²⁷Al and ²⁹Si Solid-State NMR Characterization of Calcium-Aluminosilicate-Hydrate

Abstract

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.

Incorporation of aluminium in calcium-silicate-hydrates

“…Sun et al [11] suggested that Al(V) and Al(VI) could compensate the negative charge introduced by the replacement of Si(+ IV) by Al(+ III) in the silica chain. However, the amount of Al(V) and Al(VI) does not correlate with the amount of Al(IV) in the silica chains [10,13].…”

“…The substitution of aluminium occurs primarily as tetrahedrally coordinated Al(IV) in the bridging position (Q 2 b ) [9][10][11][12], as indicated in Fig. 1.…”

“…At higher Ca/Si ratios, the relative amount of Al(IV) in C-S-H decreases, instead more octahedrally coordinated Al(VI) is observed [10,11,13,14]. The octrahedrally coordinated Al(VI) has been suggested to represent either an amorphous aluminium hydroxide or a calcium aluminate hydrate at the C-S-H surface: third aluminium hydrate (TAH) [15] or in the interlayer within the C-S-H [13].…”

“…The octrahedrally coordinated Al(VI) has been suggested to represent either an amorphous aluminium hydroxide or a calcium aluminate hydrate at the C-S-H surface: third aluminium hydrate (TAH) [15] or in the interlayer within the C-S-H [13]. In addition to Al(IV) and Al(VI), approximately 10% of the aluminium associated with C-S-H is present as pentacoordinated Al(V) regardless the Ca/Si ratio of the C-S-H [10,13,14]. Sun et al [11] suggested that Al(V) and Al(VI) could compensate the negative charge introduced by the replacement of Si(+ IV) by Al(+ III) in the silica chain.…”

The influence of ambient pH on fly ash-based geopolymer

“…The spectra were subjected to the deconvolution in a qualitative calculation [19][20][21][22]. From the NMR data of the specimen 14 days after prepared, there were three types of Na + inside the specimen.…”