Quasi-elastic neutron scattering (QENS) was used to monitor the state of water in portland cement and tricalcium silicate pastes during the first 2 days of hydration at three different temperatures. By applying a double-Lorentzian rather than a single-Lorentzian fitting function, the QENS signal from water at a given hydration time was divided into three separate populations arising from liquid water, chemically bound water, and constrained water. The constrained water population consisted of water adsorbed on surfaces and contained in very small (< 10 nm) pores, and could he associated primarily with the calcium-silicate-hydrate (C-S-H) phase. The rate of increase in the chemically bound water population closely followed the exothermic heat output, while the constrained water population increased more rapidly during the first several hours of hydration and then leveled off.