Use of organic bases in the synthesis of geopolymers

Alkali geopolymers are a facile forming route for alkali aluminosilicate ceramics because they are formed as fluids and crystallize upon heating to about 1000 °C. It is also possible to replace the alkali cations by an ion exchange process, expanding the number of ceramic compositions that can be made while taking advantage of the versatility in forming afforded by geopolymers. However, the ion exchange process is very slow for small bodies and entirely impractical for pieces thicker than a few millimeters. This thesis investigated the synthesis of geopolymers with organic bases rather than inorganic alkali hydroxides. This would avoid the use of alkali cations entirely and greatly widen the range of ceramics formable via geopolymers. Three organic bases were tested – guanidine, 1,1,3,3-tetramethylguanidine (TMG), and tetramethylammonium hydroxide (TMAOH) – for their ability to form geopolymers with Al2O3:SiO2 ratios of 1:2 and 1:4 as well as 3:2 to match the composition of mullite, a common ceramic. Guanidine was successful, causing reaction at room temperature and producing bodies with atomic structures and microstructures similar to the equivalent sodium geopolymer compositions. TMAOH was able to induce setting in Al2O3•4 SiO2 systems, but they required heating to 50 °C for 1 month, and the resultant bodies were substantially different from sodium geopolymers. An even higher cure temperature, such as 80 °C, might reduce the setting time and improve the reaction into a geopolymeric material. TMG was entirely unsuccessful, and no compositions solidified or even exhibited a substantial extent of reaction. For the organic base compositions that solidified, their crystallization upon heating was studied up to 1600 °C. The Al2O3•2 SiO2 and Al2O3•4 SiO2 compositions produced mullite + glass and the 3 Al2O3•2 SiO2 compositions became almost pure mullite. All of them had substantial amounts of porosity after firing from a combination of the geopolymer synthesis method and the firing program. The amount of porosity and crystal morphology varied with the organic base, overall composition, and choice of precursors used to reach the 3 Al2O3•2 SiO2 composition. An alternative route to casting a pre-mullite material was devised which relied only on having a solution with high pH and an amorphous mullite powder with a high fraction of five-coordinated aluminum. These compositions behaved like hydratable alumina binders and were bound together by precipitated aluminum hydroxide phases rather than geopolymer. However, it seemed to be more generally viable and all of the organic bases, including TMG, caused setting in a short time, albeit with some differences in the microstructure and density after firing. Attempts to extend this method to non-mullite aluminate compositions, using MgO•Al2O3 and 3 Y2O3•5 Al2O3 as examples, were not successful. Although the use of an alkaline solution instead of water alone did induce a greater degree of aluminum hydroxide precipitation, there was not always enough aluminum hydroxide to bind the material and setting was sometimes delayed rather than accelerated. The use of organic base geopolymers for producing non-mullite ceramics was demonstrated by the synthesis of cordierite (2 MgO•2 Al2O3•5 SiO2) from a guanidine metakaolin-based geopolymer. The field of available ceramics was expanded further by showing that geopolymer-like materials could be made using Y2O3 and ZnO as substitutes for metakaolin, giving Y2O3•2 SiO2 and 2 ZnO•SiO2 compositions, respectively.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Samuel, Devon

Contributors dc:contributor

• Kriven, Waltraud M
• Shoemaker, Daniel
• Krogstad, Jessica
• Garg, Nishant

• Geopolymer
• Organic Base
• Aluminosilicate
• Ceramic

Statement dc:rights

• Copyright 2024 Devon Samuel

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/124148

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F124148