Abstract
Properties of rock‐cemented coal gangue‐fly ash backfill (CGFB) bimaterials determine the effects of strip CGFB mining on controlling the surface subsidence in coal mines, which are affected by the coal gangue particle size in CGFB. In this paper, uniaxial compression tests were conducted on the coarse sandstone‐CGFB composite samples with different coal gangue particle sizes, and their strength, acoustic emission (AE), and failure characteristics were investigated. The uniaxial compressive strength (UCS) and elastic modulus of the composite sample decreased with the coal gangue particle size. The strength of the composite sample is mainly dependent on that of CGFB in it, affected by interactions between CGFB and coarse sandstone. The deformation of the coarse sandstone weakened the damage accumulation within CGFB, resulting in the strength of the composite sample larger than that of CGFB. The average UCS values of composite samples with coal gangue particle sizes of 0∼5 mm, 5∼10 mm, and 10∼15 mm, increased by 10.78%, 14.98%, and 12.70% compared with CGFB in them, respectively. AE event signal regularity of the composite sample was divided into three stages: rising period, calm period, and active period. The intensity and frequency of AE event signals in three periods were strengthened with the coal gangue particle size. The calm period can be taken as the precursory information for the failure and instability of composite sample under loading, whose duration became shortly with the coal gangue particle size. The rebound deformation of coarse sandstone caused the fluctuations of AE event signals at the later stage of active period. The failures of the composite sample occurred within CGFB, and no obvious failures were found in the coarse sandstone. The CGFB mainly experienced the splitting failure accompanying by varying degrees of surface spalling failures. The broken degree of CGFB increased with the coal gangue particle size, and the largest weight ratio of CGFB fragments (chips) after failure was determined by the coal gangue particle size.
Highlights
The coarse sandstone was used as the rock material, which was collected from the immediate roof in the 4338 working face of Daizhuang Coal Mine of Shandong Province, China. e coal gangue-fly ash backfill (CGFB) composed of cement, fly ash, coal gangue, and water
Strength Characteristics of Composite Samples. e uniaxial compressive strength (UCS) values and elastic moduli of coarse sandstone-CGFB composite samples with different coal gangue particle sizes are compared in Figures 6(a) and 6(b), respectively
From the failure characteristics of composite samples, their overall strength is determined by the strength of CGFB in them. e stress-strain curve can well reflect the mechanical properties of the sample under loading [42]
Summary
According to previous investigations [18, 31, 32], the mass ratio of cement to fly ash to coal gangue was 1 : 4 : 6 in CGFB, and the solid materials comprised 78% of the total weight of CGFB. Both coarse sandstone and CGFB had a same height of 50 mm in the standard composite samples (Φ 50 × 100 mm). An AG-X250 servo-controlled testing system, as the loading system, was used to carry out the uniaxial compression tests on coarse sandstone-CGFB composite samples with different coal gangue particle sizes. Compared with group A, the average UCS values of groups B and C decrease by 20.40% and 40.45%, respectively, and the corresponding average elastic moduli decrease by 10.44% and 22.09%, respectively. ese illustrate that the strength and elastic modulus of the composite sample generally decrease with the coal gangue particle size, which are consistent with the effects of the coal gangue particle size on that of pure CGFB samples [20]
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