## 1 Introduction  
Coal gagne (CG) is a solid waste byproduct of the coal mining process. The mass of CG, which is approximately 15-20% that of coal produced yearly, is one of the largest amounts of solid waste in China. Currently, 1500 gagne mountains have been formed in China, storing more than 3 billion tons of unused CG, covering an area of nearly 20,000 hectares [1]. Similarly, fly ash (FA) constitutes the majority of coal combustion products (CCPs), accounting for approximately 40-90% of the total CCPs. For every 4 tons of coal burned, 1 ton of CCPs is generated, and the worldwide production of CCPs in 2019 was approximately 1.2 billion tons per year [2, 3]. The disposal of CG and FA not only occupies a considerable amount of land, but, due to long-term weathering and leaching, it also causes the accumulation of heavy metals and water pollution in the surrounding soil [4]. Moreover, FA can also produce dust pollution [5], which potentially contaminates the atmosphere. Accordingly, part of the green mining technology consists of the cementation of solid waste resources, such as CG and FA, and using them to backfill the goal of coal mines. The solid waste is prepared as a slurry composite and pumped into the underground goat of the coal mine through backfilling pipelines [6]. The confined environment of the goat could limit, to some extent, the dispersion of any mobile contaminants leaching from the backfilling material [7, 8]. This technology can reduce hazards caused by CG and FA, as well as largely solve a series of other ecological problems, such as the destruction of the underground water system and subsidence due to the traditional method of mining. Specific binders, such as cement and lime, must be used to cement solid waste and produce backfilling materials of coal mines with a prescribed strength [9]. Nevertheless, the production of these binders is considered a serious contributor to occupational health hazards and life-threatening conditions [10; 11]. Phosphogypsum could also bind waste byproducts [12], but the leaching of backfilling materials can release phosphates into the underground water [13]. Other binders, such as polymers and mechanically activated gangue powder, are energy consuming, and their production releases a huge amount of carbon dioxide into the atmosphere [14]. Hence, many researchers have been searching for new binders to tailor more environmentally friendly backfilling materials. This is the case of microbially induced carbonate precipitation (MICP), which is a more sustainable biomineralization technology [15; 16; 17; 18], to be used as a novel binder. It is based on the enzymatic reaction of microorganisms that produce urease through cellular metabolism and catalyze the decomposition of urea to produce NH\({}_{4}\)+ and CO\({}_{3}\)\({}^{2-}\) ions, which, in turn, provide CO\({}_{3}\)\({}^{2-}\) for the mineralization reaction:  
\[\text{CO(NH}_{2})_{2}+2\text{H}_{2}\text{O}\to 2\text{NH}_{4}{}^{+}+\text{ CO}_{3}\text{${}^{2-}$} \tag{1}\]  
When calcium ions exist in the solution and meet the deposition conditions, CO\({}_{3}\)\({}^{2-}\) can be combined with them to form biological calcium carbonate of a certain crystal shape, according to the following reaction [19; 20]:  
\[\text{Ca}^{2+}+\text{CO}_{3}\text{${}^{2-}$}\to\text{CaCO}_{3}\text{(s)} \tag{2}\]  
The mechanism of microbial mineralization is shown in Figure 1[21]. This principle has great potential and good prospects to be developed in many applications. For example, to increase the durability and reduce the maintenance costs of concrete structures, micro-cracks can be continuously repaired by using MICP technology [22; 23; 24]. Some researchers studied the application of the MICP method to reinforce loose soils and improve the strength characteristics of expansive soils with low permeability [25; 26; 27]. In other studies, MICP technology reduced the release of heavy metals into the environment, because heavy metal ions can replace Ca\({}^{2+}\) ions in the biocalcite structure [28; 29; 30; 31; 32].