Given that normal filling technology generally cannot be used for mining in the western part of China, as it has only a few sources for filling gangue, the feasibility of instead using cemented filling materials with aeolian sand as the aggregate is discussed in this study. We used laboratory tests to study how the fly ash (FA) content, cement content, lime–slag (LS) content, and concentration influence the transportation and mechanical properties of aeolian-sand-based cemented filling material. The internal microstructures and distributions of the elements in filled objects for curing times of 3 and 7 days are analyzed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The experimental results show that: (i) the bleeding rate and slump of the filling-material slurry decrease gradually as the fly ash content, cement content, lime–slag content, and concentration increase, (ii) while the mechanical properties of the filled object increase. The optimal proportions for the aeolian sand-based cemented filling material include a concentration of 76%, a fly ash content of 47.5%, a cement content of 12.5%, a lime–slag content of 5%, and an aeolian sand content of 35%. The SEM observations show that the needle/rod-like ettringite (AFt) and amorphous and flocculent tobermorite (C-S-H) gel are the main early hydration products of a filled object with the above specific proportions. After increasing the curing time from 3 to 7 days, the AFt content decreases gradually, while the C-S-H content and the compactness increase.
A method of paste backfill recovery for residual room coal pillars is hereby proposed. The principles and processes of this method are systemically explained to address issues such as mining-induced earthquakes from spontaneous destabilization, surface subsidence, and low recovery rates. These are caused by the instability of residual coal pillars due to their spontaneous combustion in room-and-pillar mining in medium-to-small coalmines in the northern Shaanxi area. This method is based on the local abundance of surface aeolian sand and solid wastes to be used as paste-backfilling materials in coalmines in the northern Shaanxi area. Uniaxial compressive strength, bleeding rate, and slump tests were performed on paste-backfilled samples constituted at different ratios based on the types of materials involved in paste backfilling in the northern Shaanxi region, thereby helping to confirm the optimal ratios for paste-backfilling materials for the Ershike coal mine. A simulation was conducted to investigate the failure, goaf vertical stress distribution, and surface deformation properties of paste-backfilled pillars and coal pillars, where paste backfilling was used with paste-backfilling materials constituted at different compressive strengths. This was to verify the experimental results that would be obtained with paste-backfilling materials constituted at different ratios, and reveal the mechanism by which paste backfilling of residual room pillars can maintain the mine's surrounding rock stability. These study results are of great instructive significance to the safe recovery of residual room pillars in China's western mining areas.Sustainability 2019, 11, 478 2 of 13 leading to severe mining catastrophes such as large-area roof collapses, and even mining-induced earthquakes. Therefore, the problem of how to recover residual room pillar resources safely has become a major issue [8][9][10].Considerable research has been undertaken by scholars in China and other countries regarding the recovery of residual room mining pillar resources. Singh et al. [11] carried out an analysis of the state of coal pillar recovery in India, studying the impact factors on the stability of deep-seated room-and-pillar mine pillars under the condition of high stress; Gan et al. [6] examined the prudent allocation of mining gateways underneath room-and-pillar goafs; Ghasemi et al. [5] proposed a new coal pillar design method to reduce support stress concentration and coal pillar sizes; Haibo et al. [12] studied roof stress on room mining residual pillars and analyzed the critical factors influencing roof stability; Jixiong et al. [13,14] analyzed the impact pattern of compression ratio on coal pillar stability, establishing a mechanical model for solid-filled recovery room pillars, and designed a corresponding coal pillar recovery process; Yun et al. [14] investigated the safe recovery of residual room pillars below an aquifer, proposed a method of short wall block mining, and established a water-conducting fracture zone development heig...
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