X-Ray pair distribution function analysis of a metakaolin-based, KAlSi2O6·5.5H2O inorganic polymer (geopolymer)

The atomic structure of geopolymers is often described as amorphous with a local structure that is equivalent to that of crystalline zeolites. However, this structural relationship has never been quantified beyond a first-nearest-neighbor bonding environment. In this study, the short to medium range (∼1 nm) structural order of metakaolin-based KAlSi2O6·5.5H2O geopolymer was quantified and compared to zeolitic tetragonal leucite (KAlSi2O6) using the X-ray atomic pair distribution function technique. Unheated KAlSi2O6·5.5H2O was found to be structurally similar to leucite out to a length of ∼8 Å, but had increased medium range disorder over the 4.5 Å < r < 8 Å range. On heating to >300 °C, changes in the short to medium range structure were observed due to dehydration and removal of chemically bound water. Crystallization of leucite occurred in samples heated beyond 1050 °C. Refinements of a leucite model against the PDF data for geopolymer heated to 1100 °C for 24 h yielded a good fit.