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Article

Rheological and Mechanical Properties of Self-Compacting Geopolymer Concrete Reinforced with Short Basalt Fibres

by
Saima Ali
,
Pulkit Khanna
,
James Stewart
,
Bidur Kafle
and
Riyadh Al-Ameri
*
School of Engineering, Deakin University, Geelong, VIC 3216, Australia
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(6), 264; https://doi.org/10.3390/jcs9060264
Submission received: 15 April 2025 / Revised: 5 May 2025 / Accepted: 12 May 2025 / Published: 26 May 2025
(This article belongs to the Special Issue Mechanical Properties of Composite Materials and Joints)
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Abstract

Due to their low environmental impact, various mineral or cellulose-based natural fibres have recently attracted attention in the construction industry. Hence, the current study focused on basalt fibres and explored the changes in the physical, mechanical, and micro-structural properties of geopolymer concrete reinforced with such fibres. The current study used self-compacting geopolymer concrete, an eco-friendly concrete composed of fly ash, ground granulated blast furnace slag, and an alkali activator, in addition to the regular components of normal concrete. The self-compacting geopolymer concrete compacts under its own weight, so extra compaction is not required. The present study investigated the effect of the fibre content and length. Two different fibre lengths were considered: 12 mm and 30 mm. Three different percentages (1%, 2%, and 3% of the weight of the total mix) of the basalt fibres were considered to determine the optimum fibre content. The mix design was carried out for all the mixes with different fibre contents and fibre lengths, and the workability properties in the slump flow, T-500, and J-ring tests are presented. The effects of the fibre length and content were evaluated in terms of compressive strength (28 and 56 days) and split tensile strength. The results indicated that a higher fibre content effectively increased the compressive strength of 12 mm long fibres. In contrast, a lower fibre content was ideal for the 30 mm long fibres. In addition, the short fibres were more effective in enhancing the geopolymer concrete’s tensile strength than the long fibres. Furthermore, a detailed microscopic analysis was carried out, which revealed that fibre clustering, voids, etc., changed the strength of the selected fibre-reinforced self-compacting geopolymer concrete. Moreover, the analytical method’s predicted tensile strength agreed with the experimental results.

