Calcium aluminate cement bonded corundum castable specimens were prepared using brown fused corundum (8-5, 5-3, 3-1 mm)，white fused corundum (≤1, ≤0.045 mm)，micro-sized α-Al2O3 and microsilica as starting materials. This work focused on investigating the relationship between the bond change in the castable matrix and the strength of the castable with 5 mass% microsilica or without microsilica after heat treatment at 110, 800 and 1 000 ℃, respectively. Chemical bond changes between the microsilica and hydrates of calcium aluminate cement after drying at 110 ℃ or firing at 800 ℃ were investigated by XPS and FTIR. The results show that Si—O—Al bonds form between the microsilica and hydrates of calcium aluminate cement after drying at 110 ℃ or firing at 800 ℃. Therefore, the increased strength of castable specimens is attributed to the formation of Si—O—Al bonds from 110 ℃ to 800 ℃.