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Article

An Investigation of the Mechanical Properties of Basalt Fibre-Reinforced Graphite Tailings Cement Mortar

by
Chen Zhang
1,
Ben Li
1,*,
Ying Yu
2,
Yu Zhang
1,
Hu Xu
1 and
Wen-xue Wang
3
1
Advanced and Sustainable Infrastructure Materials Group, School of Transportation, Civil Engineering and Architecture, Foshan University, Foshan 528000, China
2
Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, School of Civil and Traffic Engineering, Shenzhen University, Shenzhen 518060, China
3
Institute of Applied Mechanics, Institute of Engineering, Kyushu University, Fukuoka City 819-0395, Fukuoka Prefecture, Japan
*
Author to whom correspondence should be addressed.
Buildings 2022, 12(12), 2106; https://doi.org/10.3390/buildings12122106
Submission received: 7 November 2022 / Revised: 21 November 2022 / Accepted: 25 November 2022 / Published: 1 December 2022
(This article belongs to the Special Issue Aggregate Concrete Materials in Constructions)
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Abstract

:
In order to solve the limitation of graphite tailings in improving the toughness of cement-based materials, this paper aims to study the effect of basalt fibres (BF) on the mechanical properties of graphite tailings cement mortar (GTCM). Basic physical and mechanical tests such as fluidity, water absorption, surface water content, flexural strength, compressive strength and modulus of elasticity were conducted on basalt fibre-reinforced graphite tailings cement mortar (BFR-GTCM), and combined with microscopic tests such as scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and mercury intrusion porosimetry (MIP) for the enhancement mechanism were deeply analysed. The results show that 0.3% BF and 20% GT are the optimal doping amounts for the mechanical response of BFR-GTCM, which especially significantly improves the flexural and crack resistance. In addition, the synergistic effect of basalt fibres and graphite tailings optimizes the spatial structure and pore distribution of the cement matrix and promoted the hydration of the cementitious material, thus improving the mechanical properties of BFR-GTCM.

1. Introduction

The continuous improvement of a country’s industrialization process will inevitably bring about the demand for population, water, transportation and solid waste treatment capacity. Thanks to the development of modern concrete technology, building materials have become the largest carrier of industrial solid waste and become an urgent problem of urban modernization and industrialization development. Among them, as a new type of non-metallic tailings, graphite tailings, whether from solid waste disposal methods or recycling methods, have a strong rationality and technicality worthy of extensive attention. Simultaneously, long-term mining has led to a large accumulation of graphite tailings and formed hundreds of millions of tons of tailings dams. Clearly, the environmental impact of this solid waste accumulation is cumulative and irreversible over time because the tailings dust billowing not only occupies a large amount of land but also causes severe air pollution [1]. With the issuance of Chinese “Guidance on Promoting the Healthy and Orderly Development of the Sand and Gravel Industry” in 2020, how to more efficiently, rationally and fully implement the comprehensive utilization of graphite tailings needs to be seriously considered.
In recent years, research on the reuse of graphite tailings to prepare ecological building materials is being understood and recognized. According to previous studies, graphite tailings are very fine sandlike in appearance and contain sufficient silica and has certain volcanic ash activity [2]. Therefore, in terms of physical appearance and chemical composition, graphite tailings have good similarity with ordinary sand and satisfy the conditions for replacing sand as a fine aggregate [3]. At present, research in this field is mainly focused on the macroscopic properties and material properties of concrete prepared by replacing natural sand with graphite tailings, clarifying the optimal content of graphite tailings and fully improving the special applicability of concrete such as frost resistance and electrical conductivity and signal shielding. Liu and Li [4,5,6,7] conducted a series of studies on the mechanical and electrical properties of mortar and concrete using graphite tailings to replace the sand. The research results show that when the replacement amount of graphite tailings does not exceed 20%, the mechanical properties (compressive strength), frost resistance and electrical conductivity of cement-based materials will be significantly improved, and the pore structure distribution will be promoted. Wang [8] concluded that graphite tailings have a uniform particle size distribution and are beneficial for the preparation of foam concrete as a light aggregate. It was found that the compressive strength of foam concrete was enhanced but still lower than that of ordinary concrete. Peng [1] studied the preparation of autoclaved aerated concrete using graphite tailings, and the results met the strength requirements. However, due to the limited improvement of interfacial bonding and occlusion between aggregate–aggregate and aggregate–mortar within the cementitious material by graphite tailings, the cementitious material still has defects such as low tensile strength and poor crack resistance [4]. Moreover, the above defects will limit the application scope and application prospect of graphite tailings cementitious materials. Therefore, how to improve the flexural and crack resistance of graphite tailings cementitious materials to meet their high flexural-crack resistance requirements in special environments is one of the key problems that still need to be solved.
It is one of the effective and feasible methods to improve the strength and toughness of graphite tailings cement-based materials by incorporating short fibres. At present, the short fibres commonly used to strengthen and toughen cement-based materials are steel fibres, carbon fibres, glass fibres, polypropylene fibres and polyethylene fibres [9,10,11,12,13,14,15,16,17]. However, steel fibres are prone to rusting in long-term service environments, which can inhibit the strength of the concrete in later stages [18,19]. Carbon fibres and polyethylene fibres are very expensive. Polypropylene fibres have low modulus of elasticity and limited flexural strength. The alkaline resistance of glass fibres is insufficient [20,21,22]. Basalt fibre stands out as a green fibre material with high tensile strength, sufficient elastic modulus, low price and good toughening ability. It also has high corrosion resistance and chemical stability [23,24,25,26,27,28,29]. Due to the excellent performance and sustainable manufacturing capacity, basalt fibre is becoming a substitute for other fibres [30,31,32,33,34,35,36,37,38]. Therefore, by combining the material characteristics and advantages of basalt fibre and graphite tailings, a new green, low-cost, high-bending, crack-resistant, solid waste cement-based material can be prepared. It not only realizes the reuse of solid waste resources but also provides theoretical guidance for the design of cement-based materials in high bending service environment. At present, the mechanism of mechanical properties of basalt-fibre reinforced graphite tailing cement-based materials still needs further study. This work will provide a theoretical basis for enriching the design and performance optimization control of solid waste materials system.
This paper evaluates the effect of basalt fibres on the physical and mechanical properties of graphite tailings cement mortar by conducting basic physical and mechanical tests on basalt fibre-reinforced graphite tailing cement mortar (BFR-GTCM), such as flowability, water absorption, compressive strength, flexural strength and modulus of elasticity. Combining the micro testing methods such as scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and mercury intrusion porosimetry (MIP) and other microscopic testing methods, the effects of BFR-GTCM’s microscopic morphology, hydration products, functional groups and pore structure on its mechanical properties were determined. In addition, the mechanism and synergistic effect of BFR-GTCM are also proposed, and the optimal proportion design considering mechanical response is proposed. At the same time, it fills the knowledge gap of mechanical properties of cement mortar under the composite system of basalt fibre and graphite tailings, and also provides a new idea for the resource utilization of solid waste of graphite tailings.

