High resolution quasi-elastic neutron scattering is used to investigate the slow dynamics of hydration water confined in calcium silicate hydrate gel in an aged cement paste at supercooled temperatures. A super-Arrhenius to Arrhenius dynamic crossover of the average translational relaxation time 〈 τ 〉 as a function of the inverse temperature is observed at TL = 231 ± 5 K, which coincides with a prominent peak in the differential scanning calorimetry cooling scan. The dynamic susceptibility chiT(t) calculated using the experimentally determined temperature dependence of the self-intermediate scattering function shows direct evidence of the enhanced dynamic fluctuations and the associated growth in size of the dynamic heterogeneity in the confined water on approaching TL.