Structural Evolution and Ceramic Formation in Metakaolin-Based Geopolymers
Bell, Jonathan Lee
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https://hdl.handle.net/2142/72163
Description
Title
Structural Evolution and Ceramic Formation in Metakaolin-Based Geopolymers
Author(s)
Bell, Jonathan Lee
Issue Date
2008
Doctoral Committee Chair(s)
Kriven, Waltraud M.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Engineering, Materials Science
Abstract
Geopolymers are increasingly being considered in a variety of refractory applications and as precursors to ceramic formation. However, fundamental details regarding their structure, thermal stability, and crystallization on heating are not well understood. In this dissertation, the structural evolution and crystallization of 4SiO2•Al2O3• M2O•11H2O (where M = K, or Cs) composition geopolymers on heating was investigated.
As hardened, unheated geopolymers have a disordered atomic structure, but formed leucite (KAlSi2O6) and pollucite (CsAlSi 2O6) glass-ceramics on heating above 1000°C and 900°C, respectively. Using the X-ray atomic pair distribution function (PDF) method, the short to medium-range order of unheated geopolymers were found to resemble that of the corresponding crystalline phases formed on heating. A high degree of medium range ordering was found in unheated Cs-based geopolymers, and is believed to arise from the presence of nuclei formed during curing. This ordering facilitated gradual pollucite crystallization at low temperature and was suggestive of a topotatic transformation mechanism. K-geopolymer exhibited less structural ordering, and leucite crystallization occurred abruptly on heating. Regardless of heating conditions, a maximum of ∼80 wt% of leucite was formed. The remaining 20 wt% constituted a glassy phase formed on heating due to sample heterogeneities created by incompletely dissolved metakaolin and free alkali. The presence of this glassy phase was confirmed by both modeling and microstructural investigations.
On heating, K and Cs-geopolymers underwent significant shrinkage, surface area reduction, and microstructural coarsening at 1200°C and 900°C respectively, due to viscous sintering. The microstructure of Cs-geopolymer consisted of ∼20-30 nm sized precipitates which coarsened on heating above 1000°C. By 1350°C the Cs-geopolymer surface attained a smooth glassy texture, although large voids from metakaolin and closed porosity remained. After heating to 1600°C, the closed porosity was removed, and the geopolymer reached ∼98% of the theoretical density of pollucite. Higher than expected levels of Cs were found near sample macropores in SEM and TEM analysis due glass formation. K-geopolymer underwent similar microstructural coarsening and the creation of closed porosity when heated above 900°C. The surface texture obtained on heating was found to be dependent on the calcination conditions, but developed a smooth, glassy appearance when heated above 1000°C. Thermal shrinkage was completed by 1100°C, and the K-geopolymer reached 99.7% of the theoretical density of tetragonal leucite.
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