Experimental Assessment of the Strength and Microstructural Properties of Fly Ash-Containing Basalt Fiber-Reinforced Self-Compacting Sustainable Concrete

Abstract

This study investigates the influence of basalt fiber on the rheological, mechanical, and microstructural properties of sustainable self-compacting concrete (SCC) incorporating fly ash and microsilica as supplementary cementitious materials (SCMs). Various SCC mixes were prepared, incorporating five different volume fractions of basalt fiber (0.05%, 0.1%, 0.5%, 1%, and 1.5%), along with a control mix. The rheological properties of fresh SCC were evaluated using slump flow and V-funnel flow tests. Subsequently, the mechanical properties, including compressive strength, splitting tensile strength, and flexural strength, were measured after 28 days of curing. Additionally, microstructural analysis was conducted using scanning electron microscopy (SEM) on fractured specimen surfaces. The results indicated that the inclusion of basalt fiber adversely affected the flowability of fresh SCC mixes, with increased fiber volume. However, the hardened concrete exhibited significant improvements in mechanical properties with the addition of basalt fibers. The optimal performance was observed in the SCC70-85/0.10 mix specimens, which demonstrated a 69.90% improvement in flexural strength and a 23.47% increase in splitting tensile strength compared with the control specimen. SEM analysis further revealed enhanced microstructural density in the concrete matrix containing basalt fiber. A two-factor analysis of variance (ANOVA) with repetitions was conducted to evaluate the effects of varying basalt fiber concentrations on the compressive, flexural, and tensile strengths of SCC mixes. The ANOVA results indicated significant effects for both SCC grade and basalt fiber concentration, demonstrating that each factor independently affected the compressive, tensile, and flexural strengths of SCC. These findings suggest that the incorporation of basalt fibers holds promise for extending building lifespans and enhancing concrete quality, representing a valuable advancement in structural engineering applications.

1. Introduction

Self-compacting concrete (SCC) represents an advanced evolution of conventional high-performance concrete, now recognized as one of the most significant technological advancements in the construction industry [1,2]. The success of SCC lies in its ability to flow under its own weight without segregation [3]. This self-consolidation ability allows SCC to fill gaps or voids that would typically require vibration when using conventional concrete [4]. SCC effectively fills spaces between congested reinforcement in heavily reinforced concrete structural elements, including beams, columns, slabs, walls, and foundations [5]. Consequently, SCC can be placed without the need for noisy vibrating equipment, promoting rapid strength development and enhancing durability [6]. These characteristics have led to SCC’s widespread adoption in various construction applications. Researchers have thus focused on optimizing SCC mixtures, emphasizing key principles such as reducing the water/cement ratio, maximizing coarse aggregate particle size, increasing binder content, and incorporating superplasticizers [7,8]. However, the high demand for cement in SCC increases costs, heat of hydration, and shrinkage [9]. This necessitates exploring alternatives to traditional cement to meet SCC requirements.

To address this challenge, industrial waste materials such as fly ash (FA), microsilica (MS), silica fume (SF), and granulated blast furnace slag (GBFS) have been utilized as supplementary cementitious materials (SCMs) in SCC, either partially or fully replacing cement to enhance cementitious composites [10,11]. Incorporating SCMs into SCC reduces cement usage, minimizes the carbon footprint, and maximizes the effective utilization of industrial by-products [12]. Using FA (Class F) in SCC enhances its fresh state properties, improving flowability, strength, and durability compared with conventional concrete [13,14,15]. For instance, Mohsen et al. [16] found that incorporating 10% and 20% FA enhanced the flexural and compressive strength of concrete, while FA replacement reduced permeability, attributed to improvements in the microstructure of concrete matrices. Similarly, Aburumman et al. [17] investigated the effect of replacing 10–50% of cement with FA in concrete mixes, reporting a 9% improvement in compressive strength with 10% FA and significant enhancements in flexural strength at various replacement levels. Bhagath Singh and Durga Prasad [18] reported that an M30-grade concrete mix with 65% fly ash (FA) achieved the required strength while consuming 59% less energy and reducing CO₂ emissions by 54% compared with ordinary Portland cement (OPC). According to the Ecoinvent database, producing one ton of cement requires approximately 4800 MJ of energy and generates around 866 kg of CO₂ equivalent emissions. In contrast, FA is considered an industrial by-product, and its use typically only accounts for transportation emissions, significantly reducing its environmental impact compared with clinker-based materials. The 54% CO₂ reduction index in this study demonstrates the effectiveness of FA in mitigating emissions and advancing sustainability in self-compacting concrete (SCC) production.

In parallel, other studies have focused on enhancing SCC properties using various fiber types and volume fractions [19,20]. Steel fibers are commonly used, but concerns about their long-term durability due to potential corrosion have prompted interest in alternative materials. Mineral-based synthetic basalt fibers have gained acceptance as a viable alternative, owing to their superior chemical, mechanical, and electrical properties [21,22]. Compared with other fibers, glass fibers are known for their high tensile strength and resistance to cracking, and are often used to enhance flexural properties. However, their susceptibility to alkali attacks in cementitious environments can limit their long-term durability [7]. Polymer fibers are also widely used to improve impact resistance and reduce shrinkage, and provide excellent workability but may have limited thermal resistance compared with basalt fibers. Recent studies, such as those by Mehmet and Arslan [23], have highlighted the benefits of basalt fibers in SCC, particularly their ability to maintain compressive strength at high temperatures. Additionally, the inclusion of small amounts of basalt fibers has been shown to improve the splitting tensile and shear strengths of concrete, especially at elevated temperatures. Çelik and Bingöl [24] further demonstrated that adding basalt fibers enhances the flexural and tensile strengths of concrete while maintaining compressive strength. They also emphasized the importance of toughness, a critical property for this type of concrete. Ozodabas [25] reported that, in general, adding basalt fibers in mixtures decreases the viscosity and increases the compressive strength of the concrete. Interestingly, adding 1% basalt fiber resulted in a slight decrease in compressive strength compared with fiber-free samples. Overall, concrete reinforced with specific ratios of basalt fibers under certain conditions has shown improved performance.

