Effect of coconut fiber on the rheological and mechanical properties of geopolymer mortar

Abstract

This study developed a coconut fiber-reinforced geopolymer mortar incorporating lead smelter slag (LSS) and investigated its rheological behavior, mechanical properties, and microstructural evolution. Fly ash and ground granulated blast furnace slag were used as precursor materials, and the effects of precursor proportion, sodium silicate modulus, and fiber content were systematically evaluated. Rheological tests and strength tests were carried out to assess fresh-state and hardened performance, while XRD and SEM were used to analyze the underlying microstructural mechanism. The results showed that LSS could be effectively incorporated into the geopolymer mortar system without causing a significant loss of workability. The mortar with a slag-to-fly ash ratio of 50:50 exhibited the most favorable comprehensive performance among the tested mixtures. Coconut fiber improved the coordination between flowability and mechanical properties at an appropriate dosage, and the optimum fiber content was 2%. Microstructural results further indicated that fiber addition did not alter the main geopolymer reaction products, and the observed performance enhancement mainly resulted from improved matrix compactness and interfacial bonding. These findings provide a feasible route for the valorization of lead smelter slag and the development of sustainable fiber-reinforced geopolymer mortars for construction applications.

1 Introduction

Geopolymers are low-carbon, environmentally friendly cementitious materials characterized by rapid strength development, high early-age strength, and strong resistance to chemical and physical degradation (Mostazid, 2023). However, the geo-polymerization reaction is highly dependent on local alkali concentration, resulting in non-uniform reaction rates and relatively low forming strength of geopolymer mortars. Consequently, geopolymer materials generally rely on aggregates as a structural skeleton to ensure the stability of the fresh mixture during casting (Artificial aggregates, 2022). Satyanarayana et al. (2007) reported that although geopolymer mortars exhibit superior durability and strength compared with Portland cement, they suffer from significant drying shrinkage and strong alkali corrosivity. Rashid (2022) further pointed out that, carbonation, alkali brittleness, and interfacial instability are the key technical challenges limiting the widespread application of geopolymer materials. Ali (2012) employed fly ash–based geopolymers to stabilize lead smelter slag, and XRD/FTIR analyses confirmed that heavy metals were immobilized through both chemical bonding and physical encapsulation, thereby significantly reducing the risk of heavy-metal leaching.

Coconut fiber, as a type of natural plant fiber extracted from the outer husk of coconuts, belongs to lignocellulosic fibers and is characterized by biodegradability and a low carbon footprint. Compared with conventional plant fibers, coconut fiber exhibits superior alkali resistance and thus better compatibility with geopolymer systems. Satyanarayana et al. (2007); Rashid (2022) provided comprehensive descriptions of the physicochemical properties of various plant fibers, including coconut fiber. Ali (2012) investigated the mechanical properties of coconut fiber and compared the effects of different surface treatments, such as alkali treatment and thermal treatment, on fiber–matrix interfacial bonding. Arunraja and Rajesh (2020) conducted a comparative study on alkali-treated and untreated coconut fibers in concrete, demonstrating that alkali-treated fibers resulted in higher compressive and flexural strengths and a moderate reduction in water absorption, with an optimal fiber content of 0.5%–1.0%. Fan et al. (2023) further revealed that the immobilization efficiencies of Pb and Cr reached 96.69% and 99.97%, respectively, while the leaching of Cu and Pb increased under acidic and alkaline environments, and Cr was predominantly immobilized through combined chemical bonding and physical encapsulation mechanisms.

Mortars generally suffer from poor workability, long strength development time, low forming strength, and weak resistance to environmental erosion (Grassl et al., 2009; Thakur, 2025). In response, numerous studies have been conducted to improve the performance of geopolymer mortars. Al Jaberi et al. (2021) investigated the flowability of fly ash–metakaolin–based geopolymer mortars and reported that increasing alkali activator content could simultaneously enhance mechanical properties and rheological performance. However, the viscosity and activation conditions were highly sensitive to construction requirements, resulting in limited overall performance improvement. Taguchi (2024) conducted acid and sulfate attack tests and demonstrated that conventional fly ash–based geopolymers exhibited poor workability due to excessively high viscosity. Elevated-temperature curing or the incorporation of high-calcium supplementary materials was shown to improve early-age strength and durability. Nevertheless, high-calcium additives, while beneficial to early strength development, significantly increase mortar viscosity and deteriorate workability, leading to an inherent contradiction between mechanical strength and rheological performance. Moreover, studies focusing on the combined use of plant fibers and lead smelter slag as a replacement for natural sand in geopolymer mortars remain scarce. Therefore, investigating coconut fiber–and lead smelter slag–reinforced geopolymer mortars is of great practical significance for expanding the application range of geopolymer mortar systems.

