Magnesium perturbation in low-pH concretes placed in clayey environment—solid characterizations and modeling

In the framework of the CI project, the interaction of two different low-pH cements (ESDRED and LAC) with Opalinus Clay (OPA) at Mont-Terri rock laboratory was studied. SEM-EDS analyses showed after 2.5 and 5 years of interaction a Mg-enriched zone associated with a decalcification of the C-S-H near the OPA contact zone of both cements. TGA and XRD investigations indicate that hydrotalcite and brucite predict classically in modeling are not formed. 29Si NMR and FTIR analyses showing a high polymerization degree of the Si-network typical of a sheet-like structure as observed for M-S-H. TEM/EDS investigations of the Mg enriched zone of both concretes indicate that the formed Mg phase exhibits a gel-like structure. New reactive transport modeling with the HYTEC code corroborates the formation of M-S-H at the interface.