The effect of a variety of doping additives on the hydration resistance of calcined materials in the MgO-CaO system was investigated. Samples were prepared from lightly calcined flotation magnesite that was mixed with dolomite, as well as from analytically pure CaO and MgO; then, the samples were doped with additives that contained cations with various valences (monvalent to tetravalent). Both the hydration rate and the powdering rate were measured. The important role of the higher-valence cations in improving the hydration resistance of the MgO-CaO materials was revealed, This behavior is believed to be due to the formation of vacancies in solid solutions of CaO or MgO with higher-valence cations, The Ti4+ cation forms a solid solution with CaO, which reduces the Ca2+ concentration and leads to the improved hydration resistance of calcined materials from the MgO-CaO system.