In this study, lead smelter slag was introduced as an additional calcium source alongside conventional fly ash and ground granulated blast furnace slag to prepare coconut fiber–reinforced geopolymer mortar incorporating lead smelter slag. On the one hand, lead smelter slag provides additional available calcium ions to promote geo-polymerization; on the other hand, the particle size of the slag used in this study is comparable to that of river sand, enabling it to serve as a substitute for standard sand in geopolymer mortar preparation. However, lead smelter slag contains substantial amounts of Pb and Zn, posing potential risks of heavy-metal leaching. During the geopolymer polycondensation process, heavy-metal ions can be physically encapsulated within the aluminosilicate gel network, thereby achieving effective hazardous waste immobilization. This study systematically investigates the rheological behavior, mechanical properties of geopolymer mortar specimens with varying dosages. Furthermore, the reaction mechanisms and microstructural evolution are elucidated through SEM and XRD analyses, providing a theoretical basis for the broader application of geopolymer mortars.

Previous studies on plant-fiber-reinforced geopolymers have mainly focused on conventional precursor systems and individual mechanical properties, while the integration of metallurgical solid waste, fresh-state rheology, and mechanism-oriented microstructural interpretation has been less systematically addressed. Studies on geopolymer mortars simultaneously incorporating coir fiber and lead smelter slag (LSS) remain limited.

The novelty of the present study lies in three aspects. First, in terms of material system, a coir-fiber-reinforced geopolymer mortar incorporating LSS was developed, in which LSS plays a dual role as both a potentially reactive solid component and a fine aggregate substitute for natural sand. Second, in terms of testing scope, the coupled effects of precursor proportion, fiber dosage, and alkaline activator modulus on rheological behavior and mechanical properties were systematically evaluated, and the macroscopic results were further linked with SEM and XRD observations. Third, in terms of practical applicability, this study focuses on the effect of plant fiber incorporation on the fresh-state workability of geopolymer mortar. Owing to the high-water absorption and moisture sensitivity of natural fibers, fiber addition may significantly reduce flowability and mixing stability. The present work therefore systematically evaluates the rheological response of coir-fiber-reinforced geopolymer mortar, with the aim of providing practical guidance for mixture optimization and future engineering application.

2 Materials and methods

2.1 Raw materials

2.1.1 Coconut fiber

Coconut fiber is classified as a rigid natural fiber material and was originally sourced from Hainan, China. The fibers were processed by Shandong Dashan Road & Bridge Group, including soaking, impurity removal, and air drying. The processed fibers exhibited a brownish appearance, with diameters ranging from 0.1 to 0.53 mm and a density of approximately 1.15 g/cm³. The original fiber lengths varied from 1 to 10 cm, as shown in Figure 1.

FIGURE 1

2.1.2 Binder and aggregate materials

Lead smelter slag (LSS) was collected from Shandong Iron and Steel Group and originates primarily from lead-rich ores. It is a high-temperature molten by-product generated during the lead smelting process, exhibiting a dark gray to black color and a dense surface structure. While LSS shows considerable potential for resource utilization, it also poses risks of heavy-metal leaching. X-ray fluorescence (XRF) analysis indicates that the main chemical components of LSS are SiO₂, Al₂O₃, and Fe₂O₃, accompanied by minor amounts of heavy-metal oxides such as Pb and Zn. The slag exhibits certain pozzolanic activity and can be utilized as a geopolymer precursor. After geo-polymerization, the former aluminosilicate tetrahedral network effectively inhibits the leaching of heavy metals. The LSS used in this study consisted of two fractions: fine powder with particle sizes ≤75 μm and granular particles with sizes ranging from 0.125 to 0.25 mm. The fine fraction was employed to enhance the cementitious properties of the geopolymer matrix, while the granular fraction was used as a substitute for river sand to form the skeletal structure of the geopolymer mortar.

Fly ash was collected from Shizuishan, Ningxia, and is classified as low-calcium Class I fly ash. Its microscopic morphology consists of spherical microspheres with particle sizes predominantly ranging from 0.5 to 300 μm, and a bulk density of approximately 700–900 kg/m³. The primary mineralogical components are mullite and quartz. In the geopolymer system, fly ash serves as a retarding agent and provides available calcium ions to facilitate the geo-polymerization process.

