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 (Al4], Al5], and Al6]) and plays multiple structural roles. Al4] occurs on the bridging tetrahedra of the drierkette Al-silicate chains, and Al5] and Al6] 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. Al4] 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 Al5] and Al6] through Al4]-O-Al5,6] 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 papers30], 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.