Degree of Hydration

The degree of hydration ($\alpha$) data are plotted in Figures 1-3 for each w/c value. In each figure there are four sets of data corresponding to the four curing periods at 100 % RH before subsequent exposure to 90 % RH. Measurements were performed in duplicate pairs and each datum is plotted on the figure. In many cases, the difference between the measured values was so small that the symbols overlap one another. Additionally, the uncertainty in the individual measurements was sufficiently small, typically less than two percent, that the presence of error bars in the figure would have been obscured by the datum symbol.

In addition to the measured DOH, in each figure there appears a solid curve, a horizontal dashed line, and a vertical dashed line. The solid curve is the DOH for SWVE specimens. The curve represents a best fit using a parabolic-hyperbolic equation [16,17,18], and the individual data have been omitted for clarity. This curve was corroborated by independent experiments using calorimetry on the same cement at the same w/c values [19,20]. The horizontal dashed line labeled "20 %" indicates the DOH at which the estimated capillary porosity is 20 %, and the capillary pores are assumed to be no longer percolated. The criterion of 20 % capillary porosity was chosen as a compromise among published values that range from 18 % to over 22 % [21,22,23]. The corresponding DOH $\alpha$ at which the porosity $\theta$ equals 0.20 can be estimated using an equation that is based upon the approach of Young and Hansen [24], and modified to account for multiple clinker phases:

$$\theta = 1 - \frac{1+1.15\alpha}{1+3.15 (w/c)}$$ (2)

The vertical dashed line labeled "DSC" denotes the age when the DSC freezing peak at -30 ºC first appears in a SWVE samples.

In Figure 1 for the 0.30 w/c specimens, there is a second dashed curve just below the solid curve. This second curve is the degree of hydration for a specimen in which the fresh paste is immediately placed inside a sealed container with no additional water (sealed specimen). This curve was generated from data reported elsewhere [19,20]. The curve for sealed specimens is not visible for the 0.40 w/c and the 0.50 w/c in Figs. 2 and 3 because it overlaps the solid curve. At these larger values of w/c, the initial volume of water within the specimen is sufficient to hydrate the specimen at a rate indistinguishable from a saturated specimen for at least 28 d.

As can be seen in Figs. 1-3, for all three w/c values, the 6 h and the 12 h specimens continue to hydrate for the first 2 d to 3 d, and then appear to almost cease hydrating. During the initial 2 d to 3 d exposure to 90 % RH, the specimens evaporate free water and come to equilibrium with the 90 % RH environment. This result is dramatic given that a 90 % RH exposure is not typically considered severe.

Figure 1: Degree of hydration $\alpha$ versus time t since mixing for 0.30 w/c pastes exposed to 90 % RH after curing at 100 % RH for 6 h, 12 h, 3 d, and 7 d. The solid curve represents the degree of hydration for a continuously saturated specimen, and the dashed curve represents the degree of hydration for a sealed specimen. The horizontal dashed line labeled 20 % refers to the capillary porosity as estimated from Eqn. 2. The vertical dashed line labeled DSC denotes the age at which the -30 º C peak first appears.

Figure 2: Degree of hydration $\alpha$ versus time t since mixing for 0.40 w/c pastes exposed to 90 % RH after curing at 100 % RH for 6 h, 12 h, 3 d, and 7 d. The solid curve represents the degree of hydration for a continuously saturated specimen. The horizontal dashed line labeled 20 % refers to the capillary porosity as estimated from Eqn. 2. The vertical dashed line labeled DSC denotes the age at which the -30 ºC peak first appears.

Figure 3: Degree of hydration $\alpha$ versus time t for 0.50 w/c pastes exposed to 90 % RH after curing at 100 % RH for 6 h, 12 h, 3 d, and 7 d. The solid curve represents the degree of hydration for a continuously saturated specimen. The horizontal dashed line labeled 20 % refers to the capillary porosity as estimated from Eqn. 2. The vertical dashed line labeled DSC denotes the age at which the -30 ºC peak first appears.

The 3 d and 7 d specimens exhibit different behavior from the 6 h and 12 h specimens. For all three values of w/c, the UWVE 7 d specimens continue to hydrate at a rate indistinguishable from a SWVE specimen. Although the 3 d specimens continue to hydrate, their rate of hydration is less than that of SWVE specimens. Of all the 3 d specimens, only the 0.30 w/c paste continued to hydrate at a rate greater than a companion sealed specimen.