Generally, self-compacting fiber-reinforced concrete (SCFRC) is more ductile and tension-resistant than conventional SCC and exhibits higher residual strength [26]. The workability of SCFRC is influenced by the type and content of fibers used, along with the SCC matrix composition [27]. Higher fiber aspect ratios (length-to-diameter ratio, L/D) and volume fractions (VF) improve the performance of SCFRC in the hardened state but can adversely affect its workability [28]. Thus, optimizing fiber length and volume fraction is crucial to maintaining the self-compacting ability of the concrete. This research aims to address this gap by investigating the use of basalt fibers as reinforcement in SCC.

This study explores the behavior of basalt fiber-reinforced self-compacting sustainable concrete mixes incorporating fly ash and microsilica as supplementary cementitious materials. While previous research has explored SCC’s mechanical and rheological properties, this study introduces the unique combination of varying basalt fiber volume fractions with a dual system of SCMs (FA and microsilica), emphasizing their synergistic effects on both rheological and mechanical performance. Furthermore, the research encompasses a comprehensive experimental program with mixes of varying grades (Grade 1, Grade 2, and Grade 3), tested under five basalt fiber volume fractions (0.05%, 0.1%, 0.5%, 1%, and 1.5%), offering new insights into the effects of fiber volume on SCC. Also, a novel aspect of this work is the inclusion of statistical analysis to quantify the impact of basalt fiber on SCC strength properties, paired with a detailed microstructural evaluation to link mechanical improvements to internal structural changes. These elements collectively establish a deeper understanding of sustainable and high-performance concrete development.

2. Experimental Program

2.1. Materials

All SCC mixes were composed of ordinary Portland cement (OPC), coarse aggregates with diameters of 4–10 mm and 10–20 mm, and natural sand with a diameter of 0–4 mm. Fly ash and microsilica, which are SCMs, along with a polycarboxylate ether superplasticizer (Epsilone HP 540, manufactured by Weber Saint-Gobain, La Défense, France), classified as an admixture according to ASTM C494, were employed as additives to enhance the properties of the SCC mixes. The basalt fibers utilized in this study were 24 mm long, chopped fibers with a diameter of 13–20 microns, manufactured by Techno Basalt-Invest LLC, Slavuta, Ukraine. The chemical compositions of fly ash and microsilica were determined using an ARL 9900 IntelliPower X-ray spectrophotometer, manufactured by Thermo Fisher Scientific Inc., Waltham, MA, USA. The moisture content and loss on ignition were determined according to the applicable testing methods as per ASTM C311/C311 [29] and all results are reported in

Table 1. Chemical and physical properties of fly ash and microsilica.

Compound	Class F Fly Ash	Microsilica
Composition (wt%)
CaO	1.34	3.5
Al2O3	18.21	0.7
SiO2	70.31	85
Fe2O3	5.26	1.1
MgO	1.00	2.7
K2O	1.08	0.001
Physical Properties
Moisture Content Max %	3	3–4
Loss on Ignition, Max %	6	2–4

.

Table 2. SCC mix components.

Material	Source	Supplier	Type/Conformity
Cement	Qatar	Qatar National Cement Company (QNCC), Doha-Qatar	OPC Class 42.5N, conformance to EN 197-1
Fly ash	India	MSTC, Kolkata-India	CLASS F, conformance to ASTM C 618
Microsilica	Qatar	Al Jabor, Doha-Qatar	Conformance to ASTM C 1240
Coarse aggregate 20 mm and 10 mm	UAE	Village Trading Group, Doha-Qatar	Crushed gabbro, conformance to BS EN 12620
Fine aggregate (washed sand)	Qatar	Qatar Primary Materials Company (QPMC), Doha-Qatar	Natural conformance to BS EN 12620
Epsilon HP 540	Qatar	Sodamco, Doha-Qatar	Polycarboxylate ether super-plasticizer, conformance to BS EN 934-2 and ASTM C 494 Type D and G
Basalt fibers	Techno basalt—Invest LLC Company, Slavuta Ukraine	Arabian Specialized Materials Co. (ASMA), Doha-Qatar	Diameter of 13-20 microns and length of 24 mm

provides an overview of the SCC mix components, including details on the source, supplier, and type.

2.2. Mixture and Sample Preparation

Epsilone HP 540 was mixed to standard design guidelines, aimed at producing a basic SCC with three distinct concrete grades. These grades were classified based on the cylindrical compressive strength of the virgin mix (concrete without basalt fibers) to cater to varying performance requirements:

• Grade 1: cylindrical compressive strength ranged from 40 to 55 MPa.
• Grade 2: cylindrical compressive strength ranged from 55 to 70 MPa.
• Grade 3: cylindrical compressive strength ranged from 70 to 85 MPa.

