Understanding the adsorption of calcium sulfate on tetracalcium aluminoferrite and tricalcium aluminate surfaces
Haoxuan Zhong, Lu Yang, Fazhou Wang
Abstract
Abstract Calcium sulfate (CaSO 4 ), an essential retarder in cement, retards the hydration of tricalcium aluminate (C 3 A) and tetracalcium aluminoferrite (C 4 AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO 4f on C 4 AF and C 3 A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO 4 on C 4 AF was stronger than that on C 3 A due to the slower CaSO 4 consumption rate, lower driving force for CaSO 4 adsorption, and surface coverage of Fe(OH) 3 gel. The adsorption of CaSO 4 hindered Ca dissolution more markedly on C 4 AF than C 3 A, which was pronounced on Fe‐free C 4 AF surfaces. The adsorption of CaSO 4 weakened the affinity of water on C 4 AF and C 3 A surfaces, lowering the driving force for H 2 O adsorption. The adsorption of H 2 O and CaSO 4 promoted the dissolution of Al on the [AlO 6 ] octahedral surface of C 4 AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO 4 on initial C 4 AF and C 3 A hydration was explained.