Slurries of quartz and slaked lime containing various crystal phases of alumina were hydrothermally reacted to synthesize aluminum-substituted tobermorite. X-ray diffraction results confirmed the formation of aluminum-substituted tobermorite during the reaction. The aluminum species were substituted at the Q² or Q³ sites in tobermorite. The amount of aluminum substituted in the tobermorite depended on the reactivity of alumina. The tobermorite formed showed fiber- and plate-like morphologies. The reactivity of the alumina used as an additive influenced the substitution site and the amount of aluminum substituted into the tobermorite structure, resulting in controlling its morphology.