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

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.