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Article

Mechanical Properties of Basalt Fiber-Reinforced Coal Gangue Coarse Aggregate-Fly Ash Geopolymer Concrete

by
Zheng Yang
1,* and
Xianzhang Ling
2
1
College of Civil Engineering and Architecture, Harbin University of Science and Technology, Harbin 150080, China
2
School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(16), 2860; https://doi.org/10.3390/buildings15162860
Submission received: 7 July 2025 / Revised: 2 August 2025 / Accepted: 5 August 2025 / Published: 13 August 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

Excellent mechanical properties are a prerequisite for the widespread application of different types of concrete in practical engineering. However, when coal gangue (CG) is used as coarse aggregate (CA) and geopolymer cement is used as auxiliary cementitious material, while reducing the demand for ordinary cement and industrial waste emissions, it has a negative impact on mechanical performance. Therefore, in response to the data gap in the study of mechanical properties of coal gangue coarse aggregate-fly ash geopolymer concrete (CG-FA-GPC), inspired by a large number of research results on fiber-reinforced concrete, this study uses basalt fiber (BF) as a reinforcing material to investigate the enhancing effect of BF on the mechanical properties of CG-FA-GPC. We selected compressive strength, flexural strength, splitting tensile strength, and stress–strain curve as evaluation indicators to compare and analyze the mechanical properties of ordinary concrete, CG-FA-GPC, and basalt fiber-reinforced coal gangue coarse aggregate-fly ash geopolymer concrete (BF-CG-FA-GPC), and to explore the reinforcement effect of BF. The results showed that with the increase in CG substitution rate, the compressive strength, flexural strength, and splitting tensile strength of CG-FA-GPC significantly decreased. A 100% CG substitution reduced the compressive strength, flexural strength, and splitting tensile strength of CG-FA-GPC by 34.5%, 43.4%, and 31.8%, respectively. The stress–strain curve reveals the dual effects of BF on the strength enhancement and deformation modification of CG-FA-GPC. With the increase in BF content, the three mechanical strengths of CG-FA-GPC show a pattern of first increasing and then decreasing, and the optimal BF content is 0.4% (volume fraction). This experiment lays the foundation for promoting research on the mechanical properties and durability of different fiber-reinforced CG-FA-GPC, advancing the feasibility of its large-scale engineering applications.

