Properties and application of thixotropic cement paste backfill with molybdenum tailings

Backfilling is considered worldwide as an effective treatment method for coal mining subsidence and mineral tailing disposal. A green thixotropic cement paste backfill with molybdenum tailing (MoT) sand was prepared which can rapidly accumulate in the coal mining cavity. A thixotropic coefficient method was also proposed to evaluate the thixotropy of the paste based on the correlation between shear stress and time. Through an orthogonal test regarding the optimal proportion design, the thixotropy, water separation rate and stone compressive strength of the thixotropic MoT-cement paste were tested. The range analysis showed that the optimal proportion of the paste was 6% mixed metal hydroxide (MMH), 0.8 water-cement ratio, and 6:4 MoT-cement ratio. The analysis in the rheological properties of the proposed paste indicated that the relationships of shear rate-shear stress of the optimal proportion conformed to the power rate rheological model. The influences of the addition amount of MMH, water-cement ratio and MoT-cement ratio on the mechanical property of the stone body of the proposed paste were tested and predicted theoretically. Through a simulated accumulation test, the relationships among the flowing distance, accumulation height and time of the thixotropic MoT-cement paste in the process of filling coal mining cavity were obtained. The cost of the thixotropic cement paste with MoT is 30–50% better economy than that with natural river sand and recycled fine aggregate whilst the thixotropic cement paste with MoT also shows good environmental performance by considering the depletion of stocked mineral tailing in coal mining cavity treatment.