Differential Scanning Calorimetry: 90 % RH Exposure- Suspended hydration and loss of freezable water in cement pastes exposed to 90 % relative humidity

The DSC data for the samples exposed to 90 % RH are shown in Figures 7-9. Each figure is composed of four graphs, one for each initial exposure time at 100 % RH prior to exposure to 90 % RH, and this age is shown at the top center of the graph.

The 6 h and the 12 h data are quite similar for all three values of w/c. The peak at -45 ºC either never forms or forms and subsequently disappears. For those specimens which the peak does occur, the age at which it appears depends upon the w/c value. The peak at -30 ºC never forms for any 6 h or 12 h specimen, regardless of the w/c value.

**Figure 7:** Freezable water, evident as heat flow (W/kg) from the sample, as a function of time for 0.30 w/c cement paste exposed to 90 % RH after curing at 100 % RH for 6 h, 12 h, 3 d, and 7 d. Individual curves are offset for comparison purposes.
$\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc30_6h}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc30_12h}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc30_3d}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc30_7d}$

**Figure 8:** Freezable water, evident as heat flow (W/kg) from the sample, as a function of time for 0.40 w/c cement paste exposed to 90 % RH after curing at 100 % RH for 6 h, 12 h, 3 d, and 7 d. Individual curves are offset for comparison purposes.
$\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc40_6h}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc40_12h}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc40_3d}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc40_7d}$

**Figure 9:** Freezable water, evident as heat flow (W/kg) from the sample, as a function of time for 0.50 w/c cement paste exposed to 90 % RH after curing at 100 % RH for 6 h, 12 h, 3 d, and 7 d. Individual curves are offset for comparison purposes.
$\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc50_6h}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc50_12h}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc50_3d}$ $\includegraphics[width=0.48\textwidth,angle=0]{GRAPHS/dsc50_7d}$

For the 0.30 w/c and the 0.40 w/c pastes, the DSC data for 3 d and 7 d exposure to 90 % RH were as expected based on the data from 100 % RH expsosure specimens. The -30 ºC peak never appears in the 3 d or the 7 d 0.30 w/c paste because it had already disappeared by this time in the 100 % RH exposure specimen. The disappearance of the -30 ºC peak in the 3 d and 7 d 0.40 w/c pastes is consistent with the behavior of the SWVE specimens.

The 3 d and 7 d exposure data for the 0.50 w/c paste revealed an effect of exposure on microstructural development. For the SWVE 0.5 w/c specimens, the -30 ºC peak appears after 14 d hydration. For samples exposed to 90 % RH at 3 d and 7 d, the peak does not appear before the end of testing at 28 d.

Although not as dramatic, the peak at -45 ºC peak appears to diminish and drift to higher temperatures over time for many of the exposed specimens. This effect is noticeable for the 6 h and 12 h exposures, and is arguable for the 3 d and 7 d specimens. Bager and Sellevold [26] observed similar effects, with the peak area decreasing noticeably for exposures below 90 % RH, and disappearing entirely for exposures below 58 % RH exposure.

Regretfully, the mass of the specimens was not determined to a meaningful precision. Very small pieces of paste would stick to the mold and fall off during handling. Minor revisions to the experimental setup are needed to avoid this problem.