1. Introduction

Global warming and its associated dangerous impacts, such as droughts, hurricanes, tsunamis, floods, glacier melting and species loss et al. are the biggest challenges faced by modern human society [1]. In the field of civil engineering and construction, the high demand for reinforced concrete is an important factor causing high CO2 emissions. This is because the CO2 generated during the concrete production process accounts for 5–8% of the total global CO2 emissions, with the majority (95%) of CO2 emissions coming from cement manufacturing. The production of one ton of ordinary Portland cement (OPC) will release nearly one ton of CO2, accompanied by the release of harmful gases SO2 and NOx [2,3]. Therefore, it is necessary to seek new materials, new methods, and new processes to reduce cement consumption in order to reduce gas emissions from various aspects, which is the responsibility of people all over the world. Many researchers have made tremendous efforts to search for environmentally friendly alternatives to cement and steel bars with low pollution and energy consumption. The extensive research on plant-based, mineral-based and animal-based natural fibers [4,5,6,7] and different supplementary cementitious materials (SCMs) [8,9,10,11,12,13,14] is in line with these goals. For example, Musa Adamu et al. [10,11] used high-volume fly ash (HVFA) as SCMs, and designed experimental mixtures using response surface methodology to study the effects of plastic waste and graphene nanoplatelets on the mechanical and durability properties of concrete. Pattharaphon et al. [12] designed high-strength concrete using coal bottom ash (CBA) as cement and fine aggregate replacements, and evaluated the properties of concrete through compressive strength, modulus of elasticity, autogenous shrinkage, and heat evolution to determine the optimal percentage of CBA.
Li et al. reported that replacing OPC as a binder for concrete composite materials can reduce greenhouse gas emissions by nearly 80% in the construction industry, which is an important component of achieving green and environmentally friendly building tasks [15]. Among the numerous alternative options, geopolymer, as a new entrant in the field of building materials, has received great attention from the scientific community. The concept of geopolymer has a long history and belongs to non-metallic materials in amorphous to semi-crystalline states. A large number of mineral waste residues and by-products of various types (blast furnace slag, fly ash, silica fume, metakaolin, rice husk ash, and wheat straw ash, etc.) can be used as raw materials for its production. After careful design, geopolymer cement can offer advantages of low cost, low energy consumption, and low emissions. The internal components of industrial waste or by-products endow geopolymer cement with excellent mechanical properties, acid and alkali resistance, fire resistance, and high temperature resistance [16,17,18,19,20].
So far, geopolymer cement, which has similar performance to OPC but obvious environmental advantages, has been regarded as the most likely alternative to OPC. The Pitcha Jongvivatsakul team had achieved some results in studying the use of industrial waste and by-products in geopolymers [21,22,23,24,25]. As they found, the addition of rice husk ash (RHA) can improve the uniformity of the microstructure of high calcium fly ash geopolymer prepared containing borax, thereby improving the compressive strength and flexural strength of geopolymer mortar containing hydrated or anhydrous borax [21]. In addition, they also found that using rice husk ash instead of nano-SiO2 to act on recycled aggregate high-calcium fly ash geopolymer concrete can effectively improve the strength of recycled aggregate geopolymer concrete (RAGC), which is due to the improved microstructure and denser matrix. However, the addition of materials rich in SiO2 has an adverse effect on the post-fire residual strength of geopolymer concrete made from recycled polymers [22]. When natural fine aggregates were partially replaced by granite waste, the slump flow rate of fresh hardened polymer concrete increased with the increase in the proportion of granite waste in the mixture. When granite waste was used to replace the natural sand by up to 50%, geopolymer concrete exhibited poor post-fire residual strength, especially when using oven-dried aggregates [23]. Hafiz et al. [24] investigated the effect of graphene nanoplatelets (GNPs) on the engineering performance of fly ash-based geopolymer concrete containing crumb rubber (CR). CR replacement significantly reduced the mechanical properties of geopolymer concrete, but the addition of GNPs can alleviate the negative impact of CR and significantly improve the mechanical properties of geopolymer concrete. Sani et al. [25] conducted a multiscale investigation on the impact of recycled plastic aggregate (RPA) replacing fine aggregates on the performance of one-part alkali-activated mortar (OPAAM). The mechanical properties of OPAAM decreased with the increase in RPA content in the mixture, but the mixture with 10% RPA exhibited the highest compressive strength, approximately 46 MPa.
The resource utilization of coal gangue (CG), fly ash (FA), and other industrial wastes poses a more severe challenge to China, which takes coal as the main energy structure, and it is necessary to seek multiple ways and aspects of the resource utilization of coal industrial wastes. In recent years, the recycling of CG for power generation and building materials is one of the effective ways of its resource utilization, among which many research results have been obtained as the coarse aggregate of concrete [26,27,28]. The properties and composition of CG are similar to those of natural gravel. Through a systematic production process, a new type of artificial gravel can be obtained, which can be used as CA in concrete materials [26]. Wang et al. [27] found that as the replacement rate of CG coarse aggregate increases, the elastic modulus and dry shrinkage rate of the corresponding concrete approximately decrease linearly. However, the magnitude of the decrease in concrete performance is closely related to the type and dosage of CG. Therefore, studying the dosage of CG used as coarse aggregate in different types of concrete has guiding significance for the application of this type of concrete. According to reports, China is the country with the largest production of FA, and the utilization rate has basically reached 67% [29]. The rise in geopolymer SCMs has also provided opportunities for the further expansion and utilization of FA. Related studies have shown that the incorporation of raw materials such as FA and slag into geopolymer manufacturing is attracting global interest, and some components of these industrial wastes determine the final performance of geopolymers [30,31]. For example, compared to OPC concrete, Ghafoor et al. [32] found that FAGPC can exhibit higher compressive strength under appropriate mix proportions and curing conditions [32]. In addition, the particle uniformity of FA can make the microstructure of its geopolymer composite material denser, thereby exhibiting better resistance to sulfate corrosion. In concrete, the quality of FA as a substitute for cement is basically limited to 15–20% of the total cementitious material [33]. Therefore, due to the consideration of increasing the utilization rate of CG and FA in the field of concrete materials, the modification of fly ash-based geopolymer concrete and the study of CG as CA are supplements to the blank demand.