To achieve the desired strength characteristics, the water-to-binder ratio was carefully calibrated for each grade: 0.40 for Grade 1, 0.35 for Grade 2, and 0.34 for Grade 3. The binder in all mixes consisted of a combination of cement, fly ash, and microsilica to enhance mechanical properties and durability. In addition to the virgin mixes, this study also evaluated the influence of basalt fiber reinforcement. Basalt fibers were added in five different volume fractions—0.05%, 0.1%, 0.5%, 1%, and 1.5%—by total weight of the concrete. This allowed for a comprehensive analysis of how varying fiber volumes affect SCC properties across different grades. Each mix was uniquely designated based on its grade and basalt fiber volume fraction. For instance, a mix in grade 40–55 MPa containing 0.5% basalt fiber by total weight of concrete was labeled as SCC 40–55/0.5. This systematic naming convention facilitated clear identification and comparison of mix performance, especially in subsequent analyses of workability, strength, and durability.

As higher fiber volume fractions negatively impact the workability of SCC, the superplasticizer dosage in the fiber-reinforced mixes was adjusted to achieve the required slump flow for self-compacting concrete while remaining within the manufacturer’s recommended range of 0.5% to 2.2% by weight of cementitious materials. However, in some trials, the desired slump flow for self-compacting ability (650 ± 50 mm) could not be achieved due to bleeding observed with the increased superplasticizer dosage (Figure 1). In such cases, the trials were discarded, and mixes with higher fiber volume fractions were not further investigated for the same grade. The concrete mix designs and proportions of the eight mixes that met the workability requirements for self-compacting concrete are presented in

Table 3. SCC mix design and proportion.

Ingredients (kg/m3)	Grade 1		Grade 2			Grade 3
	SCC 40–55/(0.00%)	SCC 40–55/(0.10%)	SCC 55–70/(0.00%)	SCC 55–70/(0.05%)	SCC 55–70/(0.10%)	SCC 70–85/(0.00%)	SCC 70–85/(0.05%)	SCC 70–85/(0.10%)
Cement	308	308	328	328	328	437	437	437
Fly ash	110	110	118	118	118	0	0	0
Microsilica	22	22	24	24	24	48	48	48
Coarse aggregate 20 mm	373	373	373	373	373	376	376	376
Coarse aggregate 10 mm	466	466	467	467	467	470	470	470
Fine aggregate (washed sand)	919	919	920	920	920	927	927	927
Water	174	174	163	163	163	161	161	161
Epsilone HP 540 (% by weight of cementitious materials)	4.0 (0.91%)	4.3 (0.98%)	4.0 (0.85%)	4.5 (0.96%)	4.5 (0.96%)	6.40 (1.32%)	7.50 (1.55%)	8.00 (1.65%)
Basalt fibers	0.0	2.8	0.00	1.4	2.8	0.0	1.4	2.8

.

2.3. Fresh Concrete Properties

The fresh concrete properties, including temperature, slump flow (as shown in Figure 2), and V-funnel measurements (as depicted in Figure 3), were evaluated for all mixtures both immediately after mixing and one hour later. The results indicated that the self-compacting concrete mixes remained within the acceptable limits throughout the testing period.

2.4. Mixing, Casting, Curing, and Transportation of Specimens

The mixing, casting, and curing of the specimens were conducted at the laboratories following these procedures:

• The dry materials (cement, fly ash, and aggregates) were mixed for 5 min to ensure uniform distribution. Basalt fibers were gradually added to the mix to prevent agglomeration, followed by the addition of water and superplasticizer. The total mixing time was 15 min.
• No vibration or compaction was required for this type of concrete.
• The specimens cast were standard cylinders, in accordance with ASTM C470 [30] and ASTM C192 [31] specifications:

  ○

  Four cylinders, each measuring 150 × 300 mm, were cast from each mix and subjected to compression testing.

  ○

  Four additional cylinders, also measuring 150 × 300 mm, were cast from each mix and tested for split tensile strength.

  ○

  Furthermore, four beams, each measuring 100 × 100 × 500 mm³, were cast from each mix and tested for flexural strength.

• After 24 h of casting, the specimens were de-molded and placed in moist curing conditions within the ready-mix company’s curing chambers, maintained at 23 ± 2 °C, for a period of 28 days.
• Upon completion of the 28-day curing period, the specimens were transported to the material lab for testing, following the transportation guidelines outlined in ASTM C31 [32].

The mixing equipment, along with the processes of mixing, casting, and curing the specimens, is depicted in Figure 4.

2.5. Hardened Concrete Properties

The compression and split-tension tests of the cylindrical specimens were conducted in accordance with ASTM C39 [33] and ASTM C496 [34] standards, respectively. Additionally, the beams were tested for flexural strength following ASTM C78 standard [35] . The test setup, along with the details of the tested cylinders and the observed flexural failures of the beams, is presented in

Table 4. Test setup and tested specimens.

	Calculated Using the Formula	Test Setup	Tested Specimens
Compressive Strength	$f_c={\displaystyle \frac{4P}{\pi d^2}}$ $f_c:$ compressive strength, MPa $P:$ failure load in compression, N $d:$ diameter of the cylindrical specimen, mm
Split-Tension Strength	$f_{ct}={\displaystyle \frac{2P}{\pi dl}}$ $f_{ct}:$ split-tension strength, MPa $P:$ failure load in split tension, N. $d:$ specimen diameter, mm. $l:$ specimen length, mm.
Modulus of Rupture	$f_r={\displaystyle \frac{PL}{b{d}^2}}$ $f_r:$  $\mathrm{modulus}\mathrm{of}\mathrm{rupture},\mathrm{M}\mathrm{P}\mathrm{a}$ $P:$  $\mathrm{maximum}\mathrm{applied}\mathrm{load}\mathrm{indicated}\mathrm{by}\mathrm{the}\mathrm{testing}\mathrm{machine},\mathrm{N}.$ $b:$  $\mathrm{average}\mathrm{width}\mathrm{of}\mathrm{specimen},\mathrm{m}\mathrm{m}.$ $d:$  $\mathrm{average}\mathrm{depth}\mathrm{of}\mathrm{specimen},\mathrm{m}\mathrm{m}.$ $\mathrm{L}:$  $\mathrm{span}\mathrm{length},\mathrm{m}\mathrm{m}.$

. Considering that the samples were cast under standardized conditions, the tests were conducted under consistent humidity levels and temperatures to prevent any potential variations in the behavior of basalt fibers due to environmental factors.

