Introduction

Concrete materials are extensively utilized in civil engineering applications. Construction sand and gravel make up approximately 70% of concrete, which are the main raw materials of the concrete skeleton. China, with an annual sand and gravel output of more than 20 billion tons, is the world’s largest producer and consumer of sand and gravel. With the depletion of natural sand and gravel resources and growing awareness of environmental protection, the contradiction between the supply and demand of natural sand and gravel has increased sharply. Therefore, the demand for alternative natural concrete aggregates in China is increasingly urgent.

With the effective implementation of China’s “Rural Revitalization Strategy”, highway and transportation construction in the southwest region has been greatly developed. The excavation of highway tunnels in this region generates a large amount of waste rock. The main components of the waste rock are limestone and dolomitic limestone. In China, the utilization ratio of untreated waste rock is relatively low and it is usually dumped directly in the quarries1. Therefore, as the waste rock stockpile increases annually, the problem that waste rock encroaching on a significant amount of land occurs. Besides, the waste rock contains carbonate or sulfate, which can easily form an aggressive solution containing CO32− or SO42− after soaking in rainwater2. Moreover, the crushing process of waste rock will yield a considerable quantity of rock powder, which is non-degradable and will form dust and haze when dry and exposed to wind. Based on the actual conditions at the construction site, the crushed waste rock may be considered to substitute for natural sand and gravel for producing manufactured sand and gravel. The quality of aggregates significantly impacts the properties of concrete, including wave velocity, porosity, and mechanical strengths3. The primary components of the waste rock studied in this paper are limestone and dolomitic limestone mixture. Currently, most scholars focus on the use of high-purity limestone in concrete4,5,6, however, the study on the aggregates containing limestone and dolomitic limestone is relatively limited. Bahulayan et al.7 investigated the impact of dolomitic limestone content in limestone aggregate on the compressive strength of concrete. The findings suggested that limestone aggregate with dolomitic limestone of less than 15% slightly decreased the strength, compared to the control concrete. However, Grattan-Bellew et al.8 reported that the expansion of dolomitic limestone crystals led to concrete expansion. The primary cause is the presence of the alkali-carbonate reaction (ACR) in the dolomitic limestone concrete9. The reaction of dolomite crystals in the aggregate with alkalis in the pore solution results in the decomposition of dolomite crystals into calcite and hydromagnesite. Furthermore, carbonate ions released during the removal of dolomite migrated into the cement paste, causing the dissolution of silicate compounds. These two negative effects lead to the expansion cracking of concrete and a further strength loss. There are significant differences among scholars regarding the application of dolomite and limestone in concrete. Therefore, further investigation can be carried out on the feasibility of limestone aggregates mixed with dolomitic limestone aggregates in concrete.

The high powder content in the aggregate adversely affects the strength of the waste rock concrete. Simultaneously, waste rock concrete exhibits poor tensile strength and cracking resistance. Therefore, many scholars have used fibers to enhance the performance of waste rock concrete10,11,12. Rawan Ramadan et al.13 improved the structural performance of reinforced concrete beams by incorporating phragmites australis fiber and waste glass additives. The mixture of 10% glass and 0.5% reed fiber achieved the highest ductility index and deflection. Madjid Gholhaki et al.14 found that the fibers in concrete reduced the crack width of the specimens after the splitting tensile test by three times. Due to their excellent tensile strength and elastic modulus, steel fiber and basalt fiber are widely used in practical projects15,16. Naraganti et al.17 reported that steel fiber can markedly improve the tensile and flexural performance of concrete. The addition of steel fiber increases the fracture energy of rubberized concrete by approximately 145% compared to the reference specimen. Holschemacher et al.18 demonstrated that the toughness and porosity of SFRC specimens were superior to the concrete without fibers. Steel fiber significantly enhanced the tensile strength and crack resistance of waste rock concrete and are extensively utilized in civil engineering applications. In addition, researchers have also extensively studied the impact of basalt fiber on concrete strength and cracking resistance19. Zheng et al.19 revealed that BFRC specimens exhibited notable advancement in mechanical properties and durability compared to common concrete, When the basalt fiber contents were 0.2%, the strength of BFRC increased by approximately 12%. Philip et al.20 indicated that the incorporation of basalt fiber significantly improved the ultimate load-bearing capacity and impermeability of concrete specimens. When the basalt fiber contents reached 0.3%, the strength and impermeability respectively increased by 27% and 22.7% compared to common concrete. Recent studies have shown that incorporating steel fiber and basalt fiber can effectively improve the mechanical properties of concrete. However, limited research has been conducted on the performance of waste rock concrete with dolomitic limestone aggregates reinforced by basalt and steel fiber. Therefore, further investigation is needed to determine the optimal dosage of steel fiber and basalt fiber in waste rock concrete. Besides, the enhancement mechanisms of steel fiber and basalt fiber may be influenced by the waste rock, which is deserved to study.

