Characterization of white Portland cement hydration and the C-S-H structure in the presence of sodium aluminate by 27Al and 29Si 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 deg. 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