LAUSR

Abstract The MgCl2 induced material degradation is the major durability issue facing the cement-based system. In this study, the degradation mechanism induced by MgCl2 attack at nanoscale is investigated for cement paste exposed to MgCl2 solution with different concentrations for 28 and 180 days by combing X-ray diffraction (XRD), 29Si and 27Al Nuclear Magnetic Resonance Spectroscopy (NMR), Scanning electron microscope-energy dispersive spectrometer (SEM-EDS) and thermodynamic modeling. The XRD, SEM-EDS and NMR results show that the cement paste exposed to 5 ~ 15 wt% MgCl2 cause disintegration of the main hydration product, calcium aluminosilicate hydrate (C–A–S–H) gel, portlandite, along with the formation of extensive Friedel’s salt, brucite precipitation and the small amount of ettringite. The phase transformation is closely related with the polymerization of the C–A–S–H gel and the de-aluminization characterized by the increasing mean silicate chain length and reduction of the Al[4]/Si ratio in the low calcium environment induced by MgCl2 attacking. Additionally, the predicted phase diagram from thermodynamic modeling matches well with the experimental analysis and gives a supplement support to the phase transformation. By correlating the phase composition change and the local structure of aluminate species and polymerization evolution, this study provides molecular insight on the fundamental MgCl2 induced degradation reactions happening in cement paste.