This study is based on the Luxi to Qubei to Guangnan to Funing Expressway (Honghe section) constructed by China First Metallurgical Group Co., Ltd, which has a total length of 27.518 km. It is in the territory of Honghe Prefecture, Yunnan Province, as shown schematically in Fig. 1. A significant amount of waste rock was generated during the construction of tunnels and bridges, which caused not only ecological protection but also posed potential risks for major engineering and geological disasters. Therefore, there is an urgent need for the green utilization of waste rock. Crushed waste rock used for on-site preparation of concrete is a good choice to reduce economic costs. This paper presents an approach to comprehensively utilize waste rock as coarse and fine aggregates for producing concrete. Furthermore, steel fiber and basalt fiber were added in the waste rock concrete to improve the physico-mechanical properties. The objectives of the study are as follows: (1) To investigate the reinforcement effects of steel and basalt fiber on waste rock concrete. (2) To reveal the influencing mechanism of the steel and basalts fiber on the mechanical properties and toughness and select the suitable fibers for concrete admixture based on fiber properties and cost. (3) To explain the reinforcement mechanism from microscopic SEM analyze. Figure 2 illustrates the procedures adopted in this study.

Fig. 1
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Schematic diagram of Expressway (Honghe section).

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Fig. 2
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Representation of experimental procedures.

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Materials and specimen preparation

Materials

According to the research requirements, the cementitious material for BFRC and SFRC is selected as cement (PO 42.5) and fly ash (class II). Tables 1 and 2 present the main performance indexes of cement and fly ash. The fly ash can improve the fluidity and strength, its smooth spherical particles helped dissolve the cement flocculation structure21.

Table 1 Properties of OPC (42.5R).
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Table 2 Properties of fly ash (Class II).
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The coarse and fine aggregates used in this study were limestone and dolomitic limestone from the tunneling site. The ratio of limestone to dolomitic limestone in the crushed sample was 16:9. The concrete aggregate particle gradation is utilized in Figs. 3 and 4. Coarse aggregates exceed 4.75 mm, while fine aggregates have a particle size smaller than 4.75 mm. The main performance indexes of the concrete aggregates are presented in Tables 3 and 4, respectively. The X-ray diffraction patterns of limestone and dolomitic limestone aggregates are shown in Fig. 5. The primary component of the limestone aggregate is calcite, and the principal components of the dolomitic limestone aggregates are calcite, dolomite, and a minor quantity of quartz.

Fig. 3
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Particle grading curve of coarse aggregate.

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Fig. 4
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Particle grading curve of fine aggregate.

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Table 3 Properties of coarse aggregate.
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Table 4 Properties of fine aggregate.
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Fig. 5
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XRD pattern of limestone and dolomitic limestone aggregate.

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Polycarboxylic superplasticizer is indispensable admixtures in the preparation of concrete due to its high water reduction and low shrinkage rate22. The properties of polycarboxylic superplasticizer are shown in Table 5.

