There have been different hypotheses about the transformation mechanisms of calcium aluminate cement hydrates and this work aims to clarify the long‐running debate about the conversion approaches. In this work, CAH10 and C2AH8 were produced from the pastes of calcium aluminate cement (CAC) cured for 24 hours at 10 and 20°C separately. And the cured pastes were continually cured at 60°C for 3 days with water and without water, respectively. The hydration of the pastes was halted by freeze‐drying, and the phases and microstructure of hydrates were investigated by XRD and SEM, respectively. The results indicate that CAH10 and C2AH8 converted into C3AH6 and AH3 in water presence at 60°C, but did not transform into C3AH6 and AH3 without water. It is confirmed that the conversion of CAH10 and C2AH8 to C3AH6 and AH3 happens through preceding solution of CAH10 and C2AH8 and subsequent precipitation of C3AH6 and AH3.

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