Composition, morphology and nanostructure of C–S–H in 70% white Portland cement–30% fly ash blends hydrated at 55 °C.

Outer product C–S–H had a mixture of fibrillar and foil-like morphology in a 28-day-old water-activated paste, and foil- or lath-like morphology in an alkali-activated paste. It was not possible to determine the chemical composition of C–S–H using SEM-EDX because of fine-scale intermixing with other phases; TEM-EDX was necessary. The C–S–H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water. The mean length of the aluminosilicate anions in the C–S–H was similar in both systems and increased with age; those in the Op C–S–H were likely to be shorter than those present in the Ip C–S–H with water activation, but longer (and more protonated) with alkali. The potassium in the alkali-activated paste was present either within the C–S–H structure charge balancing the substitution of Al3+ for Si4+, or adsorbed on the C–S–H charge balancing sulfate ions.