The degrees of hydration measured using the loss-on-ignition technique are summarized in Figs. 3 and 4, for the w/c= 0.35 and w/c=0.435 cement pastes, respectively. Three major observations can be taken from the figures. First, a higher degree of hydration is ultimately achieved in the w/c=0.435 pastes, due both to their higher water content and to the presence of more space available for the precipitation and growth of hydration products. Second, for both w/c values, sealed curing results in a lower degree of hydration at later ages.21 As would be expected, this decrease is more significant for the lower w/c cement paste. Third, in terms of achieved degree of hydration, the sealed/saturated curing condition, in which the cement pastes were cured under sealed conditions for 7 d and then saturated on top, appears to provide equivalent performance to the saturated curing condition for the thin cement paste specimens employed in this study. Any differences between the saturated and the sealed/saturated curing condition results are well within the experimental error (±0.004) in the LOI technique. This third observation leads to the natural question: does equivalency in achieved degree of hydration correspond to equivalency in the microstructure developed during that (globally equal) amount of hydration? The answer is provided in the SEM and LTC analyses to be presented next.
Figure 3− Degree of hydration via LOI technique vs. time for w/c=0.35 cement paste cured under sealed, sealed/saturated, and saturated curing conditions at 20 ºC.
Figure 4− Degree of hydration via LOI technique vs. time for w/c=0.435 cement paste cured under sealed, sealed/saturated, and saturated curing conditions at 20 ºC.