Structural characterisation of magnesium (sodium) aluminium silicate hydrate (M-(N)-A-S-H) phases by X-ray absorption near-edge spectroscopy

Abstract

Magnesium silicate hydrate phases (M-S-H) have been observed forming in low pH cements, which came in contact with Mg-rich ground waters or argillaceous rock formations, such as Opalinus Clay. Their crystallographic structure has been suggested to be a purely ordered sheet-like phyllosilicate with a tetrahedral octahedral (TOT O) layering, i.e. similarly to a talc plus brucite structure. In nature, Mg phyllosilicates with this type of layering contain several type of cations, in particular Al, as well as alkaline earth metals. This study shows that the X-ray near-edge spectra of synthesised Mg-Al- silicate hydrates and Mg-Al-Na silicate hydrates (M-A-S-H and M-N-A-S-H) show distinct differences to M-S-H, indicating the possible accommodation of Al in their structure. This is further supported by the great similarity of the spectra to natural Mg–Al containing phyllosilicates, i.e. clinochlore, having a TOT O layering. The results, furthermore, suggest that Na is not taken up in the structure due to the strong similarities of the M-N-A-S-H with the M-A-S-H phases. The comparison of the latter spectra with natural references show high resemblance not only with clinochlore, but also with sepiolite, a Mg containing phyllosilicate with water molecules bridging between broken octahedral chains. It is, therefore, suggested that the Mg in the M-(N)-A-S-H phases is bound similarly to clinochlore with a TOT O layering and that water molecules or OH groups are present between broken octahedral layers. Aluminium is possibly accommodated in the tetrahedral and/or octahedral layers.