  Spark plasma sintering (SPS) of MgAl2O4 powder was investigated at
temperatures between 1200 and 1300{\deg}C. A significant grain growth was
observed during densification. The densification rate always exhibits at least
one strong minimum, and resumes after an incubation period. Transmission
electron microscopy investigations performed on sintered samples never revealed
extensive dislocation activity in the elemental grains. The densification
mechanism involved during SPS was determined by anisothermal (investigation of
the heating stage of a SPS run) and isothermal methods (investigation at given
soak temperatures). Grain-boundary sliding, accommodated by an in-series
{interface-reaction/lattice diffusion of the O$^2$-anions} mechanism controlled
by the interface-reaction step, governs densification. The
zero-densification-rate period, detected for all soak temperatures, arise from
the difficulty of annealing vacancies, necessary for the densification to
proceed. The detection of atomic ledges at grain boundaries and the
modification of the stoichiometry of spinel during SPS could be related to the
difficulty to anneal vacancies at temperature soaks.
