Molecular statics simulations are used in conjunction with the Embedded Atom Method to study the segregation tendency of magnesium and the effect of doping on aluminum symmetric tilt grain boundaries (STGB). It is observed that a state of hydrostatic stress exists predominantly near the grain boundary and influences the segregation energy of magnesium. The origin of hydrostatic stress is explained in terms of local geometrical arrangement of atoms at the grain boundary. The energy of a given grain boundary is found to increase with addition of Mg atoms and the increase depends on the normal distance of the atom from the grain boundary.