2. Materials and Experimental Methods

2.1. Raw Materials and Mixing Proportions

The cement was made from ordinary silicate cement (P·O 42.5N) produced by the Foshan Cement Factory, and the mechanical properties are shown in Table 1. The graphite tailings were derived from the city of Jixi, Heilongjiang Province, China, and the river sand was local river sand from the city of Foshan. The material appearance, particle size distribution and particle gradation of the graphite tailings and sand are shown in Figure 1. The chemical compositions of the cement, graphite tailings and sand are listed in Table 2. The physical properties of the sand and graphite tailings are listed in Table 3. Basalt fibre with the length of 12 mm and diameter of 17 mm was used, the physical properties of the fibre are listed in Table 4, and the appearance of the fibre morphology is shown in Figure 2. In this study, graphite tailings were replaced with river sand by the volume fraction of 0%, 10%, 20%, 30%, 40% and 50% for basalt fibres. Basalt fibres were mixed in 0%, 0.1%, 0.2%, 0.3% by volume of cementitious material, respectively. The water/cement ratio (W/C) was set as 0.4, and the cement-sand ratio was 1.5. The mortar mixes are shown in Table 5, where GT indicates the graphite tailings used to replace river sand and BF is the basalt fibre added to the material system.

2.2. Specimen Casting and Curing Conditions

The specimens were prepared with reference to Chinese standards GB/T17671-1999 [39] and JGJ/T70-2009 [40]. Three samples of each mixture type were prepared. Specimens with dimensions of 40 × 40 × 160 mm (288 pieces) were used to determine the flexural and compressive strengths at 3 d, 7 d, 14 d and 28 d. The 28-day water absorption and surface water content were measured with 100 × 100 × 100 mm (72 pieces) specimens. Specimens with dimensions of 70.7 × 70.7 × 210 mm (72 pieces) were measured for the 28-day static compressive modulus of elasticity. All test specimens were conditioned at 20 ± 2 °C and a relative humidity of ≥95% until the specified age.

2.3. Experimental Methods

2.3.1. Macro Performance Testing

Workability Properties Test

Fresh mortar was vibrated in the specified vibration state using a skipping table, and the measured range of extension was used to measure its working performance according to the Chinese standard GB/T2419-2005 [41].

Water Absorption and Surface Moisture Content

According to the Chinese standard JGJ/T70-2009 [40], the specimen was dried at 105 ± 5 °C for 48 ± 0.5 h and then removed and weighed to obtain the mass m0. The specimen was then completely immersed in water for 48 ± 0.5 h and removed and weighed to obtain the mass m1. The arithmetic mean of the measured values of three specimens was taken as the water absorption rate of the mortar and is accurate to 1%. The water absorption of mortar was calculated as follows:
W x = m 1 − m 0 m 0 × 100
where Wx is the water absorption of mortar (%), m1 is the mass of a specimen after water absorption (g), and m0 is the mass of a dried specimen (g).
In addition, the surface moisture content of BFR-GTCM was measured in this paper. The specimens were removed from the water, wiped dry and then determined using a PW-2 multifunctional surface material moisture meter.

Mechanical Properties Test

Flexural and compressive strength tests were performed on specimens at curing ages of 3 d, 7 d, 14 d and 28 d using the HYZ-300-10 mortar compression and bending machine according to Chinese standards GB/T17671-1999 [39] and JGJ/T70-2009 [40]. The specimens were tested for modulus of elasticity at 28 days using a microcomputer-controlled universal pressure tester and a deformation measuring instrument. 3 specimens were tested for each group, and the arithmetic mean of the measured values was taken as the final result.

2.3.2. Macro Performance Testing

To further explore the effects of basalt fibres on the micro properties of graphite tailings cement mortar, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and mercury-pressure porosimetry (MIP) were used to analyse the microscopic morphology, pore structure distribution, hydration products and chemical bonding or molecular enhancement mechanisms of graphite tailings cement mortar specimens. The samples were dried and prepared in accordance with Chinese standards GB/T 16594-2008 [42], GB/T 30904-2014 [43], ISO 19618-2017 [44] and GB/T 21650-2008 [45]. In summary, the flow of the experimental scheme in this paper is shown in Figure 3.

3. Results and Discussion

3.1. Macro Comprehensive Performance Analysis on BFR-GTCM

3.1.1. Workability Performance of BFR-GTCM

The experimental result of the workability of BFR-GTCM is shown in Figure 4. The results show that, (1) The flowability performance of all fresh mortars varied between 228 mm and 165 mm. When graphite tailings or basalt fibres are mixed alone, the fluidity of fresh mortar decreases with the increase in graphite tailings replacement rate or basalt fibre content. (2) When compounded with basalt fibres and graphite tailings, the decreasing trend of mortar fluidity is more obvious. For example, when compounded with basalt fibres and graphite tailings, the flowability of BFGT330 decreased by 25.9% compared with BFGT000, while the flowability of BFGT030 decreased by only 10.1% compared with BFGT000 when blended with graphite tailings alone. (3) When the graphite tailings replacement rate is 40~50%, the flowability of BFR-GTCM with 0.2~0.3% basalt fibre content is lower than 180 mm, which does not meet the conditions of mortar preparation and forming. Furthermore, when the replacement rate of graphite tailings does not exceed 30%, the flowability of BFR-GTCM with 0.1~0.3% basalt fibre content is higher than 180 mm, which can meet the actual use requirements and is suitable for the step-by-step regulation of cement mortar working performance. The decrease in the flow of fresh mortar is mainly attributed to the following: (1) The specific surface area of basalt fibre is larger than that of fine aggregate and the surface is hydrophilic, which can absorb more free water and cement slurry for wrapping, resulting in a decrease in flow. (2) The reduction of fresh mortar fluidity is mainly attributed to the following factors: (1) The specific surface area of basalt fibres is larger than that of fine aggregates, and the surface of basalt fibres is hydrophilic, which leads to the fibres easily absorbing more free water and cement slurry for wrapping, resulting in reduced fluidity. (2) The fineness modulus of graphite tailings is lower than that of river sand, resulting in a higher water absorption rate of graphite tailings than river sand. Therefore, the fluidity of fresh mortar is more affected in the case of graphite tailings and basalt fibres being blended at the same time, and the negative impact on the fluidity of fresh mortar is more obvious with the increase in blending amount.

