Cs⁺ Promoting the Diffusion of K⁺ and Inhibiting the Generation of Newberyite in Struvite-K Cements: Experiments and...

Struvite-K cements, also called magnesium potassium phosphate cements (MKPCs), are applicable for particular applications, especially the immobilization of radioactive Cs⁺ in the nuclear industry. This work focuses on how Cs⁺ affects the hydration mechanism of struvite-K cements because newberyite and brucite in the hydration products are deemed to be risky products that result in cracking. Experiments and molecular dynamics simulations showed that Cs⁺ promoted the diffusion of K⁺ to the surface of MgO, which greatly facilitates the formation of more K-struvite crystals, inhibiting the formation of newberyite and brucite. A total of 0.02 M Cs⁺ resulted in a 40.44%, 13.93%, 60.81%, and 32.18% reduction in the amount of newberyite and brucite, and the Cs immobilization rates were 99.07%, 99.84%, 99.87%, and 99.83% when the ratios of Mg/P were 1, 3, 5, and 7, respectively. This provides new evidence of stability for struvite-K cements on radioactive Cs⁺ immobilization. Surprisingly, another new crystal, [CsPO₃·H₂O]₄, was found to be a dominating Cs-containing phase in Cs-immobilizing struvite-K cements, in addition to Cs-struvite.