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 (similar to 1 nm) structural order of metakaolin-based KAlSi2O6 center dot 5.5H(2)O geopolymer was quantified and compared to zeolitic tetragonal leucite (KAlSi2O6) using the X-ray atomic pair distribution function technique. Unheated KAlSi2O6 center dot 5.5H(2)O was found to be structurally similar to leucite out to a length of similar to 8 angstrom, but had increased medium range disorder over the 4.5 angstrom < r < 8 angstrom range. On heating to > 300 degrees 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 degrees C. Refinements of a leucite model against the PDF data for geopolymer heated to 1100 degrees C for 24 h yielded a good fit.