Characterization of white Portland cement hydration and the C-S-H structure in the presence of sodium aluminate by Al and Si MAS NMR spectroscopy

The effects of hydrating a white Portland cement (wPc) in 0.30 and 0.50 M solutions of sodium aluminate (NaAlO₂) at 5 and 20 °C are investigated by ²⁷Al and ²⁹Si magic-angle spinning (MAS) NMR spectroscopy. It is demonstrated that NaAlO₂ accelerates the hydration of alite and belite and results in calcium-silicate-hydrate (C-S-H) phases with longer average chain lengths of SiO₄/AlO₄ tetrahedra. The C-S-H phases are investigated in detail and it is shown that the Al/Si ratio for the chains of tetrahedra is quite constant during the time studied for the hydration (6 h to 2 years) but increases for higher concentration of the NaAlO₂ solution. The average chain lengths of “pure” silicate and SiO₄/AlO₄ tetrahedra demonstrate that Al acts as a linker for the silicate chains, thereby producing aluminosilicate chains with longer average chain lengths. Finally, it is shown that NaAlO₂ reduces the quantity of ettringite and results in larger quantities of monosulfate and a calcium aluminate hydrate phase.