  The incorporation of nanosilica in cementitious systems helps in accelerating
the cement hydration process and early-stage formation of the main hydration
product, that is, calcium silicate hydrate. Nanosilica reacts with the other
key hydration product, calcium hydroxide to form additional calcium silicate
hydrate. In this work, Terahertz spectroscopy has been employed to study the
variation in hydration kinetics of the main constituent of cement, Tricalcium
Silicate, due to the addition of different percentages of nanosilica. The
variation has been analyzed in terms of change in the absorbance as well as a
shift in the wavenumber of the key resonances. The resonance around 520, 514
cm-1, due to symmetric-asymmetric bending of silicate tetrahedral units,
decreases in intensity very rapidly as hydration progresses and more so for the
nanosilica incorporated samples demonstrating accelerated hydration.
Interestingly, the resonance around 451 cm-1, due to symmetric and asymmetric
bending of silicate tetrahedral units along with some contribution of
vibrational modes of calcium oxide polyhedral units, does not show a similar
decrease in intensity. The prominence of the resonance around 451 cm-1 even
after the onset of hydration is found to be mostly due to the contribution from
the vibrational modes of the newly formed silicates tetrahedra chains of
calcium silicate hydrate. Early-stage formation of calcium silicate hydrate due
to nanosilica incorporation has been analyzed by tracking the shift in
resonances as polymerized chains of silicates are formed. These results have
been corroborated with Scanning Electron Microscopy results which clearly show
the early changes in the morphology of the hydrated C3S due to nanosilica
incorporation.