Compared with traditional ordinary cement structures, GPC exhibits excellent sustainability. However, the lack of tensile and bending load resistance of concrete systems is the main obstacle to practical application problems, as this can lead to severe disintegration and sudden collapse of structural components without prior warning [34]. There is no doubt that the same issue of brittleness is also a research hotspot in GPC. Most research has successfully transformed the failure mode of concrete into ductile failure using industrial synthetic fibers in brittle geopolymer media [35,36,37,38,39,40]. For example, Peem et al. [38] used micro carbon fiber (CF) as a reinforcing material to improve the mechanical properties of fly ash geopolymer containing fine recycled concrete aggregate (RCA). The addition of CF can increase the nucleation sites of the geopolymerization reaction and the bridging effect of fibers. The incorporation of 0.2% CF can result in geopolymer mortar containing 100% RCA with higher compressive and splitting tensile strengths. In addition, Peem et al. [39] also found that polypropylene (PP) fibers can effectively improve the flexural performance of alkali-activated fly ash (AAF) concrete containing granite industrial waste (GW). The fiber-reinforced AAF concrete containing GW exhibited extensive cracking in the core after fire, with the residual compressive strength of 25–39% after 30 min of fire exposure. Pattharaphon et al. [40] evaluated the compressive and flexural strength of alkali-activated fly ash mortar containing superabsorbent polymers (SAPs) and polypropylene (PP) fibers. PP significantly improved the flexural capacity of cement mortar, while the compressive strength began to decrease after 28 d. Compared with samples without SAPs, samples produced using SAPs can effectively maintain residual compression and flexural strength.
However, when considering the selection of industrial fiber-reinforced GPC, special attention should be paid to the raw material development, environmental cost, self-performance, and the compatibility between fiber and matrix of industrial fiber [7]. The extensive use of industrial fibers is undoubtedly contradictory to global efforts to reduce greenhouse gas emissions. In addition, in areas where recycling and reuse are not feasible, most industrial fibers are difficult to handle, which means that the economic and environmental costs at the end of the service life of corresponding concrete structures are increased. This is also contrary to the goal of maximizing the recycling and utilization of construction demolition waste to reduce the negative impact of the construction industry on the environment. Therefore, with increasing attention to environmental issues, building materials researchers are considering using natural fibers (NFs) as reinforcing materials instead of industrial fibers for cementitious composite materials [7,41,42,43,44] to increase their ductility and tensile stress.
NF sources include plants, animals, and minerals. Plant-based natural fibers (such as coconut fiber, bamboo fiber, etc.) have many limitations, such as being significantly affected by growing environments, exhibiting large mechanical performance variability, insufficient alkali resistance (prone to degradation in high-alkali cement-based environments), and weak interfacial bonding with geopolymer matrices. Therefore, researchers have turned their attention to natural mineral fibers, among which basalt fiber (BF) is a typical example. BF is produced by melting and drawing basalt rock, making it an environmentally friendly material with high-temperature resistance, corrosion resistance, high hardness, and excellent wear resistance. In the field of construction engineering, it is considered a promising potential alternative to carbon fiber and glass fiber [30]. Compared in depth with synthetic industrial fibers, BF causes less environmental pollution. More importantly, BF raw materials are abundant, and the energy consumption of the melting and drawing process is much lower than that of synthetic fiber production. At the same time, BF also possesses unique advantages that are irreplaceable by other industrial fibers, namely its good chemical compatibility with geopolymers (rich in silicon and aluminum) due to its natural volcanic rock composition. It is considered a potential substitute for carbon fiber and glass fiber in the field of construction engineering in the future [41,42,43,44]. Zeng et al. [42] found that under the same volume fraction, BF had a better strengthening effect on the compressive toughness of concrete materials than polypropylene fiber. Punurai et al. [43] found that with the increase in BF content, the flexural strength of the mixed geopolymer slurry increased, and the drying shrinkage rate decreased. When the content of BF is 40%, its 28 d bending strength can be increased by 64%. Yang et al. [44] reported that adding 6 kg/m3 of BF to concrete can delay early cracking and reduce lateral strain of the concrete. Therefore, for concrete with different usage requirements, the optimal amount of BF incorporation requires extensive research. As Bakytzhan et al. [45] adopted a method combining experimental and numerical techniques based on the COMSOL (version 6.2) finite element method (FEM), they ultimately determined that a fiber content range of 0.25% to 0.5% can enhance material performance.
Based on the above literature, we deeply understand that researchers in the relevant field have taken a positive attitude towards reducing the demand for ordinary Portland cement and reducing greenhouse gas emissions by replacing some OPC with in situ polymer cementitious supplementary materials, and actively call for systematic research on geopolymer cementitious materials. However, when blocky coal gangue and other waste materials are used as CA for concrete, while realizing the resource utilization of industrial waste, their material deterioration has a negative impact on the mechanical properties of concrete. However, inspired by numerous research results on fiber-reinforced concrete, researchers have found that fibers can also be applied to enhance the performance of geopolymer concrete caused by replacing coarse aggregates with CG. In addition, with the increasing awareness of environmental protection, the selection of natural fiber-reinforced materials has become a research hotspot. However, the above literature review also indicates that the current research on the performance of geopolymer concrete reinforced with different natural fibers and different coarse aggregate substitutes is mainly focused on experimental research. However, in order to achieve practical engineering applications of corresponding concrete materials, a large number of multi-type fiber-reinforced geopolymer concrete combination experimental research work is urgently needed to fill the relevant data gaps for the writing of relevant specifications. Therefore, this study uses BF as reinforcing materials to conduct compressive tests, flexural tests, splitting tensile tests, and axial compressive tests. The compressive strength, flexural strength, splitting tensile strength, and stress–strain curve are used as evaluation indicators to compare and analyze the mechanical properties of ordinary concrete, CG-FA-GPC, and BF-CG-FA-GPC. The influence of CG and BF on the mechanical properties of FA-GPC is discussed in order to provide positive data supplementation for the study of natural BF reinforced FA-GPC.

