Effect of magnesium on calcium silicate hydrate (C S H)
The addition of MgCl2 to synthetic calcium silicate hydrate (C S H) with Ca/Si 0.8 lowers the pH values below 10. Experimental observations and thermodynamic calculations show the destabilization of C S H and the precipitation of magnesium silicate hydrate (M S H) In contrast, the addition of MgO to C S H increases the pH values up to 11.5 and the Ca/Si in C S H from 0.8 to 1.0. At high pH, the formation of M S H and brucite is observed experimentally. Less brucite is present at 50 than at 20 degC indicating a slow dissolution kinetic of brucite at 20 degC. Thermodynamic modelling is in good agreement with the changes in C S H and M S H formation in the presence of MgO although no brucite is predicted. M S H is observed in the presence of both MgCl2 and MgO, is stable from pH 7.5 to 11.5 and has a structure comparable to the structure of M S H prepared from MgO and SiO2 only.