Abstract: The temperature range of the stable existing and structural transformation of ettringite were studied
by DTA, XRD and SEM. The effect of ettringite structural transformation on expansive behavior of expansion cement was found. The result indicates:
(1) Under dry condition, the ettringite dehydrates quickly between 87℃ and 150℃, the structure distortion makes it extremely lose the crystal
characteristic, but it does not decompose and it can be recovered in water. The ettringite can be synthesized and stable existing, under hydrothermal
condition below 100℃. (2) under the dry condition below 100℃, the expansion cement concrete strength does not decrease but its expansive behavior can be lost even negative expansion produced,
so expansion cement and self-stress cement are not suitable under the dry condition of over 70℃ or under the dry and wet alternate conditions.

Key words: ettringite, structural stability, expansion cement, expansive behavior