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

Abstract

Hydration reactions of C3A and C4AF with calcium sulfate hemihydrate and gypsum were investigated and the kinetics of the reactions compared. The rates of C3A and C4AF 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 C4AF 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, C4AF reacts at a much higher rate than C3A. Ettringite formation occurs during the first 8 to 9 days of C3A 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 C3A and C4AF. The effects on the hydration characteristics of C4AF are significant. The hydration of C3A with gypsum in water, in saturated Ca(OH)2 solution, and in 0.3M NaOH solution were compared. Heat evolution is the lowest for hydration in 0.3M NaOH. The onset of monosulfate formation occurs prior to the complete reaction between gypsum and C3A in the NaOH solution.