Molecular Dynamics Analysis of the Mg²⁺ Structure Behavior in SiO₂–CaO–Al₂O₃–MgO Slag System

Abstract

In order to analyze the structural behavior of Mg²⁺ in the slag and the difference of structural behavior with Ca²⁺, this paper examines the variation rule of microstructure of SiO₂–CaO–Al₂O₃–MgO slag system with the increase of MgO/CaO ratio by molecular dynamics simulation. The results indicate that in the system, Mg²⁺ can exist in three coordination forms when combined with oxygen: 4, 5, and 6. It can form ionic bonds with anions to exist as a network modifier, and it can also participate in network formation by forming [MgO₄] tetrahedra through Mg–O covalent bonds. Under the condition of constant SiO₂ and Al₂O₃ content, as the MgO/CaO ratio increases, the proportion of Mg^IV in the slag first increases and then decreases, with a turning point at 30.25%. In contrast, the Mg^VI ratio shows a decrease followed by an increase, with a minimum value of 17.33%. Ca²⁺ in the system mainly exists as a network modifier, and its modifying effect first decreases and then increases with the increase of MgO/CaO ratio. The stability of the Si–O and Mg–O tetrahedral structures changes little with the increase in Mg²⁺ content, while the stability of the Al–O tetrahedral structure decreases, and Al^IV gradually transforms into higher coordinated Al. The content of bridge oxygen and the high polymerization degree structural units increase with the addition of Mg²⁺, leading to an increase in the melt polymerization degree.