Magnesium and calcium silicate hydrates, Part I: Investigation of the possible magnesium incorporation in calcium silicate hydrate (C-S-H) and of the calcium in magnesium silicate hydrate (M-S-H)

Calcium silicate hydrate (C-S-H) and magnesium silicate hydrate (M-S-H) have been described as two separate phases. This work investigates their stability domains and the possible uptake of small amounts of magnesium by C-S-H or, reversely, the uptake of small amounts of calcium by M-S-H. The phases, synthesized by co-precipitation, are characterized using a large panel of techniques (thermogravimetry analysis, X-ray diffraction, X-ray pair distribution function analysis, 29Si MAS-NMR spectroscopy, measurement of zeta potential and cation exchange capacity) while the compositions of the solutions at equilibrium are determined by ion chromatography and pH measurements. Syntheses of C-S-H samples in the presence of magnesium ((Ca + Mg)/Si = 0.80 and Mg/Si = 0.05 or 0.10) yield two separate phases: C-S-H and M-S-H. There is no experimental evidence of any uptake of magnesium by C-S-H. On the contrary, when M-S-H samples are synthesized in the presence of calcium ((Ca + Mg)/Si = 0.80 and Ca/Si = 0.05 or 0.10), the low pH of the suspension (9-10) prevents the formation of C-S-H but favors the precipitation of M-S-H with small amounts of calcium. This latter may be sorbed onto the surface of M-S-H to outbalance its negative charges and/or incorporated into the interlayer, as suggested by small structural changes.