Magnesium and calcium silicate hydrates, Part II: Mg-exchange at the interface "low-pH" cement and magnesium environment studied in a C-S-H and M-S-H model system

"Low-pH" cementitious materials have been developed in the context of nuclear wastes encapsulation to reduce the alkaline plume and the early hydration heat causing shrinkage of the Portland cement based-materials in contact with clayey rocks. This study follows the evolutions at an interface between calcium silicate hydrate (C-S-H) with a Ca/Si ratio of 0.8 and magnesium silicate hydrate (M-S-H) with a Mg/Si ratio of 0.8, while controlling the pore solution by using reservoirs. In a first step a simplified "low-pH" binder was mimicked by C-S-H with a low Ca/Si in the presence of magnesium. In a second step the impact of calcium on pure M-S-H was studied. Secondary electron microscopy observations show the fast deterioration of the C-S-H and the precipitation of M-S-H in the C-S-H disk and an uptake of calcium in the M-S-H disk together with a change of the reservoir compositions including pH values. The reactive transport modelling is in good agreement with the changes in both the solid phases and the composition of the solution reservoirs.