2. Experimental

2.1. Materials and Mixture Proportions

The cement was Conch brand P·O 42.5 OPC produced by Shaanxi Qinling Cement Company, and its physical properties and chemical composition are shown in Table 1. Fly ash (Grade I) was produced from HSBC New Materials Co., Ltd. in Panzhihua, China, and the sieve allowance was 16% by a 5 μm square hole. The basic properties and main components of FA are shown in Table 2. The alkaline excitation solution was prepared by Na2SiO3 and NaOH. Na2SiO3 (8.5% Na2O, 26.5% SiO2, and 65% H2O, wt%) was a colorless viscous liquid produced by Henan Bingrun Casting Materials Co., Ltd. in Gongyi, China, with a modulus and Baume of 3.12 and 40° Bé, respectively. NaOH was produced at the Kelong Chemical Reagent Factory in Chengdu, China. It was a solid sheet-like particle with purity ≥ 98.0% and was used to regulate the modulus of Na2SiO3. NaOH solution should be prepared at least 24 h before pouring because it will release a lot of heat in water and cause the temperature of the activator to be too high, resulting in temperature cracks in the specimen. The CA consisted of continuous graded crushed stone and CG with a size of 5–20 mm. The CG was produced in Fuyang City, Anhui Province, and was crushed by a jaw crusher and manually screened, as shown in Figure 1a. The specific parameters are shown in Table 3. The fine aggregate was selected from ordinary zone II river sand (The particle size is 0.35–0.5 mm), and the performance indicators are shown in Table 4. BF was purchased from Changzhou Bochao Engineering Materials Co., Ltd., as shown in Figure 1b. The physical and mechanical parameters were shown in Table 5, where d was the diameter of the monofilaments, l was the stub length, ρ was the density, E was the Young’s modulus, ft was the tensile strength, and δ was the ultimate elongation. The water-reducing agent was the FDN high-efficiency water-reducing agent, purchased from Shaanxi Jingcheng Water-Reducing Agent Engineering Company, and its performance parameters are shown in Table 6. The water was ordinary tap water from the laboratory.
According to references [46,47], NaOH solution (12 mol/L) was mixed with Na2SiO3 solution in a mass ratio of 1:2.5 to prepare an activator. A fly ash-based polymer concrete (FA-GPC) mix was designed with CG replacing part of the crushed stone as CA, and CG coarse aggregate was used to replace the crushed stone in equal amounts of 0%, 40%, 70% and 100% (mass fraction). Meanwhile, prepared ordinary concrete C0 according to the same liquid-solid ratio and used it as a control group. Finally, taking CG-FA-GPC concrete with a CG substitution rate of 40% as an example, the BF content to cast basalt fiber-coal gangue coarse aggregate-fly ash-based polymer concrete (BF-CG-FA-GPC), the BF content was adjusted to 0%, 0.2%, 0.4% and 0.6% (volume fraction). The detailed mix proportions of different groups of concrete are shown in Table 7. For the convenience of marking and identification during testing, the specimens were uniformly numbered FGC-xC-yB, where xC and yB represent CG and BF content, respectively.

2.2. Preparation of Test Specimens and Testing Methods

The preparation process of the concrete specimen was as follows: after the weighed cementing material, sand and stone were slowly stirred in the mixing pot for 2 min. The mixing water was poured and stirred slowly for 5 min to wet the material fully. Then, the cooled exciting solution was added to the mixing pot and stirred slowly for 2 min, and then quickly stirred for 1 min. It should be noted that to ensure the uniform dispersion of BF in the fresh mixture, the BF must be loosened beforehand. Next, the BF in 2–3 increments was gradually added into the mixing pot containing the weighed cementitious materials, sand, and aggregate. After mixing for 2 min, I added the mixing water for wet mixing to prevent the fibers from clumping prematurely due to the liquid. Throughout the mixing process, I visually assessed the dispersion of BF. If obvious fiber clumps were observed on the surface of the fresh mixture, I extended the mixing time by 1–2 min. At room temperature, the mixture was poured into the corresponding mold and vibrated on the vibrating table. I covered the plastic film and let it stand for 24 h before demoulding, and then put it in a water tank with a water temperature of (23 ± 2) °C for curing until the predetermined age.
With reference to standards such as GB/T 50081-2019 “Standard for Test Methods of Physical and Mechanical Properties of Concrete” [48] and GB/T 50082-2009 “Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete” [49], the testing plan had been formulated. The workability of fresh concrete was determined by the slump test. The slump cylinder used meets the standard specifications, with a diameter of 100 mm on the upper part and 200 mm on the lower part, and a height of 300 mm. The YAS-5000 microcomputer-controlled electro-hydraulic servo pressure testing machine (maximum pressure of 2000 kN) was used for concrete-related mechanical tests. The compressive test and splitting tensile test used cubic specimens with a side length of 100 mm, with loading rates of 0.5 MPa/s and 0.05 MPa/s, respectively. The bending strength test was carried out using prismatic specimens with dimensions of 100 mm × 100 mm × 400 mm, which were uniformly and continuously loaded at a rate of 0.05 MPa/s. The strength test consisted of three specimens in a group, and the average value was calculated as the representative strength value. Prismatic specimens with dimensions of 150 mm × 150 mm × 300 mm were selected for axial compressive test (stress–strain curve), and the DH3816 dynamic acquisition instrument, produced by Jiangsu Donghua Testing Technology Co., Ltd. in Taizhou, China, was used for data acquisition. Each set of tests includes three parallel specimens (total n = 3). After the test, the curves of the three specimens should be checked for completeness and reasonableness. If the shapes of the three curves are similar (e.g., peak stress, strain, and post-peak trends are close), the data can be considered reliable. For the same strain value, the arithmetic mean of the stresses from the three specimens is taken to generate the final curve. If the differences are significant (e.g., peak stress varies by more than 15%), it may be due to experimental errors (e.g., eccentric loading, instrument malfunction) causing obvious deviations in the data. In such cases, the outlier data should be discarded, and additional tests should be conducted. The loading mode was the hierarchical loading of load control and displacement control combined: the first stage was load control, with a loading rate of 3 kN/s~5 kN/s, loading to 100 kN (not exceeding 40% of the peak load); the second stage was load control, with a loading rate of 0.3 kN/s~0.5 kN/s, loading to the peak load of the specimen about 80%; the third stage was displacement control, with a loading rate of 0.02 mm/min. When the descending section was loaded to 85% of the peak stress, the loading continued until the specimen was damaged.

