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 ²⁹Si MAS NMR

Abstract

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.

Incorporation of aluminium in calcium-silicate-hydrates

“…At Al/ Si = 0.2, in addition to the Q 2 p the presence of stratlingite leads to a signal at −85.6 ppm [44]. The presence of aluminium in C-S-H introduces an additional peak, Q 2 p (1Al) between −82.0 and −80.5 ppm, previously observed by [10,11,16,45,46]. The 29 Si NMR signal of pairing silica, Q 2 p , is shifted by a tetrahedrally coordinated aluminium, Al(IV) present in the neighbouring bridging position of the "dreierketten" chains and forms Q 2 p (1Al).…”

“…NMR studies indicate that the presence of alkali hydroxides increases the uptake of aluminium in synthetic C-S-H [14] and white Portland cement [16].…”

Effect of Alkalis on Cementitious Materials:Understanding the Relationship between Composition, Structure, and Volume Change Mechanism

“…The amount of aluminum incorporated into the C-S-H phase is partly controlled by the available quantity of alkali ions. More specifically, previous experiments show that the alkali ions promote the incorporation of Al in the bridging sites of the silicate chains of C-S-H (Skibsted and Andersen 2013). In return, the addition of Al can increase the alkali binding capability of C-S-H (Hong and Glasser 2002).…”

Effect of temperature and aluminium on calcium (alumino)silicate hydrate chemistry under equilibrium conditions

“…The 29 Si MAS NMR spectra are dominated by intense bands at -79.4 ppm, -83.5 ppm and -85.3 ppm (Figure 6), which are characteristic of silicate species in chain-end (Q 1 ), bridging Figure 1) [52]. An additional peak is apparent between the Q 1 and Q 2 p sites in the Al/Si* = 0.1 spectra compared to the Al-free samples, which indicates that Q 2 (1Al) sites are present in the C-A-S-H systems.…”