Mechanism of the Enhanced C4AF Sintering and Chloride-Binding Capacity Caused by Magnesium Doping

This work examined the impact of Mg doping on the sintering, hydration, and chloride-binding capacity of C4AF minerals and elucidated the role of Mg doping on reactivity mechanisms by experiments and calculations. Rietveld analysis determined the solid solubility limit of Mg in C4AF is 1.6% by weight. By the density functional theory (DFT), it revealed Mg preferentially substitutes Fe sites, followed by Ca sites in C4AF. Increasing Mg doping up to 4% by weight enhanced the chloride-binding capacity of hydrated C4AF pastes by more than 50%, attributed to the increased formation of Friedel’s salt. The introduction of Mg in C4AF promoted clinker sintering upon hydration and produced katoite, which reacted with chloride. Additionally, Mg facilitated the formation of layered double hydroxides that adsorb chlorides. The changes imparted by Mg doping served to increase microstructure porosity and optimize pore size distribution. The results provide insight into the role of Mg in high-Mg, high-Fe cement, demonstrating that C4AF phase change improves resistance to chloride intrusion. This has implications for the use of high-Mg limestone and the application of this cement in marine environments.