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Lanthanum apatite is one of the most promising materials as electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFC), those operating close to 700°C. The complexity of microstructures, composition, and geometries of SOFC devices, make it necessary to have a precise control of processing parameters to obtain the desired combination of properties. This work involves the preparation and optimization of La10AlSi5O26.5 materials by reaction sintering of the raw materials (La2O3, Al2O3, and SiO2) to be used as IT-SOFCs electrolytes. Homogeneous mixed suspensions of those ceramic oxides were prepared. A heterocoagulation process was followed for ensuring a better reactivity during sintering. All the parameters involved in the process, such as deflocculant content, particle size of the initial powders, sonication time of the mixed suspensions, compaction, and sintering cycle were optimized. The effect of all these parameters were studied and discussed. Finally, La10AlSi5O26.5 electrolytes with density values of ∼88% of the theoretical density and total conductivities of 1.7 × 10−2 S/cm at 700°C were obtained. This study opens a path for the preparation of concentrated suspensions for obtaining dense thin films.