3.1.2. Water Absorption and Surface Water Content of BFR-GTCM

Near the surface of unsaturated cementitious materials, water molecules can easily intrude into the interior of the building material. Obviously, it is important to investigate the mechanisms of water transport in cementitious materials to assess their service life and to improve the mechanical properties of construction materials [46]. One of the methods to evaluate the moisture intrusion capacity of cementitious materials is to determine the pore connectivity properties between the surface and interior of the material by testing the water absorption and surface moisture content. Figure 5 shows the effect of basalt fibres on the water absorption and surface moisture content of graphite tailings cement mortars. The results show that the water absorption of BFR-GTCM tends to increase with the increase in GT replacement rate or BF content when graphite tailings or basalt fibres are blended alone. When the graphite tailings substitution rate is 10~30%, the water absorption rate increases and then decreases with the increase in basalt fibre content, in which BFGT320 has the lowest water absorption rate, which is 9.5% lower than the base group (BFGT000). However, when the graphite tailings replacement rate exceeds 30%, the water absorption of cement mortar decreases first and then increases with the increase in basalt fibre content, where BFGT350 is the highest value (9.6%) of water absorption of BFR-GTCM and increases by 14.3% compares with the base group. Therefore, the admixture of basalt fibres has an obvious inhibiting effect on the water absorption of cement mortar when the graphite tailings replacement rate is in the range of 10%~30%. In addition, the surface water content results of BFR-GTCM show that the surface water content of cement mortars with 0% to 30% graphite tailings replacement rate are all higher than 40%, while the surface water content of cement mortars with 40% to 50% graphite tailings replacement rate are all lower than 40%, with the lowest being 32% (BFGT140).
The main reasons for the changes of water absorption and surface moisture content are as follows: (1) graphite tailings are ultrafine aggregates with certain optimization effects in volume filling and space compensation, which can reduce the larger space area connectivity to achieve the effect of reducing water absorption. (2) the appropriate amount of basalt fibres is evenly dispersed in the cement matrix through sufficient mixing, which can form a more reasonable space distribution after interacting with graphite tailings and impede the internal pore connectivity, thus reducing the water absorption of cement mortar and enhancing the surface moisture content. (3) However, when the graphite tailings replacement rate exceeds 30%, more ultrafine aggregates need to combine with a large amount of free water to achieve condensation hardening. At the same time, the combination of graphite tailings and basalt fibres at larger compound admixture will significantly reduce the fluidity of cement slurry, thus causing the uneven distribution of internal space of cement mortar and forming more pore channels with larger pore permeability pressure, which eventually leads to the increase in water absorption and decrease of surface moisture content [47].

3.1.3. The Changes in Flexural Strength of BFR-GTCM

The flexural strength is one of the indexes to evaluate the mechanical properties of cement mortar, especially for the special mortar with bending and cracking resistance requirements, the flexural strength is the most important evaluation index. In this paper, the flexural strength of BFR-GTCM is investigated at different maintenance ages, and the results are shown in Figure 6. For the flexural strength of BFR-GTCM at an early age (at 3-day curing age), the flexural strength of BFR-GTCM increases and then decreases with the increase in the graphite tailings replacement rate when graphite tailings are mixed alone. Among them, the flexural strength of BFR-GTCM is higher than the base group (BFGT000) in the range of 10%~30% GT replacement rate. This is because when graphite tailings are properly mixed, the ultrafine graphite tailings can fill the interstices inside the material better and enhance the denseness of the cementitious material. At the same time, the volcanic ash activity of graphite tailings can promote the formation of a small amount of C-S-H at the early stage of hydration, which further enhances the early flexural strength of cementitious materials [48]. With the incorporation of basalt fibres, the flexural strength of BFR-GTCM is significantly improved. When the graphite tailings replacement rate is 10%~30%, 0.1%~0.3% BF content can significantly improve the flexural strength of graphite tailings cement mortar. This indicates that the composite effect of appropriate amounts of basalt fibres and graphite tailings can improve the early flexural strength of BFR-GTCM sufficiently. In particular, when the graphite tailings replacement rate is 10%, the cement mortar with 0.3% BF content has the highest flexural strength, 7 MPa, which is 14.8% higher than the baseline group. However, after the graphite tailings replacement rate exceeds 30%, the basalt fibres and graphite tailings cannot be uniformly distributed inside the matrix due to the poor fluidity of the fresh mortar at this ratio, thus leading to initial defects in the cementitious material, and the compounding effect of basalt fibres and graphite tailings will only negatively affect the flexural strength. The flexural strength development of BFR-GTCM at 7 and 14 days of curing age is basically similar to that at 3 days. The flexural strength of BFR-GTCM at 7 and 14 days of curing age is basically similar to that at 3 days. In the range of 10% to 30% GT replacement rate, the 0.3% BF content shows the most significant improvement in the flexural strength of graphite tailings cement mortar up to 8.4 MPa, where at 14 days of curing age, BFGT300, BFGT310, BFGT320 and BFGT330 increase the flexural strength over BFGT000 by 13.7%, 9.6%, 15.1% and 11.0%, respectively. Similarly, the flexural strength of BFR-GTCM shows a decreasing trend and is lower than that of the base group after the graphite tailings replacement rate exceeds 30%, with a maximum decrease of 16.4%.
The test results for the flexural strength of BFR-GTCM at 28 days of standard curing are shown in Figure 7. When the basalt fibres are not mixed, the 10% graphite tailings replacement rate has the best effect on the flexural strength of cement mortar, but only 1.3%. With the incorporation of basalt fibres, the higher flexural strength region is mainly concentrated in the range of 0.2~0.3% BF content and 10~30% GT replacement rate. Among them, when the graphite tailings replacement rate is 20%, the 0.3%BF content has the most obvious effect on the improvement of the flexural strength of graphite tailings cement mortar, which is 18.8% higher than that of the base group. This ratio is the best admixture of modified cement mortar flexural strength when basalt fibres are compounded with graphite tailings. When the replacement rate of graphite tailings exceeds 30%, the improvement effect of basalt fibres admixture on flexural strength is not obvious, and even the phenomenon of inhibiting the development of flexural strength appears. At this time, the flexural strength of the cement mortar is reduced to 6.7 MPa at most, which is 16.3% less than the base group. In addition, when the graphite tailings replacement rate is 40~50%, the basalt fibres cannot effectively improve the flexural strength development, and the flexural strength of cement mortar is all lower than that of the base group. This is similar to the results for cement mortars maintained at early ages. When graphite tailings and basalt fibres are compounded in a certain ratio, basalt fibres can fully exert their high tensile properties to compensate for the macro and micro cracks in the tensile and shear zones of cement mortar to improve the overall toughness, and the fine aggregate filling and interfacial friction of graphite tailings can also minimize the pull-out and interfacial slippage of basalt fibres after being pulled, thus reducing the internal crack width of cement mortar under bending load. The width of cracks in the cement mortar under bending load is reduced, which finally improves the flexural strength and crack resistance of the material. However, when the compound admixture of graphite tailings and basalt fibres exceeds a certain range, the fluidity of the cementitious material is significantly reduced, causing more fibre agglomeration and slurry agglomeration, hindering the uniform distribution of basalt fibres and the hydration and hardening processes of the material system, resulting in more internal cracks or defects, and eventually leading to a decrease in its toughness. The purpose is to show that the flexural strength of basalt fibre and graphite tailings reinforced cement mortar is limited to 0.1–0.3% BF and 10–20% GT.

