What are geopolymers?

So, during the ANS Student Conference, I heard a presentation from Jean-Claude from EDF, the French company which operates nuclear sites. He introduced to me the idea of using polymer materials and polymer composites in reactor environments. My roommate at the conference Travis asked if they were being introduced to high fluence environments and the response was yes but that information was sparse. In a metal-dominated industry, polymers may soon have their place in commercial reactors.
Specifically, geopolymers! These are chains or networks of mineral molecules linked with covalent bonds. They have following basic characteristics:a) Nature of the hardened material:

• X-ray amorphous at ambient and medium temperatures
• X-ray crystalline at temperatures >500°C

b) Synthesis Routes:

• alkaline medium (Na, K, Ca) hydroxides and alkali-silicates yielding poly(silicates) – poly(siloxo) type or poly(silico-aluminates) – poly(sialate) type
• acidic medium (Phosphoric acid) yielding poly(phospho-siloxo) and poly(alumino-phospho) types

As an example, one of the geopolymeric precursors, MK-750 (metakaolin) with its alumoxyl group –Si-O-Al=O, reacts in both alkaline and acidic systems (same for siloxo-based and organo-siloxo-based geopolymeric species). In the late 1970’s, Joseph Davidovits, the inventor and developer of geopolymerization, coined the term “geopolymer” to classify the newly discovered geosynthesis that produces inorganic polymeric materials now used for a number of industrial applications. He also set a logical scientific terminology based on different chemical units, essentially for silicate and aluminosilicate materials, classified according to the Si:Al atomic ratio:

Si:Al = 0, siloxo
Si:Al = 1, sialate
Si:Al = 2, sialate-siloxo
Si:Al = 3, sialate-disiloxo
Si:Al > 3, sialate link

This terminology was presented to the scientific community at a IUPAC conference in 1976. In the introduction of his book on alkali-geopolymer cement, the alkali-cement scientist John Provis, challenged the use of the word ‘sialate’ arguing that “…the term ‘sialate’ was already in use (since the 1950s) to describe any of the salts of organic sialic acid …” He simply forgot to mention that long before 1950 geology has been using extensively the term ‘sialic’, for example in ‘sialic metamorphic rocks‘, or ‘the oceanic crust is mostly basaltic and the continental crust is mostly sialic, meaning the rocks, such as granite, contain high amounts of aluminum and silica‘. Not to forget the fact that fly ashes were and still are commonly classified into three entities: calcic-, ferric- and sialic-groups; the sialic component results from the %weight of (Si02 + Al203 + TiO2). The geopolymeric ‘sialate‘ term proceeds from the same scientific logic (it is the acronym of silicon-oxo-aluminate), in contrast with the organic molecule ‘sialic acid’ that was derived from an ancient Greek word meaning ‘saliva’, with no scientific association. Geopolymers are presently developed and applied in 10 main classes of materials:

• Waterglass-based geopolymer, poly(siloxonate), soluble silicate, Si:Al=1:0
• Kaolinite / Hydrosodalite-based geopolymer, poly(sialate) Si:Al=1:1
• Metakaolin MK-750-based geopolymer, poly(sialate-siloxo) Si:Al=2:1
• Calcium-based geopolymer, (Ca, K, Na)-sialate, Si:Al=1, 2, 3
• Rock-based geopolymer, poly(sialate-multisiloxo) 1< Si:Al<5
• Silica-based geopolymer, sialate link and siloxo link in poly(siloxonate) Si:Al>5
• Fly ash-based geopolymer
• Ferro-sialate-based geopolymer
• Phosphate-based geopolymer, AlPO4-based geopolymer
• Organic-mineral geopolymer

According to the first refernce listed below, low level salt waste associated with the production of nuclear materials at the DOE facilities will be disposed of in shallow and burial sites managed by the DOE. Two of the primary contaminants of interest are Tc-99 and I-129, both soluble and long-lived. Cs-137, a short-lived, strong gamma emitter is also a soluble contaminant of concern. In addition to these, waste salt solutions can contain toxic quantities of metals as defined by EPAs RCRA. Some research is being done concerning, the durability of geopolymers obtained by alkali activation of aluminosilicate materials (i.e., metakaolin and fly ash), usefulness of geopolymers for immobilization of low-level and intermediate-level nuclear waste, exposure to sulphates, seawater attack, acidic media, alkali-silica reaction, steel corrosion, fire, and other potentially damaging environments (neutrons!), and much more.

Also discussed by Jean-Claude and fellow panelists was the materials approval process - a rigorous one when discussing nuclear reactors. The willingness for a company to use a newly proposed material is based on a relative willingness to incur some risk  (related to operating experience and confidence in performance). It was the hope of everybody present that through international cooperation efforts, some materials research might be expedited for the good of all.

References
http://www.flyash.info/2011/190-Cozzi-2011.pdf
http://www.geopolymer.org/
http://www.sciencedirect.com/science/article/pii/B9781845696269500080
http://www.sciencedirect.com/science/article/pii/B9781845694494500096
http://www.sciencedirect.com/science/article/pii/B9781845694494500084?np=y