Calcium aluminate cement (CAC) is a commonly used binder for refractory castables. There has been a long-running debate on the transformation mechanisms of calcium aluminate cement hydrates through solid-state reaction or solution precipitation. In order to demonstrate the conversion mechanism of CaO·Al2O3·10H2O (CAH10) to 3CaO·Al2O3·6H2O (C3AH6) and Al2O3·3H2O (AH3), the CAC pastes cured at 10 ℃ to produce CAH10 was further cured at 60 ℃ with and without free water. The hydration of the pastes was halted by freezing-drying, and the phases and microstructure of hydrates were investigated by XRD and SEM, respectively. The results indicate that CAH10 converts into C3AH6 and AH3 in water presence at 60 ℃, but does not transform into C3AH6 and AH3 without water, demonstrating that conversion of CAH10 into C3AH6 and AH3 does not happen through solid-state reaction, but takes place through preceding dissolution of CAH10 and subsequent precipitation of AH3 and C3AH6 from the solution.