Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO₂–Na₂O by Molecular Dynamics Simulations

Abstract

Molecular dynamics (MD) simulations have been used to study the effect of Na ions on the structure properties of CaO–SiO₂–Na₂O melts. The short-range structure and medium-range structure of CaO–SiO₂–Na₂O in this study are consistent with existing data. The replacement of Ca²⁺ with Na⁺ in CaO–SiO₂–Na₂O melts has almost no effect on the degree of polymerization and distribution of bond angles of Si–O tetrahedron. From micro perspective, Na ions enhance the mobility of CaO–SiO₂–Na₂O melts by multiple ways. Firstly, the modification effect of Na⁺ on the melt network structure is weaker than that of Ca²⁺, the Si–O tetrahedron around Na⁺ is sparser than Ca²⁺, which is more conducive to ions movement. Secondly, the diffusion capacity of Na⁺ is much greater than other ions in CaO–SiO₂–Na₂O system, which the overall diffusion capacity of the system can be improved by adding more Na⁺. Thirdly, since Na⁺ has only one charge, there is no electrostatic restraint on the depolymerized tetrahedron which happened in multivalent charges such as Ca²⁺, so that the mobility of CaO–SiO₂–Na₂O is stronger than that of CaO–SiO₂. The micro changes provide an explanation for the improvement of macro liquidity.