Outstanding problems posed by nonpolymeric particulates in the synthesis of a well-structured geopolymeric material: Fid-Bau Portal

Outstanding problems posed by nonpolymeric particulates in the synthesis of a well-structured geopolymeric material

Singh, Puyam S. / Bastow, Tim / Trigg, Mark

AbstractIn the conventional geopolymer synthesis of ternary reactants system of silicate anions [Na2O·2SiO2], metakaolin [Al2O3·2SiO2] and aqueous alkali [Na2O], the SiO2/Al2O3 molar ratio can only be varied from ∼2 to 8. For geopolymerisation to occur, the minimum mole percentage of metakaolin reactant is about 25% and the maximum mole percentage of Na2O content is about 30%. As the reduction of metakaolin and increase of alkali content is limited, the total conversion of metakaolin into polymeric material is uncertain. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible in this synthesis of a ternary reactant system even by ²⁹Si NMR, as the signal due to metakaolin is indistinguishable from a broad ²⁹Si NMR peak consisting of many resonance lines of polymer network of cross-linked Si/Al tetrahedra. With our modified synthesis method employing colloidal SiO2 as an additional component, the metakaolin content is decreased while keeping an optimum amount of Na2O, thereby increasing the compositional SiO2/Al2O3 molar ratio. In this case, the signals related to Al-substituted SiO4 tetrahedra are reduced, indicating the reduction of Al-substituted SiO4 tetrahedra in the polymer network, resulting in better-resolved ²⁹Si NMR lines. The ²⁹Si NMR signal related to metakaolin is then distinguishable in the spectra of cured products by observing a series of samples with 11, 7, 5 and 3 mol% of metakaolin reactant. ²⁹Si NMR signal related to metakaolin was only absent in the spectra of cured product when metakaolin reactant content is ≤5 mol%; that is, the compositional SiO2/Al2O3 molar ratio is ≥20.