Abstract: Calcining way is used to activated hard clay. The anhydrite was obtained by calcined and dehydration ardealite. The efficient expanding admixture of concrete is the btained by mixing anhydrite and calcining hard clay. The result indicate that expansive admixture meet requirement specified in industry standard JC476-2001. From SEM picture of 90 d cement pastes with expanding admixture, it can be seen that the formation of AFt and C-S-H with significant SO_3, Al_2O_3 and its adsorbing water are responsible for the expansion of expansive mortar of concrete.

Key words: expanding admixture%hard clay%ardealite%calcining