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.