It has been the merit of T.W. Swaddle and his team (North M.R. and Swaddle T.W., (2000), Kinetics of Silicate Exchange in Alkaline Aluminosilicate Solutions, Inorg. Chem., 39, 2661-2665) to demonstrate the existence of soluble aluminosilicate species in solution in relatively high concentrations and high pH. One major improvement in their research was that their study was carried out at very low temperatures as low as -9°C. Indeed, it was discovered that the polymerization of oligo-sialates was taking place on a time scale of around 100 milliseconds, i.e. 100 to 1000 times faster than the polymerization of ortho-silicate, oligo-siloxo units. At room temperature or higher, the reaction is so fast that it cannot be detected with conventional NMR equipment.
Five ortho-sialate solute species isolated in KOH solutions, after North and Swaddle (2000).
The hypothetical oligomers set forth in geopolymer synthesis are no longer virtual molecules. They actually exist in soluble forms in concentrated solutions at high pH. Swaddle’s study confirms the polymerization mechanisms tentatively reported earlier by Davidovits (1976) with linear oligo-sialate, oligo(sialate-disiloxo) and rings or cycles, as starting geopolymer building units.