1. Introduction

Geopolymer concrete is a popular cement-based, eco-friendly material. Geopolymer production is estimated to emit approximately 80% less CO2 than conventional Portland cement concrete, primarily due to its significantly lower energy consumption during manufacturing [1]. Hence, geopolymer concrete is increasingly used as an alternative to traditional Portland cement concrete.
An innovative geopolymer concrete has been developed for ambient curing conditions known as self-compacting geopolymer concrete [2]. The advantage of self-compacting geopolymer concrete is its ability to flow under its own weight during the compacting stage, eliminating the need for vibration. Self-compacting geopolymer concrete uses an alkaline activator (e.g., sodium metasilicate) to activate the pozzolanic materials (e.g., fly ash and ground granulated blast furnace slag) in alkali-activated cement systems. On the other hand, alkali-activated cement is not used to prepare self-compacting concrete; Ordinary Portland Cement is used.
Nevertheless, a shortcoming of using any geopolymer concrete is its low tensile strength, intense shrinkage cracks, and, consequently, its high brittleness. Additionally, integrating slag in fly ash-based geopolymer concrete mixtures at a content exceeding 30% of the binder can increase the likelihood of crack development, compromising the concrete’s strength and long-term durability [3]. Using fibres to reinforce geopolymer concrete can increases its ductility and enhances its mechanical properties [1,4].
In recent years, synthetic fibres have been extensively used in normal and geopolymer concrete to enhance tensile strength. However, synthetic fibres degrade slowly after demolition, which is not environmentally friendly; hence, natural fibres are receiving attention as reinforcement materials for concrete. For instance, basalt fibres from volcanic rocks have emerged as a compelling option due to their inherent properties, including a high strength, thermal resistance, and corrosion resilience [5]. Unlike traditional reinforcing fibres, the production process for basalt fibres consumes less energy and requires no additives, resulting in lower costs and a reduced environmental impact [6].
One of the advantages of basalt fibres lies in their exceptional strength, surpassing that of both natural and synthetic fibres [7]. The mechanical properties of basalt fibres are comparable to those of carbon fibres. However, the production cost of basalt fibres is much lower than that of carbon fibres, making them an economically attractive option for various applications [8]. Additionally, basalt fibres exhibit desirable properties, including resistance to acidic and alkaline environments and exceptional low- and high-temperature resistance [9]. Moreover, basalt fibres can enhance the toughness of the composite or geopolymer concrete when acting as a reinforcing material by forming interfacial solid bonds with concrete, preventing crack formation and propagation and thereby maintaining the tensile strength. A few researchers [10,11,12,13] studied the effects of basalt fibres as reinforcement in concrete and found that they significantly increased the composite concrete’s tensile strength.
Khan et al. [14] investigated the mechanical properties of basalt fibre-reinforced concrete with varying percentages of basalt fibres. They varied the basalt fibre content from 0.34% to 0.68% and inferred that an increase in the basalt fibre content up to 0.68% led to improved mechanical properties of basalt fibre-reinforced concrete. Zeng and Tang [15] recommended a 1% to 1.5% fibre content to enhance the compressive strength, tensile strength, flexural strength, and elastic modulus. The study emphasised that increasing the fibre content by up to 2% could improve the mechanical properties. Furthermore, Yan et al. [16] incorporated varying fibre contents (0.5%, 1.5%, 2%, and 2.5%) into a concrete mixture to assess their effects on the mechanical properties and concluded that the specimens containing 2.5% basalt fibres exhibited the highest increase in tensile strength of around 20% compared to the control geopolymer concrete. Ayub et al. [17] investigated the impact of the basalt fibre content on high-performance fibre-reinforced concrete (HPFRC). They found that adding basalt fibres slightly affected the compressive strength. However, the tensile strength significantly increased with higher fibre contents. Specimens with 1%, 2%, and 3% basalt fibre contents showed tensile strength improvements of 1.64%, 5.27%, and 23.95%, respectively, indicating 3% as the optimal fibre content.
There have been limited studies on basalt fibre-reinforced geopolymer concrete (BFRGC), indicating a relatively emerging area of research within construction materials. For instance, Timakul et al. [1] explored the effects of incorporating basalt fibres on the mechanical properties of geopolymer concrete, revealing promising enhancements in compressive strength, flexural strength, and splitting tensile strength. The study also reported notable improvements in ductility and crack resistance in the fibre-reinforced specimens. In addition, researchers [18,19] studied the effects of varying percentages of basalt fibres on the material properties of geopolymer concrete. They inferred that increasing the geopolymer concrete’s basalt fibre content could improve the geopolymer composite’s mechanical strength properties. However, the studies were confined to considering short basalt fibres when exploring the strength properties of the composite geopolymer concrete. In addition, the studies did not investigate the effects of basalt fibres on self-compacting geopolymer concrete. Furthermore, the effects of the fibre length on the mechanical properties of self-compacting geopolymer concrete were not explored in recent studies on basalt fibre-reinforced geopolymer concrete.
Short basalt fibres, which are typically 6 mm to 20 mm long, are primarily utilised to control plastic shrinkage cracking and enhance early-age mechanical properties in concrete. The effectiveness of short basalt fibres lies in developing a three-dimensional network within the concrete matrix, improving cohesion, and mitigating crack propagation. However, their reinforcing capabilities may be limited, especially regarding long-term tensile strength and durability. In contrast, long basalt fibres, which typically start at a length of 25 mm and extend to 50 mm or more, offer superior reinforcement by bridging larger cracks and distributing loads more effectively. Their higher aspect ratio enables them to withstand higher tensile stresses and improve the ductility and toughness of fibre-reinforced concrete. Additionally, long fibres exhibit an enhanced bond strength with the concrete matrix, improving load transfer mechanisms and the resistance to crack initiation and propagation [20].
The cost of long basalt fibres is higher than that of short fibres. However, long basalt fibres are worth using due to their ability to enhance concrete’s mechanical and durability properties and control macro-cracks in concrete [4,21]. Ali et al. [21] examined the effect of different basalt fibre lengths (12 mm and 24 mm) and fibre content on the compressive strength of concrete and found the improvement in compressive strength of concrete up to 10% fibre following further decrease in compressive strength.
The recent literature on incorporating basalt fibres into concrete using varying fibre contents and fibre lengths highlighted the significant scope for extending the investigations to geopolymer concrete, particularly self-compacting geopolymer concrete, which has not been explored. In addition, the workability of geopolymer concrete is expected to be affected by the decrease in the slump of geopolymer concrete, as with concrete [22]. The research on geopolymer concrete mentioned above did not mentioned specific information on changes in the workability of geopolymer concrete with varying fibre contents and fibre lengths. Moreover, no study has been conducted on the incorporation of high percentages of short and long basalt fibres (up to 3%) into geopolymer concrete to understand the changes in the rheological and mechanical properties. The limited research work on basalt fibre-reinforced geopolymer concrete has not explored the optimum fibre content and there are no comparative evaluations of the effects of short basalt fibres vs. long basalt fibres as a reinforcement material on mechanical properties like compressive strength, tensile strength, flexural strength, etc. In addition, the recent studies on basalt fibre-reinforced geopolymer concrete did not conduct microstructural analyses, which are essential to understanding the bonding between the geopolymer concrete matrix and the basalt fibres.
Hence, the current study aimed to systematically investigate the influence of varying basalt fibre contents on self-compacting geopolymer concrete’s fresh and hardened properties and its microstructure. Different basalt fibre contents as a percentage weight of the total mix (1%, 2%, and 3%) and fibre lengths (12 mm and 30 mm) were used to explore the workability and mechanical properties and to conduct a microstructural analysis of the self-compacting geopolymer concrete. Slump flow, T500, and J-ring tests were carried out to understand the changes in the workability of the geopolymer concrete with different fibre contents and fibre lengths. In addition, compression and split tensile tests were carried out to understand the changes in mechanical properties. The study also carried out a microstructural analysis of the basalt fibre-reinforced specimens. It was used to infer the optimum content and fibre length to achieve the best performance in terms of rheological and mechanical properties. Moreover, using an analytical method, the study attempted to predict the tensile strength of the control and basalt fibre-reinforced geopolymer concretes.

