Studies on the stability of the calcium sulfoaluminate hydrates. Part 1: Effect of temperature on the stability of ettringite in pure water

The stability of the ettringite phase in relation to its solubility in pure water has been studied at 30°, 60° and 100° for a time interval of 14 days. Ettringite is stable up to 60° in a solution of a pH value around 11. Its solubility increases with temperature. It converts to the low sulfate form after one hour boiling which decomposes further to gypsum after 14 days. Gypsum appears first when the pH of the solution is lowered to 9 and the aluminium concentration clearly decreases.