Chemical bond changes between microsilica and hydratable alumina were studied by X-ray photoelectron spectroscopy and Fourier transformation infrared spectroscopy. The formation of Si-O-Al bond was detected by the decreased O 1s and Si 2p binding energy and the lowered vibration frequency of hydratable alumina with the addition of microsilica after drying at 110 A degrees C and heating at 800 A degrees C. Furthermore, it was observed by scanning electron microscope that the interlocking structure developed between the particles of hydratable alumina and microsilica after drying at 110 A degrees C and the sintering took place among the particles after heating at 800 A degrees C. Therefore, it is proposed that the increased strength of the castables after drying and heat treatment at intermediate temperatures was attributed to the presence of microsilica which resulted in the formation of Si-O-Al bond and interlocking structure between hydratable alumina and microsilica particles.