DIRECT OBSERVATION OF ALUMINUM GUEST IONS IN THE SILICATE PHASES OF CEMENT MINERALS BY AL-27 MAS NMR-SPECTROSCOPY

The principal mineral phases in Portland cements are the impure forms of the calcium silicates, Ca3SiO5 and Ca2SiO4, known as alite and belite, in which the silicates are modified in composition and crystal structure by incorporation of guest ions such as Mg2+, Al3+ and Fe3+. This work reports the first direct evidence for guest-ion substitution in alite and belite by the observation of Al substitution employing Al-27 magic-angle spinning (MAS) NMR. For both minerals Al3+ is observed to substitute for Si4+ and it is shown that Al-27 MAS NMR can be used to quantify the amounts of Al substitution in the silicate phases of ordinary Portland and oilwell cements at levels well below 1 wt.%. The Al-27 quadrupole coupling parameters and isotropic chemical shift (delta) have been determined for the unique Al guest-ion site in belite, which exhibits the most deshielded chemical shift (delta = 96.1) yet reported for a tetracoordinated Al bonded to four oxygens.