3.1.4. Compressive Strength of BFR-GTCM

Among the many indicators of cementitious materials, compressive strength is one of the most basic mechanical property indicators reflecting the ability of cementitious materials to withstand compressive loads. It is not only the only basis for determining its strength class, but also the main factor for determining other mechanical properties. Figure 8 shows the variation law of compressive strength of BFR-GTCM under early curing conditions. At 3 days of curing age, the compressive strength of BFR-GTCM without basalt fibres increases and then decreases with the increase in graphite tailings. 10% graphite tailings can significantly improve the flexural strength of cement mortar by 10% over the base group. With the incorporation of basalt fibres, the flexural strength of BFR-GTCM is further improved. Among them, the 0.3% BF content has the most significant effect on the improvement of the compressive strength of BFR-GTCM. For example, BFGT310 improves 16.4% compared with BFGT000. All of these ratios are the best admixtures for the flexural and compressive strengths of BFR-GTCM at early ages. However, the reinforcing effect of basalt fibres begins to level off when the graphite tailings replacement rate exceeds 10%. Overall, basalt fibres have a positive effect on the development and enhancement of the 3-day compressive strength of graphite tailings cement mortar. As the curing age increases (7-days and 14-days curing age), the enhancement effect of basalt fibres on the compressive strength of BFR-GTCM with 0% to 30% GT replacement rate is more obvious. When the graphite tailings replacement rate exceeds 30%, the compressive strength of BFR-GTCM shows a decreasing trend with the increase in basalt fibre content.
The results of compressive strength tests of BFR-GTCM at 28 days of age are shown in Figure 9a,b. The results show that 20% GT can effectively improve the compressive strength of cement mortar by 15.9% compared with the base group when graphite tailings are blended alone. The compressive strength of BFR-GTCM is further enhanced by the incorporation of basalt fibres, in which the cement mortar prepared with 20% GT replacement rate and 0.3% BF incorporation has the highest compressive strength (55.3 MPa), which is increased by 18.7% compared with the base group. It can be seen that the composite effect of graphite tailings and basalt fibres can not only solve the problem of insufficient flexural strength of cementitious materials, but also enhance their compressive strength to the maximum extent. This may be attributed to the fact that when the mortar is mixed with an appropriate amount of basalt fibres, the three-dimensional randomly distributed basalt fibres are tightly bound in the matrix, which limits the lateral deformation of the cement matrix in compression. At the same time, the stress moves along the slurry interface toward the position of basalt fibres during the loading process, which causes internal stress redistribution. Therefore, a certain amount of basalt fibres incorporation can retard the degree of damage of BFR-GTCM under compression. However, when the graphite tailings replacement rate exceeds 30%, the compressive strength of BFR-GTCM decreases with the increase in basalt fibre content. In particular, when the graphite tailings replacement rate is 40% to 50%, the increase in basalt fibre content has a more obvious effect on the development of compressive strength of BFR-GTCM. For example, BFGT150, BFGT250 and BFGT350 decrease by 3.9%, 2.4% and 7.7%, respectively, compared with BFGT000. When the admixture of basalt fibres and graphite tailings is higher, more cement paste is required in the matrix to encapsulate the aggregates and fibres, which affects the interfacial bonding effect between cement paste-aggregate-fibre, thus leading to a negative effect of fibres on the development of compressive strength. In order to enhance the compressive strength of the material, the proportional admixture of basalt fibres and graphite tailings should be accurately controlled within a reasonable range during the design of the BFR-GTCM material system.
In addition, the compressive-flexural ratio of BFR-GTCM at 28 days of age is also investigated in this paper. As shown in Figure 9c, the compressive-flexural ratio of cement mortar when graphite tailings are mixed alone are higher than those of the base group. For example, BFGT010, BFGT020, BFGT030, BFGT040 and BFGT050 increase the flexural ratio by 5.2%~20.7% compared with BFGT000, which indicates that the brittleness of cement mortar gradually increases and the toughness gradually decreases with the increase in graphite tailings replacement rate. With the incorporation of basalt fibres, the compressive-flexural ratio of BFR-GTCM decreases significantly, and the toughness and crack resistance of cement mortar are improved. For example, the compressive-flexural ratio of BFGT320 is reduced by 17.1% compared with that of BFGT020. However, when the graphite tailings replacement rate exceeds 20%, the compressive-flexural ratio of BFR-GTCM is higher than the base group with the increase in basalt fibre content. The above results indicate that the basalt fibres can improve the toughness and crack resistance of BFR-GTCM in the range of graphite tailings replacement rate up to 20%, especially for the 20% GT replacement rate of cement mortar. The results are not accidental, but are due to the overlap effect of BF and the filling effect of GT. However, this high flexural and crack resistance of BFR-GTCM is only limited to a maximum dosing of 0.3% BF and 20% GT. Beyond this range, both effects will fail.

