29Si and 27Al high-resolution NMR characterization of calcium silicate hydrate phases in activated blast-furnace slag pastes

Within the framework of improving mechanical properties of activated blast-furnace slag cements, a set of hardened pastes of 28 days age were analyzed by 29Si and 27Al high-resolution nuclear magnetic resonance (NMR) at 9.4 T. Structural and compositional differences among C-S-H phases obtained with different activation and curing conditions were characterized by NMR. Activation of the slag was done with compounds of different alkalinity (sodium silicate, sodium hydroxide, calcium hydroxide and gypsum), and under steam and wet curing. Parameters characterizing the extent of the hydration reaction, the polymerization degree of the aluminum-silicate chains and the Al/Si ratio in C-S-H were obtained from NMR spectra. 29Si-NMR spectra indicated that connectivites of silicate tetrahedra in all pastes are compatible with the “dreierkette” structural model of C-S-H. A substantial degree of polymerization of the aluminum-silicate groups in C-S-H was observed in pastes resulting from activation with sodium silicate and gypsum/calcium hydroxide blend. Steam curing and a higher alkali concentration enhanced the incorporation of Al in C-S-H. Even with low concentration of alkali, through steam curing it is possible to obtain a degree of incorporation of Al in C-S-H as high as in the case of higher alkali addition.