Cement and Concrete Reference Laboratory cement proficiency sample 152 [12], issued in January of 2004, was used to assess the hydration rates of cement pastes cured under saturated and sealed conditions. Cement pastes with w/c=0.35 and w/c=0.45 were prepared by mixing the water and cement in a temperature-controlled high speed blender for several minutes at 20 ºC. For the water-to-solids mass ratio (w/s) of 0.45, blended cements with either a fine or a coarse limestone powder replacing 20 % of the cement by mass were also prepared. The fine and coarse limestone were obtained by classifying a commercially available material with a cutoff diameter of approximately 30 µm [13]. Based on its measured loss on ignition, the limestone powder was estimated to be 97 % CaCO3. Wafers (about 3 g to 5 g) of cement paste were placed in small capped plastic vials and cured under either saturated (water ponded on top) or sealed conditions at 20 ºC. At ages of (1, 3, 7, 28, and 92) d, specimens were removed from their vials, crushed to a fine powder, flushed with methanol, and divided between two crucibles. The non-evaporable water content (WN) of each crucible sample was determined as the mass loss between 105 ºC and 1000 ºC divided by the mass of the ignited sample, corrected for the loss-on-ignition of the dry original (blended) cement powder. Previously, the expanded uncertainty in the calculated WN has been estimated to be 0.001 g/g cement, assuming a coverage factor of 2 [2]. WN values were converted to estimated degrees of hydration based on the calculated Bogue composition of the cement and published coefficients for the non-evaporable water contents of the various cement clinker phases [14]. Further experimental data on the influence of w/c on the degree of hydration of cement pastes was taken from the literature [2, 4, 15-17].