Preparation and strength formation mechanism of surface paste disposal materials in coal mine collapse pits

Abstract

Surface paste disposal (SPD) can utilize substantial coal gangue while effectively disposing of active coal mine collapse pits. The response surface method was used for mixture ratio design, and the desirability function method was used for the multiobjective optimization of SPD materials. For the optimal mixture ratio of SPD materials, microstructure analysis was performed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). A pore (particle) and crack analysis system was utilized to detect microstructural pore characteristics with increasing curing age. The optimal characteristics are a water-to-binder ratio of 4.0, an aggregate-to-binder ratio of 16.91, and a fine gangue rate of 60%. The porosities at 1–28 d decrease from 20.50% to 2.62%. Hydration products fill the pores randomly, leading to a probability entropy of pores >0.95. Ettringite (Aft) formed by cement hydration at 3–14 d exhibits a skeleton structure with complicated pore boundaries, increasing the probability distribution index from 2.0725 to 3.1523 and fractal dimension from 2.2625 to 4.4169. In late hydration, coal gangue powder acts as a supplementary cementitious material (SCM) that works with cement to generate calcium silicate hydrate (C-S-H) gel to cover the gaps in the AFt skeleton. Reducing the number of pores and softening the pore boundaries decrease the pore probability distribution index from 3.1523 to 2.0203 and the fractal dimension from 4.4169 to 2.1983. A denser “skeleton-gel” network structure eventually forms.