Calcium silicate hydrate and its Al-substituted form synthesized by a hydrothermal process were investigated by X-ray diffraction, compositional analysis, and magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, in order to determine the mechanism of Al and Na incorporation in the tobermorite structure with varying molar ratios of Ca/Si and Al/Si. At a high molar ratio of Ca/Si, the silicate chains of tobermorite are ruptured, the degree of polymerization of the silicate chains is lowered, and the high calcium concentration lowers the content of Na2O in the structure. Solid-state Si-29 and Al-27 MAS NMR spectroscopy confirm that all Al atoms were incorporated in the silicate chains of tobermorite. The tetrahedrally coordinated Al (Al(IV)) could either act as the bridging tetrahedron (Q(B)(2)) for the dreierketten chain of tobermorite, or be present in Q(3) sites that link two dreierketten chains together. Therefore, the degree of polymerization of the silicate chains of tobermorite is increased at high molar ratio of Al/Si. Furthermore, the greater charge deficit due to the replacement of Si4+ by Al3+ ions is compensated by increased adsorption or binding of Na+.