Microstructural characterisation of geopolymers synthesised from kaolinite/stilbite mixtures using XRD, MAS-NMR, SEM/EDX, TEM/EDX, and HREM

Abstract

When crystalline aluminosilicates partially dissolve in a concentrated alkaline medium, an amorphous geopolymeric gel is formed interspersed with undissolved crystalline particles. Some aluminosilicates dissolve more readily than others to give an equilibrium ratio of aluminium to silicon in the gel. In this case study, kaolinite and stilbite mixtures were used to investigate the relative reactivity of different minerals when present in different ratios. XRD and 29Si and 27Al MAS-NMR were used to determine when a specific mineral was completely transferred into the gel phase. Electron diffraction using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM) were employed to establish the amorphous nature of the gel phase. Scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) and TEM/EDX were then used to determine the composition of the gel. By using simple mass balance assumptions, the quantity of gel and the extent of partial dissolution of an aluminosilicate could then be calculated. It was found that a geopolymer containing a higher weight percentage of CaO in its gel, a lower ratio of (average surface area)/gel, and where the undissolved crystalline particles have a higher hardness had higher mechanical strength. The method developed in this paper is also applicable to other cementitious materials.