performance, with IDI ranges of 2.80–3.02 and 2.87–3.15, respectively, surpassing that of the flax fiber composites. The enhanced performance observed in sisal fiber-reinforced geopolymer concrete can be ascribed to the fiber’s optimal combination of mechanical strength and flexibility, which enables efficient crack bridging and promotes greater energy dissipation under impact loading conditions.

Fig. 14.

Impact ductility index of GC.

Failure pattern of GC specimens

Existing literature highlights that plain GC and other types of concrete typically experience immediate failure upon the formation of the first crack, indicative of a characteristic brittle failure mode. In these materials, only a few impact blows are required to propagate the crack across the specimen’s thickness, causing subsequent brittle fracture^4,13,49,50. Under brittle failure conditions, the GC specimen underwent a sudden separation into four distinct segments with the crushing of aggregates and matrix following the initiation of the first crack. This failure mode was characterized by the formation of a dominant crack that propagated through the entire specimen, as depicted in Fig. 15a. This behavior can be ascribed to the absence of crack-bridging mechanisms, the inherently brittle nature of the material, and the effects of stress concentration, all of which contribute to the rapid propagation of cracks and premature failure. GC specimens reinforced with coir, flax, jute, and sisal fibers exhibited a distinct fracture response compared to unreinforced counterparts. Figure 15 illustrates the failure patterns observed across 13 different GC specimen configurations. The inclusion of natural fibers enhances crack resistance by facilitating stress redistribution through discrete fiber bridging, thereby mitigating crack propagation and limiting crack widening²³. As a result, the material exhibits superior impact resistance, attributed to its increased energy absorption capacity. These findings are consistent with previous research, which has established those natural fibers, when optimally incorporated, contribute to crack width control, leading to improved ductility and enhanced tensile strength⁵¹. As shown in Fig. 15 (b)-(m), the top surface of GC specimens, regardless of fiber inclusion, exhibited a high resistance to impact loads before the onset of cracking. The repeated impacts were primarily concentrated in the central region beneath the steel ball, causing localized surface fractures. The figure further illustrates that the thin matrix layers underwent crushing due to continuous impact loading, consequently exposing the embedded natural fibers. These fibers created a protective barrier that lessened the deterioration rate of the surrounding matrix, thereby enhancing the material’s resistance to progressive damage.

Fig. 15.

Failure pattern of GC under impact loading (a) G-0, (b) G-C20, (c) G-C40, (d) G-C60, (e) G-F20, (f) G-F40, (g) G-F60, (h) G-J20, (i) G-J40, (j) G-J60, (k) G-S20, (l) G-S40 and (m) G-S60.

Previous investigations^52,53 have indicated that a shadow zone develops behind the fibers due to their repeated exposure to perpendicular impact forces. The ability of these exposed fibers to dissipate impact energy minimizes stress transmission to the adjacent cementitious matrix, thereby mitigating the progression of damage within this protected region. In the case of fibrous GC, coir and jute fibers, characterized by their high ductility and elongation capacity, contribute to a more uniform distribution of cracks within the impact zone. Rather than forming a few dominant fractures, adding these fibers causing the development of multiple fine cracks, as they effectively bridge and inhibit crack propagation (Fig. 15b-c, h-j). This behavior facilitates a ductile failure mode, wherein fiber pullout serves as the predominant mechanism for energy dissipation²³. Specifically, the strong interfacial bond between jute fibers and the GC matrix improves stress transfer efficiency, thereby improving energy absorption and ensuring a gradual failure process²³. Likewise, the incorporation of coir fibers improves post-failure integrity due to their high elongation potential, which delays complete structural failure despite extensive matrix damage. In contrast, flax and sisal fibers, while possessing superior tensile strength, exhibit greater stiffness, leading to distinct failure mechanisms. Compared to coir and jute fibers, flax and sisal fibers tend to generate fewer but larger cracks, indicative of a more brittle response under impact loading (Fig. 15e-g, k-m). The failure behavior of flax fibers involves a combination of fiber pullout and rupture, where certain fibers experience tensile failure while others disengage from the matrix, resulting in moderate post-failure load retention. Conversely, sisal fibers, due to their higher rigidity, predominantly fail through fiber rupture rather than pullout, leading to a more abrupt reduction in structural integrity and a lower residual load-bearing capacity⁵⁴. The failure pattern observed in sisal fiber-reinforced GC is characterized by the existence of wider cracks and a more rapid crack propagation rate compared to other fiber-reinforced composites.

