C12A7 calcium aluminate electride (C12A7:e-) is a novel inorganic functional material with mayenite crystal structure. Because of its physical and chemical properties, C12A7:e- can find application as a catalyst carrier in various reactions, including ammonia synthesis. However, the low specific area of this material imposes limitations on its use in catalysis. This limitation can be circumvented by synthesis within carbon nanoreactors as the carbon-coated particles resist sintering. We investigate by solid-state 27Al nuclear magnetic resonance (NMR) spectroscopy the structure of C12A7 electride prepared by the carbon nanoreactor process. An accurate determination of NMR parameters via simultaneous application of both DFT calculations and solid-state NMR proved the structure of carbon-coated C12A7 electride to be identical to the one of materials synthesized by more conventional routes. It was shown that the conducting electrons of C12A7:e- do not alter the cage structure, thus leading to similar NMR parameters of electride and insulating (cement) samples. Finally, the carbon nanoreactor process led to a significant synthesis temperature decrease of C12A7 structure formation.