The incorporation of aluminum into silicate ions in alkaline aqueous solutions, studied by aluminum-27 nmr

Aluminum-27 n.m.r. spectra have been obtained at very high magnetic field (14.1 T) for mixed silicate/aluminate solutions. The assignment of the peaks to aluminate connectivities is attempted, clarifying earlier work and producing definitive information on the equilibrium between various aluminum-containing species (including, arguably, sites with four Al-O-Sl bridges). Variable temperature experiments show that chemical exchange processes are occurring between these species. The evolution of the spectra with time after mixing silicate and aluminate solutions has been studied as a function of temperature, indicating both slow and rapid changes. Two-dimensional EXSY spectra (mixing time 1 ms), obtained as a function of temperature, give further evidence of chemical exchange. It seems that the more highly connected aluminum sites are relatively stable. (C) Elsevier Science Inc. 1997.