3.1.5. Elastic Modulus of BFR-GTCM

The elastic modulus of the BFR-GTCM cured for 28 days is shown in Figure 10. The results show that the elastic modulus of BFR-GTCM increases and then decreases with the increase in GT replacement rate when blended with graphite tailings alone, and the elastic modulus of BFR-GTCM blended with 20% graphite tailings reaches the highest value, which increases by 15% compared with the base group. With the incorporation of basalt fibres, the elastic modulus of BFR-GTCM is all improved. At this time, the higher elastic modulus region is mainly concentrated between 20–30% GT replacement rate and 0.1–0.3% basalt fibre content, especially the compound blending of 20% GT replacement rate and 0.3% BF content has the most obvious positive effect on the elastic modulus of BFR-GTCM, and its maximum elastic modulus is 36.5 GPa (compared with the base group increased by 26.8%). When the right amount of basalt fibres is combined with graphite tailings, the randomly distributed fibres are interwoven to form a fibre network, which can share the stress and resist deformation together with the aggregate under load, thus reducing the stress gradient distribution and strain distribution inside the mortar, and can effectively limit the lateral development and cracking width of internal cracks, and finally further improve the elastic modulus of cement mortar. However, when the graphite tailings replacement rate exceeds 30%, the incorporation of basalt fibres has an inhibitory effect on the development of the elastic modulus of BFR-GTCM. This is due to the fact that the mixing of excessive basalt fibres and graphite tailings leads to a decrease in the fluidity of the cement mortar, which causes an inhomogeneous distribution of fibres, thus increasing the crack density and initial defects within the cementitious material. More initial defects lead to lower resistance to deformation and lower modulus of elasticity of BFR-GTCM. In summary, the elastic modulus and compressive strength of BFR-GTCM change similarly, which are determined by GT and BF. The use of 0.3% basalt fibre content and 20% graphite tailings replacement rate has an effective improvement on the mechanical properties of cement mortar.

3.2. Reinforcement Mechanism Analysis of BFR-GTCM

In order to further explore the enhancement mechanism of basalt fibres on the mechanical properties of graphite tailings cement mortar, the base group and cement mortar specimens with 20% graphite tailings replacement rate after standard curing for 28 days are selected for microscopic testing and analysis in this paper, and their specimen numbers are BFGT000, BFGT020, BFGT120, BFGT220 and BFGT320.

3.2.1. Micro-Morphological Analysis of BFR-GTCM

The results of the SEM morphology analysis of BFR-GTCM are shown in Figure 11. When graphite tailings and basalt fibres are not mixed, more pores and cracks appear in the mortar, and its internal structure is loose. When 20% graphite tailings are added, the cement mortar is filled with many gel particles and accompanied by Ca(OH)2 deposition, the internal space is filled and the pore structure is significantly reduced; at this time, the water absorption, compressive strength and elastic modulus corresponding to BFGT020 are improved. However, the internal crack density is not reduced, and the crack width is not improved. When the basalt fibres are incorporated, the pore structure of BFGT120 and BFGT220 is slightly increased due to the negative impact of fibre incorporation on cement mortar fluidity, and although the width and number of cracks inside them are significantly improved, the uneven distribution of fibres will have a certain negative impact on the mechanical properties of BFR-GTCM. In particular, for the scanning electron microscope analysis of BFGT220, the basalt fibres cannot be uniformly dispersed in the cement mortar at this time, and there is a large amount of agglomeration and agglomeration between the fibres, which affects the degree of fibre-to-fibre compactness. Additionally, more microcracks and pore structures are found around the agglomerated fibres, which affects the interfacial bonding properties and overall synergy between fibres and mortar, and finally leads to the reduction of compressive strength of BFGT220. When the basalt fibre content is 0.3%, the interfacial transition zone (ITZ) between the fibres and mortar is filled with many C-S-H gels, and the residual C-S-H gels are also attached to the fibres surface, which improves the dense degree of BFGT320. At this time, the fibres and mortar can be tightly and organically bonded, and the space microstructure around the fibres can be fully filled with hydration products. When BFGT320 is subjected to load, a higher frictional resistance can be formed in the process of pulling out the fibres inside it, thus improving the mechanical properties of BFGT320 to the greatest extent.

3.2.2. The Hydration Products of BFR-GTCM

In this paper, the hydration products of BFRGTCM are analysed by XRD, and the results are shown in Figure 12. The results show that when graphite tailings are blended alone, graphite tailings can reduce the peak intensity and envelope area of CH and CaCO3, and their envelope areas are reduced by 10.0% and 2.2%, respectively. Meanwhile, the incorporation of graphite tailings also promotes the increase in C-S-H peak intensity and envelope area, and its envelope area increases by 10.3%. This indicates that the admixture of graphite tailings contributes to the generation of C-S-H in cement mortar, thus enhancing its mechanical properties. When doped with basalt fibres, BFGT120, BFGT220, and BFGT320 show weak and lower changes in peak intensity of CH and CaCO3 than the base line group (BFGT000) compare to BFGT020. This is due to the water absorption of the blended basalt fibres, which inhibits the production of Ca(OH)2 and CaCO3; meanwhile, the lesser CH hinders the production and development of CaCO3, which also contributes to the decrease of CaCO3. In addition, the peak intensity and relative phase content of C-S-H increase with the compounding of graphite tailings with basalt fibres. For example, the C-S-H envelope area of BFGT320 increases by 69.0% compared with that of BFGT000, which indicates that the compounding of graphite tailings with basalt fibres can effectively promote the formation of C-S-H, fill the internal pores and positively affect the mechanical properties of BFR-GTCM to some extent. To sum up, the main reason for the improvement of the mechanical properties of cement mortar is due to the consumption of CH by the incorporation of graphite tailings and basalt fibres, which in turn promotes the generation of C-S-H, thus optimizing the microstructure, hydration degree and compactness of BFR-GTCM.