2.1.3 Alkaline activator

The alkaline activator mainly consisted of sodium silicate solution with a silicate modulus of 1.19 and an 8 mol/L sodium hydroxide (NaOH) solution. The mass ratio of sodium silicate solution to NaOH solution was maintained at 2.5. To minimize the influence of heat released during dissolution on the experimental results, the alkaline activator solutions were prepared 1 day prior to use.

2.1.3.1 Group A

The alkaline activator was composed of NaOH solution and sodium silicate solution. The mass ratio of NaOH solution to fly ash was 0.35, while the mass ratio of sodium silicate solution to NaOH solution was fixed at 2.5. The concentration of the NaOH solution was 8 mol/L and was prepared 1 day in advance. The mass ratio of slag to fly ash was 4:1.

2.1.3.2 Group B

The alkaline activator consisted of NaOH solution and grade D sodium silicate solution. The mass ratio of NaOH solution to fly ash was 0.75, and the mass ratio of sodium silicate solution to NaOH solution was 2.5. The NaOH solution concentration was 8 mol/L and prepared 1 day prior to mixing. The mass ratio of slag to fly ash was 1:1.

2.1.3.3 Group C

The alkaline activator was prepared using NaOH solution and sodium silicate solutions with different silicate moduli, while maintaining a constant mass ratio of sodium silicate solution to NaOH solution of 2.5. The concentration of the NaOH solution was 8 mol/L and prepared 1 day in advance. The SiO₂/Na₂O molar ratios were adjusted to 1.0, 1.2, 1.4 and 1.6. The mass ratio of slag to fly ash was 4:1.

2.1.3.4 Group D

The alkaline activator consisted of NaOH solution and sodium silicate solutions with varying silicate moduli, with a fixed mass ratio of sodium silicate solution to NaOH solution of 2.5. The NaOH solution concentration was 8 mol/L and prepared 1 day in advance. The SiO₂/Na₂O molar ratios were 1.0, 1.2, 1.4 and 1.6. The mass ratio of slag to fly ash was 1:1.

2.2 Mix proportions

The mix proportions of the geopolymer mortar are listed in Table 1. The water-to-binder ratio was fixed at 0.6. The alkaline activator had a SiO₂/Na₂O molar ratio of 1.2, with a NaOH solution concentration of 8 mol/L. Coconut shell fiber was incorporated at dosages of 0%, 1%, 2%, and 3% by weight of the binder. Based on the optimal fiber content, lead smelting slag (LSS) was further introduced at dosages of 1200 g.

Lead smelting slag Geopolymer (LSSGP) mortar was prepared using a JJ-5 planetary mixer. The detailed mixing procedure was as follows: the binder materials and LSS were first placed into the mixing bowl and mixed at low speed for 1 min. After achieving a uniform dry mixture, the pre-prepared and cooled alkaline activator solution was added, followed by mixing at medium speed for 5 min and high speed for 1 min. Subsequently, coconut shell fiber was introduced and mixed at medium speed for an additional 2 min to ensure uniform dispersion.

After mixing, the fresh mortar was cast into prismatic molds with dimensions of 40 × 40 × 160 mm. The surface was leveled using a scraper, and the specimens were then transferred to a standard curing chamber with a relative humidity of 95%. After the designated curing age, the specimens were demolded for subsequent testing.

The specimen designation was defined according to the binder composition, fiber content, and activator parameters. For example, GF8020-C0 represents a geopolymer paste based on a slag-to-fly ash mass ratio of 80:20 with 0% coconut shell fiber. GF8020-L1.0 denotes a geopolymer paste with a slag-to-fly ash ratio of 80:20 and a coconut shell fiber content of 1%, in which the combined sodium silicate and sodium hydroxide activator had a SiO₂/Na₂O molar ratio of 1.2.

2.2.1 Experimental design logic

As shown in Table 2. The experimental program was designed in two stages. First, Groups A and B were established to investigate the effects of precursor proportion and coir fiber dosage on the rheological and mechanical properties of geopolymer mortar. In these groups, the activator modulus was fixed at 1.2, the NaOH concentration was 8 mol/L, and the LSS content was kept constant at 1,200 g. Two precursor proportions, namely 80/20 and 50/50 (slag/fly ash, by mass), were selected, while the coir fiber dosage was varied from 0% to 3%.

Second, Groups C and D were designed to evaluate the effect of activator modulus under representative fiber conditions. Based on the first-stage results, a coir fiber dosage of 1% was used as the reference level. For the two precursor systems (80/20 and 50/50), the SiO₂/Na₂O molar ratio of the activator was adjusted to 1.0, 1.2, 1.4, and 1.6. In this way, the experimental design enabled a stepwise analysis of the effects of precursor proportion, fiber dosage, and activator modulus.