Microstructure characteristic

SEM analysis of GC

The microstructural characteristics, fiber-matrix interactions, and gel formation mechanisms of geopolymer composites comprising FA, GGBS, and SF, reinforced with coir, flax, jute, and sisal fibers, were examined through SEM (Fig. 16). SEM analysis (Fig. 16a) revealed that the geopolymer matrix primarily consists of N-A-S-H and C-A-S-H gels, essential for mechanical strength and durability⁵⁵. Densely packed regions indicate effective geopolymerization, while unreacted FA particles, microcracks, and porosity suggest incomplete reactions and potential durability concerns, similar to findings by Yang et al.⁵⁶. Coir fibers exhibited voids and debonding due to their hydrophilic nature, indicating weak fiber-matrix bonding, though nearby gel formation suggests partial adhesion improvement (Fig. 16b). Flax fibers showed moderate interaction, with evidence of crack bridging and matrix adherence⁵⁷indicating a somewhat effective bond (Fig. 16c). Jute fibers demonstrated strong adhesion, minimal pullout (Fig. 16d), and enhanced load transfer, though surface degradation was noted due to alkaline exposure. Sisal fibers showed the best performance, with strong mechanical interlocking, minimal degradation, and dominant N-A-S-H and C-A-S-H gel formation (Fig. 16e), enhancing crack resistance and durability.

Fig. 16.

SEM analysis of GC (a) fibre-free GC matrix, (b) coir, (c) flax, (d) jute, and (e) sisal.

XRD analysis of GC

The XRD pattern of the developed GC, as depicted in Fig. 17, confirms the coexistence of amorphous and crystalline constituents an inherent characteristic of alkali-activated aluminosilicate materials. The detected crystalline phases primarily comprise Quartz (Q), Calcite (C), Hematite (H), and Mullite (M), in addition to trace levels of unidentified minor phases. The well-defined diffraction peaks at approximately 20.8°, 26.6°, and 36.5° (2θ) are indicative of residual crystalline quartz. This mineral phase is predominantly attributed to the unreacted siliceous content in Class F FA and SF, commonly used as aluminosilicate precursors. The pronounced intensity of these peaks is reflective of quartz’s chemical inertness under alkaline activation, contrasting with amorphous silica, which undergoes dissolution and participates in geopolymerization. Consequently, quartz primarily serves as an inert filler rather than an active binder component in the geopolymer matrix. Reflections appearing at 16.4°, 26.1°, and 40.8° (2θ) correspond to Mullite, a refractory aluminosilicate phase formed during the high-temperature combustion process of FA production. The persistence of mullite in the matrix suggests its resistance to alkaline dissolution and limited involvement in the gel formation process¹². This observation implies that portions of alumina remain structurally bound within the mullite framework, thereby reducing their availability for participation in the synthesis of the geopolymeric gel and potentially influencing the mechanical integrity of the hardened matrix. Diffraction peaks near 29.4°, 32.1°, and 49.2° (2θ) are ascribed to Calcite, a phase often correlated with the existence of calcium silicate hydrate (C-S-H) in blended geopolymer systems incorporating GGBS⁵⁸. The development of C-S-H suggests a co-existing hydration mechanism in addition to the typical geopolymerization reactions, particularly in high-calcium systems. This dual-gel structure, involving both C-S-H and N-A-S-H gels, has been shown to enhance early-age mechanical strength and microstructural refinement¹². Minor diffraction signals at 33.1° and 35.7° (2θ) are attributed to Hematite, which likely originates from the iron oxide content present as impurities within GGBS or FA⁵⁹. Although Hematite does not significantly contribute to the binding network, its presence can influence certain physical properties, such as coloration, thermal behavior, and long-term durability of the geopolymer matrix.

Fig. 17.

XRD analysis of GC.

Thermogravimetric and derivative thermogravimetric analysis

Figure 18 shows the TGA and derivative thermogravimetric (DTG) curves black representing TGA and red representing DTG illustrating a detailed thermal characterization of the GC incorporating FA, GGBS and SF. The thermal behaviour of GC with FA, GGBS, and SF was analyzed using TGA and DTG over 25–1000 °C, revealing distinct weight loss stages and DTG peaks associated with the decomposition of hydration products and chemically bound phases. The initial mass loss between 33 °C and 101 °C, confirmed by a DTG peak at 101 °C, corresponds to the evaporation of physically adsorbed and weakly bound water from the surface and pore structure of the geopolymer matrix. This peak, characteristic of geopolymeric systems, signifies retained free water due to the high surface area of SF and unreacted FA particles. The 200–600 °C range corresponds to the XXXihydroxylation of hydration products, marked by gradual mass loss due to the release of –OH groups from C-S-H, C(N)-A-S-H, and related gels formed by GGBS activation⁶⁰. DTG peaks at 557 °C, 624 °C, and 689 °C indicate sequential bound water loss and possible amorphous aluminosilicate decomposition, suggesting partial crystallization or secondary reactions. The DTG peak at 689 °C indicates the decomposition of CaCO₃, likely formed through atmospheric carbonation of calcium-rich phases from GGBS. This reflects structural degradation or sintering in FA-based geopolymers. SF may have limited carbonation by densifying the matrix and restricting CO₂ penetration. Above 965 °C, minimal weight loss and a slight DTG increase suggest structural reorganization or melting in the aluminosilicate matrix. This indicates the completion of decomposition and confirms the excellent thermal stability of the geopolymer binder.

Fig. 18.

TGA and DTG analysis of GC.