3. Results and Discussion

3.1. Slump Analysis

As shown in Table 8, the slump of each group of FGC-xC-yB concrete is higher than C0. During the mixing process, more water forms a layer of water film on the surface of the aggregate, increasing the fluidity of the interface between the aggregate and the geopolymer slurry, thereby increasing the slump of FGC-xC-yB. The slump of FGC-xC decreases with the increase in CG aggregate substitution rate because CG itself has larger pores and stronger water absorption. During the mixing process, as CA, it will absorb water from the mortar, which is equivalent to reducing the effective water–cement ratio, thus reducing the fluidity of concrete. Therefore, it is expected that more high-range water-reducing admixtures will be required to achieve the same slump [50,51]. This is because the CG coarse aggregate has obvious edges and protrusions on its surface, making it difficult for the slurry to flow over the aggregate surface, thus leading to a decrease in slump. Other possible reasons include differences in other characteristics of coal gangue and fly ash, such as type, source, and purity grade, which all affect the demand for high-range water-reducing admixtures [52]. The slump of FGC-xC-yB group concrete also decreases with the increase in BF content. The reason is that after BF is added to the concrete, it is wrapped by mortar and forms a fiber network in the concrete, which reduces the fluidity of the concrete and leads to a decrease in slump. In addition, the physical presence of fibers can also restrict the movement of aggregates and increase the viscosity of the mixture, thereby impairing the fluidity of the mixture [51]. Therefore, the reduction in slump poses challenges to construction techniques, such as extending vibration time, adjusting pumping parameters, or template design, and in practical engineering, layered pouring techniques can be used to alleviate the negative impact of BF on slump.