2.3 Test methods

2.3.1 Rheological behavior

The rheological behavior of the geopolymer mortar was measured using a Brookfield DV3T rheometer (Brookfield, United States), as shown in Figure 2. An SC4-21 spindle was used, with a shear rate range of 0–200 s^-1. The test procedure was programmed, and data were collected using the Rheocalc T software. Data acquisition was performed using a multiple-point single reading method. The shear program was set as follows: first, a pre-shear stage was applied, during which the shear rate was increased to 200 s^-1 for 30 s, followed by a 30 s rest period to eliminate the material’s previous shear history. Subsequently, the shear rate was gradually increased from 0 to 200 s^-1 over 300 s using the multiple-point single reading method. Finally, the shear rate was uniformly decreased from 200 s^-1 to 0 at the same acquisition mode.

FIGURE 2

2.3.2 Compressive and flexural strength tests

The mechanical properties of the geopolymer mortar were evaluated in terms of compressive strength and flexural strength. The tests were carried out using a TYE-300E cement mortar testing machine (Wuxi Jianyi Instrument & Machinery Co., Ltd., China) with an accuracy of ±0.5%, in accordance with GBT 17671–2021, Methods of Testing Cement Mortar Strength (ISO method). Prismatic specimens with dimensions of 40 × 40 × 160 mm were first subjected to the flexural strength test. The broken halves obtained from the flexural test were then used for the compressive strength test. The loading rates for compressive and flexural testing were 2.4 kN/s and 50 N/s, respectively. For each mixture and curing age, three parallel specimens were tested, and the reported results correspond to the average values.

The laboratory mixing procedure in this study was carried out using a JJ-5 planetary mortar mixer with a bowl capacity of 5 L and a paddle width of 135 mm. The paddle rotational speed was 140 ± 5 r/min at low speed and 285 ± 10 r/min at high speed, while the corresponding planetary revolution speeds were 62 ± 5 r/min and 125 ± 10 r/min, respectively. The rated motor power of this type of mixer is generally approximately 0.37/0.55 kW. Mixing was performed in accordance with the standard operating procedure. Materials are first mixed at low speed for 30 s, followed by the prescribed sand addition, a brief stop for scraping the bowl and paddle, and subsequent mixing at high speed.

2.3.3 Microstructural characterization

Representative specimens after curing were selected for SEM and XRD analyses to investigate the microstructural features and phase composition of the geopolymer mortar with and without coir fiber.

For SEM observation, small fragments were taken from the internal fracture surfaces of the hardened specimens after mechanical testing, to avoid the influence of external contamination and surface irregularities introduced during casting. SEM analysis was used to examine the compactness of the matrix, pore distribution, fiber dispersion, and the interfacial bonding between coir fiber and the geopolymer matrix. Prior to testing, the samples were prepared according to the requirements of the instrument. SEM observation was carried out using a SEM model, manufacturer] under an accelerating voltage of [] kV.

For XRD analysis, the cured specimens at 7 d and 28 d were crushed into powder for phase identification. However, during conventional manual crushing, part of the coir fiber was found to separate spontaneously from the hardened geopolymer matrix, which could reduce the representativeness of the powder sample. To minimize this effect, an additional fiber-reinforced specimen was prepared by ball milling the cured composite prior to XRD testing. The purpose of introducing the ball-milled fiber-containing sample was to reduce the influence of fiber–matrix separation during sample preparation and to improve the reliability of the phase analysis. XRD measurements were performed using a XRD model, manufacturer] with [radiation type, e.g., Cu-Kα radiation], over a 2θ range of 0°–90°, with a step size of 1° and a scanning rate of 2°/min. The XRD results were used to identify the main crystalline phases and to determine whether the incorporation of coir fiber altered the principal phase assemblage of the geopolymer system.

3 Results and discussion

3.1 Mechanical properties

The shear stress–shear rate behavior of geopolymer mortar pastes containing different coconut shell fiber contents (0%, 1%, 2%, and 3%) was measured using a Brookfield DV3T rheometer. Table 3 shows the Bingham model fitting parameters for the rheological behavior of geopolymer mortars in Groups A and B. Figure 3 shows the variation of shear stress with shear rate for mortars with two binder ratios, 80:20 and 50:50, under the different fiber dosages.