2.6. Microstructural Analysis

The research included a microstructural analysis of fractured concrete specimens to investigate the influence of basalt fibers on the concrete’s properties, utilizing scanning electron microscopy (SEM). To obtain high-resolution images, the surfaces of the samples were first coated with a gold-palladium layer to eliminate excess charges following their drying in a vacuum chamber. The scanning and imaging were subsequently performed using an FEI-Nova Nano SEM microscope manufactured by FEI Technologies Inc., Hillsboro, OR, USA.

2.7. ANOVA Analysis

This study sought to elucidate the impact of varying basalt fiber concentrations on the mechanical properties of sustainable self-compacting concrete (SCC), thereby contributing to the advancement of high-performance environmentally sustainable construction materials. The empirical investigation involved assessing concrete samples incorporating basalt fiber in proportions ranging from 0.05% to 1.5% by weight of cement across three SCC grades, with compressive strengths of 40–55 MPa, 55–70 MPa, and 70–85 MPa, respectively. These samples underwent testing under compression, flexure, and tension to evaluate their mechanical properties.

To analyze the effects of different basalt fiber concentrations on the compressive, flexural, and tensile strengths of the SCC mixes, a two-factor analysis of variance (ANOVA) with repetitions was employed. This statistical method assesses the influence of two independent variables—namely SCC grade and basalt fiber concentration—on a dependent variable (SCC mechanical properties) while accounting for repeated measurements, thereby controlling for individual differences that might otherwise obscure the effects of the experimental factors.

The key components of two-factor ANOVA with repetitions are as follows.

• Factors: the factors under investigation are basalt fiber concentration and SCC grade, each with multiple levels representing different concentrations and grades.
• Repetitions: the repeated measures aspect involves testing several specimens under identical conditions, thereby minimizing individual variability, reducing error variance, and enhancing the statistical power of the analysis.
• Main Effects: The analysis independently evaluates the main effects of each factor. The main effect of Factor A (basalt fiber concentration) represents its impact on the dependent variable (mechanical properties), averaged across the levels of Factor B (SCC grade), and vice versa.
• Interaction Effect: The two-factor ANOVA also examines the potential interaction effect between the two factors (basalt fiber concentration and SCC grade). An interaction effect occurs when the influence of one factor on the dependent variable (SCC mechanical properties) depends on the level of the other factor. Identifying interactions is crucial, as it reveals whether the factors interact synergistically or antagonistically in ways that cannot be predicted by examining each factor in isolation.
• Assumptions:

  ○

  Sphericity: This assumption requires that the variances of the differences between all combinations of related groups (i.e., the repeated measures) are equal. Violations of this assumption can lead to inaccurate conclusions; therefore, it is typically tested using Mauchly’s test of sphericity. If sphericity is violated, adjustments such as the Greenhouse–Geisser correction are applied.

  ○

  Normality: the data within each group should be approximately normally distributed.

  ○

  Independence: while measurements within each subject are repeated, the measurements between different subjects are assumed to be independent.

3. Results and Discussion

3.1. Fresh Concrete Results

The slump flow test is one of the most critical standard procedures in construction, used to assess the workability and consistency of freshly mixed concrete. This test measures the vertical displacement of a concrete specimen after it is placed in the Abrams cone and the cone is subsequently removed. The results of the slump flow tests for the concrete mixes are presented in

Table 5. Fresh concrete properties results.

Test		Grade 1		Grade 2			Grade 3
		SCC 40–55/0.00	SCC 40–55/0.10	SCC 55–70/0.00	SCC 55–70/0.05	SCC 55–70/0.10	SCC 70–85/0.00	SCC 70–85/0.05	SCC 70–85/0.10
Temperature (°C) ASTM C 1064 [36]	Immediate	24.0	25.0	24.0	24.2	23.7	24.8	25.1	22.5
	1 h	23.4	24.6	23.4	23.9	23.2	24.05	24.3	21.1
Slump flow (mm) ASTM C 143 [37]	Immediate	725	650	740	740	650	665	645	630
	1 h	690	600	695	685	610	620	595	580
V-funnel (sec.)	Immediate	4.11	4.80	4.83	4.88	5.70	5.00	6.41	5.00
	1 h	3.77	5.20	5.78	5.69	6.90	5.41	6.48	6.30

. As previously discussed, increasing the fiber concentration had a detrimental effect on the workability of self-compacting concrete (SCC) for a given concrete grade (i.e., the same mixture composition). Consequently, to maintain the required workability and achieve self-compacting properties, the dosage of superplasticizer was increased to the maximum permissible amount before the onset of bleeding as the basalt fiber concentration in the reinforced mixes was elevated.

For certain mixes, the attempt to increase the superplasticizer dosage to meet SCC workability requirements resulted in bleeding, leading to the rejection of those mixes. Consequently, mixes of the same grade with higher basalt fiber concentrations were not pursued. Across all mix grades (Grade 1, Grade 2, and Grade 3), bleeding was observed in mixes with fiber concentrations exceeding 0.1% by weight of concrete; therefore, higher fiber concentrations were not considered for further analysis.

