Reactivity of Calcined Clay in Alite-Calcium Sulfoaluminate Cement Hydration

Alite-calcium sulfoaluminate (ACSA) cement is a low energy cement which contains both alite (C3S) and ye'elmite (C(4)A(3)$). Blending of ACSA with calcined clay would be a possibility to further decrease the CO2 balance of ACSA. Clay with 40-50 % kaolinite content was calcined in a static furnace at 750 degrees C for 30 min. The pozzolanic reaction of the calcined clay was studied using the reaction between calcite, portlandite and calcined clay under controlled pH conditions (in 0.1 and 0.3 M KOH solutions). The portlandite consumptions up to 7 days were characterized by thermogravimetric analysis (TGA) and used to determine the reaction degree of the calcined clay using thermodynamic modelling. Calcined clay showed a high reactivity during the first days. This initial high reactivity of the clay observed in the pozzolanity test agrees with the results of the ACSA-clay blends obtained by X-ray diffraction and thermogravimetric analysis, where significant portlandite consumption by added clay was observed during the first days of hydration. After 28 days the ACSA blends with 10 % calcined clay show a significant higher compressive strength than the plain ACSA.