2. Experimental Investigation

2.1. Selection of Materials

This study used a novel self-compacting geopolymer concrete that was previously developed by Rahman and Al-Ameri [2], which includes a single alkali activator that works under ambient curing conditions, as a potential new construction material. The newly formulated self-compacting geopolymer concrete (SCGC), which is devoid of cement and superplasticisers and is cured under ambient conditions, is expected to attain a strength of approximately 40 MPa after 28 days of curing, which is comparable to conventional M40 grade concrete. The following materials were used in the proposed geopolymer concrete, along with different percentages of basalt fibres (BFs):
  • Fly ash;
  • Micro fly ash;
  • Slag;
  • Fine aggregate (FA);
  • Coarse aggregate (CA);
  • Alkali activator (sodium metasilicate);
  • Water.

2.2. Mix Preparation

The control and basalt fibre-reinforced geopolymer concrete mix designs followed the guidelines outlined in AS 1012.2:2014 (Methods of Testing Concrete, Method 2: Preparing Concrete Mixes in the Laboratory) [23].
The mixing process involved mixing the alkali activator (sodium metasilicate) with fine and coarse aggregate. Then, the pozzolanic materials (fly ash, micro fly ash, and slag) were added to the mix, and the mixing continued for around 2 min. The fibres were then added, and the dry mixing was continued for another 2 min to uniformly disperse the fibres throughout the mix. After that, water was gradually added, and mixing was carried out for another 6 min. Then, the mixing process was paused for 2 min for thixotropic setting, and finally, mixing was continued for 2 min.
A total of 7 batches were mixed and cast for the two different fibre lengths (12 mm and 30 mm) and three different basalt fibre contents (1%, 2%, and 3% weight of the total mix). For every mix, nine cylinders were cast that were 200 mm in height and 100 mm in diameter (Figure 1). Table 1 summarises the components of the control mix and 12 mm long and 30 mm long basalt fibre-reinforced geopolymer concretes. The orientation of the basalt fibres was not considered in the present study.
The GPC-BF-12 mix contained 12 mm long basalt fibres, whereas the GPC-BF-30 mix contained 30 mm long fibres. The physical properties of the basalt fibres of the selected lengths are presented in Table 2.
The water-to-binder ratio was 0.43, and the activator-to-binder ratio was 0.1. The mixes were prepared following AS1012.2 [23] for the seven batches using the materials listed in Table 1. Figure 1 illustrates the mixing process of the control self-compacting geopolymer concrete mix and the basalt fibre-reinforced geopolymer concrete mixes with different fibre contents and fibre lengths. Three mixes from each batch were placed into the standard cylindrical moulds (Figure 2) and light compaction was applied (clause 7.6 AS1012.8.1 [24]). All the mix specimens were cured at 23 ± 2 °C and 50% humidity, following AS3600 [25], for 28 days after the initial 48 h of curing.

3. Results and Discussion

3.1. Rheological Properties

The rheological properties of the control geopolymer mix and the basalt fibre-reinforced geopolymer mixes were measured using standard workability tests: the slump flow test, T500 test, and J-ring test. The typical setup of the workability tests is shown in Figure 3. The EFNARC [26] requirements and the Australian AS1012.3.5:2015 standard [27] methods of testing concrete were followed when conducting the workability tests and measuring the workability parameters as the mixes were concrete-like materials.

3.1.1. Slump Flow Test

The first workability test performed in the current study was the slump flow test (Figure 3a). The slump cone, also known as Abram’s cone, was positioned at the centre of the levelled base plate and was firmly secured with its larger opening aligned with the 200 mm circle on the base plate. The mix was placed into the cone, and then the cone was lifted. The spread of the mix was measured, and the self-compactness of the mix was determined following the Australian standards [27] and EFNARC guidelines [26]. Figure 4 shows the average slump flow results for all the GPC-BF mixes mentioned in Table 1.
Figure 4 reveals that all the mixes of 12 mm and 30 mm basalt fibre-reinforced geopolymer concrete achieved slump flow values ranging from 510 mm to 700 mm and 580 mm to 700 mm, respectively. According to AS1012.3.5(2015) [27], concrete is classified as self-compacting and workable if the slump flow exceeds a spread value of 500 mm. Hence, the control and basalt fibre-reinforced geopolymer concretes (up to a 3% fibre content) can be considered self-compacting and workable.
Figure 4 shows that the control GPC achieved the highest slump flow of 700 mm and that there was a decrease in workability of the geopolymer concrete mix with an increase in the basalt fibre content, with the average results ranging from 700 to 580 mm and 700 to 510 mm for the specimens with the 12 mm long and 30 mm long basalt fibres, respectively. Studies, such as those from Ma et al. [22], Borhan [28], and Arivalagan [6], have shown that the inclusion of basalt fibres in concrete mixtures tends to reduce workability. The reductions in slump flow, which are a sign of reduced workability, might be due to the higher water demand required to coat the surfaces of the basalt fibres and the increased internal friction during mixing. However, no segregation was detected in any of the mixtures during the study, indicating that the geopolymer concrete mix maintained uniformity and cohesion even with a basalt fibre content of up to 3%. Decreases in slump values, which indicate reduced flowability, of around 3%, 13%, and 17% were found for the GPC-BF-12 mixes with 1%, 2%, and 3% basalt fibre contents, respectively. In contrast, flowability reductions of 7%, 20%, and 27% were found for the GPC-BF-30 mixes with fibre contents of 1%, 2%, and 3%, respectively. The GPC-BF-30 mixes were composed of 30 mm long basalt fibres, which have a larger surface area than the 12 mm long basalt fibres used to prepare GPC-BF-12 mixes. The larger surface area of the 30 mm basalt fibres absorbs more moisture than the 12 mm basalt fibres, which has a smaller surface area. Hence, the flowability reduction rate was higher for the GPC-BF-30 mixes compared to the GPC-BF-12 mixes.

