Formulation Optimization and Hydration Mechanism of Phosphogypsum-based Filling Cementitious Materials
Phosphogypsum is a solid waste byproduct produced in the wet phosphoric acid process,characterized by low re-activity and is a waste that is difficult to treat harmlessly.The slag produced in the ironmaking process of Jiuquan Iron and Steel Company has high reactivity,which can be mixed with phosphogypsum to prepare cementitious materials suitable for filling abandoned mining voids,thus mitigating geological hazards associated with subsidence.The optimum formula of phosphogypsum based filling cementing materials was obtained by orthogonal experiment of filling cementing material strength under different ratio and fineness.On this basis,scanning electron microscopy(SEM)and X-ray diffraction(XRD)were applied to analyze the internal structure.The results are as follows.The average particle size of slag is 15.88 μm,with a quicklime content of 5.5%～6.0%,a phosphogypsum content of 30%～33%,a NaOH content of 1.5%～2.0%,and a mirabilite content of 2.5%～3.0%.With the addition of grinding aids and early strength agents,the 3 d and 7 d strength of the filled cementing material reached 1.58 MPa and 4.10 MPa,respectively.The amount of hydrat-ed calcium silicate gel and ettringite increased gradually with the growth of age,and the structure was gradually dense.The hydrated calcium silicate gel plays a filling and bonding role in the filling cement material,and ettringite plays a supporting role in the filling cement material,thus improving the overall strength of the material.
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