Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO<sub>2</sub>–Na<sub>2</sub>O 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.