In his message posted in the topic how to separate neat geopolymer, Dr. Zhang Yunsheng (zhangys) from Southeast University, Hong Kong, China, wrote:
In your email, you said that it should be noted to avoid using general term such as METAKAOLIN. In fact, many naturally occurring and man-made Si-Al materials with low calcium have been used to synthesis various types of geopolymers in recent 3 years. However, the results showed that the mechanical, durability properties of geopolymer synthesized by other raw materials through room temperature curing are not as good as those by metakaolin. Thus I think metakaolin is a very key or necessary constituent raw material for preparing high performance geopolymers in room temperature. In addition, there are immense amounts of kaolin, especially medium or lower grade kaolin in china, which are not still applied.
Why do we have to avoid using the wording METAKAOLIN??
The word metakaolin means: another type of kaolin. It does not provide any information on the reactivity, the physico-structure, and its ability to geopolymerisation. The word metakaolin, in the industry, refers to kaolin OR Kaolinitic clays dehydroxylated, that is to say thermally treated at a temperature above 450°C. Industrial metakaolin are thermally treated either at:
500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000°C.
These products are used as fillers for organic plastics, rubber, paper, etc. or as pozzolanic additives in Portland cement.
Above 1000°C, the products are called: chamotte, and are used in the ceramic industry.
The calcined kaolinite we need for geopolymerisation is an alumino-silicate oxide with a very specific 27Al NMR spectrum, essentially with very well resonances at 55 and 20-25 ppm, assigned to Al(IV) and Al(V) coordination with O.
In order to emphasize the fact that we are selecting a material containing the mineral KAOLINITE (not the kaolin), i.e. clays which contains this poly-silico-aluminate, such as: kaolin, kaolinitic clays, dickite clays, nacritic clays, and perhaps also halloysite clays, I am using the term MK-750 [MetaKaolin-750°C] (or KANDOXI, an acronym for KAaolinite-Nacrite-Dickite-OXIde). This oxide is the one, which has the NMR spectrum with Al(IV-V).
In my book Geopolymer Chemistry & Applications I am using the wording metakaolin MK-750, or MK-750 alone..
Prof. Dr. Joseph Davidovits