Chemical optimisation of the compressive strength of aluminosilicate geopolymers synthesised by sodium silicate activation of metakaolinite

High strength cements can be synthesised by alkali activation of materials rich in Al2O3 and SiO2. In this study, amorphous aluminosilicate polymers produced by sodium silicate activation of metakaolinite were studied, with particular reference to chemical optimisation of the compressive strength according to the relative concentrations of Si, Al and Na in the polymer. The sodium silicate was manufactured from silica fume and sodium hydroxide. The compressive strengths of polymers with Si:Al molar ratios of 1.0-3.0 and Na:Al molar ratios of 0.5-2.0 were considered. The polymers were cured at 75degreesC for 24 h and their compressive strengths measured after aging for 7 days. The strength was found to depend systematically on the relative amounts of Si, Al and Na, with the maximum being 64 +/- 3 MPa for an Si : Al : Na molar ratio of 2.5:1:1.3. X-Ray diffraction/scattering data indicate qualitatively that the bonding network in the amorphous aluminosilicate alters systematically with composition.