3.2.3. Analysis of Functional Group Changes in BFR-GTCM

Figure 13 shows the main functional group evolution pattern of BFR-GTCM. It can be found that the incorporation of 20% graphite tailings mainly causes the contraction vibration of -OH (3640) functional group in Ca(OH)2 without the incorporation of basalt fibres. With the addition of basalt fibres, the absorption peaks of -OH (3640) functional group vibrations changes from strong to weak, indicating that graphite tailings and basalt fibres inhibit the formation of Ca(OH)2 in cement mortar [49]. This may be caused by the hydrophilic nature of graphite tailings and basalt fibres, resulting in insufficient bound water in the mortar during hydration; it may also be reflected laterally by the change in the bending vibration of H-O-H (1640) to lower the absorption peak of the -OH (3640) functional group. In addition, the absorption peak of Si-O (815) functional group increases significantly with the increase in basalt fibre admixture and graphite tailings replacement rate, indicating the increase in C-S-H generation in the cementitious material. At the same time, this result also validates the conclusion drawn by SEM. In summary, the combination of basalt fibres with graphite tailings significantly promotes more Ca(OH)2 consumption and conversion to C-S-H formation in BFR-GTCM [50], and ultimately optimizing the mechanical properties of BFR-GTCM.

3.2.4. The Pore Structure Distribution of BFR-GTCM

This paper characterises the changes in pore structure within the mortar by mercury-pressure testing. Figure 14a and Table 6 show the test results of pore size distribution, porosity, cumulative pore volume, total pore volume and total pore area of BFR-GCTM, where the cumulative pore volume percentages are classified according to the principles of harmless pores (<20 nm), less harmful pores (20–100 nm), harmful pores (100–1000 nm) and multi-harmful pores (>1000 nm) [51]. The cumulative pore volume percentages are shown in Figure 14b. The results show that for cement mortar without basalt fibres, the incorporation of 20% graphite tailings resulted in a shift of the main peak position of the pore structure distribution from 51.8 nm to 40.3 nm and a decrease in the porosity from 13.277% to 13.003%, or 0.274%. At the same time, the cumulative pore volume curve of pores larger than 1000 nm (multi-harmful pores) shifts downward, and the cumulative pore volume percentage decreases from 34.53% to 24.56%. This indicates that graphite tailings have an optimization effect on the pore structure of cement mortar, which leads to the improvement of water absorption, compressive strength and elastic modulus in its macroscopic properties. With the increase in basalt fibre admixture, the main peak decreases from 0.0625 mL/g to 0.0524 mL/g, and the cumulative pore volume curve of greater than 1000 nm (multi-harmful pores) shifts first upward and then downward to the lowest, and the cumulative pore volume increases and then decreases to 15.860%, 16.985% and 12.960%. At 0.3% basalt fibre content, the percentage of multi-harmful pores (>1000 nm) in the cumulative pore volume percentage decreases to the lowest level of 17.62%, corresponding to the macroscopic performance of BFGT320, which is also improved to a larger extent. However, when the basalt fibre content is 0.2%, the envelope area of the pore structure distribution of BFGT220 increases to the maximum, the cumulative pore volume curve shifts to the uppermost end, and the pore size is mainly distributed at >10,000 nm. In addition, the cumulative pore volume percentage of harmless pores (>1000 nm) in BFGT220 is at the highest value, 45.67%, which is 32.26% higher than that of the reference group. This unreasonable pore structure distribution eventually led to the abrupt decrease in the mechanical properties of BFGT220.
In summary, the incorporation of 0.3% basalt fibres can improve the pore structure distribution to some extent, increasing the percentage of pore structure with less harmful pores (20–100 nm) and reducing the percentage of more harmful pores (>1000 nm), because the organic combination of 0.3% basalt fibres and 20% graphite tailings will promote the generation of the main hydration products of cement mortar, and can effectively fill the internal space and thus improve the pore structure distribution. However, when 0.2% basalt fibres are incorporated, the agglomeration of fibres inside the cement mortar occurs, which hinders the generation and development of hydration products of cementitious materials. Simultaneously, free water exists at the end of the fibres, which is released back into the mortar by migration, thus forming more harmful pores and multi-harmful pores, and finally reducing the mechanical properties of BFR-GTCM.

3.3. The Synergistic Enhancement Mechanism of BF and GT to BFR-GTCM

The basic mechanical property test of BFR-GTCM shows that basalt fibres can improve the strength and toughness of graphite tailings cement mortar, especially using 0.3% basalt fibre content and 20% graphite tailings replacement rate, basalt fibres and graphite tailings can give full play to their synergistic reinforcing effect. Figure 15 reveals the strengthening and toughening mechanism of basalt fibres to graphite tailings from a microscopic perspective, taking the flexural strength test of BFR-GTCM as an example. The positive effects generated by the composite effect of basalt fibre and graphite tailings are produced by three main aspects: (1) the bridging effect of basalt fibres and cement matrix. Basalt fibres are randomly distributed in the cement matrix, and the fibres can withstand most of the lateral tension when subjected to lateral loads, slowing down the stress concentration and effectively preventing the expansion of cracks, thus producing good toughness. (2) the enhancement of adhesion and occlusion force. As graphite tailings belong to superfine sand, the number of particles and obvious angles, the friction force generated by its filling action can enhance the adhesion and occlusion force between fibres and hydration products and limit the deformation of mortar. At the same time, the randomly distributed discrete fibres interweave rows into a spatial mesh structure under the action of adhesive and occlusal forces, which provides an important influence on the stress distribution and deformation coordination of cement mortar under the action of load. (3) the reduction of porosity. Basalt fibres are easily broken into fine filaments during the mixing process, and these small-diameter fibres can fill the capillary pores in the matrix and improve the internal compactness and interfacial bond strength. (4) the promotion of the generation of major hydration products and groups. The volcanic ash activity of graphite tailings facilitates the bonding of [-OH] and [Si-O] functional groups during the hydration process, leading to more Ca(OH)2 being consumed for conversion to form C-S-H, thus improving the microstructure of the mortar. In summary, the bridging effect can significantly reduce the brittleness and deformation of cement mortar. The enhancement of adhesion and occlusion can delay the time of cement mortar fracture, and the filling capillary effect can improve the internal pores of cement mortar and the denseness is enhanced. However, it is worth noting that poor flowability will indirectly lead to fibres agglomeration, which directly affects the enhancement of its mechanical properties, so better workability is a prerequisite. Considering the workability, mechanical properties (especially bending strength), microstructure morphology and hydration product composition of BFR-GTCM, 0.3% basalt fibre content and 20% graphite tailings replacement rate are selected as the optimal admixture under the mechanical response of BFR-GTCM.