FIGURE 3

For the scatter plots of Groups A and B, the shear stress–shear rate data were fitted using both the Herschel–Bulkley (hereafter HB) model and the Bingham model. The correlation coefficients of the Bingham model were consistently higher than those of the HB model, indicating a better fit. Equation 1 is example 1 of an equation:

In the equation, c represents the degree to which the rheological behavior of the fluid deviates from linearity. A value of c > 0 indicates that the material exhibits shear-thickening behavior, whereas c < 0 corresponds to shear-thinning behavior. When c = 0, the model reduces to the Bingham model, in which the material begins to flow only when the applied stress exceeds the yield stress, after which the flow follows Newtonian viscosity:

Although the HB model can better describe mixtures with pronounced nonlinearity, the Bingham model was retained as the main reference model in the present study because it provides a simpler engineering interpretation of yield stress and plastic viscosity, which is more suitable for comparing the workability-related behavior of different mixtures in an application-oriented context.

The experimental results indicate that both Groups A and B are well described by the Bingham model, suggesting that the proportions of fly ash and slag, as well as the dosage of coconut shell fibers, are not the primary factors affecting the rheological behavior of the geopolymer. For Group B, under the same mix proportion, the incorporation of coconut shell fibers can moderately modify the rheological properties of the geopolymer mortar, making its behavior shift from a shear-thickening Herschel-Bulkley (HB) model towards a closer fit to the Bingham model. Comparing Groups A and B, it is evident that Group B more closely follows the HB model, indicating that a higher slag content increases the viscosity of the paste and enhances its shear-thickening characteristics. In low-modulus LSS geopolymer mortar systems, slag exhibits a thickening effect.

Furthermore, the yield stress of Group A is significantly lower than that of Group B. Specifically, the yield stress for Group A at 0%, 1%, 2%, and 3% fiber content is 2.59106, 7.47791, 9.71629, and 12.21197 Pa, respectively, which is substantially higher than the corresponding values of 0.79395, 0.175, 2.51655, and 6.21141 Pa in Group B. This suggests that the effect of coconut shell fibers on yield stress is influenced by the proportion of slag and fly ash in the mix, the higher the proportion and calcium content, the greater the influence of the fibers.

To further verify the trends, Groups C and D were designed to investigate the effect of the sodium silicate modulus on the rheological behavior of LSS geopolymer mortar. Table 4 shows Bingham model fitting parameters for the rheological behavior of geopolymer mortars in Groups C and D. Figure 4 presents the scatter plots of shear stress versus shear rate for geopolymer mortars prepared with sodium silicate moduli of 1.0, 1.2, 1.4, and 1.6, respectively.

FIGURE 4

Figure 4 shows that, with increasing LSS content, the fitted rheological equations of Groups C and D are no longer well described by the Bingham model but gradually tend toward the Herschel–Bulkley (HB) model. When the LSS content is 1.0 and 1.2, the geopolymer mortars still exhibit an approximately linear relationship between shear stress and shear rate, and the difference between the two mix designs remains relatively small. However, when the LSS content increases to 1.4 and 1.6, the discrepancy between the two geopolymer mortar formulations becomes more pronounced, and the rheological behavior is better fitted by the HB model, with R² values of 0.9868 and 0.9911, respectively.

In addition, it can be observed that the fitted flow behavior index n of the GF8020 group is consistently greater than 1, whereas that of the GF5050 group is consistently less than 1. This indicates that when the slag-to-fly ash ratio is 4, the geopolymer mortar exhibits shear-thickening behavior, while a ratio of 1 result in shear-thinning behavior. Combined with the characterization of raw material compositions, it can be concluded that a higher calcium content in the geopolymer precursor leads to more pronounced non-Newtonian behavior of the slurry. Conversely, when the calcium content is relatively low, the slurry behavior tends to conform to that of a Bingham plastic.

3.2 Mechanical properties

Based on the above conclusions, compressive and flexural strength tests were conducted to verify the mechanical performance of Groups A, B, C, and D, and the results are presented in Figure 5.

FIGURE 5

Specifically, panels (a), (b), (c), and (f) show the compressive strength of Groups A, B, C, and D at curing ages of 3, 7, and 28 days, respectively, while panels (d), (e), (g), and (h) present the corresponding flexural strength results at different curing ages for each group.

3.2.1 Compressive strength

As shown in Figure 6, the incorporation of coconut shell fiber significantly reduces the damage degree of the specimens after failure. The results indicate that, for Group A, the incorporation of coconut shell fiber slightly enhances the compressive strength of the specimens. However, when the fiber content exceeds 3%, the increment in compressive strength becomes less pronounced. Notably, the 3-day compressive strength of the GF8020-C3 mixture is significantly higher than that of other mixtures with the same precursor composition, indicating that coconut shell fiber contributes positively to the early-age strength development of geopolymer mortars. In contrast, for Group B, the addition of coconut shell fiber exhibits a negligible influence on compressive strength, and the overall strength level of Group B is markedly lower than that of Group A. Similar trends have been reported in previous studies, where the incorporation of coconut (coir) fibers was shown to improve the mechanical performance of geopolymer composites at low dosages, while excessive fiber addition resulted in limited or even adverse effects on compressive strength development due to reduced matrix continuity and workability.

