Study on the activation of ternesite in CaO·Al2O3 and 12CaO·7Al2O3 blends with gypsum for the development of low-CO2 binders

Abstract This study aimed primarily to broaden the limited knowledge presently at hand about ternesite, 5CaO·2SiO2·SO3 (shorthand C5S2 S ¯ , ter), reactivity in calcium sulfoaluminate-calcium aluminate (CSA-CAC) blended cements by exploring the hydration of the constituents of simplified CaO·Al2O3 (CA) + 5CaO·2SiO2·SO3 (ter) + CaSO4·2H2O (gypsum, gyp) or 12CaO·7Al2O3 (C12A7) + 5CaO·2SiO2·SO3 (ter) + CaSO4·2H2O (gyp) systems. According to earlier research, ternesite, a mineral constituent in some eco-efficient calcium sulfoaluminate cements, may be activated during the hydration of Al(OH)3 or other calcium aluminates. The extent to which such activation may be affected by the presence of calcium sulfate (gypsum) in CSA cements is not currently known, however. The present findings showed that ternesite is activated by both C12A7 and CA in the presence and absence of gypsum, although less intensely in its presence. Calorimetric analysis revealed that ternesite favours calcium aluminates reactivity with water in the presence and absence of gypsum because its surface serves as a nucleation site where aluminate hydrates cluster just minutes after the reaction begins. The mineralogical characterisation of the aluminate pastes bearing gypsum and ternesite was consistent with thermodynamic modelling, which predicted the same hydration products in both, although with different stability ranges.