Location of Aluminum in Substituted Calcium Silicate Hydrate (C‐S‐H) Gels as Determined by ²⁹Si and ²⁷Al NMR and EELS

Abstract

Solid-state "A1 and 29Si magic angle spinning NMR spectroscopy has been combined with electron energy loss spectroscopy carried out in the transmission electron microscope to determine the location of A1 substituting in a semicrystalline C-S-H gel present in a hydrated synthetic slag glass. The gel is found to contain mainly pentameric silicate chains in which the central silicon is substituted by aluminum.

EELS fingerprint of Al-coordination in silicates

“…Differing amounts of plural scattering will undoubtedly affect our results, however in all cases sample thicknesses were sufficiently similar for a qualitative comparison to be made. These results therefore suggest that Al occurs predominantly in tetrahedral sites in the outer product region, where it substitutes for silicon in the linear silicate chain structure as revealed by 29 Si magic angle spinning nuclear magnetic resonance (MASNMR) studies [19]. The splitting of peaks (a) and (b) in the Al K-ELNES (Fig.…”

“…Both these increases were found to be correlated with an increase in the relative concentrations of Mg and Al. These observations may be explained by the existence of two phases [19], however, the added benefits of high spatial resolution in determining local site symmetry by EELS can not be stressed too highly, especially in multiphase systems.…”

“…This is the reverse of the calculation (Fig. 2b) [19]. For our purposes we may envisage the hydrated cement sample as being divided into two regions: the inner and the outer product -so named since they refer to the location of the original unhydrated grains in the sample.…”

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

“…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. These species lead to bands centred at -81.9 ppm in the deconvoluted spectra [53]. Cross-linked Q 3 (1Al) and Q 3 sites are also evident at -91.9 ppm and -96.6 ppm respectively in the spectrum of the Al/Si* = 0.1, 80°C sample, which is the only spectrum that contains clearly visible resonance signals for these sites.…”

(Micro)-structural comparison between geopolymers, alkali-activated slag cement and Portland cement

“…These have been previously assigned, respectively, to SiQ 2 and SiQ 2 (1Al) units by a lot of authors who published similar spectra for alkaliactivated slags. 2,3,5,7,26 So, most of the silicate tetrahedra are chain mid-members and important Al substitution for Si occurs in the silicate chains. The general characteristic of the AAS sample spectrum compared to the PC sample is a shift towards more negative chemical shifts, which reflects significantly longer aluminosilicate chains than in Portland cement.…”