FIGURE 6

Compared with fly ash, slag contains a higher calcium content, which promotes the formation of calcium-centered tetrahedral structures under alkaline conditions, resulting in a denser molecular network. This structural densification accelerates the reaction rate and enhances the hardened strength of the material. The beneficial effect of coconut shell fiber on early-age strength is mainly attributed to its physical characteristics rather than chemical interactions. As shown in Figure 5, plain geopolymer paste undergoes abrupt disintegration upon plastic failure, whereas specimens reinforced with coconut shell fiber exhibit improved toughness. After failure, the fiber-reinforced specimens can partially retain their pre-failure physical integrity due to the bridging effect of the fibers. This behavior can be attributed, on the one hand, to the interwoven network structure formed by randomly distributed chopped coconut shell fibers during mixing, and on the other hand, to the stress transfer mechanism during failure. After compressive failure, a triangular crushed wedge zone tends to form near the loading point in fiber-reinforced specimens, allowing them to retain a certain residual compressive capacity. Further discussion of this mechanism is beyond the scope of this study.

The results indicate that, for the GF8020 mixtures in Group A, the compressive strength of the specimens exhibits a decreasing trend with increasing SiO₂/Na₂O ratio. In contrast, for the GF5050 mixtures in Group B, the compressive strength increases as the SiO₂/Na₂O ratio increases.

These results suggest that a higher calcium ion content leads to poorer compatibility with sodium silicate–based alkaline activation systems. This phenomenon is primarily attributed to the differences in precursor composition between Groups A and B. Group A is dominated by high-calcium slag, whereas Group B mainly consists of low-calcium fly ash. As the calcium content increases, the viscosity of the alkali-activated slurry increases accordingly, which reduces the compactness and homogeneity of the geopolymer paste, ultimately resulting in a decrease in compressive strength.

In contrast, due to the relatively lower calcium content in Group B, higher sodium silicate content promotes the formation of a greater amount of gel particles adhered to the particle surfaces, leading to increased surface roughness and a pronounced increase in yield stress. In this case, sodium silicate enhances the mechanical performance of the specimens by modifying the physicochemical properties of the surface polymerization products. Consequently, when the calcium content of the precursor is low, a higher SiO₂/Na₂O ratio results in increased compressive strength. Conversely, when the calcium content is high, an excessive SiO₂/Na₂O ratio deteriorates the rheological properties, hinders proper molding, and ultimately leads to a reduction in compressive strength.

3.2.2 Flexural strength

Figure 7 shows the flexural strength results of the specimens in Groups A–D. For Groups A and B, the incorporation of coir fiber generally improved the flexural strength of the geopolymer mortar, and the difference between the two precursor systems became less pronounced after fiber addition. This trend suggests that the contribution of coir fiber is more clearly reflected in crack control and bending resistance than in compressive resistance.

FIGURE 7

For Groups C and D, the effect of the activator modulus on flexural strength was less pronounced than that on compressive strength. In the 80/20 system, the flexural strength first increased and then decreased with increasing SiO₂/Na₂O ratio, reaching a maximum at 1.2. In the 50/50 system, the flexural strength showed a generally increasing trend within the investigated range. Compared with compressive strength, the flexural response was more sensitive to the dispersion state and distribution uniformity of coir fiber in the matrix. Therefore, the observed improvement in flexural strength is more likely associated with fiber bridging, crack deflection, and post-cracking toughening, rather than with significant changes in the primary reaction products.

Taken together, the results indicate that coir fiber has a more pronounced influence on flexural strength than on compressive strength within the investigated range. This finding is consistent with the expected role of short natural fibers in enhancing the bending resistance and crack-arresting capacity of brittle geopolymer matrices.

Based on a comprehensive comparison of the experimental results from Groups A, B, C, and D, it can be concluded that, like cement-based materials, geopolymer specimens exhibit almost no tensile capacity in the absence of fiber reinforcement. The incorporation of chopped coconut shell fibers markedly enhances the tensile strength of the specimens, with the reinforcing effect being dominated by physical mechanisms. In contrast, the influence of the SiO₂/Na₂O modulus on the tensile strength of geopolymer specimens is negligible.

