New attempt to produce red mud-iron tailing based alkali-activated mortar: Performance and microstructural characteristics

Highlights

• An alkali-activated mortar composed of 75% iron tailing and 16% red mud was produced.

• The alkali-activated mortar showed good environmental stability performance up to 21-days exposure.

• The products generated in the alkali-activated mortar were Ca₃AlFe(SiO₄)(OH)₈ and C-A-S-H gel.

• The products mainly existed as SiQ²(1Al) structure with relative content of 68.12%.

Abstract

This work evaluated the mechanical strength, environmental stability performance, and microstructure of red mud-iron tailing based alkali-activated mortar (AAM) when iron tailing was blended in two ways (as aggregate or mineral admixture) with two kinds of red mud (sintering-process or Bayer-process). The work revealed that the best performance was for the AAM composed of 75% iron tailing as aggregate and 16% Bayer-process red mud, which attained the 28-day compressive and flexural strengths of 33.5 and 6.4 MPa, respectively. This 21-days exposed AAM was resistant to acid, alkali, and sulfate attacks. Its compressive strength attained 34.43, 49.61, and 38.26 MPa after 21-days soaking in 2% HCl, 5% NaOH, and 5% Na₂SO₄ solutions, while its flexural strength attained 6.8, 6.7, and 6.3 MPa, respectively. Moreover, this AAM was resistant to freezing and thawing. After 150 freeze-thaw cycles, its compressive and flexural strengths attained 40.57 and 7.3 MPa, respectively. It is found that the main products generated in this 28-days cured AAM were C-A-S-H gel and Ca₃AlFe(SiO₄)(OH)₈, in which the four-coordinate and six-coordinate Al was 51.56% and 48.44%, respectively, and [SiO₄] structure was mainly existed as SiQ²(1Al) with relative content of 68.12%. This AAM production was deemed to be feasible, which provided a technical foundation for the efficient utilization of iron tailing and red mud.

Introduction

Red mud is a solid waste generated in the process of extracting alumina from bauxite. About 35–40% bauxite enters into the tailing, which is discharged in the form of high alkaline red mud slurry with pH 10.0–12.5. The volume content of solid phase in the red mud slurry is around 15–40% [1,2]. Generally, 1.0–2.5 tons of red mud are discharged for every 1 ton of alumina extracted [3]. The global emission of red mud has reached 120 million tons annually [4,5]. As the alumina industry developed rapidly in China, red mud has accumulated over 300 million tons with an annual discharge of 90 million tons [6]. Depending on the availability of land space near the alumina plant, red mud is usually stockpiled in a dam or discharged directly into the nearby sea [3,7,8]. Red mud has characteristics of high specific surface area, strong water absorption, and high alkalinity. The discharge and storage of vast red mud not only require high treatment cost, but also bring a potential risk to pollute the surrounding environment, such as surface and groundwater pollution, soil alkalization, dust pollution, and suspended particles pollution in the sea [7,9]. Hence, efficient utilization of red mud has become a top priority for alumina extraction industry to meet the objective of sustainable development. Considering that the annual discharge of red mud is tremendous, using red mud for producing building materials is a promising way to largely consume this solid waste, so as to solve the problems of land occupation and potential environmental pollution of red mud. However, there is 2.0–10.0% Na₂O in the red mud, which is existing in the form of Na⁺ in pore solution [10]. It results in a certain degree of strong alkalinity. The high alkali content limits the use of red mud in traditional building materials.