Table 5 Properties of polycarboxylic superplasticizer.
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A review of related literature23 showed that 24 mm basalt fiber and 35 mm hook-end steel fiber had a better reinforcement performance in BFRC and SFRC, respectively. Therefore, 24 mm basalt fiber and 35 mm hook-end steel fiber were selected as test materials in the present study (Fig. 6). Table 6 presents the physical properties of basalt fiber and steel fiber. Basalt fiber exhibits a brown metallic luster and filamentous morphology, demonstrating good compatibility with concrete. The ends of steel fiber are specially processed into a hooked structure, which enhances their bonding with the concrete matrix through mechanical anchorage.

Fig. 6
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Macroscopic morphology of fibers.

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Table 6 Physical properties of fibers.
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Mix proportions

The proportion ratios of concrete mix were determined based on the Chinese standard JTG 3370.1-201824. The fiber contents were obtained according to the Chinese standard JGJ/T 221-201025. This study aims to determine the effect of fibers on the characteristics of concrete. The mix design of the concrete is presented in Table 7.

Table 7 Proportioning ratios and expected fiber admixture of concrete (unit: kg/m3).
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In this study, the waste rock was used to prepare C50 concrete. The C50 concrete with no fibers was selected as the control group and was named JF50. The concrete with basalt fiber contents at 1 kg/m3, 2 kg/m3, and 3 kg/m3 were named as BF01, BF02, and BF03, respectively. Besides, the concrete with steel fiber contents at 20 kg/m3, 40 kg/m3, and 60 kg/m3 were named SF20, SF40, and SF60, respectively.

Specimen preparation

The cement, waste rock coarse and fine aggregates, polycarboxylic superplasticizer, and basalt fiber or steel fiber were poured into the mixer following the design proportion. First, the mixture was dry-mixed for 30 s. Thereafter, the polycarboxylic superplasticizer and 80% of the water were added and then mixed for 120 s. Second, the remaining water was added to the mixer and then mixed for 60 s. Finally, the fresh concrete mixture was poured out of the ground, and it still needed to be stirred with a shovel for 60 s. Therefore, fresh concrete with uniform mixing was obtained.

Upon completion of the mixing process, the slump of fresh concrete was measured. Then, the well-mixed fresh concrete should be poured into the molds (100 × 100 × 100 mm3 and 100 × 100 × 400 mm3). Subsequently, the molds full of fresh concrete were placed on a vibrating table and secured. Vibration should be maintained continuously until the surface is fully covered in slurry or no air bubbles overflow. Once the casting process was complete, the specimens were numbered, demolded, and weighed. To replicate the natural curing conditions of the project site, the specimens were then placed in a curing room for 7 d and 28 d, respectively. In addition, sprinkling water on the specimens every 12 h is needed to keep the moist of the concrete. The principal apparatus and methodology employed in these tests are illustrated in Fig. 7.

Fig. 7
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Main test apparatus and procedure.

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Test method

(1) Slump test

The slump test was carried out following the Chinese standard GBT 50080-201626. The fresh concrete mixture should be uniformly loaded into the slump cylinder in three layers. A bullet-nosed steel tamping rod is used to compact each concrete layer. Once the fresh concrete has been filled, the excessive fresh concrete on the bottom plate of the cylinder edge should be removed. The difference between the concrete’s height and the slump cone height is denoted as the total slump height.

(2) P-wave velocity test

The non-metallic acoustic wave detector (RMS-SY5(T)) was used to measure the P-wave velocity of hardened concrete after curing to specified times. To reduce errors, three specimens with a size of 100 × 100 × 100 mm3 were utilized for the P-wave velocity test.

(3) Mechanical test

The specimens with the size of 100 × 100 × 100 mm3 were utilized for the uniaxial compressive strength (UCS) test and split tensile strength (STS) test. The specimens with size of 100 × 100 × 400 mm3 were employed for the flexural strength (FS) test. The UCS, STS, and FS of concrete were tested according to EN 1239027. The loading rates for the UCS, STS, and FS tests were 0.8 MPa/s, 0.08 MPa/s, and 0.08 MPa/s, respectively. The schematic diagram of the mechanical test is presented in Fig. 8.

Fig. 8
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Schematic diagram of mechanical tests (a) UCS, (b) STS, (c) FS.