3.2.3 Correlation analysis

To further examine the relationships among fiber dosage, compressive strength, and flexural strength, a correlation analysis was performed using the average values of the tested mixtures as shown in Figure 8. The Pearson correlation coefficient was adopted to quantify the linear relationships among the selected variables. The variables included compressive strength and flexural strength at different curing ages, together with coir fiber dosage. The correlation matrix was visualized in the form of heatmaps using [software name, e.g., Python/Origin]. It should be noted that the correlation analysis in this study was used only as an auxiliary statistical tool to identify association trends, rather than as direct evidence of causality or mechanism.

FIGURE 8

Figure 7 presents the results of the heatmap-based correlation analysis, where panel (a) represents the relationship between compressive strength and flexural strength, and panel (b) illustrates the correlation between coconut shell fiber content and the 28-day compressive and flexural strengths.

As shown in panel (a), the compressive strengths at different curing ages exhibit strong correlations with each other, whereas the correlation between compressive strength and flexural strength for the same specimens at the same curing age is relatively weak. Under normal circumstances, compressive strength and flexural strength of the same specimen tend to exhibit an approximately linear correlation. However, due to the incorporation of coconut shell fiber, the fiber-induced network structure significantly enhances the flexural strength of the specimens and enables them to retain their physical integrity even after failure. As a result, the correlation between compressive and flexural strengths in this study is weakened. This statistical trend is consistent with the interpretation that coir fiber contributes mainly through physical reinforcement and crack-control effects; however, the correlation analysis itself does not constitute direct mechanistic evidence.

4 Microstructural characterization

4.1 SEM

As observed from the micrographs, the geopolymer material incorporating coconut shell fiber exhibits a gel phase like that of conventional geopolymers, with no new hydration products being formed.

The matrix of the plain geopolymer paste is relatively loose, containing a certain number of voids, while the flocculated gel structures appear locally dense. Owing to limitations in mixing precision, variations in the mixing uniformity between the alkaline solution and the geopolymer paste result in differences in reaction rates, leading to the formation of numerous microcracks on the matrix surface.

As shown in Figure 9, after the incorporation of coconut shell fiber, the randomly distributed chopped fibers overlap and interconnect to form a network structure. When propagating cracks encounter the fibers, the fiber network effectively impedes crack extension. In addition, the rough surface of the coconut shell fibers is coated with a substantial amount of geopolymer matrix, indicating good interfacial bonding between the fibers and the geopolymer matrix. The enhanced compactness of the fiber-reinforced matrix contributes to improved early-age strength development.

FIGURE 9

4.2 XRD

Before the XRD analysis, it was observed that the coir fiber was not completely integrated with the geopolymer paste matrix after curing. During manual crushing of the fiber-reinforced specimens, part of the coir fiber tended to separate spontaneously from the hardened paste, which may lead to poor representativeness of the powder sample used for XRD measurement. To reduce this effect and improve the reliability of phase identification, an additional fiber-containing sample was prepared by ball milling the hardened composite prior to XRD testing. Therefore, the XRD analysis in this study included both the conventionally crushed fiber-reinforced sample and the ball-milled fiber-containing sample. The purpose of introducing the ball-milled sample was not to define a new material system, but to verify whether the phase composition obtained from the fiber-reinforced specimen was affected by sample heterogeneity caused by fiber–matrix separation during powder preparation.

XRD patterns of the geopolymer mortar with and without coir fiber show highly similar diffraction features, and no new obvious crystalline peaks are observed after fiber incorporation. This indicates that the addition of coir fiber does not significantly alter the phase assemblage of the geopolymer matrix. The broad diffuse hump within 20°–38° further suggests that the reaction products are still dominated by amorphous gel phases, which is consistent with the typical structural characteristics of geopolymer materials. The identifiable crystalline peaks are mainly attributed to residual mineral phases from the raw precursors, and their limited variation after fiber addition confirms that the overall reaction pathway remains essentially unchanged.

Therefore, the enhancement induced by coir fiber cannot be reasonably attributed to the generation of new crystalline products. Instead, combined with the mechanical performance results, the improvement is more likely associated with the physical reinforcing role of the fiber, including crack bridging, stress transfer, and pull-out energy dissipation during loading. In this sense, the role of coir fiber in the present system is mainly reflected in microstructural stabilization and mechanical toughening rather than phase transformation. The diffraction profile of the ball-milled coir fiber also does not exhibit clearly distinguishable independent crystalline features compared with the matrix specimens, implying that the contribution of the fiber cannot be fully captured by XRD alone. Accordingly, XRD mainly supports the conclusion that the phase composition remains stable after fiber incorporation, whereas the performance enhancement should be interpreted as arising primarily from physical reinforcement and microstructural optimization of the existing gel network.

