Synthesis and Thermal Treatment of Lithium- and Magnesium-Containing Geopolymers

Geopolymers are a class of cementitious aluminosilicate materials that are receiving an increasing amount of attention due to their potential applications in toxic waste remediation and as construction materials. They are composed of a network of crosslinked silicate and aluminate tetrahedra with charge-balancing alkali cations and are therefore similar in composition to alkali aluminosilicate zeolites. They are, however, x-ray amorphous.¹⁻⁴ They are formed by the dissolution of a solid aluminosilicate in a solution of alkali hydroxide or alkali silicate to form aluminosilicate ions which subsequently polymerise.  The effects of adding magnesium to metakaolin geopolymer systems was examined. Magnesium was added as soluble magnesium salts and as magnesium oxide and hydroxide. When added as a soluble salt, an amorphous magnesium (alumino)silicate with a lower degree of silicate polymerisation than a geopolymer is formed. When added as the oxide or hydroxide, hydrotalcite is formed. In both cases, the product is produced alongside a separate geopolymer phase. A magnesiumcontaining geopolymer phase was not found in either. When heated to 1200°C, geopolymers with magnesium oxide added bloated to form lightweight foams.  Lithium analogues of conventional metakaolin geopolymer systems with a range of lithium, aluminium, silicon and water contents were examined. Systems with molar ratios similar to those of commonly studied sodium and potassium metakaolin geopolymers produce self-pelletised lithium zeolites. The zeolite formed was Li-EDI, the lithium analogue of zeolite F. This is the first reported synthesis directly from metakaolin. True lithium geopolymers are found not to form in the systems examined. The zeolite bodies react to form β-eucryptite and β-spodumene at temperatures from 800 – 1350°C.  The use of aluminium hydroxide and amorphoud silica rather than aluminosilicates as raw materials for the formation of potassium geopolymers was found to produce geopolymers with embedded grains of unreacted silica and aluminium hydroxide.