The internal relative humidity of the hydrating cement pastes was evaluated using a Rotronic Hygroscope DT 2 equipped with four measuring cells . Before and after each experimental run, the equipment was calibrated using salt solutions with RHs between 75 % and 100 %. The Rotronic equipment is contained in a thermostatically controlled box with the temperature controlled to within ± 0.1 ºC.
The fresh cement paste was simply placed in a sample holder that fit tightly into the sealed Rotronics cell. Typically, the volume of cement paste placed in the holder was a cylinder 42 mm in diameter and about 7 mm high. For most experiments, the freshly mixed paste was placed directly into the Rotronics cell, and the change in relative humidity monitored immediately. However, for one experiment (with the 254 m2/kg cement), to assess the contribution, if any, of bleeding to the RH readings, a companion sealed specimen was first rotated for 12 h (well beyond the setting time when any bleeding would cease) and then crushed and placed in a measuring cell. Bleeding was a concern for these experiments because of the fact that any expansion or lack of shrinkage observed for the coarser cements might possibly be due to the readsorption of bleed water by the hydrating cement paste. The readings for this rotated/crushed specimen were virtually identical to those obtained on the virgin specimen, suggesting that, as expected, little or no bleeding was occuring for these w/c=0.35 cement paste specimens. The maximum standard deviation in RH between readings made on companion specimens was 0.48 %, with average standard deviations for the four finenesses being in the range of 0.07 % to 0.23 %.