The Complex Structure of Geopolymers

W. M. Kriven, J.L. Bell and P. Sarin
Department of Materials Science and Engineering
University of Illinois at Urbana-Champaign, IL 61801

Geopolymers are rigid, alumino-silicate, pre-zeolitic, hydrated gels containing group I, charge-balancing cations and made under ambient temperatures. The gel is an amorphous, cross-linked, impervious, acid-resistant structure, which is 40 % nanoporous (3 nm pore radius) and nanoparticulate (5-10 nm). Geopolymer gels are being used for a number of refractory applications such as molds for metal casting, temperature-resistant panels, and high-temperature adhesives. Despite this, little information is available regarding their crystallization, thermal expansions, and microstructural evolution with temperature. In this study, the formation of pollucite (CsAlSi2O6) and leucite (KAlSi2O6) from geopolymer precursors was studied using dilatometry, thermogravimetric analysis (TGA), differential scanning calorimetery (DSC), high temperature in-situ synchrotron X-ray diffraction (HTXRD), and a variety of microstructural characterization techniques (MIP, BET, SEM, EDS, TEM, SAD, HREM, STEM, nanodiffraction, etc.). It was found that crystallization temperatures and activation energies are lower using geopolymer gels compared with most other ceramic preparation techniques. Both phases were found to crystallize at around 900-950oC, and the resultant ceramic phase consisted of a crystalline phase surrounded by an amorphous matrix. Leucite composition geopolymers crystallized up to 80 wt% KAlSi2O6) , which is much higher than one can obtain via traditional dental porcelain routes. The microstructures of porous geopolymer bodies were converted to ceramics and examined by a variety of microstructural characterization techniques. A novel technique for in situ, in air, high temperature (2000oC) X-ray, synchrotron diffraction, using a Quadrupole Lamp Furnace (QLF) has recently been developed. In addition, a new Curved Image Plate Detector (CIPD) has been designed for rapid (30 sec) collection of X-ray diffractometry spectra, followed by Rietveld analysis of the data. With this technique the thermal expansion coefficients of geopolymer-derived compositions have been measured and found to be tailorable, depending upon composition, specifically cation species and Si/Al ratio.