Abstract
To research the properties of cemented coal gangue‐fly ash backfill (CGFB) exposed to different concentrations of sodium sulfate solutions under drying‐wetting cycles, the mass changes, uniaxial compressive strengths, sulfate ion contents at different depths, and microstructures of CGFB samples were measured in this study. The results show that the CGFB samples were damaged by salt crystallization in the dry state and attacked by the expansive products in the wet state. The sulfate ion contents in CGFB samples increased with the sulfate concentrations and drying‐wetting cycles and decreased from the surface to the inside of the samples. The damage process of CGFB samples evolved from the surface to the inside. In the early stage of corrosion, sulfate ions adsorbed to the surface of CGFB samples and consumed nonhydrated particles to form acicular ettringite and other products that filled the material pores. For this stage, the driving force of sulfate ions to enter into the CGFB samples was the highest for the samples immersed in 15% sodium sulfate solution, and the masses and strengths increased the fastest. As the drying‐wetting cycles continued, the nonhydrated particles inside the samples were nearly completely hydrated, and the samples were constantly damaged by salt crystallization and dissolution. The corrosion ions entered into the samples and consumed portlandite to produce a large amount of prismatic ettringite and aggravated the internal corrosion of CGFB samples. At the fifteenth drying‐wetting cycle, the higher the salt concentration of the immersion solution was, the faster the masses and the strengths of CGFB samples decreased. Moreover, the surface spalling and failure of CGFB samples were more severe.
Highlights
Coal takes the dominant position in China’s energy market and energy consumption structure [1]
Four components were used for coal gangue-fly ash backfill (CGFB) sample preparation: cement, fly ash, coal gangue, and water. e cement was the 42.5 slag Portland cement (Second Cement Plant, Yishui, Shandong Province), and the grade II fly ash was selected with a lightgray color (Huangdao Power Plant, Qingdao, Shandong Province). e coal gangue came from the solid waste produced in Daizhuang Coal Mine (Zibo Mining Group, Shandong Province) and was carried by secondary crushing
CGFB samples generated white dot-like products at the corners after immersion in all sodium sulfate solutions except the 0% solution, which indicated that salt crystallization occurred on the surface. is was because of the rough surface of CGFB samples providing sites for the retention of sulfate ions
Summary
Coal takes the dominant position in China’s energy market and energy consumption structure [1]. E properties of cemented coal gangue-fly ash backfill (CGFB) are the basis of paste filling to determine the effect of controlling ground subsidence. The level of underground water in coal mines varies with rainfall, seasons, reservoir storage and drainage, etc., and the environment of the goaf would be either dry or wet. E CGFB would suffer from the effect of drying-wetting cycles of mine water, which in turn would influence the safety and stability of the goaf. Liu et al [8,9,10] studied the influence of high sulfate solutions on the properties of cement-based materials and found that the strengths of the materials improved in the early stage of immersion and decreased with
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