The incorporation of aluminum in the calcium–silicate–hydrate (C–S–H) phases formed by hydration of three different white Portland cements has been investigated by 29Si MAS NMR. The principal difference between the three cements is their bulk Al2O3 contents and quantities of alkali (Na+ and K+) ions. 29Si MAS NMR allows indirect detection of tetrahedral Al incorporated in the silicate chains of the C–S–H structure by the resonance from Q2(1Al) sites. Analysis of the relative 29Si NMR intensities for this site, following the hydration for the three cements from 0.5 d to 30 weeks, clearly reveals that the alkali ions promote the incorporation of Al in the bridging sites of the dreierketten structure of SiO4 tetrahedra in the C–S–H phase. The increased incorporation of Al in the C–S–H phase with increasing alkali content in the anhydrous cement is in accord with a proposed substitution mechanism where the charge deficit, obtained by the replacement of Si4+ by Al3+ ions in the bridging sites, is balanced by adsorption/binding of alkali ions in the interlayer region most likely in the near vicinity of the AlO4 tetrahedra. This result is further supported by similar 29Si MAS NMR experiments performed for the white Portland cements hydrated in 0.30M NaOH and NaAlO2 solutions.
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