Experimental Study on the Durability of Fly Ash‐Based Filling Paste in Environments with Different Concentrations of Sulfates

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. e 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. e 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. e 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.

Experimental Study on Diffusion Process of Sulfate Ion in Cemented Gangue Backfill Material

“…At 180 days, when the concentration of the external solution Na 2 SO 4 solution is 3%, 5%, and 10%, the SO 2− 4 concentration is 0.4634%, 0.5215%, and 0.6712% from the surface of the specimens at 3 mm. SO 2− 4 of high concentration in external solution can accelerate the ion diffusion, which indicates that the reaction product is increased, so that the internal pores are expanded in CGBM [29].…”

“…at is because a small amount of Mg 2+ enters into CGBM in the early age and reacts with the hydration product to form brucite [13,25]. ese are all water-insoluble precipitate, which can fill the pores of CGBM to improve its compressive strength and prevent SO 2− 4 to penetrate [29,30]. As time goes on, SO 2− 4 penetrates the pores gradually and reactants increase, which leads to internal crack expansion in CGBM [31].…”

“…It can be observed from Figure 7(c) that there is much columnar crystal in CGBM after corrosion with SO 2− 4 and Mg 2+ for 180 d [28], which is arranged orderly. At the same time, a large amount of amorphous matter is wrapped around the columnar crystal [29], its main products are Mg(OH) 2 , gypsum, ettringite, and M-S-H. e diffraction peaks intensity of M-H-S and Mg(OH) 2 is low in the low concentration solutions. It is verified that the formation of Mg(OH) 2 can improve sulfate resistance of CGBM in the early composite solutions, so the compressive strength is increased.…”

Study on Damage Model and Damage Evolution Characteristics of Backfill with Prefabricated Fracture under Seepage‐Stress Coupling

“…From (2) and 3, it can be deduced that the loaded damage constitutive model of the backfill under pseudotriaxial conditions is as follows:…”

“…Backfill coal mining technology is an effective method to improve the utilization of coal resources and control surface subsidence. It has been widely used in China and has become one of the main means of green coal mining [1][2][3][4][5][6][7][8]. e stability of the backfill is an important factor to prevent the instability of the goaf and the subsidence of the ground surface [9,10].…”