The discrete variational X alpha method has been conducted to elucidate the formation of a heterobridging bond (HBB), Ca-O-Si, when a Si-O pendant bond of SiO2 comes close to the Ca atom on the surface of calcium oxide or hydroxide. A decrease in the coordination number For Ca in CaO or Ca(OH)(2) increases the electron population density (PD) between Ca and. O, and decreases the PD between Si and O. These simultaneous changes in PD favor the formation and stabilization of a Ca-O-Si HBB. The calculated results agree well with the experimental observation of precursor formation toward calcium silicate during milling of a mixture of Ca(OH)(2) and SiO2.