Alkaline activation of metakaolinite-silica mixtures: Role of dissolved silica concentration on the formation of geopolymers

The alkali activation of metakaolinite permits the preparation of new amorphous cementitious materials denoted as “geopolymers”. The formation of these aluminosilicates depends on the thermal activation of kaolinites and the amount of silica in the activating solutions. In this work, the influence of Si:Al ratios on the geopolymer's formation is investigated for short reaction times. Structural and morphological transformations were followed with the XRD, SEM, IR and MAS NMR (29Si, 27Al, 23Na and 1H) techniques. For kaolinites (SiAl =1) heated at 550 °C, geopolymers with tectosilicate networks easily formed; however, in kaolinites heated at 800 °C, the formation of geopolymers was delayed as a consequence of silica segregation. The addition of silica (Si:Al= 3) favoured the formation of geopolymers particles surrounded by sodium-silicates with tubular morphology. In this association, kinetic and thermodynamic parameters play an important role. For prolonged alkaline activations, zeolites formed from the geopolymers. The quantitative analysis of the NMR spectra made possible the study of the formation of zeolites from amorphous geopolymers.