Steel industry is an important pillar industry of national economy in China. With the increasing demand of iron metal and the decreasing grades of iron ore resources, vast tailings are generated in the iron ore beneficiation process [11]. The accumulation of iron tailings reached 5 billion tons in 2015, and the annual emission is over 600 million tons [12]. Most of iron tailings are disposed by stacking, which brings a large number of tailing dams and results in some adverse impacts, such as significant operation and management cost for the tailing dams and harmful elements migration from iron tailings [13,14]. Moreover, iron tailing dams occupy a large amount of arable land, and the negative effects can aggravate to be a further threat towards the surrounding ecological environment and public safety [15]. Therefore, it is very meaningful to realize the sustainable development of iron and steel industry, including the ecological environment protection and efficient utilization of iron tailings. The composition and components of iron tailings vary largely due to the different iron ore origins and beneficiation processes. However, the chemical composition of iron tailing mainly comprises oxides of silicon, aluminum, iron, calcium, and magnesium [14,16], and its main mineral composition contains quartz, feldspar, pyroxene, garnet, and amphibole [14]. As iron tailing is rich in silica and alumina, and its mineral composition is somewhat similar to that of natural sand, it provides a precondition for the general utilization of iron tailing to produce construction materials [11,14,16]. One of the applications is using iron tailing as fine aggregate instead of river sand in concrete [17,18], which can not only solve the problem of iron tailings accumulation, but also alleviate the shortage of river sand. Zhao et al. [17] reported that using iron tailing to completely replace fine aggregate significantly reduced the workability and compressive strength of concrete, and the mechanical properties were not reduced when the content of iron tailing in the fine aggregate was controlled within 40%. Shettima et al. [18] also found that using iron tailing as aggregate instead of river sand reduced the workability of concrete, but it improved the strength and elastic modulus within a proper replacement ratio. Overall, the main problems of using iron tailing instead of river sand in concrete include the finer particle size, smaller fineness modulus, and larger apparent density of iron tailing. It is thought that the iron tailing needs to be modified or mixed with river sand when it is used as fine aggregate in the concrete.

Alkali-activated cement (AAC) is a promising alternative to Portland cement-based materials [19]. It is generally made of aluminosilicate materials and alkaline activator [20]. For the past few years, some scholars have successively prepared AAC by using silica-alumina based solid wastes, such as iron tailing, red mud, blast furnace slag, and fly ash. Duan et al. [21] combined fly ash and iron tailing to prepare AAC, in which a mixed solution of water glass and NaOH was used as stimulant. It was found that the setting time and workability of AAC were significantly affected when the iron tailing content exceeded 20%. Liu and Cui [22] used fly ash and iron tailing as raw materials with the activator of NaOH to prepare AAC. They found that the optimal content of iron tailing was 20–35%, and a complex network structure was generated in the AAC, in which [SiO₄] and [AlO₄] were polymerized to form dimers, trimers, and polymers. Since the alkali content in red mud limits its addition in Portland cement, more and more research has been focused on preparation of AAC by using red mud. Ye et al. [23] used strong alkaline water glass solution to stimulate red mud and blast furnace slag for preparation of AAC. They found that aluminosilicate substances were dissolved in the alkali solution to form nanometer sized particles, which were mainly C-(A)-S-H gels with poor crystallinity and amorphous polymer gels. Lu [24] found that the early strength of AAC prepared by red mud and blast furnace slag increased with the increasing of water glass modulus and the content of water glass, and this AAC was a non-crystalline amorphous material with three-dimensional network structure.

It is noticed that most of the AAC research related to red mud and iron tailing mainly focused on the use of red mud or iron tailing separately to prepare AAC, but the investigation on the combination of red mud and iron tailing to prepare AAC was rarely mentioned. This paper studied the feasibility of using red mud and iron tailing together to produce alkali-activated mortar (AAM), in which the iron tailing was blended in two ways (as aggregate or mineral admixture) with two kinds of red mud (sintering-process or Bayer-process). The mechanical properties and environmental stability performance including resistances to acid, alkali, and sulfate attacks as well as freeze-thaw cycling of the AAM were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and magic-angle spinning nuclear magnetic resonance (MAS NMR) were used to analyze the hydration products and microstructure of the red mud-iron tailing based AAM. This work could provide a useful method for the efficient utilization of red mud and iron tailing.