In an aluminous cement, which mainly consists of CA, the stable phases arising during hydration are C(3)AH(6) and AH(3). This communication presents a chemical route for the synthesis of C(3)AH(6) which is based on hydration of C(3)A. Mixtures of CA, C(3)AH(6) and AH(3) are characterised by ultrasonic testing. The ultrasonic velocity obtained on these mixtures is lower than what is observed in hydrated aluminous cement of similar chemical composition. Interfaces are thought to play a significant role in the ultrasonic response of aluminous cements.