The crystal structure of Magnesium Silicate Hydrate (M-S-H) and its relation with talc-like clay mineral
Résumé
In the framework of geological disposal of radioactive waste, cement materials are foreseen to be used in the mechanical stability of the installations and to limit radionuclide transfer through the rock formation. In order to fill these requirements, the behavior of various concrete formulations is currently studied. In this context, low pH cementitious materials are considered as alternatives to classical high performance concretes in order to minimize chemical interactions at the interfaces between cement materials and the clay surrounding rock formations and/or engineered clay materials. The additive of supplementary cementitious materials such as blastfurnace slag and fly ash induces pozzolanic reactions with the precipitation of low Ca/Si (C/S) ratio of calcium silicate hydrates (C-S-H) to reach a lower equilibrium pH of the pore solution (pH ~10).
It follows that the control of the pH will be governed by the C-S-H with lower C/S ratio. However, if in Ordinary Portland Cement (OPC) and/or standard concrete formulations the hydration products are well characterized with well constrained kinetic/thermodynamic models; for low pH formulation, the mineralogical control of elements in solution have to be discussed, if we consider that concentrations of elements in solution like Aluminum and magnesium are higher than for OPC. In particular for the hydration of MgO, which is mainly introduced by the BFS, Zhang, Cheeseman and Vandeperre have mentioned the precipitation of brucite (MgOH2) which reacted with the silica fume to produce a magnesium silicate hydrate (M-S-H). The calculated pH in equilibrium with this mineralogical assemblage is around 10.5 and satisfies the requirement of low pH condition.
The occurrence of M-S-H has been mentioned in many environments, and was described as low crystalline phases according to the low intensity and broad peaks signals. Despite this abundance of M-S-H evidences and characterizations, the structure of M-S-H is not well known. Some authors have tried to explain the precipitation of such Mg-silicate hydrates according to Ca/Mg isomorphic substitution in the calcium silicate hydrates (C-S-H). The present study aims at determining this structure, to define their nature (cement phase or phyllosilicate). Two low temperature synthesis of M-S-H with a Mg/Si ratio of 0.6 and 1.2, close to the talc composition (Mg3Si4O10OH2) and consistent to the Ca/Si of the C-S-H phases. Crystal chemistry of M-S-H was determined by combining TGA, electron probe micro-analysis (EPMA), TEM, NMR and powder X-raydiffraction. Characterizations of these poorly crystalline Mg-silicates reveal a 2:1 magnesium phyllosilicate-like structure with short range stacking order, described as a talc-like structure. All the talc structure crystallographic evidences described in previous studies are in agreement with the characterization of the talc synthesis performed for short synthesis time. In that case, the M-S-H particles are displaying a low cristallinity and a small size. XRD patterns were successfully modeled according the modeling approach developed by, thus providing meaningful and accurate structural information, including structure defects, despite the weak modulation of the profiles. A full structure model is thus proposed for M-S-H.