The fundamental unit within a geopolymer structure is a tetrahedral complex consisting of Si or Al coordinated through covalent bonds to four oxygens. The geopolymer framework results from the cross-linking between these tetrahedra, which leads to a 3-dimensional aluminosilicate network, where the negative charge associated with tetrahedral aluminium is balanced by a small cationic species, most commonly an alkali metal cation. These alkali metal cations are often ion-exchangeable, as they are associated with, but only loosely bonded to, the main covalent network, similarly to the non-framework cations present in zeolites.