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.
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.