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A number of models have been presented to explain the role of MgO on sintering and grain growth of alumina. In view of recent experimental results showing that abnormal grain growth (AGG) is related with a small amount of glass-forming impurities, the beneficial role of MgO has to be re-evaluated in conjunction with the microstructural evolution of alumina in the presence of such impurities, particularly Ca and Si. With a fixed amount of Si, and thus for a known critical grain size for AGG, varying amounts of MgO were introduced to determine how MgO influences the critical grain size required for AGG. The mobilities of dry and wet boundaries were also determined as a function of MgO concentration with the fixed Si concentration. The experimental results revealed that the primary role of MgO is to reduce the migration rate of wet boundaries. Little effect on the migration rate of dry grain boundaries has been observed.