Al(IV)-(V)-(VI) coordination in calcined kaolinite

You answered my question about kaolinite calcination. You said me to see your patent 5,288,321. I read it and I understood that you obtained a metakaolinite, calcined between 700°C and 800°C, with aluminium coordination number of IV, V and VI. In this temperature I would obtain less aluminium VI than IV or V. In your others patents about Geopolymer you always emphasized that its synthesis must start with Al(IV) and/or Al(V), but never with Al(VI). My questions are: 1) Can I have some Al(VI) in the metakaolin used in the geopolymer synthesis or not? 2) Do you think that there is some calcining conditions that allows all Al(VI) be converted in Al(IV) and/or Al(V)?
Valeria Figueiredo Felisbino Barbosa, Instituto Militar de Engenharia, DE/4, Rio de Janeiro, Brazil

When, in 1978-1979, I made the first (Na,K)-PSS GEOPOLYMITE binder, scientific knowledge was based on X-Rays investigation methods.

It was generally accepted that the coordination of Al in calcined kaolinite was Al(IV). See for example the paper by A.J. Leonard, “Structural Analysis of the Transition Phases in the Kaolinite-Mullite Reaction Sequence”, J. Am. Ceram. Soc., 60 37-43 (1977). The use of MAS-NMR spectroscopy had not been developed, so far. The first paper on that topic was published in 1985 by K.J.D. MacKenzie et al., “Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27 Al Solid-State Nuclear Magnetic Resonance: I, Metakaolinite”, J. Am. Ceram. Soc., 68 293-97 (1985). Their study showed that 800°C calcined kaolinite contained about 10% of the total Al sites in Al(VI) coordination. This was a big surprise. In 1988, J. Sanz et al. published a paper that introduced the new discovered and strongly debated Al(V) coordination, “Aluminum-27 and Silicon-29 Magic-Angle Spinning Nuclear Magnetic Resonance Study of the Kaolinite-Mullite Transformation”, J. Am. Ceram. Soc., 71 C-418-C-420 (1988). This later paper is important for your understanding of the calcination parameter.

I always introduced in my patent description the newest knowledge in terms of Al coordination. It is therefore not wise to compare claims and descriptions dating back to 1979-1980 with those filed later in the 1990’s.

You’ll find recent detailed explanation in my newest book (2008) Geopolymer Chemistry & Applications, Chapter 8.

Prof. Joseph Davidovits