Heat flow calorimetry and SEM investigations to characterize the hydration at different temperatures of different 12CaO·Al2O3 (C12A7) samples synthesized by solid state reaction, polymer precursor process and glycine nitrate process

Mayenite was synthesized using the conventional solid-state reaction and two low temperature synthesis methods a self-combustion method and a polymeric precursor process. Using the low temperature methods the metastable phase 5CaO·3Al2O3 (C5A3) crystallizes at 1173 K (2 h) instead of C12A7. After forming pure crystalline 12CaO·7Al2O3 (C12A7) at 1373 K (2 h) the hydration was monitored at 283 K, 288 K, 293 K, 298 K and 301 K by heat flow calorimetry. During hydration the first calorimetric peak correlates with the formation of layers around the C12A7 grains and the second peak corresponds to further hydration reactions and crystallization of lamellar calcium aluminate hydrates C2AH8±x showing different hydration steps of 8.2H2O, 8.0H2O and 7.5H2O depending on temperature. At higher temperatures the formation of hydrate shells is increased and consequently the further hydration reaction is hindered.