Effect of different gypsum dosage on the chloride binding properties of C4AF hydrated paste

To reveal the mechanism of the high chloride-binding capacity for high ferrite Portland cement, ferrite phase (C4AF) was prepared and the effects of different gypsum dosages (0%, 5%, 10%, 20%, 30%) on the chloride binding performance, phase composition, microstructure and pore structure of C4AF hydrated paste were investigated by XRD, TG-DTA, SEM-EDS and nitrogen adsorption analysis. The results show that the pure C4AF hydrated paste had a strong chloride binding ability, in which the chemical binding chloride ion ability reached 28.3 mg/g. When the gypsum dosages increased from 5% to 30%, the chemical binding chloride ion ability decreased by 7.4%, 13.4%, 52.2% and 74.5%, respectively, compared with the pure C4AF hydrated paste. With the increase of gypsum dosages, the content of C3(A,F)H6 in the C4AF hydrated paste decreased, while a large amount of monosulfate (AFm) was generated and the specific surface area of hydration products was greatly reduced. During chloride ion erosion, mainly C3(A,F)H6 reacted chemically with chloride salt to generate Friedel’s salt, and the chemical chloride binding ability showed a linear correlation with the content of C3(A,F)H6 in the C4AF hydrated paste.