About geopolymerization

In 1937 W. L. Bragg published a method for classifying all kinds of silicates and their crystal structures based on the concept of the ionic theory by L. Pauling. The fundamental unit is a tetrahedral complex consisting of a small cation such as Si⁴⁺, or Al³⁺ in tetrahedral coordination with four oxygens (Pauling’s first rule). Many textbooks explain the geometry of the SiO^4- tetrahedron and other mineral structures as determined by the relative sizes of the different ions.
This ionic coordination representation is no longer adapted to the requirements of geopolymer chemistry that is governed by covalent bonding mechanisms.

Not ionic (tetrahedral) but covalent !

In the figure below, six atomic arrangements are used to illustrate the silicate ionic structure on the one hand, and the siloxonate/sialate covalent construction on the other hand.

a) Electrons distribution: in the external layer of the atoms Si, O, Al and Na. The electrovalence rules command the creation of the octet (8 electrons in the external shell) either by donating electrons (donator) or by receiving electrons (receptor), as follows:
- Si has 4 electrons. It is a donator or an acceptor (tetra-valence).
- O has 6 electrons. It is a receptor (di-valence).
- Al has 3 electrons. It is a donator in acidic medium (tri-valence) and a receptor in alkaline solution (tetra-valence).
- Na has 1 electron. It is a donator (mono-valence).

b) Ionic concept tetrahedron, coordination: The ions (Si⁴⁺, 4O^-) build the single tetrahedron. Si donates 4 electrons to the 4 oxygens and turns into a small cation Si⁴⁺. Si is tetracoordinated with the 4 oxygens. To achieve anionic stability, each oxygen needs an eighth electron supplied by a metal (Na, K, Ca, Mg, Fe, etc.) or another Si, not shown on the Figure.

c) Ionic concept: By the mutual sharing of one oxygen anion O^2-, two or more tetrahedra may link to form polyanionic groups.

d) Covalent concept: The molecule (SiO₄) results from the co-sharing of electrons between one Si atom and the four surrounding oxygens yielding Si-O covalent bonds (tetravalence). The ortho-siloxonate molecule (SiO₄)^4- requires additional metallic ion donators (Na, K, Ca, Mg, Fe) not shown on the figure.

e) Covalent concept: The polycondensation into di-siloxonate and higher polymeric siloxonates occurs by additional co-sharing of electrons between Si and O. The di-siloxonate molecule (Si₂O₇)^6- requires additional metallic ion donators (Na, K, Ca, Mg, Fe).

f) Covalent concept: The formation of the ortho-sialate molecule with the covalent bond Si-O-Al- occurs in alkaline medium. The Al atom takes the single electron pertaining to a metalloid (Na for example) and becomes tetra-valent, like Si, with an additional negative electrostatic charge. The Na+ cation is strongly attached to the sialate molecule and balances the negative charge.

The differences between the ionic concept (coordination) and the covalent macromolecular bonding are profound. The double tetrahedron in structure (c) is sharing one oxygen anion O^2-, whereas in the di-siloxonate molecule of structure (e), the covalent bond is achieved through Si and O co-sharing only one electron. This results in stronger bond within the latter structure.