Direct observation of aluminium guest ions in the silicate phases of cement minerals by 27 Al MAS NMR spectroscopy

Abstract

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 guestion substitution in alite and belite by the observation of Al substitution employing ²⁷Al magic-angle spinning (MAS) NMR. For both minerals Al³⁺ is observed to substitute for Si⁴⁺ and it is shown that ²⁷Al 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 ²⁷Al quadrupole coupling parameters and isotropic chemical shift (δ) have been determined for the unique Al guest-ion site in belite, which exhibits the most deshielded chemical shift (δ= 96.1) yet reported for a tetracoordinated Al bonded to four oxygens.