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Figure 1 shows a comparison of degree of hydration achieved under saturated and sealed conditions for the two cements studied, with w/c=0.3. In both cases, the sealed conditions are observed to reduce the achieved hydration, particularly at times exceeding 7 days (168 hours). At early times, the newly created empty porosity (from the chemical shrinkage) has only a minor effect on the hydration kinetics as a significant volume fraction of water-filled capillary pore space still exists. For a w/c=0.3 system, initially, about one half of the volume is filled with water. However, as more and more water is consumed by the hydration reactions, the fraction of empty pore space relative to that which is water-filled becomes significant, and a substantial reduction in the hydration rate is observed. The predicted results based on the NIST cement paste microstructural model are seen to closely follow the experimental values, particularly for Cement 115. Thus, it appears that the model is able to account successfully for the effects of self-dessication on hydration kinetics and may be useful as a predictive tool for anticipating the effects of different curing regimens on achievable hydration and physical properties, as the model has already been applied to predicting 7 and 28-day compressive strengths from 3-day values [14].
It should also be noted that simulations performed for w/c=0.4 for the two cements have indicated much smaller differences in the hydration kinetics for saturated and sealed conditions. For Cement 115, the ``final'' achieved degrees of hydration after 5000 cycles of the hydration model were 0.88 and 0.83 for the saturated and sealed conditions, respectively. The corresponding values for Cement 116 were 0.90 and 0.85. In addition, the differences to be expected after 28 days of curing, for example, were less than for the w/c=0.3 systems (on the order of 0.01 for Cement 115 and 0.03 for Cement 116). Thus, the effects of sealed curing become more prominent at lower w/c (< 0.4) ratios.
Figure 1a: Comparison of saturated and sealed (self-dessicated) hydration for
Cement 115 with w/c=0.3.
Figure 1b: Comparison of saturated and sealed (self-dessicated) hydration for Cement 116 with w/c=0.3.