Abstract: The analysis of XRD indicates that slag is similar with minerals in the initial sediments. However, the proportion of bauxite Al₂O₃, hematite Fe₂O₃ has a big difference. The electric structure, elastic constants of Al₂O₃ and Fe₂O₃ were calculated using the ultra soft pseudo-potential plane-wave method based on the first-principles. The results show that from the structure of material, the accumulation mode of Al³⁺ and O^2- in Al₂O₃ crystal is sparser than that of Fe³⁺ and O^2- in Fe₂O₃, which is the reason for their different properties. For their chemical bonds in the direction of <100> and in tangential, Fe-O is more easy to deformate and fracture than Al-O, leading to Fe₂O₃ deposition in the clean water wall. For Fe₂O₃, tangential surface plays a key role for its deposit.