Direct measurement of the kinetics of geopolymerisation by in-situ energy dispersive X-ray diffractometry

In-situ energy dispersive X-ray diffractometry (EDXRD) using synchrotron radiation has been used to directly observe the kinetics of formation of a geopolymeric gel from a metakaolin precursor. The use of a purpose-built hydrothermal cell with polychromatic radiation from a wiggler source enables collection of a full diffraction pattern approximately every 150 s. This provides sufficient time resolution to observe the collapse of the metakaolin structure as it dissolves in the activating solution, accompanied by the reprecipitation of the geopolymeric gel binder phase from the now-supersaturated solution. Measurements taken on a limited set of samples of different composition (Si/Al ratio) show a clear trend in the rate of reaction with composition, and also a distinctly different mechanism of reaction in the most highly alkaline systems compared to those containing higher levels of dissolved silica in the activating solution. This corresponds to the results of previous microscopic observations showing significantly different microstructures in these systems, and confirms the value of this technique in analysis of the kinetics of geopolymerisation.