Abstract
In order to study the effects of different concentrations of sulfate on the strength of fly ash‐based coal mine filling paste, using variable control, mechanical analysis, and other means, the changes in the uniaxial compressive strengths of filling paste blocks soaked in different concentrations of sodium sulfate solution for different durations are studied, and their stress‐strain curves are discussed. The hydrated products of each block are analyzed at different stages by XRD, and the results indicate that different concentrations of sodium sulfate solution have different effects on the strength of the filling paste after soaking for different durations. A sodium sulfate solution with a concentration of 5% had an activator effect on the fly ash‐based filling paste and enhanced the strength of the filling paste. A sodium sulfate solution with a concentration of 10% and 15% increased the early strength of the paste test block faster, but after 60 d, the strength decreased. The stress‐strain curves for these blocks show that the elastic moduli of the filling paste test blocks change irregularly, and it was found that with the increase in soaking time, the blocks soaked in the 10% and 15% sodium sulfate solutions developed fissures in the later stage that adversely affected the strength of the filling paste. The XRD results show that the filling paste test block hydration products are hydrated calcium silicate (C‐S‐H) based and that ettringite (AFt), beneficial to strength of the filling paste in proper quantities, appeared in the main product of the filling paste test blocks that were soaked in the sodium sulfate solution. With the increase in the concentration of the sodium sulfate solution, the AFt is generated in larger quantities, and gypsum crystals begin to appear, which is not conducive to the filling paste block strength.
Highlights
Coal mine paste filling technology is an important part of green coal mining: it plays a significant role in supporting the overlying strata, preventing or reducing surface subsidence of the coal mine [1,2,3,4,5], and provides an effective solution the problem of “three under, one above” mining [6, 7]
A series of filling paste test blocks were soaked in varying concentrations of sodium sulfate solutions for varying durations. e uniaxial compressive strengths of the blocks were determined, and XRD analyses of the constituent materials of the pastes were conducted to identify the products present in the paste material. e testing program resulted in the following conclusions: (1) In the early stages of immersion, the uniaxial compressive strength of the filling paste test blocks soaked in 10% sodium sulfate solution was the highest
With the increase in soaking time, the uniaxial compressive strengths of the test blocks soaked in clean water and the 5% sodium sulfate solution were improved, while the strength of the filled paste test blocks soaked in the 10% and 15% sodium sulfate solutions decreased after soaking for 60 d
Summary
Coal mine paste filling technology is an important part of green coal mining: it plays a significant role in supporting the overlying strata, preventing or reducing surface subsidence of the coal mine [1,2,3,4,5], and provides an effective solution the problem of “three under, one above” mining (mining under a building, under a railway, under a water body, or above confined water) [6, 7]. Guo et al [25] investigated the microstructures of gypsum and fly ash adhesives by using X-ray diffraction, scanning electron microscopy, and pore structure analysis, and it is proved that desulfurization gypsum can effectively destroy Si-O and Al-O bonds in fly ash particles and produce AFt and other substances, so as to improve the compressive strength and tensile bond strength of fly ash-cemented material He and Wei [26] studied the effect of sulfate on the performance of fly ash-cement adhesives with different fly ash contents using strength tests, and the results show that in a certain sulfate range, the greater the amount of activator is, the better the effect of fly ash will be. Microstructural and compositional changes in the fly ash-based filling paste after sulfate attack were investigated using micro-XRD analysis
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