The highest slump values were recorded for the control mixtures at each grade, where no fiber was added, indicating a relatively high level of workability and flowability in these concrete mixtures. However, the incorporation of basalt fibers at concentrations of 0.05% and 0.1% by weight resulted in a slight decrease in slump values compared with the control mixes, as illustrated in Figure 5 and Figure 6. This reduction can be attributed to several factors. The large surface area of the basalt fibers tends to absorb available water, leading to a marginal decrease in flow values, particularly when the fiber content exceeds 0.05%. Additionally, the presence of fibers may introduce friction between the fibers and the cement and aggregate during the mixing process. This interparticle friction hinders the movement of the mixture and increases its temperature, which may result in greater water evaporation during mixing, as depicted in Figure 7. Consequently, this increased friction, combined with the larger surface area of the fibers, necessitates a higher amount of water in the mixtures.

Similar conclusions were drawn by Ma et al. [19], who found that the workability of concrete reinforced with basalt fibers decreases as the size and length of the fibers increase. The consistency of these findings across different studies highlights the significant impact of basalt fibers on cementitious composites.

3.2. Mechanical Properties

The mechanical properties of the concrete samples are summarized in Table 5, which presents the experimental results of all batches after 28 days of curing, along with the percentage variation in compressive strength, splitting tensile strength, and flexural strength (modulus of rupture) relative to the control concrete samples (without basalt fibers) for each grade.

As shown in

Table 6. Fiber basalt impact on concrete mechanical properties.

Mix ID.	Compressive Strength (MPa)				Split-Tension Strength (MPa)				Flexural Strength (MPa)
	S1	S2	S3	S4	S1	S2	S3	S4	S1	S2	S3	S4
SCC 40–55/0.00	51.22	52.54	49.16	48.96	4.44	4.07	4.33	3.73	4.36	4.93	6.05	4.24
	Average				Average				Average
	50.47				4.14				4.895
SCC 40–55/0.10	51.99	52.62	53.66	53.14	4.79	4.19	4.67	3.74	5.47	5.91	6.56	6.18
	Average				Average				Average
	52.85				4.35				6.03
	Change (%)				Change (%)				Change (%)
	+4.71				+5.07				+23.30
SCC 55–70/0.00	66.56	67.89	62.76	61.8	3.87	4.32	4.10	3.36	5.21	5.74	5.56	6.12
	Average				Average				Average
	64.75				3.91				5.66
SCC 55–70/0.05	66.88	69.51	68.02	67.67	3.87	4.32	4.35	3.81	6.12	5.8	5.65	6.44
	Average				Average				Average
	68.02				4.09				6.00
	Change (%)				Change (%)				Change (%)
	+5.05				+4.60				+6.18
SCC 55–70/0.10	69.39	68.49	69.58	70.58	3.89	4.49	4.59	4.65	6.38	6.21	5.92	6.95
	Average				Average				Average
	69.51				4.41				6.37
	Change (%)				Change (%)				Change (%)
	+7.35				+12.79				+12.46
SCC 70–85/0.0	78.07	79.96	82.85	83.64	4.56	5.00	4.91	4.47	5.00	5.40	4.73	5.46
	Average				Average				Average
	81.13				4.73				5.15
SCC 70–85/0.05	81.9	82.19	81.63	81.95	5.05	5.09	5.33	4.52	8.33	7.4	7.59	7.48
	Average				Average				Average
	81.92				4.99				7.70
	Change (%)				Change (%)				Change (%)
	+0.97				+5.50				+49.51
SCC 70–85/0.10	84.68	86.62	84.05	85.04	5.95	5.84	5.58	6.00	9.71	8.89	8.84	7.54
	Average				Average				Average
	85.10				5.84				8.75
	Change (%)				Change (%)				Change (%)
	+4.89				+23.47				+69.90

, for the same concrete grade, the hardened concrete properties of mixes reinforced with basalt fibers were consistently higher than those of the unreinforced mixes. The enhancement in mechanical properties, such as compressive, tensile, and flexural strengths, increased with the concentration of basalt fibers. This improvement can be correlated with the fresh concrete properties discussed in the previous section, where it was observed that the slump flow of the reinforced mixes was lower than that of the unreinforced mixes for the same concrete grade. Furthermore, the slump flow decreased as the concentration of basalt fibers increased, not due to a reduction in water content, but rather due to challenges in fiber dispersion and the entanglement of fibers, which reduced workability. This entanglement likely contributed to the observed improvement in strength properties by enhancing the mechanical interlocking and overall density of the concrete matrix.

3.2.1. Compressive Strength

Figure 8 illustrates the impact of basalt fiber on the compressive strength of self-compacting concrete across all batches, as measured at 28 days, revealing varying levels of effectiveness.

In the first group, consisting of concrete with a grade of 40–55 MPa, the introduction of 0.10% basalt fiber resulted in an increase in compressive strength to 52.85 MPa, representing a 4.71% improvement compared with the reference mix of the same strength grade. Similarly, in the second group, with concrete mixtures designed for a grade of 55–70 MPa, the incorporation of 0.05% and 0.10% basalt fiber led to compressive strength increases of 5.05% and 7.35%, respectively, compared with the control mix.

In the third group, consisting of grades 70–85 MPa, the addition of 0.05% and 0.10% basalt fiber resulted in average compressive strengths of 81.92 MPa and 85.10 MPa, respectively. This corresponded to increases of 0.97% and 4.89%, respectively, when compared with the control mix for this grade.

These findings highlight the beneficial effect of basalt fiber on the compressive strength of self-compacting concrete, particularly as the fiber concentration increases across different strength grades.