3.2. Mechanical Property

Table 9 lists the compressive strength (fcu), flexural strength (ff), and splitting tensile strength (ft) of all concrete specimens under each mix proportion. The median value in the table is the average of the test values of three specimens in each group. Compared to OPC concrete (C0), the mechanical strength of FGC-xC-yB concrete in each group has been improved under the same conditions and age. The reason for this is that the setting and hardening process of geopolymer cementitious materials is a reaction process in which the silicon oxygen bonds and aluminum oxygen bonds in the raw materials break and then recombine under the action of alkaline catalysts. This process makes the bonding force of GPC to aggregates much higher than that of silicate concrete. This finding is basically consistent with the research conclusions of Talha Ghafoor [32] and Mousavinezhad et al. [51]—the basic mechanical strength of concrete prepared with an appropriate amount of fly ash-based geopolymer as a supplementary cementitious material will increase. Therefore, in GPC, the interface transition zone is no longer a weak area, and the strength of aggregates can be better utilized, thereby exhibiting better mechanical properties in GPC. In addition, the density of fly ash-based polymer cementitious materials is lower than that of OPC, and there may also be microaggregate reactions during its setting and hardening process, thereby increasing the density of the rough structure network in the interface area and reducing the microdefects in the transition area. Therefore, the mechanical strength of FA-GPC is higher than that of OPC concrete.
The fcu data of FGC-xC group shows that with the increase in CG coarse aggregate substitution rate, the fcu of the cubic specimens of FA-GPC shows a significant decrease at different ages. As shown in Figure 2a, among all FGC-xC groups, the FGC-0C group and FGC-100C group had the highest and lowest 7 d strength, with values of 55.79 MPa and 44.94 MPa, respectively, which increased by 24.1% compared to the lowest. The highest and lowest 28 d strengths were also found in the FGC-0C group and FGC-100C group, with values of 66.6 MPa and 49.50 MPa, respectively, with a maximum increase of 34.5% compared to the lowest. This result is consistent with numerous studies [27,28,53] on coal gangue coarse aggregates, which is attributed to the inherent material inferiority of CG. However, the fcu of the FGC-100C group at different ages is higher than C0, which is the result of the effect of FA after excitation. We must admit that the internal structure of CG is loosely layered, with strong water absorption and low strength, making it prone to fracture. In addition, there are certain cavities at the bonding interface between cementitious materials and CG coarse aggregates. As the amount of CG coarse aggregate increases, its structural drawbacks will become more apparent, directly reflected as the compressive strength of FGC-xC group concrete decreases with the increase in CG substitution rate. However, when considering the addition of BF to improve the brittleness of concrete, the fcu of FGC-xC-yB shows a change in first increasing and then decreasing with the increase in BF dosage, and the fcu at a dosage of 0.4% is not significantly different from that of the FGC-0C group, as shown in Figure 2b. It can be seen that adding an appropriate amount of BF improves the compressive strength of concrete, which may be due to the improvement of microstructure densification. However, when the fiber volume fraction exceeds a certain threshold, problems such as uneven fiber dispersion and agglomeration may occur, resulting in a decrease in the compactness of the concrete and a gradual decrease in fcu.
The ff and ft are both used to evaluate the crack resistance and tensile strength of concrete. Table 9 shows that the overall variation pattern of the ff and ft of FGC-xC-yB is similar. Considering the core content and experimental cost of this study, only the FGC-40C group with excellent mechanical properties was subjected to BF reinforcement. The 28 d ff and ft of each group of FGC-xC concrete (as shown in Figure 2c) show a very significant and regular decrease, while for FGC-40C-yB, their ff and ft show a trend of first increasing and then decreasing with the increase in fiber content, as shown in Figure 2d. Compared to FGC-0C, the ff of FGC-40C and FGC-100C decreased by 34.0% and 43.4%, respectively, and the ft decreased by 13.3% and 31.8%, respectively. The ff and ft of FGC-40C-0.4B are both the highest, at 4.56 MPa and 1.82 MPa, respectively. This is consistent with the findings of Punurai et al. [43], Yang et al. [44], and Maleki et al. [54], indicating that the appropriate incorporation of BF can effectively enhance the tensile strength and splitting tensile strength of concrete materials. It can be seen that the addition of 0.4% (volume fraction) BF has a positive effect on enhancing the crack resistance and tensile strength of FGC-40C.
What we are quite clear about is that the ff and ft of concrete largely depend on the interface transition zone strength [54]. However, the adverse effects of CG defects on the ff and ft of concrete are more pronounced than fcu. In addition, the ff and ft of C0 are higher than those of GPC, which may be related to the three-dimensional layered structure of GPC. Layered stacked structures can withstand significant pressure but are more prone to tearing and cracking when subjected to tension, resulting in lower ff. However, the difference in ff and ft between the FGC-40C-0.4B group and the FGC-0C is very small. Analyzing the effect of BF, a small amount of BF dispersed in the form of monofilament can suppress the initial cracking of concrete. The strong tensile strength of the fibers themselves and the strong bonding strength with the concrete matrix enable the concrete specimens that produce cracks to still withstand a certain tensile force until the basalt fibers are pulled out and the specimens are damaged. However, excessive addition of BF will form a weak link between the fibers and the concrete, increasing the internal defects of the concrete.
In addition, under the splitting tensile load, FGC-40C-yB concrete specimens are mainly subjected to transverse tensile stress, and the fibers crossing the cracks can share the matrix stress, effectively restraining crack propagation and limiting specimen deformation, thereby improving the ft of FGC-40C-yB. Excessive addition of BF can lead to issues such as poor dispersion of BF and reduced fluidity of the concrete. This is because an excess of BF significantly increases the internal frictional resistance of the mixture, lowering the slump. Insufficient fluidity makes it difficult to achieve full compaction during pouring, introducing macroscopic air pores (especially for high-strength materials like FGC-40C-yB, which are more sensitive to porosity). Additionally, excessive BF can become entangled during mixing, forming clusters that disrupt uniform distribution. These clusters create localized weak zones in the matrix, hindering the dense filling of cement paste and leading to increased microporosity. This analysis is basically consistent with the cause analysis reported by Yang et al. [44] and Maleki et al. [54]. Furthermore, when the BF content exceeds the critical threshold, the cement paste cannot fully coat each fiber, resulting in reduced fiber-matrix contact area. This weakens both the mechanical interlocking and chemical bonding at the interface. Consequently, the reinforcing effect of BF diminishes, manifesting as a decline in the splitting tensile strength of the concrete.

