Effect of magnesium on calcium silicate hydrate (C-S-H)

Ellina Bernard , Barbara Lothenbach (1) , Fabien Le Goff , Isabelle Pochard (2, 3) , Alexandre Dauzeres (4, 5)

1 Laboratory for Concrete & Construction Chemistry
2 ICB - Laboratoire Interdisciplinaire Carnot de Bourgogne
3 UTINAM - Univers, Transport, Interfaces, Nanostructures, Atmosphère et environnement, Molécules (UMR 6213)
4 IRSN/PRP-DGE/SRTG/LETIS - Laboratoire d'Etude et de recherche sur les Transferts et les Installations dans les Sols
5 IRSN/PSE-ENV/SEDRE/LETIS - Laboratoire d'étude et de recherche sur les transferts et les interactions dans les sous-sols

The addition ofMgCl2 to synthetic calciumsilicate hydrate (C-S-H) with Ca/Si=0.8 lowers the pH values below 10. Experimental observations and thermodynamic calculations show the destabilization of C-S-H and the precipitation of magnesium silicate hydrate (M-S-H). In contrast, the addition of MgO to C-S-H increases the pH values up to 11.5 and the Ca/Si in C-S-H from 0.8 to ~1.0. At high pH, the formation of M-S-H and brucite is observed experimentally. Less brucite is present at 50 than at 20 °C indicating a slowdissolution kinetic of brucite at 20 °C. Thermodynamic modelling is in good agreement with the changes in C-S-H and M-S-H formation in the presence of MgO although no brucite is predicted. M-S-H is observed in the presence of both MgCl2 and MgO, is stable from pH 7.5 to 11.5 and has a structure comparable to the structure of M-S-H prepared from MgO and SiO2 only.

Domaines: Chimie

Ellina Bernard, Barbara Lothenbach, Fabien Le Goff, Isabelle Pochard, Alexandre Dauzeres. Effect of magnesium on calcium silicate hydrate (C-S-H). Cement and Concrete Research, 2017, 97, pp.61-72. ⟨10.1016/j.cemconres.2017.03.012⟩. ⟨hal-02065347⟩