As the second most important metal after steel, aluminum and its alloys are widely used in construction, transportation, electrical appliances, machinery, and other industries due to their excellent properties . At present, 95% of the world’s aluminum companies use the Bayer process to treat bauxite ore in order to produce alumina and then to obtain aluminum metal through electrolysis; the main solid waste produced is called Bayer process red mud (BRM). Therefore, BRM is a type of red, silty, and strong alkaline main industrial solid waste generated in the process of alumina industry . Statistically, every ton of electrolytic aluminum produced will discharge 2.0–3.6 tons of BRM. At present, the total output of BRM in the world has reached 90 million t/a, the total amount of surface storage has exceeded 2 billion tons, and the annual production will continue to increase in the next ten years .
Traditionally, fresh BRM would be directly discharged and stored in the open-air storage yard without any further treatment . Due to the large amounts of free alkaloids, chemically bound alkaloids, fluoride, and heavy metal ions in BRM, the open surface storage of BRM not only occupies a large amount of land but also easily generates dust and groundwater pollution, and causes ecosystem destruction . The alkaline liquid leached during long-term storage penetrates the ground, polluting water sources and causing soil alkalization . In addition, the exposure of BRM to the air forms dust, which severely pollutes the atmosphere . In 2010, a BRM dam burst in Hungary, and approximately 100,000 cubic meters of highly alkaline BRM leaked, polluting 1017 hectares of agricultural land and making it unsuitable for crops . Taneez  analyzed the significant threat to soil and vegetation posed by BRM due to its high alkalinity and fine grain size resulting from the Bayer process. With the implementation of environmental protection policies and the emphasis on sustainable development, the disposal of BRM has become a critical bottleneck restricting the development of the alumina industry, and placing a heavy burden on socio-economic development and environmental protection . Therefore, research on the safe disposal and comprehensive utilization of BRM has become a new research hotspot.
In view of the potential hydration properties of red mud during sintering and combination, it can be used in the production of cement, bricks for building, glass, and special ceramics, as well as in additives or auxiliary materials for asphalt materials, roadbed materials, thermal insulation materials, and other building materials [11,12,13,14]. BRM with a large specific surface area shows excellent adsorption capacity for metal ions and radioactive elements, which can be used as an adsorbent for environmental restoration, such as waste gas treatment, wastewater treatment, and soil remediation [15,16,17]. In addition, BRM is used to produce coal-burning desulfurizers, polymer water-purifying agents, and siliceous calcium agricultural fertilizers [18,19]. However, there are a large number of alkaline compounds (Na2O and K2O) that are difficult to eliminate, as well as fluorine, aluminum, and many other impurities, which pose an environmental risk in the preparation of building or functional materials . Thus, the contradiction between technology and economy, as well as safety and environmental protection issues, means the comprehensive utilization of BRM is still at the experiment and research stage, limiting its industrial application and large-scale popularity . Currently, the emissions of BRM in China are as high as 600 million tons, while the comprehensive utilization rate is only 4% . Therefore, it is exceptionally urgent to minimize the hazards of BRM and achieve its multichannel and large-scale utilization.
Cemented backfill mining involves a well-proportioned mixture of solid wastes, cementing materials, and water, which are transported from a surface backfilling station to the underground mining goafs by a pipeline; excellent superiority is shown in ground pressure management after consolidation and hardening . As one of the main industrial solid wastes, phosphogypsum contains large amounts of strong acidic phosphates and sulfates, which are difficult to remove but easily pollute the groundwater. The Kaiyang mine in Guizhou province has used phosphogypsum as a backfilling aggregate for more than ten years, thus demonstrating a safe means of disposal of main industrial solid wastes . Coal gangue and fly ash are two other main industrial solid wastes; the Suncun mine in Shandong province has reused them as backfilling aggregate materials for nearly 20 years, which is not only beneficial in ensuring the safety of mining but also provides a new means of solid waste disposal . Zhu  used red mud from the sintering process as a partial replacement of binders in cemented backfill mining practices. However, owing to the unique physicochemical properties of BRM, to date, no studies have been conducted on red mud-based cemented backfill (RMCB) while using such types of BRM . Therefore, it is innovative and meaningful to explore the feasibility of reusing BRM as a backfilling aggregate for the large-scale backfill treatment of mining goafs.
Due to the large number of contaminants in BRM, the open surface storage of BRM may easily generate groundwater and soil pollution, and microorganisms and plants destruction. In order to realize the large-scale industrial application of BRM as a backfilling aggregate for underground mining and simultaneously avoid polluting groundwater, the most important aspect is to prevent the transfer of BRM contaminants from surface storage to the underground goafs. Since there is little soil or microorganisms but lots of hard stones and groundwater in the underground mining goafs, then the main environmental safety problem caused by BRM for backfilling is groundwater pollution. In this paper, the material characteristics of BRM were analyzed through physical, mechanical, and chemical composition tests. The optimum cement–sand ratio and solid mass concentration of the backfilling were obtained based on a large number of mixture proportion tests. Using the results of bleeding, soaking, and toxic leaching experiments, a fuzzy comprehensive evaluation method was used to comprehensively evaluate the environmental impact of BRM on groundwater systems.
2. Materials and Methods
Unlike lateritic bauxite ores in other countries, bauxite ores in China are mainly of the ancient weathering crust type, and nearly 50% are distributed in Shanxi province. Due to the similar minerogenetic conditions and mineral processing methods, BRM in Shanxi province shows similar physical and chemical features . Taking an alumina plant in Shanxi province as an example (see Figure 1), the Bayer process mainly includes the crushing and pulping of bauxite ore, high-pressure digestion of aluminum oxide, separation and washing of red mud, and other production processes. The principle of the Bayer process is dissolving the alumina in bauxite ore with strongly basic NaOH at a high temperature; a sodium aluminate solution and BRM are generated and separated, and then the sodium aluminate decomposes and generates aluminum hydroxide at a low temperature. Finally, the alumina product is obtained after washing and calcining . The Bayer process can be simplified as Equation (1)
Figure 1. Satellite image of an alumina plant in Shanxi province.
About 2 tons of fresh BRM were packed and transported to the backfilling laboratory of Central South University (see Figure 2). The particle size, oxide content, and mineral composition analysis of BRM were commissioned by Changsha Research Institute of Mining and Metallurgy Co., Ltd. which is located in Changsha City, Hunan Province, China by using a Laser Particle Size Analyzer (Mastersizer 2000), an X-Ray Fluorescence Spectrometer, and an X-Ray Diffractometer, respectively. BRM is similar to ultrafine soil particles with its median particle size of 3.248 ?m and a specific surface area of 2940 m2/kg, which are much higher than those of ordinary Portland cement 42.5 (PO 42.5) and slag powder. The specific gravity of BRM is 2.424, the plasticity index is 17.0–30.0, and the permeability coefficient is 3.35 × 10?5 cm/s. There are large amounts of free alkaloids, chemical binding alkaloids, fluoride, and heavy metal ions in BRM, which are difficult to remove and easily pollute the surface environment as a result of open storage (see Table 1)…
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