3.1.2. T500 Test

The second workability test in the current study, following [27], was the T500 test. The test used a similar slump cone and stopwatch as the previously mentioned workability test. The stopwatch was used to record the time for the control or fibre-reinforced geopolymer concrete mix to spread over the 500 mm base plate of the slump cone apparatus [26]. Islam et al. [29] recommended an acceptable range for T500 results of 2–7 s.
Figure 5 shows that the mixes with a higher basalt fibre content had an increased segregation tendency according to the T500 criteria. Figure 5 shows that the T500 values for the concretes with the 30 mm long basalt fibres were higher than those with the 12 mm long basalt fibres. This could be due to the formation of a fibre network within the geopolymer concrete, the weight of the long fibres, and the increase in the surface area of the fibres, which increases the viscosity and water absorption capacity and resulted in the reduced flow resistance of GPC-BF-30 mixes. In addition, the interlocking tendency of the fibres further reduced the workability. The T500 values of the 30 mm basalt fibre-reinforced geopolymer concrete mixes containing 3% basalt fibres exceeded 7 s, indicating that segregation likely occurred.
A T500 value above 7 s was found for the GPC-BF-30 mix with a 3% fibre content, which might be due to an increased viscosity, which reflects the expected difficulties in handling and placing composite geopolymer concrete. However, the geopolymer concrete can be workable and self-compacting if the other test results meet the required criteria. The T500 test is generally less reliable due to the uncertainties caused by human error when recording the time using a stopwatch. Hence, Al-Rousan et al. [30] prioritised the slump flow test over the T500 test in evaluating the workability of GPC.

3.1.3. J-Ring Test

The third workability test carried out in the current study was the J-ring test, which analyses the flow of geopolymer concrete with reinforcement. The standard range for a J-ring test passing score recommended by EFNARC [26] is 0 mm to 10 mm. Figure 6 presents the J-ring test results for all the geopolymer concrete mixes with different fibre contents.
Figure 6 shows that the J-ring test value increased with the basalt fibre content for the selected fibre lengths. The control GPC flowed smoothly through the J-ring apparatus, with a J-ring test value of 5 mm, indicating good flowability. However, as the basalt fibre content increased, the flowability decreased. The J-ring test values were 5 mm, 6 mm, and 9 mm for the reinforced geopolymer concrete with 12 mm long basalt fibre contents of 1%, 2%, and 3%. Higher J-ring test values were found for the 30 mm long basalt fibre-reinforced geopolymer (7 mm and 10 mm for the concretes with 1% and 2% fibre contents, respectively). The GPC-BF mix with a 3% fibre content was found to have exceeded the acceptable range of J-ring test values.
The formation of fibre networks within the geopolymer concrete could be the reason for the increase in the J-ring values, reflecting a reduced workability. Furthermore, increased water absorption due to the surface area of the basalt fibres and the friction between the fibres and the binder matrix could be the reason for the increased J-ring values. Consequently, geopolymer concrete with a higher basalt fibre content might require adjustments in the concrete mix design to maintain the desired flow properties for the proper placement and compaction of the mix during construction [31].

