Influence of crystallinity and surface area on the hydration kinetics of CA2

The hydration of CA2 (CaAl4O7) is usually a slow process. However, we found that CA2 is easily grindable and that the BET surface area and the amount of X-ray amorphous phase highly influence the hydration kinetics. Tempering of ground samples significantly decreased the BET surface area and the amount of X-ray amorphous phase. The hydration of tempered CA2 started later and proceeded much more slowly. A different degree of crystallinity of CA2 explains the wide range of different hydration kinetics and especially the occurrence of the often observed exothermic heat flow peak prior to the CA2 main reaction. By different grinding procedures and tempering we were able to prepare samples that were completely hydrated within 24 h and such samples which did not react within 24 h. Using heat flow calorimetry, in-situ XRD and in-situ 1H-TD-NMR we show that X-ray amorphous CA2 reacts prior to crystalline CA2.