TRIPLE-QUANTUM 2-DIMENSIONAL AL-27 MAGIC-ANGLE NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE ALUMINUM INCORPORATION IN CALCIUM SILICATE HYDRATES

Triple-quantum two-dimensional Al-27 magic angle spinning nuclear magn etic resonance (Al-27 3Q-MAS NMR) was used to characterize the substit ution of Si4+ by Al3+ into the Te-Oc-Te structure of calcium silicate hydrates (C-S-H). This substitution was studied with C-S-H having an O c/Te ratio of I and in equilibrium with Al(OH)(3) in aqueous suspensio ns. In the absence of NaOH, no substitution into the C-S-H structure o ccurred. Addition of NaOH in the preparation increased the concentrati on of Al(OH)(4)(-) and favored substitution. The deficit of charge res ulting from this substitution was compensated by the accommodation of sodium in the interlayer space of the C-S-H. Increasing levels of subs tituted silicon correspond to higher alkaline and lower calcium conten ts in the interlayer space. Two substitution sites were distinguished, corresponding to the bridging and nonbridging positions in the chains of tetrahedra. A high Al-tetra(Si+Al-tetra) ratio indicated a redistr ibution of the aluminum tetrahedral sites to stabilize the substituted structure favoring aluminum in the bridging position.