The recorded findings demonstrate significant improvements in the average compressive strength of samples containing higher proportions of basalt fiber compared with those without fiber addition. It can be concluded that, as the percentage of basalt fiber content increased, there was a corresponding enhancement in the compressive strength of the self-consolidating concrete. These results suggest a substantial positive influence of basalt fibers on the microstructure of self-consolidating concrete, particularly as the fiber dosage is augmented. This improvement is likely due to the fibers’ bridging effect and their ability to fill voids within the concrete matrix [38].

These findings are consistent with the study by Zhou et al. [39], which also reported that the incorporation of basalt fiber into concrete significantly improved compressive strength and reduced cracking compared with control specimens.

3.2.2. Flexural Strength

The flexural strength of self-compacting concrete (SCC) reinforced with basalt fiber exhibited a trend similar to that of its compressive strength, as illustrated in Figure 9. The addition of basalt fiber significantly enhanced flexural strength. In the concrete group with grades 40–55 MPa, the flexural strength increased by approximately 23.3% at a fiber content of 0.10% compared with plain concrete. Notably, the type of failure observed in the reference concrete involved splitting into two pieces during the flexural strength test, whereas the fiber-reinforced concrete displayed the continuous development of multiple cracks [40].

In the second group, the enhancement in flexural strength reached 6.18% and 12.46% for batches containing 0.05% and 0.10% fiber, respectively, compared with the control mix. The third group demonstrated that the optimal basalt fiber dosage for concrete grades 70–85 MPa was 0.05% and 0.10%, resulting in the highest strength increases of 49.51% and 69.90%, respectively, compared with the control mix.

These findings suggest that the incorporation of basalt fiber into self-compacting concrete has a more pronounced impact on flexural strength than on other mechanical properties. The superior efficiency of basalt fiber in enhancing flexural strength is attributed to its high pull-out resistance, which is due to the fiber’s bridging effect. This effect significantly contributes to the improvement in concrete’s bending strength, as reported by Meddah et al. [41].

3.2.3. Splitting Tensile Strength

The tensile strength of the self-consolidating concrete (SCC) was determined following the ASTM C496 standard [34]. The inclusion of basalt fibers in the concrete mix has been shown to significantly enhance the splitting tensile strength, corroborating findings from prior research [42]. As illustrated in Figure 10, the results of the tensile strength tests were uniformly positive across all test groups. Specifically, the tensile strength exhibited a maximum increase when 0.10% basalt fiber content was incorporated, with improvements of 5.07%, 12.79%, and 23.47% for the first, second, and third groups, respectively, relative to their respective control mixtures. Additionally, a direct correlation was observed between the tensile strength and compressive strength of all the concrete mixes tested. This enhancement can be attributed to the bridging effect of the fibers, which contributes to a denser microstructure and an overall increase in strength [43]. The mechanical properties of basalt fibers, with tensile strengths ranging from 2600 to 4840 MPa and an elastic modulus between 80 and 115 GPa, further support their effectiveness [44]. Moreover, the optimal dosage of basalt fibers for tensile strength was found to be consistent with the optimal dosages for flexural and compressive strengths, as reported in Table 5. This consistency underscores the potential of basalt fibers to effectively improve both the mechanical properties and microstructural integrity of SCC. Based on the findings of this and previous studies, which are in agreement, the optimal fiber content for SCC mixtures is suggested to range between 0.05% and 0.10% [44,45].

3.3. Microstructural Analysis Results

Scanning electron microscopy (SEM) analysis is a powerful tool for examining the morphology of hydrated cement-based composites. To assess the impact of basalt fibers on the microstructure of self-consolidating concrete (SCC) at 28 days, SEM images were captured for samples from mixes 40–55/0.00 and 40–55/0.10, as shown in Figure 11 and Figure 12, respectively. The SEM image in Figure 12 reveals a densely hydrated cement matrix near the fiber surface, which may initially appear to indicate a bond between the fiber and matrix after 28 days. However, the fiber surface in Figure 12 is clean, suggesting that no direct chemical bonding occurred between the basalt fiber and the cement matrix. Instead, the dense hydration observed around the fiber likely improved the mechanical interaction through a friction-based mechanism rather than a strong chemical bond. Furthermore, the numerous uniformly dispersed micro-bubbles evident in the matrix contributed to an optimized pore structure by reducing the presence of large voids and gaps. This enhancement in the pore structure likely increased the overall density and integrity of the concrete matrix [44].

The SEM images further corroborate these observations, as the voids (depicted as black areas) in the 40–55/0.10 reinforced specimens are notably fewer compared with those in the unreinforced 40–55/0.00 specimens. This reduction in voids is likely attributable to the complementary effects of fly ash and basalt fibers. Fly ash is known to enhance the long-term properties of concrete through its pozzolanic reaction, which leads to a denser matrix. When combined with basalt fibers, the resulting concrete exhibits improved performance due to the synergistic properties of both materials. Fly ash contributes to the densification of the matrix over time through both filler effects and the gradual progress of pozzolanic reactions, which enhance the overall matrix density. The visible fly ash spheres in the SEM images may represent partially or unreacted particles at this stage of curing, as pozzolanic reactions require time to fully occur. Nonetheless, the incorporation of basalt fibers reinforces this matrix, improving the mechanical properties and crack resistance of the concrete by providing physical restraint and enhancing its toughness [16,46].