3.2. Mechanical Properties

3.2.1. Compressive Strength

The compressive strengths of the mixes were measured using a 3000 kN (Instron, Norwood, MA, USA) compression testing machine. A loading rate of 33 MPa per second was applied to three specimens of each mix to obtain the average compressive strength.
Figure 7 illustrates the 28-day compressive strengths of the control and composite GPC mixes with various 12 mm and 30 mm basalt fibre contents.
Figure 7 shows a fluctuation trend with increased fibre content for the 12 mm length basalt fibre-incorporated GPC mixes. The compressive strength decreased slightly with a 1% basalt fibre content compared to the control GPC. This reduction might be due to the insufficient compaction of the GPC-BF-12-1 mixes, which developed voids. However, a significant increase (22%) in compressive strength was found for the GPC-BF-12-2 (2% fibre content) mix compared to the control GPC, which was around 34 MPa. However, no further improvement in compressive strength with a 3% basalt fibre content, possibly due to improper fibre dispersion during the mix preparation. Hence, incorporating a basalt fibre content of 3% into the GPC mix might be adequate for the fibre length of 12 mm in order to obtain an adequate compressive strength.
On the other hand, Figure 7 shows a decreasing trend with increased fibre content for the 30 mm length basalt fibre-incorporated mixes. The compressive strength was found to decrease by around 3%, 10%, and 28% for the GPC-BF-30-1 (1% fibre content), GPC-BF-30-2 (2% fibre content), and GPC-BF-30-3 (3% fibre content) mixes, respectively, compared to the control GPC mix. This might be due to fibre clustering of the long basalt fibres. This trend aligns with the existing literature, such as Yan et al. [16], who reported a 15% reduction in the 28-day compressive strength with a 2.5% basalt fibre content, and Borhan [28], who observed a 12% reduction with a 0.5% fibre content. Dias and Thaumaturgo [32] also noted a 26.4% reduction with a 1% basalt fibre content due to fibre clustering, the formation of voids, and the absorption of excess water by the fibres, leading to hydration issues and weak bonding. In addition, Sivanantham et al. [33] observed a decrease in the compressive strength of steel fibre-reinforced self-compacting concrete with an increasing aspect ratio (length/diameter) of the fibre. Hence, the short fibres (12 mm) were superior to the long fibres (30 mm) in terms of enhancing the compressive strength of the GPC mixes.
The reduction in compressive strength with increased basalt fibre content could be due to several other factors. Firstly, adding a large amount of basalt fibres increases the porosity of the geopolymer concrete matrix and reduces the density and uniformity of the geopolymer concrete. This increased porosity adversely impacts the compressive strength of the geopolymer concrete. Secondly, the mechanical strength of the geopolymer concrete is significantly affected by the orientation and distribution of the basalt fibres. If the fibres are not correctly oriented or aligned, the geopolymer concrete might not be able to effectively resist compressive forces, resulting in a decrease in the compressive strength. These observations highlight the intricate relationship between the basalt fibre content, porosity, fibre orientation, and compressive strength of geopolymer concrete [34].
All mixes were also cured for 56 days to observe the impact of the fibre content on the strength development of geopolymer concrete at early ages. Figure 8 presents the variation in the compressive strength for the control mix and GPC mixes with different fibre contents after 56 days. The bar chart shows a steady strength development, with an average change of 1 MPa between 28 and 56 days for all the mixes.

3.2.2. Split Tensile Strength

The split tensile strengths of the mixes were measured by placing the cylinders horizontally in the testing machine, as mentioned in the previous section. A loading rate of 785 N per second was applied, and the indirect tensile strength for three specimens of each mix was recorded to obtain the average tensile strength.
The 28-day split tensile strength results are shown in Figure 9, which showed an increasing trend in the tensile strength with increased fibre content for both the 12 mm fibres and 30 mm fibres. Hence, the split tensile strength test results for the GPC mixes revealed a significant improvement in the tensile strength of the GPC-BF mixes with an increasing fibre content, which aligns with the research findings on fibre-reinforced concrete studies by Jalasutram et al. [10], Bheel [13], and Revade [35]. The graphs also show that the enhancement in tensile strength was more significant for the GPC-BF-12 mixes than for the GPC-BF-30 mixes. It can be seen in Figure 9 that the inclusion of the short fibres (GPC-BF-12-1, GPC-BF-12-2, and GPC-BF-12-3) considerably increased the tensile strength by 17.8%, 42.5%, and 54.9%, respectively, compared to the control GPC. The increase in tensile strength for the GPC-BF-30 mixes was 20%, 32%, and 38% with fibre contents of 1%, 2%, and 3%, respectively.
The enhancement in tensile strength can be attributed to several factors. Initially, the matrix in the fibre-reinforced concrete predominantly bears the load under stress. As the strain on the fibres increases, they experience more stress and delay the initial cracking. Once cracking initiates, the fibres act as bridges across the cracks, redistributing the load and stabilising crack propagation. This stress redistribution underlies the concrete’s load-bearing capacity, resulting in higher peak tensile stress. Moreover, secondary cracks may develop alongside the primary crack due to stress redistribution facilitated by the basalt fibres, preventing centralised cracking within the concrete section.
The modes of failure (Figure 10) observed during this test provided valuable insights into the ductility of the GPC-BF mixes. It was noted that, unlike the control specimen, the basalt fibre-reinforced geopolymer concretes showed a delayed failure following the initial cracking. This effect can be attributed to the improved ductility provided by the fibres.
The findings of this study are consistent with the previous research by Ma et al. [22] and Arivalagan [6], highlighting the influence of basalt fibres on concrete strength. Denser basalt fibres enhanced the indirect tensile strength by resisting crack propagation. The GPC-BF samples after tensile strength testing showed a uniform aggregate distribution across the entire specimen, which adheres to the EFNARC guidelines. There are no signs of segregation in Figure 10.

