The Effect of Alkali Ions on the Incorporation of Aluminum in the Calcium Silicate Hydrate ( C – S – H ) Phase Resulting from Portland Cement Hydration Studied by 29 Si MAS NMR

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 29 Si MAS NMR . The principal difference between the three cements is their bulk Al 2 O 3 contents and quantities of alkali ( Na + and K + ) ions. 29 Si MAS NMR allows indirect detection of tetrahedral Al incorporated in the silicate chains of the C – S – H structure by the resonance from Q 2 (1 Al ) sites. Analysis of the relative 29 Si 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 SiO 4 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 Si 4+ by Al 3+ 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 AlO 4 tetrahedra. This result is further supported by similar 29 Si MAS NMR experiments performed for the white Portland cements hydrated in 0.30 M Na OH and NaAlO 2 solutions.