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(4) SEM test

This paper aims to examine the microstructure of the JF50 group and the BFRC experimental group, aiming to understand the interaction between fibers and cementitious materials. Samples with dimensions of approximately 10 mm × 10 mm × 10 mm were prepared by crushing the crushed concrete specimens following the UCS test for SEM (Zeiss Gemini Sigma 300 VP SEM) tests. To obtain a clearer image, the surface of the sample will be sprayed with gold to improve its electrical conductivity before the SEM test.

Results and discussion

Flow properties

The concrete slump is presented in Fig. 9. The slump of JF50 is the maximum (27.5 cm), showing the best flowability. Besides, the concrete slump decreases with increasing basalt fiber and steel fiber contents. For example, the BF03 group has the smallest slump of 189 mm, which is 31.3% lower than the JF50. In addition, the SF60 group has the smallest slump of 185 mm, which is 32.7% lower than the JF50. The reasons for the decrease in slump are as follows. First, the incorporation of fibers prevents the sliding between concrete aggregates, which enhances the internal friction of concrete28. Second, fibers form a dense network structure in the concrete, causing a reduction in the flowability. Overall, all the slump results meet the requirement of a C50 concrete slump (≥ 18 cm) according to JTG 3370.1-201824.

Fig. 9
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Concrete slump.

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P-wave velocity

The P-wave velocity evolution patterns are presented in Fig. 10. With increasing curing age from 3 to 28 d, the P-wave velocities of all concrete groups keep rising. As the curing age increases, the main hydration products (C-S–H gel and Ca (OH)2) exhibit a gradual increase in the concrete. This results in a denser internal structure for the concrete specimens, thereby enhancing the wave velocity. Besides, BF01 shows the best velocity performance in the BFRC test group, and the velocity is 5058 m/s at 28 d, which is 2.0% higher than the JF50. In the SFRC test group, SF40 exhibits the best velocity performance throughout the entire curing period, and the velocity is 5101 m/s at 28 d, which is 2.8% higher than the JF50. The suitable increase in fiber contents results in the formation of a denser, crosslinking network structure within the concrete, which increases the P-wave velocity29. However, when the dosage of basalt fiber exceeds 1 kg/m3 or the dosage of steel fiber exceeds 40 kg/m3, the fiber aggregation will cause a decrease in P-wave velocity.

Fig. 10
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P-wave velocity of concrete specimens.

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Mechanical properties

Uniaxial compression strength

Figure 11 illustrates the UCS of concrete specimens. It can be observed that the UCS of BFRC exhibits a gradual decline with the increase of BF contents. BF01 shows the best UCS in the BFRC test group at 28d, and the UCS is 58.7 MPa, which is 9.3% higher than the JF50 group. Besides, the UCS of SFRC initially rises and then declines with the increase of steel fiber contents, consistent with previous research findings30,31. SF40 shows the best UCS in the SFRC test group at 28d, and the UCS is 63.4 MPa, which is 18.1% higher than the JF50 group. The variation pattern of UCS of concrete specimens aligns with that of P-wave velocity (Fig. 10).

Fig. 11
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UCS of concrete specimens.

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The reasons for improving the UCS of the concrete by adding basalt fiber and steel fiber are as follows. First, the basalt and steel fiber both exhibit high tensile strength and elastic modulus (Table 6), which can absorb more strain energy and show a smaller deformation in the concrete. Second, the basalt and steel fiber have good compatibility with the concrete matrix, which significantly improves the friction to facilitate the bridging effect. The significant bridging effect effectively limits the formation and expansion of the cracks to further increase the overall UCS of the concrete. Third, basalt fiber has relatively good individual dispersion, which makes it easy to form a three-dimensional network structure in concrete32. Fourth, steel fiber has a relatively long length and two end hooks, which further remarkably enhances the bridging effect. However, when the fiber contents are excessive, the fibers agglomerate together to increase the number of pores and ultimately reduce the UCS33,34.

