ON THE CHEMISTRY OF DELAYED ETTRINGITE FORMATION

摘要

Basic research was carried out on the chemistry of the calcium sulfoaluminate hydrates in their pure form as well as during the hydration of C_3A in the presence of gypsum, lime, alite, alkali hydroxide and alkali sulfates at temperatures up to that of boiling water. The results revealed the existence of the ettringite phase with a non stoichiometric amount of sulfate i.e. with a sulfate concentration lower or greater than 3 moles. The decomposition of the monosulfate hydrate to a sulfate-deficient ettringite is accelerated by a calcium ion concentration in solution of 0.6 g/l and is delayed by hydroxyl ions and portlandite. The calcium sulfoaluminate hydrates are decomposed by boiling water to calcium sulfate phases and alumina gel. In sodium hydroxide solutions hydrogarnet and portlandite form, the aluminate and sulfate being retained in solutions as alkali salts. A mechanism for DEF is proposed.