Kinetics of Tricalcium Aluminate and Tetracalcium Aluminoferrite Hydration in the Presence of Calcium Sulfate

Abstract

Hydration reactions of C 3 A and C 4 AF with calcium sulfate hemihydrate and gypsum were investigated and the kinetics of the reactions compared. The rates of C 3 A and C 4 AF hydration, as determined by heat evolution, vary depending on whether the sulfate‐containing reactant is gypsum or calcium sulfate hemihydrate. The following sequence of reactions involving C 4 AF occurs when hemihydrate is the reactant: gypsum formation during the first hour, ettringite formation between 20 and 36 hours, and the conversion of ettringite to monosulfate over a period of about 12 hours. Monosulfate formation initiates prior to the complete consumption of gypsum. The onset of this conversion occurs at a shorter hydration time when hemihydrate is a reactant and the total amount of heat evolved is lower. The hydration reactions in saturated calcium hydroxide solution occur more slowly than those in water. Based on heat liberation, C 4 AF reacts at a much higher rate than C 3 A. Ettringite formation occurs during the first 8 to 9 days of C 3 A hydration. Once the gypsum is consumed, ettringite converts to monosulfate during two additional days. Compared to gypsum, hemihydrate decreases the rates of hydration of both C 3 A and C 4 AF. The effects on the hydration characteristics of C 4 AF are significant. The hydration of C 3 A with gypsum in water, in saturated Ca(OH) 2 solution, and in 0.3 M NaOH solution were compared. Heat evolution is the lowest for hydration in 0.3 M NaOH. The onset of monosulfate formation occurs prior to the complete reaction between gypsum and C 3 A in the NaOH solution.