Differential Scanning Calorimetry: 100 % RH Exposure

The DSC data for the SWVE specimens is considered first as a basis for analyzing the specimens exposed to 90 % RH. The DSC data for the SWVE specimens are shown in Figures 4-6 for the three w/c values. Each figure shows a series of curves, each with a label indicating the age of the specimen. The curves are offset vertically from one another and plotted on the same ordinate scale to facilitate direct comparison of both the relative peak heights and the corresponding freezing temperature.

Each figure gives a qualitative picture of microstructure formation during hydration under 100 % RH conditions, indicated by the three peaks that may appear during hydration. Figure 4 shows that, chronologically, the first peak appears (during the first few hours) at -20 ºC, and is due to water in capillary pores. Figures 5 and 6 reveal that the next peak forms (after 1 d hydration) at -45 ºC and is due to water freezing in pores with entryways only few nanometers wide. The last peak forms near -30 ºC and is due to pores of an intermediate size that will be discussed subsequently. For this last peak at -30 ºC, the time of appearance, the time of subsequent disappearance, and the freezing temperature appear to be a function of the w/c value (initial solids fraction). As mentioned previously, because the pore size distribution in cement paste is unimodal [15], the presence of distinct peaks can only be attributed to the formation of reservoirs, composed of relatively large pores, completely surrounded by passageways composed of relatively smaller pores.

Figure 4: Freezable water, evident as heat flow (W/kg) from the sample, as a function of age for a 0.30 w/c cement paste cured at 100 % RH. The data labels indicate the age of the specimen since the time of casting. Individual curves are offset for comparison purposes.

Figure 5: Freezable water, evident as heat flow (W/kg) from the sample, as a function of time for a 0.40 w/c cement paste cured at 100 % RH. The data labels indicate the age of the specimen since the time of casting. Individual curves are offset for comparison purposes.

Figure 6: Freezable water, evident as heat flow (W/kg) from the sample, as a function of time for a 0.50 w/c cement paste cured at 100 % RH. The data labels indicate the age of the specimen since the time of casting. Individual curves are offset for comparison purposes.

Figures 4-6 also include estimates for the corresponding equilibrium relative humidity, shown along the upper x-axis. The relationship between freezing temperature and equilibrium relative humidity has been calculated by Fagerlund [25]. Based on the Fagerlund derivation, to evaporate water from pores with a freezing peak at -30 ºC, the external relative humidity at 25 ºC would have to fall below 75 % RH. This relation, however, is only a physically motivated approximation. Moreover, because the peaks indicate the size of the pores surrounding the reservoirs, the relative humidity corresponds to the equilibrium RH for the smaller pores surrounding the reservoir, and not the equilibrium RH for the reservoir itself, which would be a higher RH.

An important feature of these DSC results for 100 % RH exposure specimens is the time-dependent behavior of the freezing peak at -30 ºC. This peak forms after the peak at -45 ºC, indicating that the pore throats that freeze at -30 ºC are the result of hydration. Moreover, the time at which the peak at -30 ºC forms and disappears is a function of the w/c value (initial solids fraction). The appearance of the -30 ºC peak occurs, roughly, near the age at which the capillary pores are assumed to be no longer percolated (approx. 20 % capillary porosity). The disappearance of the -30 ºC peak occurs after 2 d and 7 d hydration for the 0.30 w/c paste and the 0.40 w/c paste, respectively. For the 0.50 w/c paste, the -30 ºC peak formed between 7 d and 14 d, and remained beyond 28 d.

In general, these results are consistent with previous results of Bager and Sellevold [26,27] on cement pastes aged for four months prior to DSC measurements. In that experiment, the freezing peaks located at -25 º C and -45 ºC correspond to the -30 ºC and -45 ºC peaks, respectively, seen here; the difference is attributed to the slower freezing rate in the Bager and Sellevold experiment. The Bager and Sellevold data also show an absence of a -25 ºC (corresponding to -30 ºC peak here) for 0.35 w/c and 0.40 w/c pastes at 4-months age. Similarly, the same peak remained after 4 months for pastes having a w/c equal to or greater than 0.45.