The stability of ettringite

Ettringite was formed during the hydration of C₃A with gypsum at temperatures between 25 and 80°C. Ettringite formation was retarded when hydration was carried out in KOH solutions whose concentrations were as high as 2·0M. Solutions with KOH concentrations between 0·5 and 1 . OM have the greatest retarding effect. In spite of the retarding effect of KOH, ettringite was observed to form at 80°C in 2·OM KOH. Hydration in solutions with KOH concentrations greater than 0·5M also produced syngenite. When C₃A is hydrated with gypsum in the presence of C₃S, ettringite, C-S-H and CH were formed in the absence of KOH. Hydration of 80% C₃S, 10% C₃A and 10% gypsum in 0·5M KOH solution was adequate to essentially eliminate the formation of ettringite. Rather, calcium hydroxide and C-S-H containing sulphate and aluminate were formed. Thus the preferred phases when hydration occurs in KOH are C-S-H and CH; whereas the preferred phases for hydration in water are C-S-H, CH and ettringite. Thus, the elimination of ettringite formation by high-temperature curing requires the presence of hydrating C₃S. The observations made in studying these model systems are used as the basis for the development of a mechanism of delayed ettringite formation in cement. They are also used in describing a relationship between delayed ettringite formation and the alkali-silica reaction.