Taken together, the XRD results at Figure 10 (7 d) and Figure 11 (28 d) show that the specimens share highly similar diffraction features, with the main peak positions and overall profiles remaining essentially unchanged and no new obvious crystalline peaks being detected. This indicates that the major reaction products of the geopolymer system were largely established at the early curing stage, while the overall phase assemblage remained relatively stable during subsequent curing. Compared with the specimen without fiber, the coir-fiber-reinforced specimen did not exhibit any distinctly new diffraction peaks at either curing age, suggesting that the incorporation of coir fiber did not significantly alter the principal mineralogical composition of the matrix. Although limited differences in local peak intensity and fine spectral features were still observed between 7 d and 28 d, these variations are more reasonably attributed to the continued refinement and stabilization of the existing gel structure rather than to the formation of new crystalline phases. Therefore, the further improvement in mechanical performance at later ages should not be simply ascribed to phase transformation but is more likely related to the progressive densification of the gel network, the enhancement of fiber–matrix interfacial interaction, and the improvement of internal structural continuity. In this sense, the XRD results support the conclusion that the beneficial effect of coir fiber in geopolymer mortar is governed primarily by physical reinforcement and microstructural optimization, rather than by significant changes in phase composition. It should also be noted that the absence of obvious new crystalline peaks in XRD does not completely exclude subtle interfacial interactions at the fiber–matrix scale, but it does indicate that no major phase transformation was induced by fiber incorporation within the investigated range.

FIGURE 10

FIGURE 11

5 Conclusion

References

• 1

  Al JaberiL. A.AliA.Al JadiriR. S.Al KhafajiZ. (2021). Workability and compressive strength properties of (Fly Ash Metakaolin) based flowable geopolymer mortar. Electron. J. Struct. Eng.23 (4), 46–51. 10.56748/ejse.23436

  • CrossRef
  • Google Scholar
  • View reference in article

• 2

  AliM. (2012). Coir fiber and its composites: a review. Compos. Interfaces19 (2), 169–187.

  • Google Scholar
  • View reference in article

• 3

  Artificial aggregates (2022). Geopolymer-based artificial aggregates: a review on methods of producing, properties, and improving techniques. PMC.

  • Google Scholar
  • View reference in article

• 4

  ArunrajaS.RajeshR. (2020). Characterization of alkali treated and untreated coir fiber reinforced concrete. Mater. Today Proc.33, 624–628.

  • Google Scholar
  • View reference in article

• 5

  FanJ.YanJ.ZhouM.XuY.LuY.DuanP.et al (2023). Heavy metals immobilization of ternary geopolymer based on nickel slag, lithium slag and metakaolin. J. Hazard Mater.453 (5), 131380. 10.1016/j.jhazmat.2023

  • Pubmed Abstract
  • CrossRef
  • Google Scholar
  • View reference in article

• 6

  GrasslP.WongH. S.BuenfeldN. R. (2009). Influence of aggregate size and fraction on shrinkage induced micro cracking of mortar and concrete. Comput. Mater. Sci., 44.

  • Google Scholar
  • View reference in article

• 7

  MostazidMd. I. (2023). Acid resistance of geopolymer concrete – literature review, knowledge gaps, and future development. J. Brill. Eng. (BEN)4 (4), 1–8. 10.36937/ben.2023.4875

  • CrossRef
  • Google Scholar
  • View reference in article

• 8

  RashidM. H. (2022). A review on properties of natural fibers and its bio-composites: effect of alkali treatment. Mater. Today Proc.62, 2883–2890.

  • Google Scholar
  • View reference in article

• 9

  SatyanarayanaK. G.GuimarãesJ. L.WypychF. (2007). Studies on lignocellulosic fibers of Brazil. Part I: source, production, morphology, properties and applications. Compos. Part A Appl. Sci. Manuf.38 (7), 1694–1709. 10.1016/j.compositesa.2007.02.006

  • CrossRef
  • Google Scholar
  • View reference in article

• 10

  TaguchiG. R. A. (2024). Mechanical properties of geopolymer concrete incorporating supplementary cementitious materials as binding agents. Discov. Civ. Eng.121.

  • Google Scholar
  • View reference in article

• 11

  ThakurR. (2025). Mitigating strength loss in mortar due to application delay: quantified impact of additional water and cement. J. Eng. Appl. Sci.12.

  • Google Scholar
  • View reference in article