29Si NMR Study of Structural Ordering in Aluminosilicate Geopolymer Gels

Abstract

A systematic series of aluminosilicate geopolymer gels was synthesized and then analyzed using ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR) in combination with Gaussian peak deconvolution to characterize the short-range ordering in terms of T−O−T bonds (where T is Al or Si). The effect of nominal Na₂O/(Na₂O + K₂O) and Si/Al ratios on short-range network ordering was quantified by deconvolution of the ²⁹Si MAS NMR spectra into individual Gaussian peaks representing different Q⁴(mAl) silicon centers. The deconvolution procedure developed in this work is applicable to other aluminosilicate gel systems. The short-range ordering observed here indicates that Loewenstein's Rule of perfect aluminum avoidance may not apply strictly to geopolymeric gels, although further analyses are required to quantify the degree of aluminum avoidance. Potassium geopolymers appeared to exhibit a more random Si/Al distribution compared to that of mixed-alkali and sodium systems. This work provides a quantitative account of the silicon and aluminum ordering in geopolymers, which is essential for extending our understanding of the mechanical strength, chemical and thermal stability, and fundamental structure of these systems.