4. Results and Discussion: Optical Microscopic Analysis

A microscopic analysis was conducted on the basalt fibre-incorporated geopolymer concrete samples with different fibre lengths and contents. The microscopic analysis explored the bonding between the basalt fibres and the geopolymer concrete matrix and the changes with the addition of the different basalt fibre contents and lengths. The samples studied using an optical microscope were 100 mm in size. Following proper grinding and polishing, one cylinder from each batch was studied using an Olympus SZ61 (Olympus, Tokyo, Japan) microscope under 4.5× magnification.
The microscope images of the GPC-BF specimens (Figure 11) showed cracks surrounding the periphery of the basalt fibres. These microcracks around the basalt fibres indicate weak bonding between the geopolymer binder and basalt fibres [36]. This weak bonding could be the main reason for the reduced compressive strength with increased basalt fibre content. The strength of GPC primarily relies on the bond strength between the binder and aggregates. Therefore, any weakness in this bond can significantly impact the mechanical properties of composite GPC.
Basalt fibres can improve the mechanical properties of GPC. However, an excessively high fibre content may not bond effectively with the geopolymer matrix. The fibres might not contribute optimally to load transfer mechanisms beyond a certain threshold, reducing the compressive strength as the fibres do not adequately integrate into the matrix [36].
In addition, the microscopic analysis showed that the samples with a higher fibre content exhibited an increased presence of voids and fibres adjacent to these voids, as shown in Figure 12.
These voids likely resulted from excess water at the fibres’ surface that did not reacted with the binder, which then evaporated, resulting in voids. These voids significantly reduce the strength of the geopolymer concrete, as air voids diminish the effective area of the cylinders, thereby reducing their load-bearing capacity [37]. Furthermore, incorporating large amounts of fibres into geopolymer concrete mixes may induce fibre clustering, where the fibres aggregate within the matrix, resulting in localised stress concentrations and compromising the structural integrity of the fibre-reinforced geopolymer concrete. Consequently, the compressive strength of the geopolymer concrete might be adversely affected, as fibre clusters act as points of weakness, exacerbating crack initiation and propagation. Figure 13 illustrates an instance of fibre clustering within the matrix.
The fibre clustering was found to be more pronounced for the GPC-BF-30 specimens than the GPC-BF-12 specimens, as longer basalt fibres have a higher tendency to cluster, leading to a heterogeneous matrix, reduced bond strength, and diminished mechanical and fracture properties [36].
Moreover, as shown in Figure 14, the microstructural observations provided a detailed view of the interaction between the basalt fibres and the geopolymer matrix. The dark colouration of the matrix, as shown in Figure 14, suggests a significant presence of unreacted or partially reacted materials, such as fly ash or other aluminosilicate precursors, which did not fully participate in the hydration process of the geopolymer concrete. This incomplete reaction can result in a weaker matrix, thereby affecting the mechanical strength of the composite geopolymer concrete [2].
The microstructural analysis revealed critical insights into the interaction between the basalt fibres and the geopolymer matrix. The decrease in compressive strength with increasing fibre content may have resulted from poor fibre dispersion, weak matrix–fibre bonding, and ineffective crack bridging.

5. Tensile Strength Prediction Using Analytical Method

Incorporating basalt fibres into geopolymer concrete was found to have a significant effect on enhancing the tensile strength of the composite geopolymer concrete. The current study used standard analytical formulas to predict the tensile strength of the control geopolymer concrete and basalt fibre-reinforced geopolymer concretes with different fibre contents and fibre lengths. The study used the compressive strength values obtained from the test results to predict the tensile strength of the control and basalt fibre-reinforced specimens. A few formulas to predict the tensile strength of concrete using the compressive strength values obtained from the tests are available in the literature. The uniaxial tensile strength (fct) is equal to approximately 90% of the split tensile strength (fct.sp) [25], and the relationship between the uniaxial tensile strength and compressive strength (f’c) is fct = 0.36 × √f′c. Hence, the relationship between the split tensile strength and compressive strength of normal concrete is
fct.sp = 0.4 × √f′c
Figure 15 compares the measured and predicted values of the split tensile strength of the geopolymer concrete with 0% (control), 1%, 2%, and 3% basalt fibre contents using the equation developed for concrete (Equation (1)) based on the compressive strength.
Figure 15 indicates that the predicted tensile strengths agree with the experimental tensile strengths for the control GPC with a 0% basalt fibre content. Significant differences in the predicted and experimental tensile strengths were observed for the GPC-BF specimens with z fibre content of up to 3%, revealing that the equation for normal concrete cannot be used for fibre-reinforced concrete.
Hence, the current study proposed an analytical formula from the literature [38] to predict the tensile strength of the basalt fibre-reinforced concrete. The analytical formula (Equation (2)) uses the correlation between the splitting tensile strength and cylindrical compressive strengths for basalt fibre-reinforced concrete following the American design code (ACI 318-11):
f’sp = αsp × (f’c/αc)βsp
where αsp and βsp are recommended to be 0.21 and 0.8, respectively. The American Design Code (ACI 318-11) [38] recommends a value of αc of between 0.76 and 0.85.
However, for basalt fibre-reinforced concrete, the αc should be within the range of 0.86~0.98 according to Fang et al. [39] based on the test results. Figure 16 shows the validity of the tensile strength prediction method for the GPC-BF specimens.
The tensile strength prediction formula (Equation (1)) adopted for normal cement concrete was used for control geopolymer concrete specimens. In contrast, the tensile strength prediction method (Equation (2)) suggested for basalt fibre-reinforced concrete was used for predicting the tensile strength of all the GPC-BF specimens in the current study. Figure 16 shows a good agreement between the predicted and experimental tensile strengths of the control and 12 mm basalt fibre-reinforced geopolymer concrete specimens. However, the predicted tensile strength of the basalt fibre-reinforced geopolymer concrete specimens with a higher fibre content (3%) was lower than the experimental values. One reason could be the reduced compressive strength and fibre clustering with a 3% fibre content. The prediction model was based on the correlation of the tensile strength with the compressive strength of the basalt fibre-reinforced concrete. Similarly, a good agreement was observed between the predicted and experimental tensile strengths of 30 mm basalt fibre-reinforced geopolymer concrete, except for the specimens with a higher basalt fibre content (3%). Improved dispersion techniques might help eliminate fibre clustering and allow for better agreement between the experimental and predicted values of the tensile strengths of basalt fibre-reinforced concrete. Hence, Equations (1) and (2) may be able to predict the tensile strength of control and basalt fibre-reinforced geopolymer concretes.