As discussed earlier, these improvements in microstructure facilitate the full realization of the self-compacting capacity of SCC, resulting in significant enhancements in both the rheological and mechanical properties of the mixtures. These findings align with those of Zhang et al. [47], who observed that high-strength concrete exhibits a better bonding interface with fibers, as evidenced by their analysis of the bonding interface between fibers and cement paste, which also improves the interface performance between the cement paste and aggregate. Consequently, basalt fibers are effective in enhancing the microstructure of fine-pore concrete [47].

This study demonstrates that incorporating basalt fibers can significantly enhance the mechanical properties of self-compacting concrete (SCC), including tensile and flexural strengths, without adversely affecting its workability. This provides a practical solution for producing high-performance concrete with improved durability and crack resistance.

In summary, the combination of fly ash and basalt fibers in concrete creates a synergistic effect that enhances the material’s overall performance. The pozzolanic reaction of fly ash contributes to the formation of additional calcium silicate hydrate (C-S-H) gel, which fills the voids within the concrete matrix, resulting in a denser and more durable structure. Concurrently, the addition of basalt fibers provides mechanical reinforcement, effectively bridging micro-cracks and enhancing the tensile strength, flexural strength, and crack resistance of the composite [48].

This synergy between fly ash and basalt fibers leads to a concrete material with improved mechanical properties, reduced permeability, and enhanced resistance to environmental stressors. Fiber-reinforced SCC mixes are well suited for high-performance structures, such as bridges, marine structures, and industrial floors, where superior mechanical properties and durability are essential. These mixes also align with environmentally sustainable construction practices by utilizing fly ash, a by-product material, and reducing reliance on conventional cement. Additionally, the enhanced crack resistance and long-term durability make this composite an ideal candidate for demanding applications, particularly in infrastructure subjected to extreme environmental conditions or dynamic loading [49].

3.4. ANOVA Analysis Test

Table 7. SCC sample compressive strength results.

SCC Grade	Basalt Fiber Percentage (%)
	0.0	1.4	2.8
Grade 2	66.56	66.88	69.39
	67.89	69.51	68.49
	62.76	68.02	69.58
	61.80	67.67	70.58
Grade 3	78.07	81.90	84.68
	79.96	82.19	86.62
	82.85	81.63	84.05
	83.64	81.95	85.04

,

Table 8. SCC sample flexural strength results.

SCC Grade	Basalt Fiber Percentage (%)
	0.0	1.4	2.8
Grade 2	3.87	3.87	3.89
	4.32	4.32	4.49
	4.10	4.35	4.59
	3.36	3.81	4.65
Grade 3	4.56	5.05	5.95
	5.00	5.09	5.84
	4.91	5.33	5.58
	4.47	4.52	6.00

and

Table 9. SCC sample tensile strength results.

SCC Grade	Basalt Fiber Percentage (%)
	0.0	1.4	2.8
Grade 2	5.21	6.12	6.38
	5.74	5.80	6.21
	5.56	5.65	5.92
	6.12	6.44	6.95
Grade 3	5.00	8.33	9.71
	5.40	7.40	8.89
	4.73	7.59	8.84
	5.46	7.48	7.54

summarize the compressive, flexural, and tensile strength results for SCC samples, respectively.

It is important to note that the ANOVA analysis (t-statistical test method) involves several key parameters, which include the sum of squares (SS), the degrees of freedom (df), the mean square (MS), the F-value, and the p-value. The sum of squares represents the variation between and within groups, while the degrees of freedom indicate the number of independent pieces of information. The mean square is calculated by dividing the sum of squares by the degrees of freedom. On the other hand, the F-value serves as the test statistic for the ANOVA test and measures the ratio of the variance between groups to the variance within groups. Meanwhile, the p-value indicates the probability of obtaining such results by chance alone. An EXCEL sheet was created for the ANOVA analysis and the significance levels were set at 0.05.

A small p-value, typically less than 0.05, suggests that the observed differences between groups are statistically significant. The two factors considered in the ANOVA analysis are SCC grades (Grade 2 and Grade 3) and basalt fiber percentages (0, 0.05, and 0.1).

Table 10. Compressive strength ANOVA analysis results.

ANOVE Two-Factors with Replication
Source of Variation	SS	df	MS	F	p-Value	F-Crit
SCC Grade	1402.2460	1	1402.2460	455.7085	3.12 × 10−14	4.4139
Basalt Fiber	76.2517	2	38.1259	12.3903	0.000413	3.5546
Interaction	6.4204	2	3.2102	1.0433	0.372656	3.5546
Within	55.3872	18	3.0771
Total	1540.3050	23

provides an ANOVA analysis summary for compressive strength results. The breakdown of the findings is as follows.

• The ANOVA results indicate a significant effect of the SCC grade factor. The F-value is 455.71, and the corresponding p-value is always 0, which is much lower than the significance level (usually 0.05). This suggests a statistically significant difference in the means between Grade 2 and Grade 3.
• There is also a significant effect of the basalt fiber quantity. The F-value is 12.39 with a p-value of 0.0004, indicating a significant difference between basalt fiber quantities (0, 1.4, and 2.8).
• The interaction between the grade and the basalt fiber factors is not statistically significant. The F-value is 1.043, and the p-value is 0.3727, which is greater than 0.05.
• In summary:
  • There is a significant main effect for both grade and basalt fiber factors, indicating that both factors independently influence SCC compressive strength.
  • No significant interaction effect is found, suggesting that the impact of one factor does not depend on the levels of the other factor.

Similarly,

Table 11. Flexural strength ANOVA analysis results.

