Relationships between composition, structure and strength of inorganic polymers

'Inorganic polymers', or geopolymers, are novel synthetic binders produced by reactions between alkali silicate solutions and solid aluminosilicates. In Part 1 of this study, 12 metakaolin-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four most important oxide components of inorganic polymers (Na2O, SiO2, Al2O3 and H2O) was assessed by electron microscopy and by strength testing. Additionally, the effect of the type of alkali cation was determined. In general, the results followed expected trends and there were clear correlations between composition, microstructure and strength. It was found that high strength was related to low porosity and a dense, fine grained microstructure. Such a structure was found in inorganic polymers with high alkali contents (Na2O/Al2O3 = 1.2) and low water contents (H2O/Al2O3 = 12). High silica and low alumina contents (SiO2/Al2O3 = 3.5-3.8) also produced this structure, however, there was a limit beyond which the strength deteriorated. In relation to the effect of alkali cations, sodium was found to give higher resin strength than potassium. The results of the study further confirm that the selection of precursor raw materials remains a critical factor to initial strength development. The relationship between different resin formulations and resulting microstructures are discussed.