The damage pattern of concrete specimens under the UCS test is shown in Fig. 12. JF50 specimens exhibit distinct shear and brittle failure modes (Fig. 12a). With the addition of basalt fiber and steel fiber, the bridging effect of the fibers causes the cracks in the concrete specimens to become curved and thin, with crack propagation exhibiting no specific directionality (Fig. 12b, c). The network structure formed by the random distribution of fibers in concrete effectively exerts the hoop effect and enables the concrete specimens to maintain their integrity after being subjected to uniaxial stress.

Fig. 12
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Damage pattern of concrete specimens under UCS test.

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Split tensile strength

The STS of concrete specimens is shown in Fig. 13. With the increase of fiber contents, the STS of BFRC and SFRC initially increases and then decreases. BF02 shows the best STS (6.61 MPa) in the BFRC test group at 28d, which is 20.1% higher than the JF50 group. In addition, SF40 shows the best STS (6.72 MPa) in the SFRC test group at 28d, which is 22.2% higher than the JF50 group. The strengthening mechanism of basalt fiber and steel fiber on the STS is shown in section “Uniaxial compression strength”.

Fig. 13
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STS of fiber concrete.

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The split damage pattern of JF50, BF02, and SF40 specimens under the STS test is shown in Fig. 14. The JF50 control group exhibits direct splitting damage through phenomenon (Fig. 14a). The BFRC test group shows more cracks compared to the JF50 control group, and the BFRC specimens still show splitting damage through at near ultimate load (Fig. 14b). Meanwhile, the addition of steel fiber alters the crack propagation direction in the SFRC specimens, enhances their integrity, and improves the ductility of the SFRC specimens (Fig. 14c). Zhu et al.35 ffound that the external mixing of basalt fiber had an ameliorating effect on the initial cracking stress of concrete. This indicates that basalt fiber effectively bears the majority of the STS in concrete, which enhances the crack resistance and stress distribution of concrete specimens36. Furthermore, steel fiber can impede the expansion of visible cracks on the macro level, delay the expansion of microcracks, enhance the flexibility following the formation of microcracks, and effectively enhance the STS of concrete. This results in better performance of SFRC than that of BFRC in terms of STS.

Fig. 14
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Damage pattern of concrete specimens under STS test.

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Flexural strength

The FS of the concrete specimens is illustrated in Fig. 15. The FS of the concrete exhibits an initial increase followed by a decline with the increment in fiber contents. BFRC and SFRC specimens exhibit higher FS than those in the JF50 group. The change in FS also verifies that the optimum fiber contents is consistent with other experimental results, respectively. The increase in strength is 17.5% and 27.4% over the JF50 control group. The FS of the concrete specimens initially increases and then decreases with the increase in fiber contents, consistent with previous research findings32,36. The strengthening mechanism of basalt fiber and steel fiber on the STS is shown in section “Uniaxial compression strength”.

Fig. 15
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FS of fiber concrete.

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The flexural damage pattern of JF50, BF02, and SF40 specimens under the FS test is shown in Fig. 16. Under the pressure of a concrete testing machine, penetration of the JF50 specimens occurred (Fig. 16a). The addition of basalt fiber resulted in the formation of more cracks compared to JF50 (Fig. 16b). Near the ultimate load, SF40 specimens’ cracks will appear without penetration, indicating the presence of multi-crack damage and fiber pullout (Fig. 16c). Their dense network structure renders concrete impervious to penetration and capable of withstanding greater FS.

Fig. 16
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Damage pattern of concrete specimens under FS test.

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Previous studies show the relationship between the UCS, STS and FS of concrete can be expressed as follows37.

$$f_{STS/FS} = a \times (f_{UCS} )^{b}$$
(1)

where a and b are constants, fSTS/FS and fUCS are STS, FS and UCS, respectively.

In addition, various national standards and studies have proposed different constants a and b, as shown in Table 838.

