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In the present work, the pore structures of the cement paste and mortar at different degrees of hydration were measured by mercury intrusion porosimetry (MIP). Each type of mix was measured in duplicate at five different degrees of hydration between 45 and 70 %. Hearn and Hooton  showed that using crushed samples can introduce an external porosity peak and suggested the use of small, saw cut prisms as an alternative. Specimens for MIP were cut from the cylinder cured in water at the time corresponding to the estimated degree of hydration, and then stored in isopropyl alcohol to remove the free water. The solvent was replaced after the first three days and storage in solvent was continued for several weeks. One month before the MIP measurements, the samples were removed from the isopropyl alcohol and dried to a constant mass in a vacuum oven at 55ºC.
The critical radius was taken to be the inflection point on the volume intrusion versus radius curve (or the maximum of the dV/dp curve). Katz and Thompson  suggested that this point corresponds to the smallest pore size diameter of the subset of the largest pores which creates a connected path through the sample. Figures 1 and 2 demonstrate the changes of the critical radius of paste and mortar samples with hydration as determined by MIP.
Figure 1: Critical pore radius changes with continuing hydration for 0.40 w/c ratio paste and mortars as determined by MIP.
Figure 2: Critical pore radius changes with continuing hydration for 0.50 w/c ratio paste and mortars as determined by MIP.