Hydration of tricalcium silicate in hydrothermal conditions in the presence of magnesium oxide has shown changes in the formation of CSH gel structure (Calcium silicate hydrates). The new CSH incorporates magnesium ions, brucite, but a weak presence of portlandite. The magnesium oxide would hinder the precipitation of portlandite. The characterization of CSH gel by Si-29 MAS-NMR with various CaO/SiO2 ratios would point out that: (1) A dreierketten structure of the CSH for low CaO/SiO2 < 1, with some defects (Q(3) defect and Q(2)) in its structure is confirmed. Some magnesium ions are incorporated in the octahedral sites, in the interlayer space of the dreierketten pattern. (2) For the CSH gels with CaO/SiO2 ratios > 1, magnesium ions would be incorporated in the silicate chains of the CSH gel in a tetrahedral coordination. Although, the low MgO/CaO ratios of CSH gels indicate that the magnesium incorporation in CSH chain is low.