4. Conclusions

In this paper, basic mechanical tests and microscopic analysis of the mechanical properties of BFR-GTCM were performed, and the following conclusions are obtained.
(1) The performance of BFR-GTCM with a graphite tailings replacement rate of up to 30% can meet the practical requirements, and the flexural and compressive strengths of BFR-GTCM are essentially the same at 3, 7, 14 and 28 d of age. In addition, the optimum basalt fibre content and graphite tailings replacement rate can effectively prevent the infiltration of external moisture; minimize the water absorption of BFR-GTCM and maximize the flexural strength, compressive strength and modulus of elasticity.
(2) Combined with SEM, XRD, FTIR and MIP microstructure analysis shows that the uniformly dispersed basalt fibres have good bridging and filling effects, which can effectively enhance the bonding between the fibres and the matrix, optimise the pore structure distribution, promote the generation of the main hydration products C-S-H and increase the phase content, thus making the mechanical properties of the cement mortar (especially flexural and cracking toughness) significantly improved.
(3) Based on the data obtained in this study, GT shows great potential for application in the resource reuse of solid waste. In conclusion, 0.3% basalt fibre content and 20% graphite tailings replacement rate are the optimal dosing for the mechanical response of BFR-GTCM under the conditions. Its excellent flexural properties provide guidance for the service of practical structures in special environments.

Author Contributions

C.Z.: Software, Formal software, formal analysis, Investigation, data investigation, data curation, Conceptualization, Methodology, conceptualization, methodology writing—original draft; B.L.: Methodology, Formal methodology, software, formal analysis, Investigation, data investigation, data curation, Visualization. Y.Y.: data resources, data curation, writing—review and editing. Y.Z.: data curation, writing—review and editing; H.X.: data curation, writing—review and editing. W.-x.W.: data curation, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by 2021 Guangdong Provincial Department of Education, Guangdong University Scientific Research Project-Special Project for Young Innovative Talents, grant number 2021KQNCX083 and Key Project of Natural Science Foundation of Heilongjiang Province, grant number JJ2022ZD0097.

Data Availability Statement

Our study did not report any data.