Table 8 Reference standards for mechanical strength relationship.
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Based on the mechanical results, a statistical regression analysis was conducted to derive two formulas:

$$f_{STS} = 0.2 \times (f_{UCS} )^{0.85}$$
(2)
$$f_{FS} = 0.26 \times (f_{UCS} )^{0.85}$$
(3)

Figures 17 and 18 compare the experimental and predicted values of the STS and FS of fiber-reinforced concrete. The proposed prediction formulas have an error of 10%. The comparison reveals that the prediction equations for STS and FS based on UCS are significantly different from previous standards. This is due to the uneven contribution of fiber incorporation to the three mechanical strengths and the significant impact of fiber type, dosage, and length on strength. Therefore, traditional concrete standards may have lower accuracy. Moreover, additional experimental data can provide more accurate statistical verification for various influencing factors. Further extensive research is needed to validate Eqs. 2 and 3.

Fig. 17
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The experimental versus predicted STS of fiber-reinforced concrete.

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Fig. 18
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The experimental versus predicted FS of fiber-reinforced concrete.

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In summary, the optimal dosage of steel fiber is 40 kg/m3, and basalt fiber is 2 kg/m3. At these dosages, both fiber-reinforced waste rock concrete meets the strength requirements, with the strength of SFRC exceeding that of BFRC. However, the incorporation of steel fiber also introduces certain drawbacks, such as increased carbon emissions, reduced corrosion resistance, increased surface roughness, and higher dead loads in engineering applications, all of which are inherent to the material properties of steel fiber. Basalt fiber exhibits better workability with concrete and excellent corrosion resistance.

CO2 emissions and cost analysis

Based on relevant studies39,40, the CO2 emissions during the production of raw materials are shown in Table 9, with transportation emissions being 1.29 × 10−4 kgCO2/(kg * km) and unit concrete production emissions being 2.9 kgCO2. The unit CO2 emissions of concrete prepared with natural aggregates (T50) and waste rock concrete with different mix proportions are presented in Table 10. It can be concluded that the concrete prepared with waste rock (JF50) reduced CO2 emissions by 70.1% compared to T50. Although the addition of fiber increased its CO2 emissions, it remained significantly lower than T50, with a reduction of at least 60.2%. Considering its mechanical properties, the CO2 emissions per unit mechanical strength of waste rock concrete were calculated (Table 10). The BFRC experimental group reduced the CO2 emissions per unit mechanical strength. Specifically, the CO2 emissions per unit STS and the CO2 emissions FS in the BF02 group decreased by 16.2% and 14.2%, respectively, compared to the JF50 group. This demonstrates that the incorporation of basalt fiber not only enhances the strength of concrete but also reduces its carbon dioxide emissions per unit mechanical strength. In the SFRC experimental group, excessive steel fiber dosage led to an increase in CO2 emissions per unit mechanical strength. In the SF40 group, CO2 emissions per unit STS increased by 0.2%, while CO2 emissions per unit SF decreased by 4.0%. In comparison, the incorporation of basalt fiber proved to be more effective in reducing the CO2 emissions of concrete.

Table 9 Carbon emissions required in the raw material acquisition stage.
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Table 10 CO2 emissions of producing concrete.
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Furthermore, the cost of producing concrete is calculated as shown in Table 11. After using waste rock aggregate, the production cost of concrete decreased by 25.5%. Even at the highest basalt fiber content (BF03), costs remain 12.6% lower, whereas the lowest steel fiber addition (SF20) increases costs by 39.7%. Considering the cost per unit mechanical strength, the BF02 group decreased the cost per unit STS and the cost per unit FS by 7.3% and 5.2%, respectively, compared to JF50. In contrast, the SF40 group increased the cost per unit STS and the cost per unit FS by 125.9% and 115.4%, respectively. Considering both cost and emission characteristics, the dosage of 2 kg/m3 of basalt fiber is the optimal solution for reducing carbon emissions and cost savings.