3.3. Stress–Strain Curve

Based on the experimental time and material cost, this article only focuses on the effect of BF reinforced CG-FA-GPC and analyzes the stress–strain curve of FGC-40C-yB group concrete, as shown in Figure 3. In the early stage of the experiment, as the load continued to increase, the deformation value of the FGC-40C specimen approached the ultimate tensile deformation, and the increase in load value was no longer significant. During the loading process, when visible fine cracks appeared on the surface of the specimen, the phenomenon of rapid crack development can be observed with a “bang” sound under continuous loading and finally presented as a crack that runs through the upper and lower parts, dividing the specimen into two parts. This is because the strength of the CG coarse aggregate itself is low, and under ultimate load, bond fracture and aggregate fracture occur between the aggregate and the mortar.
However, taking FGC-40C-0.4B as an example, when the deformation value of the specimen approaches the ultimate deformation value, the load growth rate slows down, and the load response value shows small range fluctuations. When the load value approaches the tensile ultimate load, its growth rate becomes slower. After observing multiple small cracks visible to the naked eye on the surface of the specimen, there will be a slight brittle sound coming out from inside the specimen, which is judged as BF pulling out and debonding. When the load reaches its peak, as the displacement increases, the load steadily decreases. At this time, the length and width of a single crack on the surface of the specimen gradually extend, as shown in Figure 3a. To develop the stress–strain curve relationship of concrete, the stress–strain curve is plotted based on the strain corresponding to the axial compressive strength (fcp) of concrete, as shown in Figure 3b. Observing Figure 3b, the FGC-40C line exhibits significant elasticity, with a stress range of approximately (0.2~0.65) fcp in the elastic stage and (0.65~0.8) fcp in the elastic-plastic stage. When BF is added, the brittleness and ductility of concrete are significantly improved. The stress range in the elastic stage is about (0.2~0.5) fcp, and the stress range in the elastic-plastic stage is about (0.5~0.8) fcp. Afterwards, the stress reached its peak and the specimen reached its ultimate bearing capacity.
With the increase in BF content, it can be observed visually that the maximum crack width of concrete gradually decreases, and in the later stage of initial cracking, the width does not expand rapidly with the increase in external load. When the load reaches its ultimate bearing capacity, parallel microcracks develop simultaneously, leading to the penetration and failure of a main crack. This is mainly because BF also has a restraining effect on FA-GPC. After the initial crack occurs, the bridging effect of BF on the crack is greater than the stress at the crack tip, which promotes the further expansion of other microcracks in the concrete and bears more external loads. Therefore, multiple micro parallel cracks appear on the surface of the specimen, but their final failure mode is a single crack failure mode. In addition, a reasonable amount of BF can form a saturated three-dimensional mesh structure in concrete, which has the best effect on improving the deformation resistance of concrete. Observing the crack width of FGC-40C-0.4B, the maximum crack width is smaller than the fiber length, which may be due to the excellent tensile strength of BF and good compatibility between with fly ash based cementitious materials and BF, ensuring good bonding performance between fibers and fly ash based materials, and preventing the bridging effect of fibers on cracks from being damaged. Al-Zu’bi et al. [55] and Maleki [54] both reported similar observations regarding the reinforcing effect of BF. They indicated that due to its excellent inherent material properties and compatibility with cementitious materials, BF can replace steel bars and positively impact the flexural strength and other properties of reinforced concrete components. However, the crack width during the failure of the FGC-40C-0.6B specimen is slightly larger. This is because BF can actually be classified as hydrophilic fibers. When the volume fraction of BF is large, its specific surface area is larger, which can bond more mortar, resulting in a decrease in the cement slurry on the surface of CG aggregates, thereby reducing the bonding strength between aggregates. The adverse effect of the bonding effect of the fiber surface slurry on concrete production gradually offsets the fiber reinforcement effect. At the same time, due to the large volume fraction of fibers, fibers form clusters inside the concrete. The probability of uneven dispersion increases, leading to a decrease in the compactness of the concrete. When BF clusters are loaded and fail, they are pulled out, and the debonding effect is significant. However, on the macro level, the BF addition group shows a basically consistent failure pattern. Therefore, based on the above strength analysis, when BF acts as a reinforcing material on CG-FAGPC, a reasonable dosage has a positive impact on the mechanical properties of concrete, which can compensate for the material performance defects caused by the inherent drawbacks of CG coarse aggregate.

4. Conclusions

This article designs and studies the mechanical properties of BF-CG-FA-GPC. Through the analysis of its compressive strength, flexural strength, splitting tensile strength, and stress–strain curve, the following specific conclusions are obtained:
(1)
Compared with ordinary OPC concrete, FA-GPC has better basic mechanical properties. The negative impact of CG coarse aggregate on the mechanical strength of CG-FA-GPC is significant. Due to the inherent performance defects of CG, the mechanical strength of CG-FA-GPC decreases with the increase in CG substitution rate. A large amount of modification work on CG itself needs to be carried out to expand the utilization rate of CG.
(2)
CG-FA-GPC has the disadvantage of high brittleness. BF, as a reinforcing material, can effectively improve its mechanical properties and the ability to resist deformation to compensate for the material deterioration caused by CG. BF has a significant effect on improving the flexural strength and splitting tensile strength of coal gangue coarse aggregate-fly ash geopolymer concrete, and greatly improves the compressive deformation and failure mode of concrete, showing a significant strengthening and toughening effect. The recommended dosage of BF in the article is 0.4% (volume fraction).
(3)
Fiber-reinforced fly ash-based geopolymer concrete has great potential as a high-performance concrete material. However, there are numerous choices of natural fibers that can be used as reinforcing materials, along with many influencing factors such as the modulus of the activator, raw material composition, and curing conditions. More macroscopic tests and microscopic techniques (such as microscopic imaging tests, XRD analysis, etc.) are needed to fill additional research gaps.
(4)
To further explore the effects of BF, microscopic analysis (SEM, XRD, in situ CT, etc.), load displacement curve analysis, fracture energy calculation, etc., will be deeply cultivated as important research contents in the future, which will also become an important supplement to the database in this research field.