6. Conclusions

This study explored the behaviour of basalt fibre-incorporated self-compacting geopolymer concrete by conducting several standard tests to investigate the rheological and mechanical properties. Two different lengths (12 mm and 30 mm) of basalt fibres were used and added at different concentrations (fibre contents of 1%, 2%, and 3%). Workability tests (slump flow, T500, and J-ring tests) were performed to examine the physical properties. Compressive strength and split tensile strength tests were conducted to measure the mechanical properties. In addition, an optical microscope was used to observe the bonding between the geopolymer binder and basalt fibres. In addition, the present study applied an analytical approach to predict the tensile strength of short and long basalt fibre-reinforced geopolymer concrete. The results are summarised below.
  • A self-compacting geopolymer concrete (GPC) mix incorporating basalt fibres was successfully developed to meet self-compacting standards.
  • The workability of the fibre-reinforced GPC mixes decreased with an increase in the basalt fibre content. Using a higher amount of fibre was found to have detrimental effects on the T500 results of the GPC-BF mixes. Concrete with the selected fibre lengths and fibre contents met the slump flow and J-ring requirements.
  • The compressive strength for the 12 mm length basalt fibre-incorporated GPC mixes showed an insignificant initial drop in compressive strength. However, a significant increase in compressive strength was found for the GPC-BF mix with a higher content (up to 3%) compared to the control specimen.
  • On the other hand, the compressive strength of the 30 mm fibre-incorporated geopolymer concretes decreased with increasing basalt fibre content. The highest reduction in compressive strength was found to be around 28% for the GPC-BF mix with a 3% fibre content compared to the control GPC mix. The short fibres (12 mm) were more effective compared to the long fibres (30 mm) in terms of increasing the compressive strength.
  • The changes in compressive strength between 28 days and 56 days of curing time were insignificant for all the GPC-BF mixes.
  • The enhancement in tensile strength was more significant for the GPC-BF-12 mixes (12 mm fibres) than the GPC-BF-30 mixes (30 mm fibres). The short (12 mm) basalt fibre-incorporated GPC showed a significant increase in tensile strength of around 26%, 74%, and 121% for fibre contents of 1%, 2%, and 3%, respectively, compared to the control GPC.
  • The microscopic images of the GPC-BF specimens showed cracking around the basalt fibres in all the specimens. Microcracks radiating from the boundary of the basalt fibres indicated a weak bond strength between the geopolymer binder and the basalt fibres. For both selected lengths of basalt fibres, a higher fibre content led to fibre clustering, causing localised stress concentrations and a reduced compressive strength.
  • The microscopic images also showed the presence of more voids in the geopolymer concrete mix as the basalt fibre content increased. The presence of these voids might have been caused by excess water at the surface of the basalt fibres, which could not react with the binder and, in turn, evaporated, leaving behind voids. Thus, the compressive strength decreased with increasing basalt fibre content.
  • Considering the workability and mechanical properties, the optimal fibre content for self-compacting geopolymer concrete was 1% for the 30 mm length basalt fibres. In contrast, up to 3% can be recommended for the 12 mm long basalt fibres. These mixes provided balanced performance, maintaining good workability characteristics and a satisfactory mechanical strength.
  • The proposed analytical method was found to be able to predict the tensile strength of the basalt fibre-reinforced geopolymer concrete specimens.

Author Contributions

Conceptualisation, S.A., B.K. and R.A.-A.; methodology, S.A., B.K. and R.A.-A.; formal analysis, S.A.; investigation, P.K. and J.S.; resources, S.A.; data curation, P.K. and J.S.; writing—original draft preparation, S.A.; writing—review and editing, B.K. and R.A.-A.; visualisation, S.A., B.K. and R.A.-A.; supervision, S.A., B.K. and R.A.-A.; project administration, S.A., B.K., R.A.-A., P.K. and J.S.; funding acquisition, S.A., B.K. and R.A.-A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank our technicians at Deakin University, Leanne Farago, Uli Bauer, Lube Veljanoski, and Michael Shanahan, for their tremendous support and commitment towards completing this project.

Conflicts of Interest

There are no financial or personal conflicts of interest in publishing the present research paper.

