Reutilisation of coal gangue and fly ash as underground backfill materials for surface subsidence control
Introduction
During coal mining and coal washing, as much as 10%–15% of the mass of produced coal is generated as coal gangue (Jiang et al., 2019, Qi et al., 2018, Wu et al., 2015a). The accumulated coal gangue (Fig. 1(a)) not only occupies land, affects the atmospheric environment and damages groundwater resources but is also a potential source of landslides, debris flows and spontaneous combustion (Li et al., 2019, Ma et al., 2019, Mjonono et al., 2019, Wu et al., 2019), which restricts development around coal mines (Adibee et al., 2013, Li and Wang, 2019). Meanwhile, as shown in Fig. 1(b), after burning coal in power plants, a large amount of fly ash is generated that accumulates on the surface (Capasso et al., 2019, Deng et al., 2017), which occupies large areas of otherwise cultivated land. Furthermore, hazardous trace elements threaten the surrounding water or soil resources through leaching and migration (fly ash is also one of the main sources of aerosols, which are air pollutants).
In fact, coal gangue and fly ash are not merely wastes but are important (and readily available) resources. As such, scholars have conducted a series of studies on the reutilisation of coal gangue and fly ash. Qiu et al. (2010) carried out calcination tests on mixtures of gangue with medium-calcium limestone and pyrite slag and revealed that released trace elements and geological energy accelerate the curing of raw cements and reduce sulphide pollution in the calcining process. Long et al. (2019) analysed the selenium content in coal gangue over different periods of time in each region of China and suggested that coal gangue is a potential source of selenium-rich fertiliser. Jia (2005) calcined coal gangue in a nitrogen atmosphere and synthesised SiC to determine the influence of temperature on SiC products prepared with coal gangue. Sarangi et al. (2001) improved paddy soils using different ratios of fly ash. The experimental results demonstrated that fly ash significantly promotes the growth of organisms. Using fly ash as an embankment material, Sekine and Sunaga (1991) tested a full-sized embankment and studied the protocols and stability around the implementation of fly ash. Shekhovtsova et al. (2018) described the characteristics of fly ash when applied to an alkali-activated cement system and established a method to determine the reactions of low-calcium fly ash in alkali-activated systems to characterise the strength of alkali-activated fly ash used as a binder. In addition, solid wastes, i.e., gangue and fly ash, have been widely used in industrial brick making, land reclamation in subsidence areas of coal mines and roadbed backfill (Fan et al., 2014, Hu and Xiao, 2013, Karan et al., 2016, Wu et al., 2015b).
To date, gangue and fly ash are not utilised on a large scale, and the comprehensive rate of utilisation remains low, so environmental pollution problems caused by coal gangue and fly ash are a pressing issue (Xu et al., 2017). To dispose of such solid wastes more efficiently, scholars have proposed methods that involve backfill such wastes into an underground goaf. In the method of backfill mining with solid wastes, backfill materials are delivered into the mine to fill the goaf formed after coal mining with the purpose of supporting the overlying strata to effectively control surface subsidence. Backfill materials mixed with fly ash and gangue are dispersed with different gap sizes between particles. For these reasons, the compression ratio of mixed backfill materials is an important factor that influences the control effects of strata movement and surface subsidence; this compression ratio is mainly determined by the ratio of gangue to fly ash.
By taking coal gangue and fly ash as the research objects, we tested the characteristics of the compressive deformation of samples with different gangue and fly ash ratios by utilising a servo-motor controlled testing machine and a homemade compression apparatus. Moreover, the analysis and discussion were performed based on the stress-strain relationships and changes in the porosity obtained from the tests, which allowed optimising the material ratio. The deformation resistance mechanisms of the gangue and fly ash as backfill materials were revealed. Finally, based on the prevailing conditions at the Coal Mine, the underground goaf was backfilled using the optimal material ratio. The surface subsidence above the working face was then monitored to verify the effects on strata movement and surface subsidence when using gangue and fly ash as backfill materials.
Section snippets
Raw materials and sample preparation
Scanning electron microscopy (SEM) images of the coal gangue and fly ash are shown in Fig. 2. The gangue has dense particles with uneven surfaces and spaces filled with small particles, indicating that it can be used as a skeleton structure. The accumulated fly ash has an irregular sheet structure with a fluffy interior and high plasticity, so it can be used as a material to fill the gaps between gangue particles.
The coal gangue and fly ash used in the experiments were collected from a gangue
Stress-strain relationships
The stress-strain relationships of the gangue and fly ash samples at different mixing ratios are plotted in Fig. 7. From Fig. 7, when the compressive stress increased to 25 MPa, the stress-strain curves of the mixed gangue and fly ash samples for each ratio were distributed in an exponential form. As the compressive stress increased, the compressive strain in the samples increased, while the deformation rate decreased. Taking the ratio of 1:1 as an example, when the compressive stress increased
Site conditions
Coal mining under the buildings situated above the Coal Mine is an extreme challenge that has affected the production and replacement of coal mines in the region. Furthermore, a large amount of gangue and fly ash is discharged from the mines each year and directly accumulates on the ground, which affects the environment in and around the mining region. To address these problems, the backfilling of solid wastes into the goaf was performed in the this Coal Mine. The research considered the
Conclusions
In this paper, we characterised the compressive deformation of samples under different gangue-to-fly ash ratios and analysed the stress-strain relationships and changes in the sample porosity. In addition, this research revealed the mechanisms for the deformation resistance of gangue and fly ash as backfill materials from a macro and meso perspective, and the optimal ratio of gangue to fly ash was found. The main conclusions are as follows:
- (1)
The ratio of the samples affects the compressive
Author contributions section
All the authors contributed to this paper. Meng Li prepared the manuscript and perform the experiments. Jixiong Zhang provided theoretical and methodological guidance in the research process. Ailing Li participated in the literature search and data processing. Nan Zhou participated in revising the manuscript and figures plotting.
Declaration of competing interest
The authors declare that the work described has not been published before; that it is not under consideration for publication anywhere else; that its publication has been approved by all co-authors; that there is no conflicts of interest regarding the publication of this article.
Acknowledgements
This work was supported by the National Postdoctoral Program for Innovative Talents [grant number BX20180361]; the National Natural Science Foundation of China [grant number 51874287]; the National Science Fund for Distinguished Young Scholars [grant number 51725403] and the China Postdoctoral Science Foundation [grant number 2018M642366].
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