What is the mechanistic implications of the higher concentration of precursors in the case of geopolymer formation? In other words how does a higher precursor concentration affect the normal mechanism of zeolite formation (nucleation and crystal growth) in order to prevent crystal growth and create a geopolymer and not just an amorphous zeolite? My first guess was that because of the high concentration the monomeric species could not orientate themselves in a ordered crystalline framework, but I am still unclear about what does exactly happen when geopolymers form and how this can be explained in terms of molecular kinetics?
In terms of geo-chemistry, how would you distinguish between a crystalline product and a polymer made up of the same building blocks (i.e. Si and Al tetrahedrons) if you have to graphically represent their structural models? Will both not have the same repeating structural unit, depending on along which axis one looks?
Excerpt from Davidovits US Patent 5,342,595, col. 7, lines 11-31:
In the geopolymeric resin, the molar ratio K2O:Al2O3 is generally comprised between or equal to:
K2O:Al2O3 1:1 and 1.6:1
When the molar ratio K2O:Al2O3 = 1:1, the poly(sialate-disiloxo) (K)- PSDS Kn(- Si-O-Al-O-Si-O-Si-O -)n consists of SiO4 and AlO4 from type Q4, as determined by MAS-NMR
spectroscopy. The geopolymer structure is entirely tridimensional.
When the molar ratio K2O:Al2O3 is in the order of 1.3:1 or higher, the obtained geopolymer is less reticulated and contains SiO4 from type Q3 only. It is assumed that the film-forming properties
of the geopolymeric resin are due to this geopolymer type.
When the molar ratio K2O:Al2O3 is too high, this can lead to the production of Kn(- Si-O-Al-O-Si-O-Si-O -)n geopolymers with a linear structure where SiO4 is of type Q2; with aging, these linear
geopolymers can depolymerise and migrate throughout the matrix and occasionally induce surface defects on the manufactured ceramic items.
Prof. Joseph Davidovits