Author Contributions

Z.Y., responsible for the overall design of the experiment and the writing of manuscripts; X.L., responsible for literature retrieval and experimental guidance. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Heilongjiang Provincial Key Research and Development Program (2023ZX01A10).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Buildings 15 02860 g001
Figure 1. CG (a) and BF (b).
Figure 1. CG (a) and BF (b).
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Figure 2. Mechanical strength of all specimens: (a,b) fcu; (c,d) ff and ft.
Figure 2. Mechanical strength of all specimens: (a,b) fcu; (c,d) ff and ft.
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Figure 3. The failure mode (a) and stress–strain curve (b) of FGC-40C-yB.
Figure 3. The failure mode (a) and stress–strain curve (b) of FGC-40C-yB.
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Table 1. Physical properties and chemical components of P O42.5 OPC.
Table 1. Physical properties and chemical components of P O42.5 OPC.
Chemical Components (% by Weight)Physical Properties
SiO2Al2O3Fe2O3CaOMgOSO3R2Oρ/(kg/m3)ff/MPafc/MPa
22.155.633.7963.841.622.740.613.083 d28 d3 d28 d
5.829.3527.849.8
Content of Composite Oxides (% by Weight)
C3SC2SC3AC4AF
50–7015–305–105–15
setting time (min)residue on 45 μm square hole sieve (%)requirement of normal consistencystability
Initialfinal
1202063.927.6qualified
Table 2. Basic properties and main components of Grade I FA.
Table 2. Basic properties and main components of Grade I FA.
Density/(g/cm3)Bulk Density/(g/cm3)Loss on Ignition/%Water Content/%Al2O3/%SiO2/%SO3/%CaO/%
2.551.122.80.8524.245.12.15.6
Table 3. The parameters of CA.
Table 3. The parameters of CA.
CAParticle Size/mmWater Absorption/%Density/(kg/m3)Crushing Index/%
crushed stone10~202.227658.5
CG10~203.5223514.1
Table 4. The parameters of river sand.
Table 4. The parameters of river sand.
TypeDensity/(kg/m3)Bulk Density/(kg/m3)Fineness ModulusMud Content/%
medium262814752.75<2%
Table 5. Physical properties of BF.
Table 5. Physical properties of BF.
d (μm)1 (mm)ρ (g/cm3)E (GPa)ft (MPa)δ/%
15182.7065.201290<3.5
Table 6. Physical properties of FDN.
Table 6. Physical properties of FDN.
StateWater Reduction Rate (%)Total Alkalinity (%) Density (g/cm3)
Brownish-red liquid≥18≤4.01.16 ± 0.02
Table 7. Mixture proportions of different concrete/(kg/m3).
Table 7. Mixture proportions of different concrete/(kg/m3).
SpecimensOPCFANaOHNa2SiO3CGStoneSandBFWater
C0536.800001058.8454.40161.1
FGC-0C-0B0536.87017501058.8454.400
FGC-40C-0B0536.870175423.52635.28454.400
FGC-70C-0B0536.870175741.16317.64454.400
FGC-100C-0B0536.8701751058.80454.400
FGC-40C-0.2B0536.870175423.52635.28454.44.420
FGC-40C-0.4B0536.870175423.52635.28454.48.840
FGC-40C-0.6B0536.870175423.52635.28454.413.260
Table 8. Slump of all specimens.
Table 8. Slump of all specimens.
SpecimensSlump/mmSpecimensSlump/mm
C0112FGC-100C-0B125
FGC-0C-0B201FGC-40C-0.2B146
FGC-40C-0B185FGC-40C-0.4B122
FGC-70C-0B159FGC-40C-0.6B108
Table 9. Mechanical strength of all specimens.
Table 9. Mechanical strength of all specimens.
Specimensfcu/MPaff/MPa (28 d)ft/MPa (28 d)
7 d28 d
C030.6435.933.531.28
FGC-0C55.7966.604.471.73
FGC-40C49.8957.832.951.50
FGC-70C45.2348.652.671.32
FGC-100C44.9449.502.531.18
FGC-40C-0.2B54.3663.163.681.63
FGC-40C-0.4B57.2165.624.561.82
FGC-40C-0.6B50.6858.974.281.69
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Yang, Z.; Ling, X. Mechanical Properties of Basalt Fiber-Reinforced Coal Gangue Coarse Aggregate-Fly Ash Geopolymer Concrete. Buildings 2025, 15, 2860. https://doi.org/10.3390/buildings15162860

AMA Style

Yang Z, Ling X. Mechanical Properties of Basalt Fiber-Reinforced Coal Gangue Coarse Aggregate-Fly Ash Geopolymer Concrete. Buildings. 2025; 15(16):2860. https://doi.org/10.3390/buildings15162860

Chicago/Turabian Style

Yang, Zheng, and Xianzhang Ling. 2025. "Mechanical Properties of Basalt Fiber-Reinforced Coal Gangue Coarse Aggregate-Fly Ash Geopolymer Concrete" Buildings 15, no. 16: 2860. https://doi.org/10.3390/buildings15162860

APA Style

Yang, Z., & Ling, X. (2025). Mechanical Properties of Basalt Fiber-Reinforced Coal Gangue Coarse Aggregate-Fly Ash Geopolymer Concrete. Buildings, 15(16), 2860. https://doi.org/10.3390/buildings15162860

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