Low-temperature synthesized aluminosilicate glasses .3. Influence of the composition of the silicate solution on production, structure and properties

The low-temperature reaction between an aqueous sodium or potassium silicate solution and metakaolinite yields a solid aluminosilicate. The influence of the molar ratios H2O/R2O (between 6.6 and 21.0) and SiO2/R2O (between 0.0 and 2.3) of the silicate solution (R = Na or K) on the aluminosilicate's production, on the reaction stoichiometry and on the aluminosilicate's molecular structure Is studied with differential scanning calorimetry, Al-27 and Si-29 magic angle spinning nuclear magnetic resonance (MAS NMR), cross-polarization MAS NMR, Fourier transform infrared spectroscopy and X-ray diffractometry. The reaction stoichiometry is determined by a one to one ratio for R/Al. H2O/R2O has no influence on the molecular structure of the aluminosilicate. Aluminium in the aluminosilicate is four-fold coordinated for the whole range of silicate solutions investigated. Moreover, Si and Al are homogeneously distributed and the ratio AI;Si in the aluminosilicate is the same as in the reaction mixture if the stoichiometric one-to-one ratio for R/Al is used. If SiO2/R2O in the Na-silicate solution is equal to or higher than 0.8, the low-temperature reaction yields an amorphous aluminosilicate or ''inorganic polymer glass''. For smaller values of SiO2/R2O the Na-aluminosilicates are partially crystalline. Thermomechanical analysis and dynamic mechanical analysis indicate that a variation in the composition of the amorphous aluminosilicates can shift the glass transition over a few hundreds of degrees, with a minimum value of 650 degrees C.