On the coordination of Al in ill-crystallized C-S-H phases formed by hydration of tricalcium silicate and by precipitation reactions at ambient temperature

Abstract High-resolution solid-state 27 Al MAS NMR measurements suggest that Al incorporated into C-S-H phases, prepared by precipitation reactions from sodium silicate and calcium chloride solutions and by hydration of tricalciumsilicate (C 3 S), may occur tetrahedrally (Al [4] ) as well as octahedrally coordinated (Al [6] ). The amount of which depends on the composition of the C-S-H phase. With increasing CaO/SiO 2 -ratio the portion of Al [6] increases and that of Al [4] decreases. The products of paste hydration of C 3 S contain about 80 % of the Al as Al [6] and 20 % as Al [4] .