Influence of calcium to silica ratio on aluminium uptake in calcium silicate hydrate

Abstract Aluminium uptake in calcium silicate hydrate (C–S–H) was studied at different Ca/Si ratios from 0.6 to 1.6 at 20 °C. Aluminium incorporation in C–S–H depends on the total amount of aluminium present in the sample. At low Al/Si ratio (≤ 0.05), the major part of the aluminium is taken up in C–S–H while at higher Al/Si ratio, the precipitation of stratlingite and/or katoite limits the Al/Si ratio in C–S–H to ≈ 0.15, regardless of the Ca/Si ratio. A strong correlation between the aqueous aluminium concentration and the aluminium uptake in the solid phase is observed. At high Ca/Si ratios, aluminium in C–S–H is observed mainly as octahedrally coordinated Al(VI) in TAH and the aluminium uptake increases with the aqueous aluminium concentration. At low Ca/Si ratio (≤ 0.8), aluminium is observed mainly as tetrahedrally coordinated Al(IV) and low aqueous aluminium concentrations (below detection limit) indicate a high affinity of aluminium towards C–S–H.