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Figure 1. Mixing of control and fibre-reinforced geopolymer concretes. (a) Control and concretes with (b) 1% BFs, (c) 2% BFs, and (d) 3% BFs.
Figure 1. Mixing of control and fibre-reinforced geopolymer concretes. (a) Control and concretes with (b) 1% BFs, (c) 2% BFs, and (d) 3% BFs.
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Figure 2. Fresh specimens of control GPC and GPC-BF mixes.
Figure 2. Fresh specimens of control GPC and GPC-BF mixes.
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Figure 3. Workability test set up. (a) Slump flow test; (b) T500 test; (c) J-ring test.
Figure 3. Workability test set up. (a) Slump flow test; (b) T500 test; (c) J-ring test.
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Figure 4. Average slump flow results for all mixes.
Figure 4. Average slump flow results for all mixes.
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Figure 5. Average T500 test results for all mixes.
Figure 5. Average T500 test results for all mixes.
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Figure 6. Average J-ring test results for all mixes.
Figure 6. Average J-ring test results for all mixes.
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Figure 7. Compressive strength of control and GPC-BF mixes with different fibre lengths.
Figure 7. Compressive strength of control and GPC-BF mixes with different fibre lengths.
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Figure 8. Compressive strength of control and GPC-BF mixes at 28 days and 56 days.
Figure 8. Compressive strength of control and GPC-BF mixes at 28 days and 56 days.
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Figure 9. Tensile strength of control and GPC-BF mixes with different fibre lengths.
Figure 9. Tensile strength of control and GPC-BF mixes with different fibre lengths.
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Figure 10. Samples with varying BF contents after split tensile strength test. (a) Control; (b) 1%, (c) 2%, and (d) 3% contents.
Figure 10. Samples with varying BF contents after split tensile strength test. (a) Control; (b) 1%, (c) 2%, and (d) 3% contents.
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Figure 11. Microscopic image of GPC with BF content of 2%.
Figure 11. Microscopic image of GPC with BF content of 2%.
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Figure 12. Microscopic image showing voids in samples with 30 mm basalt fibres with (a) 2% and (b) 3% contents.
Figure 12. Microscopic image showing voids in samples with 30 mm basalt fibres with (a) 2% and (b) 3% contents.
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Figure 13. Microscopic image of samples with (a) 2% and (b) 3% basalt fibre contents showing fibre clustering.
Figure 13. Microscopic image of samples with (a) 2% and (b) 3% basalt fibre contents showing fibre clustering.
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Figure 14. Geopolymer mixes with increasing basalt fibre content from top to bottom.
Figure 14. Geopolymer mixes with increasing basalt fibre content from top to bottom.
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Figure 15. Tensile strength prediction using cement-concrete formula.
Figure 15. Tensile strength prediction using cement-concrete formula.
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Figure 16. Tensile strength prediction using the proposed formula.
Figure 16. Tensile strength prediction using the proposed formula.
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Table 1. The components of the control, and 12 mm long and 30 mm long basalt fibre-reinforced geopolymer concretes (for 1 cubic metre).
Table 1. The components of the control, and 12 mm long and 30 mm long basalt fibre-reinforced geopolymer concretes (for 1 cubic metre).
MaterialControl (GPC)GPC-BF12-1/GPC-BF30-1GPC-BF12-2/GPC-BF30-2GPC-BF12-3/GPC-BF30-3% in Total Mix
Fly Ash (kg)8.148.148.148.1416
Micro Fly Ash (kg)2.042.042.042.044
Slag (kg)6.16.16.16.112
Sodium Metasilicate (kg)1.61.61.61.632
CA (kg)11.511.511.511.522
FA (kg)12.912.912.912.925
Water (kg)8.18.18.18.116
BFs (kg)0---0
BFs (kg)-0.5--1
BFs (kg)--1-2
BFs (kg)---1.53
Table 2. Physical properties of basalt fibres.
Table 2. Physical properties of basalt fibres.
DesignationLength (mm)Diameter (µm)Density (g/cm3)Tensile Strength (MPa)Elongation (%)
BF 1212132.6–2.810002.4–3.15
BF 3030132.6–2.810002.4–3.15
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MDPI and ACS Style

Ali, S.; Khanna, P.; Stewart, J.; Kafle, B.; Al-Ameri, R. Rheological and Mechanical Properties of Self-Compacting Geopolymer Concrete Reinforced with Short Basalt Fibres. J. Compos. Sci. 2025, 9, 264. https://doi.org/10.3390/jcs9060264

AMA Style

Ali S, Khanna P, Stewart J, Kafle B, Al-Ameri R. Rheological and Mechanical Properties of Self-Compacting Geopolymer Concrete Reinforced with Short Basalt Fibres. Journal of Composites Science. 2025; 9(6):264. https://doi.org/10.3390/jcs9060264

Chicago/Turabian Style

Ali, Saima, Pulkit Khanna, James Stewart, Bidur Kafle, and Riyadh Al-Ameri. 2025. "Rheological and Mechanical Properties of Self-Compacting Geopolymer Concrete Reinforced with Short Basalt Fibres" Journal of Composites Science 9, no. 6: 264. https://doi.org/10.3390/jcs9060264

APA Style

Ali, S., Khanna, P., Stewart, J., Kafle, B., & Al-Ameri, R. (2025). Rheological and Mechanical Properties of Self-Compacting Geopolymer Concrete Reinforced with Short Basalt Fibres. Journal of Composites Science, 9(6), 264. https://doi.org/10.3390/jcs9060264

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