Effects of MgSO₄ on calcium-silicate-hydrate

Magnesium sulfate (MgSO₄) is a salt that has the potential to damage portland cement-based materials. This paper argues that the mechanism of damage is dependent on the concentration of MgSO₄ and reports the results of the change in the micromechanical properties of calcium-silicate-hydrate (C-S-H) when exposed to different MgSO₄ concentrations . Different concentrations were selected starting from the concentration found in seawater and increasing up to 20 g/l, which is the concentration considered for accelerated tests. The micromechanical properties of C-S-H were probed using nanoindentation; X-ray diffraction measurements were also performed on C-S-H specimens. A Mori-Tanaka homogenization scheme was employed to upscale these results, yielding two homogenization levels. At low MgSO₄ concentrations (2.2 and 4 g/l) the formation of brucite and gypsum crystals in the pore solution contributes to the overall magnitude of the elastic modulus of the specimen, while at higher concentrations of MgSO₄ (10 and 20 g/l), decalcification of C-S-H is observed, which results in the degradation of C-S-H micromechanical properties.