Effects of silica reactivity on the nature and formation of Al-substituted 1.1 nm tobermorite

The effects of the reactivity of the silica source on the formation and morphology of Al-substituted 1·1 nm tobermorite in a hydrothermally treated CaO-Al2O3-SiO2-H2O system were studied. Colloidal silica and fine quartz were used as the silica source and gibbsite, containing only octahedrally coordinated Al, was the aluminous source used in this investigation. From a combination of solid-state 27Al MAS NMR, XRD, DTA-TGA and SEM, unequivocal evidence can be found that, with increasing silica reactivity, tetrahedrally coordinated Al in Al-substituted 1·1 nm tobermorite prevails over octahedrally coordinated Al species. Reaction sequences in this system as a function of the reactivity of the silica source are presented.