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)
Résumé
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, Xray
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.
Domaines
Chimie
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