Castables were prepared using tabular corundum,alumina powder,and calcium aluminate cement as raw materials. The effects of curing humidities(RH=30% and RH=80%)on demoulding strength of castables were studied. The results show that:1)The demoulding strength of CAC bonded castables is higher when cured at lower humidity(RH=30%) than cured at higher humidity(RH=80%);2)The main hydration products generated are C2AH8 and AH3 cured at low humidity (RH=30%),the interlocked structure of interlaced network formed by flaky C2AH8 can provide demoulding strength for castable;3)When curing humidity increases to RH=80%,the hydration product C2AH8 gradually transforms to C3AH6 and AH3,the significant increase in voids between the granular hydration product C3AH6 and the internal hydration product of the sample,leads to a decrease the demoulding strength of castable under high humidity conditions.