A radioisotope technique was used for the determination of the rate of movement of Ca²⁺ and Fe³⁺ ions in MgO added with or without a small amount of SiO₂, Al₂O₃, etc. As the tracers RI⁴⁵Ca and RI⁵⁹Fe were used. The relationship between the rate of movement of these ions and the sintering behaviors of the mixtures were also studied.
The results obtained were summarized as follows:
(1) The movement of these ions is caused by thermal diffusion and draining. In MgO added with CaO and SiO₂, the rate of movement of Ca²⁺ ion increased with increasing the molar ratio CaO/SiO₂, reached to a maximum when CaO/SiO₂≅1, and then decreared; no movement was observed when CaO/SiO₂_??_2. Additions of Al₂O₃ and Fe₂O₃ to MgO together with CaO and SiO₂, however, produced different results, i.e., the rate of movement of Ca²⁺ in this mixture increased with increasing the ratio (CaO/SiO₂) without having a maximum.
(2) Additions of oxides such as TiO₂, Cr₂O₃, MnO₂, B₂O₃, etc., which were found, by the author's previous works, to be effective in promoting sintering of MgO and the growth of periclase crystals, always increased the rate of movement of Ca²⁺.
(3) Comparing with Ca²⁺ ions, Fe³⁺ ions in MgO had a low rate of diffusion. Draining occured only when Ca²⁺ coexisted in MgO.
(4) X-ray analysis showed that the compounds, CaO⋅MgO⋅SiO₂, 2CaO⋅Fe₂O₃, 4CaO⋅Al₂O₃⋅Fe₂O₃, etc., were produced in MgO by the movement of Ca²⁺ and Fe³⁺ ions.
(5) It was suggested that when the above compounds melt at high temperatures producing the liquid phase, the loard-bearing capacity of MgO is lowered.