The role of Al in C–S–H: NMR, XRD, and compositional results for precipitated samples

X-ray diffraction, compositional analysis, and 29Si and 27Al MAS NMR spectroscopy of Al-substituted tobermorite-type C–S–H made by precipitation from solution provide significant new insight into the structural mechanisms of Al-substitution in this important and complicated phase. Al occurs in 4-, 5-, and 6-coordination (Al[4], Al[5], and Al[6]) and plays multiple structural roles. Al[4] occurs on the bridging tetrahedra of the drierkette Al–silicate chains, and Al[5] and Al[6] occur in the interlayer and perhaps on particle surfaces. Al does not enter either the central Ca–O sheet or the pairing tetrahedra of the tobermorite-type layers. Al[4] occurs on three types of bridging sites, Q3 sites that bridge across the interlayer; Q2 sites that are charge balanced by interlayer Ca+2, Na+, or H+; and Q2 sites that are most likely charge balanced by interlayer or surface Al[5] and Al[6] through Al[4]–O–Al[5,6] linkages. Although the data presented here are for relatively well-crystallized tobermorite-type C–S–H with C/S ratios ≤ 1.2, comparable spectral features for hydrated white cement pastes in previously published papers[30], [31] and [32] [M.D. Andersen, H.J. Jakobsen, J. Skibsted, Incorporation of aluminum in the calcium silicate hydrate (C–S–H) of hydrated Portland cements: a high-field 27Al and 29Si MAS NMR investigation Inorg. Chem. 42 (2003) 2280–2287; M.D. Andersen, H.J. Jakobsen, J. Skibsted, Characterization of white Portland cement hydration and the C–S–H structure in the presence of sodium aliminate by 27Al and 29Si MAS NMR spectroscopy, Cem. Concr. Res. 43 (2004) 857–868; M.D. Andersen, H. J. Jakobsen, J. Skibsted, A new aluminum-hydrate phase in hydrated Portland cements characterized by 27Al and 29Si MAS NMR spectroscopy, Cem. Concr. Res., submitted for publication.] indicate the presence of similar structural environments in the C–S–H of such pastes, and by implication OPC pastes.