In contact with natural waters, concrete can be exposed to the action of magnesium. Mg-attack on the cement paste leads to dissolution of cementitious phases and formation of brucite, hydrotalcite and/or magnesium silicate hydrates (M-S-H). The knowledge of the properties of the latter is limited. In binders with supplementary cementitious materials, M-S-H formation is favoured over that of brucite because of the lower contents of portlandite. To investigate the effects of a magnesium attack on such binders, a low Ca/Si model cement paste was immersed in a 5 mmol/L MgCl2 solution for several months. Energy-dispersive spectrometry analyses coupled with X-ray diffraction and electron probe microanalyses showed Ca leaching, Si preservation and Mg enrichment of the altered zones of the pastes corresponding to the dissolution of C-S-H and formation of M-S-H and amorphous silica. The Mg-enriched zone showed a lower residual Young's modulus than the sound zone as measured by microindentation.