Fourier transform infrared analysis of GC

Figure 19 presents the FTIR spectrum of the GC, revealing distinct absorption bands associated with the structural evolution of the binder. The band at 418 cm⁻¹ corresponds to Si–O bending vibrations, signifying the creation of Si–O–Si and Si–O–Al linkages and the onset of geopolymerization⁶¹. A peak at 669 cm^–1 reflects Al–O bending, suggesting aluminum incorporation into the amorphous gel network, typical of N–A–S–H and/or C–A–S–H gels⁶⁰. The prominent band at 1010 cm⁻¹, ascribed to asymmetric stretching of Si–O–T (T = Si or Al), signifies the transformation of unreacted FA into an amorphous aluminosilicate network, with the observed shift from ~ 1090 cm^–1 indicating increased disorder and cross-linking. The 1415 cm⁻^–1band, corresponding to C–O stretching, is associated with carbonation, particularly in calcium-rich GGBS-based systems. Minor peaks at 2164 cm^–1 and 2360 cm^–1 suggest the presence of adsorbed CO₂, further supporting post-curing carbonation. Overall, the spectral features confirm successful geopolymer formation alongside secondary carbonation processes that may influence long-term durability.

Fig. 19.

FTIR analysis of GC.

Challenges and environmental considerations in scaling fibrous GC

The industrial-scale use of coir, flax, jute, and sisal fibers in GC faces several challenges related to material variability, environmental concerns, and processing limitations. These natural fibers exhibit inconsistencies in size, strength, and moisture content due to varying agricultural and processing conditions, complicating standardization, and quality control. The lack of universal classification standards and their susceptibility to biodegradation further hinder large-scale application. Additionally, their hydrophilic nature necessitates surface treatments that are not yet optimized for industrial use. While these fibers offer environmental benefits such as renewability and biodegradability, large-scale cultivation may lead to land-use conflicts, water consumption issues, and pollution from fiber extraction processes. From a processing standpoint, achieving uniform fiber dispersion in geopolymer matrices is difficult due to fiber entanglement and the high viscosity of the binder, which often results in poor mechanical performance. Conventional mixing equipment may be insufficient, requiring additional treatments and energy-intensive processes. Overcoming these technical and environmental barriers is essential for the sustainable integration of natural fibers in geopolymer-based industrial applications.

Conclusions

This study examines the impact resistance of GC incorporating FA, GGBS, and SF, and reinforced with sisal, jute, coir, and flax fibers at various lengths. The results of the experiment led to the following findings.

Flax fiber-reinforced GC (G-F20) achieved the highest compressive strength (45.87 MPa, + 6.50%), followed by sisal, jute, and coir. Increasing fiber length from 20 mm to 60 mm consistently reduced compressive strength across all types. For impact resistance, coir mixes showed a significant J1 increase (up to 66%) and a 171.43% rise in J2, indicating improved energy absorption, though longer fibers reduced workability. Flax at 40 mm (G-F40) offered optimal impact performance (J1: +21.74%, J2: +75.82%), while 60 mm lengths reduced effectiveness due to dispersion issues. Jute at 40 mm (G-J40) yielded the highest J1 gain (36.95%) and doubled J2, outperforming shorter and longer lengths. Similarly, 40 mm sisal (G-S40) provided balanced gains (J1: +50%, J2: +117.58%), whereas G-S60 achieved the highest J2 (135.16%) but a slightly lower J1. Ductile fibers like coir and jute enhanced crack dispersion and energy absorption, promoting gradual failure, while stiffer flax and sisal fibers led to wider cracks and more brittle behavior.

SEM analysis confirmed the formation of both N-A-S-H and C-A-S-H gels, indicating effective activation of FA, GGBS, and silica fume. While dense matrix regions were observed, unreacted particles and microdefects suggested uneven geopolymerization. Fiber–matrix bonding varied: coir showed weak adhesion, flax moderate, jute strong with surface wear, and sisal exhibited excellent interlocking, enhancing crack resistance. XRD identified amorphous and crystalline phases, including Quartz, Mullite, Calcite, and Hematite. Quartz and Mullite remained inert, while Calcite indicated the presence of both C-S-H and N-A-S-H gels from GGBS, contributing to early strength and matrix densification. FTIR confirmed geopolymer formation through Si-O-Si and Si-O-Al bands (418, 669, 1010 cm⁻¹), while peaks at 1415, 2164, and 2360 cm⁻¹ indicated carbonation, likely from calcium-rich GGBS, potentially affecting long-term durability.

Author contributions

Author contributionsA.B: Validation, Visualization, Writing – review & editing, Investigation, Formal analysis, Conceptualization, Formal analysis, Methodology, Software, Validation. G. M: Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Conceptualization. Y.A: Writing – original draft, Validation, Methodology, Investigation, Supervision, Formal analysis, Conceptualization. Z.I: Writing – original draft, Validation, Supervision. M.K: Writing – review & editing, funding. H. S. A: Writing – review & editing. S.A: Writing – review & editing, S.D: Writing – review & editing.

Funding

No funding is available for this research work.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Competing interests

The authors declare no competing interests.