Acknowledgments

The authors thank the team members from ASIM Group, China, the support from Foshan Intelligent Land and Ocean Engineering Materials Engineering Technology Research and Development Center, Foshan, China.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Material appearance, (b) particle size distribution, (c) granular gradation of GT and sand.
Figure 1. (a) Material appearance, (b) particle size distribution, (c) granular gradation of GT and sand.
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Figure 2. Appearance and morphology of basalt fibres.
Figure 2. Appearance and morphology of basalt fibres.
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Figure 3. Flowchart of experimental methods.
Figure 3. Flowchart of experimental methods.
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Figure 4. Effect of basalt fibres on workability of graphite tailings cement mortar.
Figure 4. Effect of basalt fibres on workability of graphite tailings cement mortar.
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Figure 5. Effect of basalt fibres on Water absorption and surface moisture content of graphite tailings cement mortar. (a) water absorption; (b) surface moisture content.
Figure 5. Effect of basalt fibres on Water absorption and surface moisture content of graphite tailings cement mortar. (a) water absorption; (b) surface moisture content.
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Figure 6. Test results of early age BFR-GTCM flexural strength: (a) flexural strength at 3 days of age; (b) flexural strength at 7 days of age; (c) flexural strength at 14 days of age.
Figure 6. Test results of early age BFR-GTCM flexural strength: (a) flexural strength at 3 days of age; (b) flexural strength at 7 days of age; (c) flexural strength at 14 days of age.
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Figure 7. Test results of 28-day age BFR-GTCM flexural strength: (a) flexural strength at 28 days of age; (b) change in flexural strength at 28 days of age (based on BFGT000).
Figure 7. Test results of 28-day age BFR-GTCM flexural strength: (a) flexural strength at 28 days of age; (b) change in flexural strength at 28 days of age (based on BFGT000).
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Figure 8. Test results of early age BFR-GTCM compressive strength: (a) compressive strength at 3 days of age; (b) compressive strength at 7 days of age; (c) compressive strength at 14 days of age.
Figure 8. Test results of early age BFR-GTCM compressive strength: (a) compressive strength at 3 days of age; (b) compressive strength at 7 days of age; (c) compressive strength at 14 days of age.
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Figure 9. Test results of 28-day age BFR-GTCM compressive strength: (a) compressive strength at 28 days of age; (b) change in compressive strength at 28 days of age (based on BFGT000); (c) compression-flexural ratio of BFR-GTCM at 28 days of age.
Figure 9. Test results of 28-day age BFR-GTCM compressive strength: (a) compressive strength at 28 days of age; (b) change in compressive strength at 28 days of age (based on BFGT000); (c) compression-flexural ratio of BFR-GTCM at 28 days of age.
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Figure 10. Test results of 28-day age BFR-GTCM elastic modulus: (a) elastic modulus at 28 days of age; (b) change in elastic modulus at 28 days of age (based on BFGT000).
Figure 10. Test results of 28-day age BFR-GTCM elastic modulus: (a) elastic modulus at 28 days of age; (b) change in elastic modulus at 28 days of age (based on BFGT000).
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Figure 11. Micromorphology of BFR-GTCM for 28 days: (a) BFGT000; (b) BFGT020; (c) BFGT120; (d) BFGT220; (e) BFGT320.
Figure 11. Micromorphology of BFR-GTCM for 28 days: (a) BFGT000; (b) BFGT020; (c) BFGT120; (d) BFGT220; (e) BFGT320.
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Figure 12. Hydration product analysis of BFR-GTCM for 28 days: (a) XRD diffraction pattern; (b) partial enlargement of the hydration product CH; (c) partial enlargement of the hydration product CaCO3; (d) partial enlargement of the hydration product AFt; (e) partial enlargement of the hydration product C-S-H.
Figure 12. Hydration product analysis of BFR-GTCM for 28 days: (a) XRD diffraction pattern; (b) partial enlargement of the hydration product CH; (c) partial enlargement of the hydration product CaCO3; (d) partial enlargement of the hydration product AFt; (e) partial enlargement of the hydration product C-S-H.
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Figure 13. Functional group analysis of BFR-GTCM for 28 days.
Figure 13. Functional group analysis of BFR-GTCM for 28 days.
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Figure 14. Pore structure distribution analysis of BFR-GTCM for 28 days: (a) pore size distribution; (b) cumulative pore volume percentage.
Figure 14. Pore structure distribution analysis of BFR-GTCM for 28 days: (a) pore size distribution; (b) cumulative pore volume percentage.
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Figure 15. The synergistic enhancement mechanism of BF and GT to BFR-GTCM.
Figure 15. The synergistic enhancement mechanism of BF and GT to BFR-GTCM.
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Table
Table 1. Mechanical properties of Portland cement.
Table 1. Mechanical properties of Portland cement.
Flexural Strength (MPa)Compressive Strength (MPa)FinenessSetting Time (min)
3 days28 days3 days28 days1.2Initial settingFinal setting
4.27.119.443.6173268
Table
Table 2. Chemical properties of Portland cement, sand and graphite tailings (%).
Table 2. Chemical properties of Portland cement, sand and graphite tailings (%).
MaterialsChemical Compositions
CementCaOSiO2Al2O3Fe2O3MgOSO3Loss
59.6522.475.804.323.242.082.44
GTCaOSiO2Al2O3Fe2O3MgOSO3V2O5K2OLoss
15.5562.5010.215.072.330.540.422.261.12
SandSiO2Loss
99.880.12
Table
Table 3. Properties of sand and graphite tailings.
Table 3. Properties of sand and graphite tailings.
Physical PropertiesFine Aggregate
TypeNatural standGraphite tailings
Size (mm)0.17–50.2–0.4
Apparent density (kg/m3)26202855
Bulk density (kg/m3)16301540
24-h water absorption (%)16.8037.10
Fineness modulus2.490.90
pH value7.0010.30
Table
Table 4. Physical properties of basalt fibres.
Table 4. Physical properties of basalt fibres.
FibreLength (mm)Diameter (μm)Density (g/cm3)Elastic Modulus (GPa)Tensile Strength (MPa)Elongation at Break (%)
BF12172.910033003.1
Table
Table 5. Mixing proportions of mortar mixtures (kg/m3).
Table 5. Mixing proportions of mortar mixtures (kg/m3).
Sample NumberCement (kg/m3)Sand (kg/m3)Graphite Tailings (kg/m3)Water (kg/m3)Fiber Content (%)
BFGT000772.141158.210308.860
BFGT100772.141158.210308.860.1
BFGT200772.141158.210308.860.2
BFGT300772.141158.210308.860.3
BFGT010772.141042.39126.21308.860
BFGT110772.141042.39126.21308.860.1
BFGT210772.141042.39126.21308.860.2
BFGT310772.141042.39126.21308.860.3
BFGT020772.14926.57310.09308.860
BFGT120772.14926.57310.09308.860.1
BFGT220772.14926.57310.09308.860.2
BFGT320772.14926.57310.09308.860.3
BFGT030772.14810.75378.63308.860
BFGT130772.14810.75378.63308.860.1
BFGT230772.14810.75378.63308.860.2
BFGT330772.14810.75378.63308.860.3
BFGT040772.14694.93463.28308.860
BFGT140772.14694.93463.28308.860.1
BFGT240772.14694.93463.28308.860.2
BFGT340772.14694.93463.28308.860.3
BFGT050772.14579.11579.11308.860
BFGT150772.14579.11579.11308.860.1
BFGT250772.14579.11579.11308.860.2
BFGT350772.14579.11579.11308.860.3
Notes: GT is the graphite tailings used to replace the river sand, and BF is the basalt fibre added to the material system. BFGT110 is the basalt fibres-graphite tailing cement mortar in which 0.1%BF + 10% GT.
Table
Table 6. Porosity, total pore size and area of BFR-GTCM.
Table 6. Porosity, total pore size and area of BFR-GTCM.
SpecimenPorosity (%)Total Intruded Volume (mL/g)Total Surface Area (m2/g)
BFGT00013.2770.03633.3622
BFGT02013.0030.03994.0772
BFGT12015.8600.04493.0707
BFGT22016.9850.04304.6107
BFGT32012.9600.03952.6285
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Zhang, C.; Li, B.; Yu, Y.; Zhang, Y.; Xu, H.; Wang, W.-x. An Investigation of the Mechanical Properties of Basalt Fibre-Reinforced Graphite Tailings Cement Mortar. Buildings 2022, 12, 2106. https://doi.org/10.3390/buildings12122106

AMA Style

Zhang C, Li B, Yu Y, Zhang Y, Xu H, Wang W-x. An Investigation of the Mechanical Properties of Basalt Fibre-Reinforced Graphite Tailings Cement Mortar. Buildings. 2022; 12(12):2106. https://doi.org/10.3390/buildings12122106

Chicago/Turabian Style

Zhang, Chen, Ben Li, Ying Yu, Yu Zhang, Hu Xu, and Wen-xue Wang. 2022. "An Investigation of the Mechanical Properties of Basalt Fibre-Reinforced Graphite Tailings Cement Mortar" Buildings 12, no. 12: 2106. https://doi.org/10.3390/buildings12122106

APA Style

Zhang, C., Li, B., Yu, Y., Zhang, Y., Xu, H., & Wang, W.-x. (2022). An Investigation of the Mechanical Properties of Basalt Fibre-Reinforced Graphite Tailings Cement Mortar. Buildings, 12(12), 2106. https://doi.org/10.3390/buildings12122106

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