Calcium silicate hydrate (C-S-H) is the main hydrate phase in Portland cements. Its composition
varies depending on the Ca/Si ratio and on the presence of other ions such as aluminium (called CA-
S-H) or alkalis.
This thesis aimed to study the composition and the structure of C-S-H. An experimental data base
was realised on the effect of different Ca/Si and Al/Si ratios, alkali concentrations (potassium and
sodium hydroxide) and of different temperatures on C-S-H after equilibration times up to 1 year.
The composition of the solid phases was analysed by TGA, XRD/Rietveld analysis and 29Si and
27Al MAS NMR. The solutions were analysed by ionic chromatography and pH measurements.
In the absence of alkali, only C-A-S-H is formed at Al/Si ≤ 0.1. At higher Al/Si ratios, katoite,
stratlingite and/or AH3 precipitate in addition to C-A-S-H, such that a part of the aluminium is
consumed and limits the Al/Si ratio in C-S-H to 0.15±0.05 regardless of the Ca/Si ratios. A strong
correlation between the aqueous aluminium concentration and the aluminium uptake in the solid
phase is observed. Aluminium in C-S-H is observed mainly as tetrahedrally, Al(IV) or octahedrally
coordinated, Al(VI) aluminium. At high Ca/Si ratios, Al(VI) is favoured and the aluminium uptake
in C-S-H increases with the aqueous aluminium concentration. At low Ca/Si ratio (≤ 0.8),
aluminium is observed mainly as Al(IV) and the low aqueous aluminium concentrations (below
detection limit) indicate a higher affinity of aluminium towards C-S-H. The uptake of aluminium
increases slightly the mean basal spacing slightly at low Ca/Si ratio and up to ≈2Å at higher Ca/Si
ratios due to incorporation of aluminium and calcium in the interlayer.
The presence of alkali hydroxide increases the pH leading (i) to a destabilisation of stratlingite and
(ii) to higher aqueous silicon and aluminium concentrations and to lower calcium concentrations.
As the aluminium uptake in C-S-H is strongly related to the aqueous aluminium concentrations, the
aluminium uptake in the C-S-H increases significantly in the presence of alkali hydroxides, as
aluminium is more soluble at higher pH.
The increase of the temperature from 7 to 80 °C increases the phase-purity and long-range order of
the C-S-H. The mean chain length remains stable up to 50°C, while at 80°C chain length increases
and crosslinking is observed in the presence of aluminium. At all temperatures studied, the presence
of aluminium has little effect on the calculated solubility of C(-A)-S-H.
The incorporation of alkali ions in C-S-H increases with the initial concentration of alkali and with
the decrease of Ca/Si ratio. The aqueous calcium concentration play a dominant role in alkali
uptake, as calcium competes with the alkali ions to charge balance the negatively charged silanol
groups of the C-S-H. A low concentration of calcium favours the alkali uptake in C-S-H. The
uptake of alkali ions shortens the silica chain length as calcium is redistributed from the interlayer
to the main sheet. The alkali uptake is not significantly influenced by the presence of aluminium in
C-S-H, the hydration time or the type of cation (sodium or potassium).
EPFL2014