Table 11 Cost of producing concrete.
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Microstructure

Based on the research findings, the micro-reinforcement mechanism of basalt fiber and its failure mechanism under uniaxial compression were further investigated by using SEM. The JF50 group and the BFRC group were subjected to SEM tests to further investigate the microstructure inside the concrete specimens (Fig. 19). In the JF50 group, the distribution of cracks was clear. The addition of fibers results in a retardation of crack extension within the concrete matrix and an enhancement of the degree of compactness Meanwhile, SEM microstructural analysis revealed that the cracks in the basalt fiber-reinforced specimens were smaller, and the internal structure of the concrete appeared denser41. Furthermore, the incorporation of fibers provides nucleation sites, thereby facilitating the hydration reaction of the cement (Fig. 20). In the BF01 group, the fiber content is minimal, and its bridging effect is weak. In the BF02 group, the bridging effect of fibers and the 3D network structure in the waste rock concrete matrix were significant, with the fiber content increasing29. However, the phenomenon of fiber agglomeration occurs when the degree of doping in fibers is higher, thereby generating a greater number of pores between fibers, as illustrated in the microscopic image of the BF03 group.

Fig. 19
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SEM micromorphology images of JF50 and BFRC.

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Fig. 20
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Hydration reaction at nucleation sites on fibers.

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Therefore, incorporating an appropriate amount of basalt fiber into concrete enhances the mechanical strength of waste rock concrete. Basalt fiber effectively bridges existing pores and microcracks in the concrete matrix, resulting in a denser and more compact structure. The fracture of the fiber constitutes the predominant form of basalt fiber destruction subsequent to the application of stress in this case42. Furthermore, when the fiber dosage exceeds a certain range, the bonding effect between the fibers and the matrix is diminished, thereby affecting its bonding performance with the concrete matrix, constraining the subsequent enhancement of concrete strength, and consequently reducing the mechanical strength.

Conclusion

This study mainly investigates the different fiber contents of basalt fiber and steel fiber on the slump, P-wave velocity, and mechanical properties of waste rock concrete. The incorporation of basalt fiber or steel fiber can enhance the overall performance of concrete within a suitable fiber content range. Although the mechanical performance of basalt fiber is lower than that of steel fiber, considering the overall CO2 emissions and cost analysis, incorporating basalt fiber remains the optimal solution. This improvement helps achieve the production of sustainable building materials with reduced environmental harm. Furthermore, this provides theoretical support and reference value for its application in engineering projects. The incorporation of fiber into the preparation of waste rock concrete significantly enhances the utilization of waste rock resources, which can minimize environmental impact for sustainable development. The key findings are as follows:

  1. 1.

    The slump of concrete after adding basalt fiber or steel fiber decreases with the increase of fiber contents and still meets the Chinese standard GBT 50080-2016, which is greater than 180 mm. The variation trends in mechanical properties for BFRC or SFRC generally show an initial increase in strength followed by a decrease as the fiber contents increase, except for the UCS of BFRC, and the UCS of BFRC exhibits a gradual decrease as the basalt fiber content increases. At the optimal basalt fiber dosage, UCS, STS, and FS reach 57.03 MPa, 6.61 MPa, and 8.53 MPa, respectively. For the optimal steel fiber dosage, these values reach 63.4 MPa, 6.71 MPa, and 9.25 MPa, respectively.

  2. 2.

    Basalt fiber and steel fiber have a significant bridging effect and cracking resistance in the waste rock concrete specimens. Basalt fiber and steel fiber both possess high elastic modulus and tensile strength, as well as good compatibility with concrete matrix. Besides, basalt fiber exhibits good individual dispersion, which is easier to form a network structure. In addition, steel fiber has a relatively long length and two end hooks, which is beneficial to enhance the bridging effect.

  3. 3.

    The SEM test results demonstrated the reinforcing effect of basalt fiber on waste rock concrete. The incorporation of basalt fiber has been demonstrated to enhance adhesion to the waste rock concrete matrix. This, in turn, increases the available formation and sites within the concrete, thereby promoting the hydration reaction and improving the internal structure of the waste rock concrete.

In addition, while SF is more effective than other additives in enhancing the strength of concrete, it is susceptible to corrosion in the natural environment. Consequently, the evolution law of the long-term performance of externally mixed concrete with SF and BF remains unclear, and further research is necessary. Furthermore, the alkali-aggregate reaction in concrete incorporating dolomite aggregates remains an area requiring further investigation.