Study on the material characteristics and barrier mechanism of magnesium potassium phosphate cement/hydroxyapatite cutoff walls for fluoride contamination in phosphogypsum waste stacks

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• Magnesium potassium phosphate cement using hydroxyapatite as admixture was prepared as a kind of new cutoff wall material. • The cutoff walls have good adsorption performance for fluorine and could meet the engineering requirements. • Internal structural parameters were analyzed through the pore-scale modeling. • The effect of the cutoff walls for controlling fluorine at 7300 days was simulated by Visual MODFLOW. Large quantities of fluoride were leakaged from the phosphogypsum wastes with rainfall and weathering, which has affected the groundwater environment. As a common engineering material, magnesium potassium phosphate cement (MKPC) with high strength and low permeability was applied in barriers to control the diffusion of contaminants in the groundwater. As an effective fluoride adsorbent, this paper selected hydroxyapatite (HAP) as the admixture for magnesium potassium phosphate cement (MKPC) to prepare the cutoff walls. To explore the feasibility of the cutoff walls applying for fluoride contaminants control, studies were carried out through investigating the appropriate material ratio, adsorption property, migration characteristics, microstructure and engineering performance of MKPC-HAP. Based on the permeability coefficient, compressive strength and adsorption property, the molar mass ratio of magnesium to phosphorus in MKPC and the addition of HAP in the total dry material were determined to be 3:1 and 20 %, respectively. The compressive strength after curing for 7 days was 27.51 MPa and permeability coefficient was 1.904 × 10 −9 m/s. Furthermore, the effective diffusion coefficient and the seepage velocity of fluoride with concentration of 100 mg/L were 7.376 × 10 −8 cm 2 /s and 2.21 × 10 −8 m/s, respectively. The Lattice Boltzmann method (LBM) was applied to simulate the distribution of flow field inside the MKPC-HAP cutoff walls and the connected porosity of MKPC-HAP was 5.12 %. Finally, the service life varied from 12 to 22 years when the fluoride concentration being 50 mg/L with the 1 m cutoff wall thickness. Visual Modflow simulation showed that the migration distance of fluoride was 820 m without the cutoff walls, while it was 402 m after setting the cutoff walls over 7300 days.