Publication
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