ANOVE Two-Factors with Replication
Source of Variation	SS	df	MS	F	p-Value	F-Crit
SCC Grade	6.6993	1	6.6993	67.7797	1.62 × 10−7	4.4139
Basalt Fiber	2.7352	2	1.3676	13.8367	0.000229	3.5546
Interaction	0.4428	2	0.2214	2.2398	0.135318	3.5546
Within	1.7791	18	0.0988
Total	11.6563	23

presents the ANOVA analysis for the flexural strength results. Below is a detailed summary of the findings.

• The two factors considered in the analysis are grades and basalt fiber values (0, 1.4, and 2.8).
• The results indicate a significant effect of the grade factor. The F-value is 67.78, and the p-value is almost zero, which is far below the standard significance level (0.05).
• There is also a significant effect of the basalt fiber factor. The F-value is 13.84 with a p-value of 0.0002, indicating significant differences between the column levels (0, 1.4, and 2.8).
• The interaction between the grade and column factors (interaction) is not statistically significant. The F-value is 2.24, and the p-value is 0.135, which is greater than 0.05. This indicates no significant interaction effect between the grade levels and basalt fiber levels on the SCC flexural strength variable.
• In summary:
  • Both the grade and basalt fiber factors show significant main effects, indicating that they independently influence the flexural strength variable.
  • The lack of a significant interaction effect suggests that the impact of one factor does not depend on the levels of the other factor.

Furthermore,

Table 12. Tensile strength ANOVA analysis results.

ANOVE Two-Factors with Replication
Source of Variation	SS	df	MS	F	p-Value	F-Crit
SCC Grade	8.4847	1	8.4847	32.6082	2.05 × 10−5	4.4139
Basalt Fiber	19.2731	2	9.6365	37.0349	4.17 × 10−7	3.5546
Interaction	9.1273	2	4.5637	17.5389	5.93 × 10−5	3.5546
Within	4.6836	18	0.2602
Total	41.5687	23

outlines the ANOVA analysis for the tensile strength results. Below is a detailed summary of the findings:

• The results indicate a significant effect of the grade factor. The F-value is 32.61, and the p-value is very close to zero, which is well below the standard significance level (0.05).
• There is also a significant effect of the column factor. The F-value is 37.03 with a p-value of a value very close to zero, indicating significant differences between the basalt fiber levels (0, 0.05, and 0.1).
• The interaction between the grade and basalt fiber factors (interaction) is statistically significant. The F-value is 17.54, and the p-value is very close to zero, which is lower than 0.05.
• In summary:
  • There are significant main effects for both the grade and basalt fiber factors, indicating that both factors independently influence the tensile strength of SCC.
  • The significant interaction effect suggests that the impact of one factor depends on the levels of the other factor, meaning the effect of grades on the dependent variable varies depending on the column levels.

It is worth noting that the interaction between the SCC grade effect and the effect of basalt fibers’ dosage is found significant only for the tensile strength, while it is negligible for the compressive and flexural strengths. This may be justified as the effect of basalt fibers is mainly related to the cracks’ bridging and such cracks are propagating much faster under tension. The tension cracks will be less for higher grades of concrete and the effect of the basalt fibers’ dosage will be dependent on the extent of such cracks that are linked to the mix grade.

4. Conclusions

The primary objective of this experimental study is to investigate the influence of basalt fibers on the rheological and mechanical properties, along with the microstructural behavior of self-compacting concrete (SCC) containing fly ash. The findings from the experimental tests lead to the following conclusions:

• The incorporation of basalt fibers adversely affected the rheological properties of fresh SCC mixtures. Specifically, the slump flow values of concrete decreased as the volume fraction of basalt fibers increased.
• While the addition of basalt fibers had a minimal impact on the compressive strength of SCC specimens, it significantly enhanced both the tensile strength and the flexural strength.
• The optimal ratio of basalt fibers was identified in the SCC70-85/0.10 mix specimens, which exhibited a 69.90% increase in flexural strength and a 23.47% increase in splitting tensile strength compared with the control specimen.
• Increasing the dosage of basalt fibers in concrete mixes led to a reduction in efficiency. Higher fiber content resulted in the formation of additional voids within the concrete matrix, which introduced air and subsequently diminished the concrete’s effectiveness.
• Scanning electron microscope (SEM) analysis revealed an improvement in the microstructural density of the concrete matrix containing basalt fibers. This enhancement was attributed to the bridging effect of the fibers and their ability to fill pores.
• Basalt fibers enhanced the overall performance of concrete, offering a competitive alternative to traditional fibers due to their superior mechanical and chemical properties and their cost-effectiveness.
• A two-factor analysis of variance (ANOVA) with repetitions was conducted to evaluate the effects of varying basalt fiber concentrations on the compressive, flexural, and tensile strengths of SCC mixes. This method examined the impact of two independent variables—SCC grade and basalt fiber concentration—on SCC mechanical properties, accounting for repeated measurements to control individual variability. The ANOVA results indicated significant main effects for both SCC grade and basalt fiber concentration, demonstrating that each factor independently affected the compressive and flexural strengths of SCC. However, it was found that there was a significant interaction between the effect of SCC grade and basalt fiber concentration on the tensile strength results.

Future Recommendations

Future research could explore a broader range of fiber concentrations, alternative fibers, and long-term durability under varied environmental conditions. Additionally, practical implementation of basalt fibers requires optimizing mix design and considering cost-effectiveness. Further studies could focus on developing guidelines for applying basalt fiber-reinforced SCC in real-world projects, including recommendations on mix proportions, construction techniques, and performance evaluation.

The authors propose future investigations that incorporate mechanical testing at earlier and later stages to examine the strength gain patterns and the durability performance of fiber-reinforced SCC. This approach will offer a more comprehensive understanding of the role of fibers in influencing the time-dependent mechanical properties of concrete.