ALBERT

Description: The present study investigates the combined capacity of a newly developed slag-blended cement (MC) and fly ash (FA) as a sustainable solution towards improving the mechanical performance of the cemented paste backfill (CPB) system of a copper-gold underground mine. A total of thirteen mix designs consisting of three MC-treated and ten MC + FA-treated blends were examined. Samples were prepared with a solids content of 77% (by total mass), and were allowed to cure for 7, 14, 28, 56 and 128 days prior unconfined compression testing. Scanning electron microscopy (SEM) studies were also carried out to observe the evolution of fabric in response to MC and MC + FA amendments. The greater the MC content and/or the longer the curing period, the higher the developed strength, toughness and stiffness. The exhibited improvements, however, were only notable up to 56 days of curing, beyond of which the effect of curing was marginal. The performance of 4% Portland cement or PC (by total dry mass) was found to be similar to that of 1.5% MC, while the higher MC inclusions of 2.5% and 3%, though lower in terms of binder content, unanimously outperformed 4% PC. The use of FA alongside MC improved the bonding/connection interface generated between the tailings aggregates, and thus led to improved mechanical performance compared with similar MC inclusions containing no FA. Common strength criteria for CPBs were considered to assess the applicability of the newly introduced MC and MC + FA mix designs. The mix designs “3% MC” and “2.5% MC + 2–2.5% FA” satisfied the 700 kPa strength threshold required for stope stability, and thus were deemed as optimum design choices.

Description: The present study examines rubber’s capacity of improving the swell–shrink potential of expansive clays. Two rubber types of fine and coarse categories with different geometrical features were considered. The test program consisted of standard Proctor compaction and cyclic wetting–drying tests. Scanning electron microscopy (SEM) analysis was also performed to identify the soil–rubber amending mechanisms, and to observe the evolution of fabric in response to alternate wetting and drying. Cyclic wetting–drying led to the reconstruction of the soil/soil–rubber microstructure by way of inducing aggregation and cementation of the soil grains. The greater the number of applied cycles, the lower the swell–shrink features, following a monotonically decreasing trend, with the rubberized blends holding a notable advantage over the virgin soil. The tendency for reduction, however, was in favor of a larger rubber size, and more importantly the rubber’s elongated form factor; thus, predicating a rubber size/shape-dependent amending mechanism. The soil–rubber amending mechanisms were discussed in three aspects—increase in non-expansive content, frictional resistance generated as a result of soil–rubber contact, and mechanical interlocking of rubber particles and soil grains. The swell–shrink patterns/paths indicated an expansive accumulated deformation for the virgin soil, whereas the rubberized blends manifested a relatively neutral deformational state, thereby corroborating the rubber’s capacity to counteract the heave and/or settlement incurred by alternate wetting and drying.

Description: Minerals, Vol. 8, Pages 381: Effects of Superplasticizer on the Hydration, Consistency, and Strength Development of Cemented Paste Backfill Minerals doi: 10.3390/min8090381 Authors: Jian Zhang Hongwei Deng Abbas Taheri Junren Deng Bo Ke The strength and consistency of cemented paste backfill (CPB) are of key concerns in the stope stability and cost control for underground mines. It is common practice to use additives, such as superplasticizer, to improve the performance of CPB. This study mainly focuses on the effects of superplasticizer on the hydration, consistency, and strength of CPB. In this study, a polynaphtalene sulfonate was used as the superplasticizer. The binder is a mix of 33.3% ordinary Portland cement and 66.7% fly ash. The CPB specimens with a tailings-binder ratio of 3:1 and a solid concentration of 70% were then tested by a low field nuclear magnetic resonance system after different hydration times. Effects of polynaphtalene sulfonate on the hydration, fluidity, and strength were investigated. Results showed that the polynaphtalene sulfonate has a strong influence on short-duration hydration, which may contribute to the strength increase of CPB. It has been demonstrated that the polynaphtalene sulfonate improved the fluidity of the CPB mixture. With the increased dosage of polynaphtalene sulfonate, the slump increased. It was also found that the polynaphtalene sulfonate dosage has a negligible effect on the 1 day (d) strength while it has a strengthening effect on the 7 d, 14 d, and 28 d strength of CPB specimens.

Description: Using gangue backfilling in underground coal mining not only controls the roof deformation in the gob area but also reduces the amount of mining waste rock. However, due to the limitations of the complicated engineering conditions, backfilling mining in the steep coal seam is not widely applied. In this study, a long-distance backfilling technology with a scraper winch for a steep coal seam was proposed and applied in a flexible shield supporting working face in Datai Mine, Beijing. Aiming at the problem of the decreasing backfilling ratio in field practice, numerical simulation was carried out to research the moving law of gangue in the goaf. The gangue mainly experienced four stages: gangue landslide stage, small-scale subsidence stage, funnel-shaped subsidence stage, and large-scale subsidence stage. The moving area of the gangue could be divided into five areas including a motionless area, a landslide area, a subsidence area, a funnel-shaped subsidence area, and a to-be-backfilled area. With the increase of the inclined length of the working face, the moving time of the gangue increased gradually. Based on the simulation results, the scheme of backfilling and mining in Datai Mine was optimized, for which the inclined length of the working face was shortened, and a higher backfilling ratio was obtained.

Description: The strength and consistency of cemented paste backfill (CPB) are of key concerns in the stope stability and cost control for underground mines. It is common practice to use additives, such as superplasticizer, to improve the performance of CPB. This study mainly focuses on the effects of superplasticizer on the hydration, consistency, and strength of CPB. In this study, a polynaphtalene sulfonate was used as the superplasticizer. The binder is a mix of 33.3% ordinary Portland cement and 66.7% fly ash. The CPB specimens with a tailings-binder ratio of 3:1 and a solid concentration of 70% were then tested by a low field nuclear magnetic resonance system after different hydration times. Effects of polynaphtalene sulfonate on the hydration, fluidity, and strength were investigated. Results showed that the polynaphtalene sulfonate has a strong influence on short-duration hydration, which may contribute to the strength increase of CPB. It has been demonstrated that the polynaphtalene sulfonate improved the fluidity of the CPB mixture. With the increased dosage of polynaphtalene sulfonate, the slump increased. It was also found that the polynaphtalene sulfonate dosage has a negligible effect on the 1 day (d) strength while it has a strengthening effect on the 7 d, 14 d, and 28 d strength of CPB specimens.

Description: Coal is the basic resource underpinning energy generation in China, however, constant, large-scale mining of coal results in many problems such as ecological destruction of mining areas. As a result, backfilling of solid waste underground is proposed to control strata and surface subsidence and to protect the environment. At present, these materials, such as granular material, cemented material and high-water-content materials are mainly used for backfilling. This study summarised the types of backfill materials that are used in coal mines in China along with the backfilling process. Moreover, distribution and characteristics of mines backfilled with these backfill materials were obtained and analysed. Considering the socio-environmental aspects that affect backfilling, this research proposed a guideline for the selection of backfill materials and then analysed specific engineering cases of three backfill materials. In addition, the future development of backfill materials was discussed. With extensive extraction of shallow coal resources in China and, therefore, rapid depletion of coal resources in eastern regions of China, coal mining depth is increasing significantly. As a result, it is required to investigate new backfill materials suited for the deep high-stress environment.