Different hydrated high alumina cement (H-HAC) mixtures containing mainly CAH 1 0 , C 2 AH 8 or C 3 AH 6 obtained at different temperatures were studied with respect to their performance as the Al-bearing component in HAC-based expansive additives. Free expansion and ettringite formation potentials of expansive cement using the different H-HAC preparations were reported. It was shown that H-HAC containing predominantly CAH 1 0 and C 2 AH 8 reacted more rapidly to form ettringite in HAC-based expansive cement systems than anhydrous HAC or H-HAC containing C 3 AH 6 . The expansion of the expansive cement containing CAH 1 0 and C 2 AH 8 developed mostly during the first day hydration. The presence of C 3 AH 6 in the expansive cement appeared to delay the expansion and decrease the ultimate expansion value significantly.