The role of magnesium during the hydration of C3S and C-S-H formation. Scanning electron microscopy and mid-infrared studies

Mid-infrared (IR) spectra of a series of calcium silicate hydrates (C-S-H) gels synthesised from C3S in the presence of magnesium oxide show many similarities with the mid-IR spectra of pure C-S-H gels. However, the presence of the vibration bands attributed to brucite is observed in all spectra of C-S-H synthesised with MgO, although only the sample with the highest CaO/SiO2 ratio (1·8) reveals a large shoulder centred at 530 cm−1, which has been assigned to the presence of magnesium with a four-fold coordination inserted in the silica chains of the C-S-H gel. The postulated formation of magnesium silicate hydrates (M-S-H) gel was detected by scanning electron microscope and energy dispersive analysis of X-rays in some areas of the C-S-H gel with the lowest CaO/SiO2 ratio (0·66).